Title	:	FDA Responds to Citizen Petition on BSTFood and Drug Admin
Source	:	Kansas State University
Author	:	Harner, Murphy, Smith
Date	:	4/21/2000
Content	:

FDA Responds To Citizen Petition On BST

U.S. Food and Drug Administration Center For Veterinary Medicine April 21, 2000 SOURCE: http://www.fda.gov/cvm/CVM_Updates/cpetup.html

On April 20, 2000, FDA responded to a Citizen Petition (Docket No. 99P-4613) from Mr. Robert Cohen concerning Posilac^®, the only FDA-approved recombinant bovine growth hormone (rbGH) product for increasing milk production in dairy cattle. FDA’s Center for Veterinary Medicine (CVM) approved Monsanto Company’s rbGH product, Posilac in November 1993 after a comprehensive review of the product’s safety and efficacy, including human food safety.

The petition requested that FDA rescind the approval of Posilac, and immediately remove it from the market based on “new evidence” that the product poses “serious health consequences for human consumers.” Later, Mr. Cohen amended this petition, most recently on December 2, 1999. As amended, the petition raised three primary issues in support of the request for withdrawal of Posilac. These issues are as follows: (1) that a recently reported increase in serum levels of insulin-like growth factor-1 (IGF-I) in humans following milk consumption represents absorption of dietary IGF-I, invalidating a basic premise of FDA’s safety assessment and proving that IGF-I in milk represents a hazard to human health; (2) that Monsanto changed the manufacturing process for rbGH after the studies supporting the New Animal Drug Application (NADA) were completed, thereby invalidating the research used to support the approval; and (3) that the 90-day toxicology study and/or the information derived from the additional 90 days of the study demonstrate both that rbGH is absorbed and that it is not safe.

In response to Mr. Cohen’s petition, FDA said that the Agency believed that these arguments do not demonstrate any human food safety issue related to the use of Posilac. Therefore, the petition requesting withdrawal of the approval of Posilac was denied.

FDA provided detailed scientific information in response to Mr. Cohen’s Citizen Petition. Highlights from the Agency’s response to the petition are as follows:

(1) The safety of IGF-I

FDA has previously maintained and continues to maintain that levels of IGF-I in milk, whether or not from rbGH supplemented cows, are not significant when evaluated against levels of IGF-I endogenously produced and present in humans. IGF-I is normally found in human plasma at concentrations much higher than those found in cow’s milk. Reported percentage increases in IGF-I concentrations in milk of rbGH supplemented cows can be misleading because the levels of IGF-I in milk are so low prior to any increase. IGF-I is a normal, but highly variable, constituent of bovine milk with the concentration depending on the animal’s stage of lactation, nutritional status, and age. While some studies indicate that levels of IGF-I may statistically increase in the milk of rbGH supplemented cows relative to unsupplemented cows, reported increases are still within the normal variation of IGF-I levels in milk. The Agency pointed out that even if all of the IGF-I in milk was absorbed, and there is insufficient evidence that it would be, the levels of IGF-I in human plasma would not rise by 1%.
(2) The manufacturing process for rbGH
FDA was fully aware of the change in the manufacturing process prior to approval of Posilac, and the Agency believed that the change did not result in a different product such that the research done with the product prior to the manufacturing change was invalid. However, to reaffirm that the conclusion the Agency reached in this case was correct, FDA re-examined information previously submitted by Monsanto to support the approval of the rbGH. We also made a site visit to the sponsor to examine batch records that are not required to be submitted to the new animal drug files. Based on this examination, FDA reaffirmed its conclusion that the manufacturing changes resulted in only biologically inconsequential variations in the product used in the safety and effectiveness studies, and therefore, the rbGH product we approved is the same as the product used in the studies.
(3) The fate and effects of rbGH in milk
Like most dietary proteins, rbGH is degraded by digestive enzymes in the gastrointestinal tract and not absorbed intact.In the response, FDA discussed in depth a study that was conducted by Richard, Odaglia, and Deslex, where rats were administered rbGH by oral gavage or subcutaneous injection. The Agency reiterated that no adverse effects of rbGH were observed following 90 continuous days of oral administration or following an additional 90 days of recovery after the cessation of drug administration in this study.

An electronic copy of FDA’s response to Mr. Cohen’s petition is available on the CVM Home Page . Individuals who do not have access to the Internet, may file a Freedom of Information (FOI) request for this response to: Food and Drug Administration, Freedom of Information Staff (HFI-35), 5600 Fishers Lane, Rockville, MD 20857. FOI requests also may be sent via fax to: (301) 443-1726.

Source: Kansas State University
Author: Harner, Murphy, Smith

J. Dairy Sci. 85:1218–1226

American Dairy Science Association, 2002.
Dairy Herd Management Practices that Impact
Nitrogen Utilization Efficiency1
J. S. Jonker,*,2 R. A. Kohn,* and J. High
Department of Animal and Avian Sciences
University of Maryland, College Park, 20742
Lancaster Dairy Herd Improvement Association
Manheim, Pennsylvania, PA 17545

ABSTRACT

Improving the efficiency of feed N utilization by dairy cattle is the most effective means to reduce nutrient losses from dairy farms. The objectives of this study were to quantify the impact of different management strategies on the efficiency of feedNutilization for dairy farms in the Chesapeake Bay Drainage Basin. A confi- dential mail survey was completed in December 1998 by 454 dairy farmers in PA, MD, VA, WV, and DE. Nitrogen intake, urinary and fecal N, and efficiency of feed N utilization was estimated from survey data and milk analysis for each herd. Average efficiency of feed Nutilization for milk production by lactating dairy cows (N in milk/N in feed ? 100) was 28.4% (SD = 3.9). On average, farmers fed 6.6% more N than recommended by the National Research Council, resulting in a 16% increase in urinary N and a 2.7% increase in fecal N. Use of monthly milk yield and component testing, administration of bovine somatotropin (bST), and extending photoperiod with artificial light each increased efficiency of feed N utilization by 4.2 to 6.9%, while use of a complete feed decreased efficiency by 5.6%. Increased frequency of ration balancing and more frequent forage nutrient testing were associated with higher milk production, but not increased N utilization efficiency. Feeding protein closer to recommendations and increasing production per cow both contributed to improving efficiency of feed N utilization. (Key words: nitrogen pollution, milk urea nitrogen, dairy cattle protein requirements) Abbreviation key: MUN = milk urea N, 3? milking = three-times daily milking.

Introduction

Nitrogen losses from agriculture to water resources present a major environmental challenge for the Chesapeake Bay Drainage Basin (Thomann et al., 1994). Dairy farming is a large agricultural enterprise in the region, making dairy farms a major contributor to the nonpoint N loading of the bay. Kohn et al. (1997) used a simple mathematical model to evaluate which management practices had the greatest impact on reducing N losses from the farm: dairy herd feeding and management, soil and crop management, or manure storage and handling. This model suggests that improving herd management is the most effective means to reduce nutrient losses to the environment. Improving herd nutrient utilization efficiency by 50% was calculated to reduce nitrogen losses to water by up to 40%, but improving manure utilization efficiency by 100% only reduced N losses to water by 10 to 14%. Other authors determined the effect of several management practices, such as animal grouping (St-Pierre and Thraen, 2001), use of bST, milking three times daily (3? milking) or artificial lighting (Dunlap et al., 2000), on nutrient utilization efficiency and nutrient excretion in research dairy herds. However, the variation in herd nutrient utilization efficiency on commercial dairy farms is not known. Jonker et al. (1998) developed and evaluated a model to estimate N excretion, N intake, and N utilization efficiency for lactating dairy cows. The model requires knowledge of milk production per cow, milk protein percentage, and milk urea N (MUN). The first objective of this study was to determine current N utilization efficiency of dairy herds in the Chesapeake Bay Drainage Basin. The second objective was to identify factors that contribute to variation from herd to herd in nitrogen utilization efficiency. With a better understanding of current management practices and their effect on potential N loading to the environment, opportunities to improve overall management may be identified.

Materials and Methods

A confidential mail survey was conducted in December 1998 with the Maryland and Virginia Milk Produc ers Cooperative (West Reston, VA). An introductory letter was mailed 1 wk prior to the survey, and a reminder letter was sent 1 wk after the survey. The cooperative had 1156 members located throughout most of the Chesapeake Bay Drainage Basin, including Delaware (n = 23), Maryland (n = 432), Pennsylvania (n = 519), Virginia (n = 172), and West Virginia (n = 18). Participants were offered monthly bulk tank milk analysis of MUN for 6 mo as an incentive to return the survey. The survey included information on dairy herd characteristics, milk production, crop production, feed inputs, management characteristics, and MUN knowledge and use. Herd characteristics included information regarding breed, number and distribution by parity of milking animals, and number and age distribution of replacement heifers. Milk production included volume and compositional data. Crop production and management included types and acreage of crops grown and use of a nutrient management plan (NMP). Feed inputs included types of feeds routinely fed and frequency of ration balancing and nutrient composition testing. Management characteristics indicated the use of various technologies (bST, increased milking frequency, etc.).

MUN Sampling and Analysis

Bulk tank MUN analyses were performed monthly for 6mo for dairy farms from the Maryland and Virginia Milk Producers Cooperative (n = 1156) from December 1998 through May 1999. Only the December samples are used in the current paper because other results were affected by our correspondence with farmers. Milk samples were collected weekly from Environmental Systems Services (College Park, MD) after routine milk component analyses were performed for cooperative members. One sample for MUN analysis was analyzed per herd per month. The fresh milk samples were treated with an antimicrobial preservative (Broad Spectrum Microtabs II, D & F Control Systems, San Ramon, CA). The milk samples were then shipped to Lancaster DHIA (Manheim, PA) for MUN analysis using the Bently Chemspec autoanalyzer (Chaska, MN).

Modeling and Data Analysis

The mean and standard deviation in N feeding parameters were calculated based on model predictions (Table 1) from the survey data and December milk analysis. Nitrogen intake, urinary and fecal N, and N utilization efficiency were determined for each herd using the model of Jonker et al. (1998), except prediction of urinary N was equal to 0.0259 times body weight times MUN, as recommended by Kauffman and St-Pierre (2001). Average BW for the cows in each herd was predicted as the weighted mean for all cows, where each cow’s body weight was assigned according to breed as follows: Holstein and Brown Swiss, 600 kg; Ayrshire and Guernsey, 500 kg; and Jersey, Milking Shorthorn, and Dutch Belted, 400 kg. Estimates of body weights were made based on DHIA data summarized by Dunlap et al. (2000). Crossbred animals were assumed to weigh the average of the breeds crossed. Crude protein requirements were determined using the National Research Council (NRC, 1989) recommendations for dairy cattle, assuming a one-group TMR was fed (Jonker et al., 1999). The protein required was assumed to be that needed by the 83rd percentile cow with respect to protein requirements for the entire milking herd (Stallings and McGilliard, 1983). Excess N feeding was determined as the difference between observed N intake, estimated using the model described previously (Table 1), and that predicted to be required. Thus, negative values represented underfeeding and decreased the average estimate of overfeeding. The accuracy ofNfeeding was calculated by taking the absolute value of observed minus required N, so that the average represents both overfeeding and underfeeding. Statistical analyses were performed using the software package JMP (SAS, 1995). Herds were excluded from the analysis whenever incomplete survey answers resulted in missing data for a variable in the model to predict N intake or utilization efficiency. Treatment means for each observed or calculated value were compared using ANOVA for discrete variables or regression for continuous variables. Observations were excluded when either X or Y values were missing (i.e., missing data were not estimated by the model). When more than two discrete variables were compared (e.g., frequency of diet formulation), the Tukey-Kramer t-test was used to compare each pair. The environmental and economic impact of overfeeding dairy herds was estimated based on summarized results. Excess N fed in the watershed was calculated by multiplying the number of cows in the watershed during the study (n = 758,347 [United States Department of Agriculture, 1998]) by the fraction of farms overfeeding N and the average excess N per overfed cow. The N losses to water resources that result from overfeeding were calculated by accounting for losses from manure during storage and application. The total excess N fed was assumed to be excreted into manure. Assuming that 25% of manure N excreted eventually becomes available to crops, excess feed N was multiplied by 0.75 to estimate manure N losses (Kohn et al., 1997). There would be additional losses of N from the production of crops. However, imported soybean meal was assumed to provide the excess feed N, so the N losses would not have occurred in this watershed. The cost of feeding excess N was estimated assuming that soybean meal (44%) could be replaced by corn grain to decrease N content. The 5-yr average prices (1996 to 2000) for soybean meal ($0.210/kg) and corn grain ($0.097/kg) were used (Bridge Information Systems, Inc., 2000).

JONKER ET AL.

Table 1. Prediction equations.

Variable	Equation
Urinary N (UN), g/d	0.0259 · milk urea N (mg/dl) · BW (kg)
N Intake (NI), g/d	(Predicted UN+ milk N+ 97)/0.83
Fecal N, g/d	Predicted NI+ predicted UN− milk N
N utilization efficiency, %	(Milk N · 100)/predicted NI
DMI, kg/d	(Predicted NI · 6.25)/dietary CP percentage

RESULTS AND DISCUSSION

A total of 472 dairy farmers responded to the survey, for a 40.8% rate of return. However, nine farms stopped shipping milk shortly after completing the survey and were therefore excluded, and 91 farms were excluded because of incomplete data (usually rolling herd average or milk protein percentage was missing). For the final data, the largest number of surveys were from Pennsylvania (n = 165), followed by Maryland (n = 139), Virginia (n = 56), West Virginia (n = 6), and Delaware (n = 6). A large range in farm size and production was represented in the survey (Table 2). Average FCM was 28.3 kg/d per cow (SD = 4.2) with 3.74% (SD = 0.24) fat and 3.25% (SD = 0.15) true protein. The average farm surveyed had 109 cows (93 milking and 16 dry cows)and 86 replacement heifers (Table 2). Several farms reported not raising any replacements. Nearly every farm (>98%) reported having Holstein cows. Jersey cattle were the second most predominant breed—reported on 11.7% of the farms—and made up 3.7% of all dairy cattle. Other breeds represented less than 1% of total dairy cattle.

The farmers participating in the program generally appeared to represent the range of farmers in the Chesapeake Bay Drainage Basin. The average milk production reported by participants was 28.3 kg/d per cow, compared to the average of 29 kg/d per cow reported for Lancaster DHIA members between July 1996 and April 1998 (Dunlap et al., 2000). Herd distribution (Table 2) was also similar to results reported for those records. The mean MUN for participating farms was 12.8 mg/dl (Table 3), compared to 12.4 mg/dl for all farms in the cooperative (Jonker et al., 2002). Higher MUN may have resulted from a tendency of participants to have higher milk production or to feed higher CP diets than nonparticipants, but the error imposed by nonrandom participation of farmers compared to all cooperative members would be 3.2% of MUN.

The model of Jonker et al. (1998) enables calculation of the variance in N utilization efficiency for a large number of herds in the field. The mean and standard deviation in N utilization parameters for lactating cows across all herds are given in Table 3. These calculations do not include dry cows or heifers, which would otherwise add to excreted N and decrease N utilization effi- ciency for the herd. Observed parameters differed significantly from recommended levels for all parameters. Observed MUN was 12.7 mg/dl, but feeding according to NRC (1989) and allowing for variation within the herd by feeding the 83rd percentile cow would have resulted in a MUN of 11.0 mg/dl.

As with any measurement, the variance can be attributed to both errors in measurement and true variance within the population. The root mean square prediction error (RMSPE) for the model used in this analysis was 16.9% of mean urinary N prediction (Jonker et al., 1998). A similar prediction error for the current study would result in prediction error accounting for 40% (100 ? RMSPE2/SD2) of the total variance (SD2) among farms reported in Table 3. The RMSPE for prediction of N utilization efficiency was 11% of prediction (Jonker et al., 1998). A similar prediction error in the present study would explain 64% of total variance in utilization efficiency among farms. Most of the model prediction error used for these calculations was associated with lab and cow variation (Jonker et al., 1998), and these would be reduced by using a single lab that uses wet chemistry and bulk tank samples representing an average of 109 cows. Therefore, we do not have adequate data to accurately estimate model prediction error under the circumstances in which the model was used in the present study. Nonetheless, the model prediction errors reported previously provide an upper limit of model prediction error.

Table 2. Milk production and distribution of cows from surveyed

farms.

Range1
	Mean	SD	10th percentile	90th percentile
Production
FCM, kg cow-1d-1	28.3	4.2	22.4	33.6
Fat, %	3.74	0.24	3.50	4.00
Protein, %	3.25	0.15	3.10	3.40
Cows
Total	109	103	40	200
Milking	93	88	34	173
Dry	16	16	4	30
1st lactation	35	35	9	70
2nd lactation	31	29	7	56
Mature	42	46	14	76
Heifers
Total	86	80	23	173
<1 yr	42	41	11	85
>1 yr	44	41	11	90
1Reported range of surveyed dairy farms (n = 372).

Source: Feedstuffs Magazine
Author: Michael Howie

Cows Given BST Remain in Herds, Stay as Healthy as Non-supplemented Cows

Michael Howie, Feedstuffs Staff Editor April 12, 1999 A study by two Cornell University researchers that examined eight years of data on the effects of bovine somatotropin (BST) has indicated that cows stay as healthy and remain in herds as long as cows in non-supplemented herds. The researchers, in a paper in the proceedings from the 1999 Cornell University Cooperative Extension Winter Dairy Management meetings, said scientific studies have given “remarkably consistent” results, showing that BST enhances milk yield and increases productive efficiency. The goal of their current study was to look at BST response under field conditions, paying particular attention to response to BST over the lactation cycle and response over the four years since approval. Researchers on the project were D.E. Bauman and R.W. Evertt of the department of animal science at Cornell University, and R.J. Collier of Monsanto Co. In order to pull together necessary information for the study, Dairy Herd Improvement (DHI) records were utilized from Northeast DHI, Vermont DHI Assn. and Pennsylvania DHI Assn. and were matched at the university with a list of BST users and nonusers supplied by Monsanto. BST users were defined as producers who used BST on or before June 1994 and had continuous use through March 1998 and supplemented at least 50% of the eligible cows in milk. From all this data, the researchers identified 340 herds with a total of more than 27,000 cows in milk. Of these, 176 were control herds (non-BST users) and 164 were BST users. They said only Holstein cows were included in the analysis. Table 1 shows the average of production parameters on test day for control and BST herds. The researchers said the parameters in Table 1 are the average of more than 2 million cow test-day records analyzed for the 340 herds. They said control herds were slightly smaller than BST herds and were lower in production before the use of BST (”pre” - defined as January 1990 through February 1994) and after BST was available (”post” - defined as July 1994 through March 1998). Data available to the researchers included milk, fat, protein, somatic cell count linear score (SCC), age in days on test day and days in milk (DIM) on test day. The researchers analyzed the data using the test-day model (TDM) for each herd for both the pre and post periods. TDM is designed to account for systematic biological and environmental changes that occur in a set of data, they said. The researchers said two sets of solutions, pre-BST and post-BST, were available from each herd for analysis of management on test day, age, DIM in the first lactation, DIM in the second lactation, month fresh and days carried calf. They said the pre and post solutions were compared to determine changes in traits caused by BST, assuming all other changes were the same in the two groups of herds. “While there were undoubtedly some changes within individual herds in both groups, this would tend to balance out with the large number of herds,” they said. The experimental design defines the response to BST as the difference pre and post in the control and BST herds. The researchers said these differences can be estimated from management on test day, the shape of the lactation curves or any other component of TDM. The researchers said any change in herd production due to weather, feed supply, education, milk price, etc., should be similar for both groups or should change at the same rate in both herds. Because of this, they said any additional differences would “logically be associated with BST supplementation.” Table 2 shows the changes in management milk related to management improvements in the control herds over the period studied. The researchers said TDM adjusts test-day production such that management milk represents production as if the cows were the same age, stage of pregnancy, stage of lactation and freshened all in the same month on all test days. Therefore, the researchers said, Table 2 shows the improvement in produciton achieved without the use of BST. Overall, from 1990 to 1998, daily production per cow increased 6.97 lb. of milk while SCC linear score decreased 0.208 units. Table 3 shows the changes in BST herds for the same period of time as in the control herds in Table 2. The researchers said changes in Table 3 represent the result of BST plus the normal management improvements that caused changes in Table 2. Therefore, they said, results in Table 3 minus results in Table 2 should show the response due to BST. These results are shown in Table 4. The researchers said the results presented in Table 4 are the response in production due to BST supplementation and represent the average per cow that is milking on test day. “Thus, the data represent an average for all milking cows in the herd with the assumption being all cows in the herd are supplemented on every test day,” they said, adding that in reality, only a percentage of the herd received supplemental BST and producers do not treat every eligible cow with BST. They said 1993 was set to zero in Tables 2-4 to provide a comparison of the changes that occurred pre- and post-BST. The researchers also plotted lactation curves to estimate the response of BST. They said the data clearly showed no difference between the control milk production first lactation curves for the two time periods studied. “This indicates that management changes in the two time periods in Table 2 did not change the lactation curves in the control herds,” said the researchers. However, BST use changed the shape of the curve. The researchers said response to supplementation appeared to increase with increased days in milk. The response peaks at 8 lb. of milk per cow per day and declines in late lactation — although the researchers said this assumes that all the first lactation cows were supplemented with BST, which was not true, so this response should be considered the “minimum.” For cows in their second or later lactation, the researchers said the spread was the same — with 8 lb. of milk response being gained by supplementing BST. In fact, the researchers said if it was assumed that 75% of cows were supplemented following day 60 of lactation, the response would peak at nearly an additional 11 lb. of milk per cow per day. As for the effect of BST on SCC, the researchers plotted lactation curves for SCC and compared pre- and post-BST periods. The researchers found that SCC varies over the lactation cycle, being high in early lactation, decreasing during the first 100 days postpartum and then progressively increasing over the rest of the cycle. This was a similar pattern for both the control and BST periods. The pattern was also similar for cows in their second or later lactation.

Source: Feedstuffs Magazine
Author: Michael Howie

Frequently Asked Economic Questions

Does it pay?

As a rule of thumb, a cow needs to eat about 4 pounds extra feed on a dry matter basis, to produce an extra 10 pounds of milk. Feed costs are typically between 6 and 8 cents per pound of dry matter ($120 to $160 per ton of dry matter).

Revenue: 10 pounds of milk/day @ $12.00/cwt =	$1.20
Expense: 4 lbs of additional feed/day @ 7 cents/lb dry matter =	-.28
Expense: POSILAC 1 STEP @ $5.20/14 days =	-.37
Expense: Labor =	-.02
Net income per cow per day =	$0.53
Net income per dose (14 days) =	$7.42
Return on Investment =	79%

What is the breakeven milk production response?

The breakeven milk production response depends on four major variables:

Mailbox milk price.Feed costs supporting the additional milk production.Labor costs to administer POSILAC 1 STEP.Cost of POSILAC 1 STEP.

With a milk price of $12.00/cwt, feed cost of 7? per pound of dry matter, labor costs of $15/hour and POSILAC 1 STEP at $5.20 per dose, the breakeven milk production response is about 4.2 pounds of additional milk produced/cow/day through the application of POSILAC 1 STEP. With these same feed and labor costs and a 10-pound daily milk production response, the milk price would have to drop to $6.63/cwt to breakeven. Typically, milk price would have to drop below $7 before questioning POSILAC profitability.

What is the impact on cash flow?

With a 10-pound daily milk production response, milk price at $10.00, $12.00, or $14.00/cwt, labor @ $15/hr, POSILAC 1 STEP at $5.20/dose, feed at 7?/lb of dry matter and a 13 month calving interval, the additional cash flow per cow per year is:

Mail box milk pric	$10.00/cwt	$12.00/cwt	$14.00/cwt
Additional Revenue	$273/cow/year	$320/cow/year	$382/cow/year
POSILAC 1 STEP cost	($100)/cow/year	($100) /cow/year	($100)/cow/year
Feed & Labor cost	($80)/cow/year	($80) /cow/year	($80)/cow/year
Additional Net Income	$93/cow/year	$140/cow/year	$202/cow/year

Source: Monsanto Dairy Group

FDA Analysis Of DGXXIV Report On Public Health Aspects Of BST

U.S. Food And Drug Administration April 13, 1999 SOURCE: HTTP://WWW.FDA.GOV/CVM/INDEX/UPDATES/DG24UP.HTML

FDA approved Monsanto Company’s recombinant bovine somatotropin (rbST) product, POSILAC 1 STEP® , in November 1993 after a comprehensive review of the product’s safety and efficacy, including human food safety. Recently, FDA reviewed the European Commission Directorate General XXIV “Report on Public Health Aspects of the Use of Bovine somatotropin — 15 -16 March 1999.”

The conclusions of the DGXXIV report with respect to the safety of IGF-1 do not appear to be consistent with the current state of scientific knowledge. Specifically, the report states that establishing an in vivo quantitative dose-effect relationship for IGF-1 is virtually impossible because of the diverse biological effects attributable to the intrinsic activity of IGF-1. In fact, there are standard hazard assessment procedures for assessing the hazard associated with all types of compounds that exert a broad variety of metabolic effects. These procedures have been applied to determine the safety of vitamins, food additives, and drugs, including hormones, for over twenty-five years.

Numerous independent researchers and scientific committees have examined the data on the dietary exposure of IGF-1 and related proteins present in milk. The data provide ample evidence that the amount of IGF-1 and truncated forms excreted in milk following the administration of rbST to dairy cows is safe for all consumers, including infants. Additional exposure data are not necessary.

FDA’s determination that food products from cows treated with rbST are safe for consumers has been supported by numerous scientific and regulatory bodies including the Joint Food and Agricultural Organization/World Health Organization Expert Committee on Food Additives (JECFA), an international panel of experts in the field of toxicology and chemistry of animal drug residues that meets to evaluate the safety of animal drugs. In 1992, the JECFA concluded that “the lack of oral activity of rbST and insulin-like growth factor I (IGF-1) and the low level and non-toxic nature of the residues of these compounds, even at exaggerated doses, results in an extremely large margin of safety for humans consuming dairy products from rbST-treated cows.” In 1998, JECFA reaffirmed the safety of milk and meat from rbST-treated cows.

Recently, FDA reviewed the issues raised in Health Canada’s report on the food safety of rbST, and concluded that there were no biologically significant effects. Based on the current body of science relative to the safety of rbST, FDA has reaffirmed that meat and milk from rbST-treated cows are safe for human consumption.

The full FDA report, “Report on the Food and Drug Administration’s Review of the Safety of Recombinant Bovine somatotropin,” is available on CVM’s Internet Home Page. A copy of this report may also be obtained by calling or writing CVM’s Communications Staff at FDA/Center for Veterinary Medicine, HFV-12, 7500 Standish Place, Rockville, MD 20855, 301-594-1755. Please include a self-addressed adhesive label to assist in processing your request.

Source: Food & Drug Administration

Update On Human Food Safety Of BST

U.S. Food And Drug Administration Center For Veterinary Medicine February 5, 1999

SOURCE: HTTP://WWW.FDA.GOV/CVM/INDEX/UPDATES/BSTSAFUP.HTML

FDA’s Center for Veterinary Medicine (CVM) has reexamined the human food safety of recombinant bovine somatotropin (rbST) in response to recent inquiries about the safety of this product. FDA’s CVM approved Monsanto Company’s rbST product, POSILAC 1 STEP® in November 1993 after a comprehensive review of the product’s safety and efficacy, including human food safety. CVM has issued a detailed report based on a careful audit of the human food safety sections of this approval. CVM’s finding upholds the Agency’s original conclusion that milk from cows treated with rbST is safe for human consumption.

The new concerns about the safety of POSILAC , currently the only rbST product approved for increasing milk production in dairy cattle in the U.S., were stimulated by the product’s review for approval in Canada. In April 1998, while the review process was underway, the Health Protection Branch (HPB) of Health Canada prepared an internal memorandum, entitled “rbST (Nutrilac) ‘GAPS Analysis’ Report,” which was critical of the review method used by the HPB, and identified areas of human food safety concern.

In particular, the Canadian report claimed that a 90-day oral toxicity study in rats had been “misreported” by FDA, and cited allegations of significant absorption of oral rbST based on serum antibody levels in the rats, and toxicity to the rats. Both the memorandum and the circumstances under which it was made public became highly controversial in Canada.

Following the publication of the Canadian document several groups and individuals in the United States raised questions about the safety of milk from rbST-treated cows. In response to these concerns, CVM “Report on the Food and Drug Administration’s Review of the Safety of Recombinant Bovine somatotropin.” The Report affirmed the original review of the 90-day rat oral toxicity study, which concluded that there were no biologically significant observed effects in either the thyroid or the prostate.

In addition, CVM conducted a review of the report cited by Health Canada of the antibody response to oral rbST. While CVM concurred that oral exposure to high doses of rbST results in antibody production, there is no evidence for biologically significant absorption of intact rbST from the gastrointestinal tract.

The “Report on the Food and Drug Administration’s Review of the Safety of Recombinant Bovine somatotropin” is available on the CVM’s Internet Home Page, which is located at http://www.fda.gov/cvm

A copy of this report may also be obtained by calling or writing CVM’s Communications Staff at FDA/Center for Veterinary Medicine, HFV-12, 7500 Standish Place, Rockville, MD 20855, 301-594-1755. Please include a self-addressed adhesive label to assist in processing your request.

Source: Food And Drug Administration

BOVINE SOMATOTROPIN AND STRESS IN DAIRY CATTLE

Dr. Robert Collier, University of Arizona

A common misconception regarding use of recombinant bovine somatotropin (rbST) in dairy cows is that animals are “stressed” by the higher levels of milk yield they achieve when being supplemented. This is an erroneous assumption, which can be easily demonstrated. Stress is normally measured by the degree of “strain” it exerts on a system. In biological systems, the degree of strain is typically measured through its impact on metabolic rate, level of production, function of the hypothalamic-pituitary-adrenal axis, and immune system. This paper will discuss the effect of rbST on these commonly used indicators of stress.

An environmental stress is typically associated with a change in metabolic rate. For example, the degree of heat or cold stress can be characterized by the change in basal metabolic rate induced by the stress. In the case of somatotropin, careful bio-energetic studies have demonstrated that basal metabolic rate of somatotropin-supplemented cows is unaltered and that nutritional requirements of bST-supplemented cows are the same as those for non-supplemented cows and are a function of the animal’s maintenance requirement, body condition and requirements for milk synthesis (12,16). This is in clear contrast to the impact of thyroid compounds (e.g. thyroprotein) which do change metabolic rate. Milk yield increases from thyroid compounds are associated with losses in body condition and eventually milk yield (3).

Another way that stress is measured in domestic animals is to evaluate production. Typically, animals that are sick or suffering discomfort demonstrate clear decreases in production and or production efficiency. This is clearly not the case with somatotropin supplementation which results in increases in milk production and feed intake of animals. Speculation that use of somatotropin would cause cows to “burn out” were based on erroneous assumptions regarding the mechanism of action. As pointed out by Bauman and McGuire (3), “Metabolic disorders would most likely occur the first few days of bST supplementation when milk yield has increased but intake has not. Suffice it to say, there is not a single mention of clinical ketosis or milk fever occurring during the first weeks of bST supplementation in any of the hundreds of published studies”. Even when cows are not adequately fed, we do not see development of a disease state with onset of somatotropin supplementation. Underfed cows which do not have available body stores to increase production when supplemented with bST demonstrate a negligible milk yield response (4,5,9). Thus, cows are not “forced” to produce milk if they are not nutritionally capable of responding to somatotropin supplementation.

An additional method of assessing impact of potential stressors on domestic animals is to examine the function of the hypothalamic-pituitary axis. This is based on evidence that under both acute and chronic stress the central nervous system of mammals evokes physiological responses that culminate in changes in secretion rate of hormones of the sympatho-adrenal axis (11). Typically, one can measure increases in secretion of Adrenal Corticotrophic Hormone (ACTH) and glucocorticoids of the adrenal. The general endocrine status of somatotropin-supplemented animals was evaluated in the acute and chronic toxicology studies as well as an endocrine challenge study during a fourth consecutive lactation of bST supplementation (1,2). In all of these studies, there was no evidence of increased secretion of glucocorticoids in animals supplemented with somatotropin. In fact, the only consistent evidence was a slight decrease in glucocorticoid concentration in somatotropin-supplemented animals that was attributed to metabolic adaptation to increased gluconeogenesis in supplemented animals. In addition to the hypothalamic adrenal axis the pituitary challenge study indicated that 4 consecutive lactations of somatotropin supplementation did not alter the ability of the pituitary to respond to endocrine challenge.

Finally, chronic stress is recognized to be immune suppressive (11). Animals that are immune suppressed are more susceptible to disease. The exact cause of the immune suppression still remains undefined but is believed to be related in part to increased secretion of adrenal glucocorticoids which suppress many immune functions. Since glucocorticoids are not increased in somatotropin-supplemented animals, one prerequisite for immune suppression would appear to be missing.

Immune function of somatotropin-supplemented animals has been measured using a variety of approaches. The immune system is sensitive to somatotropin stimulation with a number of studies demonstrating improved immune function in somatotropin-supplemented animals. The effects of somatotropin include improved thymus weight and thymosin concentration in plasma (8), improved primary lymphoid tissue in bone marrow, stimulated activation of peripheral lymphocytes and macrophages (6), improved cytokine responses to challenge (7), improved antibody synthesis (14), improved IgG level (10), augmented production of superoxide anion by macrophages and neutrophils (13). Burvenich and co-workers (15, 17) demonstrated improved production recovery from E. coli and Strep uberis mastitis in lactating dairy cows when cattle were supplemented with bovine somatotropin. In spite of these results, their laboratory reported no positive effect on neutrophil function (18) and Elvinger et al. (19) detected no effects of somatotropin on migration or chemotaxis of polymorphonuclear leukocytes. Thus, all of the evidence gathered to date does not support a negative effect of somatotropin on immune function. Quite to the contrary, the accumulated evidence supports a stimulatory role of somatotropin in regulation of the immune system. Evaluation of the mastitis incidence and duration of somatotropin treated cows indicates that duration of mastitis was not altered in cows supplemented with rbST. Thus, there is also no evidence that acute or chronic supplementation of lactating dairy cows results in suppression of the immune system.

Summary of the Effects of rbST on the Measures of Stress

References

1. 1. Adriaens, F.A., D.L. Hard, M.A. Miller, R.H. Phipps, R.H. Sorbet, R.L. Hintz and R. J. Collier. 1994. Pituitary response to thyrotropin, corticotropin and gonadotropin releasing hormones in lactating cows treated with sometribove for a fourth consecutive lactation. Submitted: Domestic Animal Endo.
2. 2. Adriaens, F.A., M.A. Miller, D.L. Hard, R.F. Weller, M.D. Hale and R.J. Collier. 1992. Long-Term effects of sometribove in lactating cows during a fourth consecutive lactation of treatment: Insulin and somatotropin responses to glucose infusion. J. Dairy Sci. 75:472-480.
3. 3. Bauman, D.E., and M.A. McGuire, 1994. Paradox of BST: Why Cows Don’t Burnout. In Proceedings Minnesota Dairy Health Conference. pp 27-40.
4. 4. Bauman, D.E. and R.G. Vernon. 1993. Effects of exogenous somatotropin on lactation. Ann. Rev. Nutr. 13:437-461.
5. 5. Chalupa, W. and D.T. Galligan. 1989. Nutritional implications of somatotropin for lactating cows. J. Dairy Sci. 72:2510-2524.
6. 6. Elsasser, T.H., and N.C. Steele., 1992. Growth hormone directed nutrient partitioning: Immune system-pituitary gland communication. In: Pennington Cent. Nutr. Ser., 2, Science of Food Regulation. pp. 164-186.
7. 7. Exon, J.H., J.L. Bussiere, and J.R. Williams. 1990. Hypophysectomy and growth hormone replacement effects on multiple immune responses of rats. Brain Behav. Immun. 4:118-128.
8. 8. Gelato, Marie C., 1993. Growth hormone, insulin like growth factor I and immune function. Trends endocrinol. Metab., 4:106-110.
9. 9. Hoogendoon, C.J., S.N. McCutcheon, G.A. Lynch, B.W. Wickham and A.K.H. MacGibbon. 1990. Production responses of New Zealand Friesian cows at pasture to exogenous recombinantly derived bovine somatotropin. Anim. Prod. 51:431-439.
10. 10. Marsh, J.A., W.C. Gause, S. Sandhy and C.G. Scanes. 1984. enhanced growth and immune development in dwarf chickens treated with mammalian growth hormone and thyroxine. Proc. Soc. Exp. Biol. Med., 175:351-360.
11. 11. Minton, J.E., 1994. Function of the hypothalamic-pituitary-adrenal axis and the sympathetic nervous system in models of acute stress in domestic farm animals. J. Anim. Sci. 72:1891-1898.
12. 12. National Research Council. 1994. Metabolic Modifiers: Effects on the Nutrient Requirements of Food Producing Animals. National Academy Press, Washington, D.C. pp 23-29.
13. 13. Snow, C.E., 1985. Insulin and growth hormone function as minor growth factors that potentiate lymphocyte activation. J. Immunol., (Suppl. 2): 776S-778S.
14. 14. Takada, Y., H. Bando, Y. Miyamoto, M Kosaka and T. Sano. 1991. Effect of growth hormone on immune function in normal and hypophysectomised rats. Nippon Naibunpi Gakkai Zasshi. 67:1162-1177.
15. 15. Vandeputte-Van Messom, G. and C. Burvenich. 1993. Effect of somatotropin on changes in milk production and composition during coliform mastitis in periparturient cows. J. Dairy Sci. 76:3727-3741.
16. 16. Vernon, R. G. 1989. Influence of somatotropin on metabolism. In: K. Sejrsen, M. Vestergaard, and A. Neimann-Sorensen, eds. Use of Somatotropin in Livestock Production. Elsevier applied Science, NY, pp 31-50.
17. 17. Hoeben, D., C. Burvenich, P.J. Eppard and D.L. Hard. 1999. Effect of recombinant bovine somatotropin on milk production and composition of cows with Streptococcus uberis mastitis. J. Dairy Sci. 82:1671-1683.
18. 18. Hoeben, D., C. Burvenich, E. Smits, P.J. Eppard and D.L. Hard. 1999. Effect of bovine somatotropin on neutrophil function and clinical signs during Streptococcus uberis mastitis. J. Dairy Sci. 82: 1465-1481.
19. 19. Elvinger, F., P.J. Hansen, H.H. Head and R.P. Natzke. 1991. Actions of bovine somatotropin on polymorphonuclear leukocytes and lymphocytes in cattle. J. Dairy Sci. 74:2145-2152.

POSILAC is a registered trademark of Monsanto Technology LLC.

Source: University of Arizona
Author: Robert Collier, DVM

Milk And Meat From BST Treated Cows Presents No Danger To Humans, Says Committee Report Released By The UN Food And Agriculture Organization

Food & Agriculture Organization Of The United Nations Press Release 98/17 SOURCE: http://www.fao.org/waicent/ois/press_ne/presseng/1998/pren9817.htm

Rome, March 5 — After examining new evidence, an independent scientific committee has reconfirmed that treating cows with the hormone bovine somatotropins, known as BST, to increase milk production is safe, according to a technical report released today by the UN Food and Agriculture Organization (FAO). The joint FAO- World Health Organization (WHO) committee concluded that “there are no food safety or health concerns related to BST residues in products such as milk and meat from treated animals.” The use of BST increases a cow’s milk production by 10 to 15 percent.

Disagreement over use of BST has complicated trade in dairy products between the United States, where BST is widely used, and the European Union, which has opposed use of the hormone.

The Joint FAO/WHO Expert Committee on Food Additives (JECFA), determines the safety of residues from veterinary drugs in food and establishes acceptable daily intakes (ADIs) and maximum residue limits (MRLs) for certain drugs when they are used on food-producing animals in accordance with good animal husbandry practices.

In the area of maximum residue limits (MRL) for BST, the Committee found that available data on the identity and concentration of residues of the veterinary drug in animal tissues provide a wide margin of safety for consumption of residues in food when the drug is used according to good practice in the use of veterinary drugs. The Committee concluded that the presence of drug residues in animal products does not present any health concerns.

In arriving at its conclusions on BST, JECFA considered possible problems such as the chances of an increase in the udder disease, mastitis, in BST-treated cows which could lead to contamination of milk with antibiotics used to treat mastitis. The Committee concluded that the use of BST will not result in a higher risk to human health due to the use of antibiotics to treat mastitis and that the increased potential for drug residues in milk could be managed by practices currently in use by the dairy industry and by following label directions for use.

Another concern the Committee examined involved the risk of insulin-dependent diabetes mellitus (IDDM). Studies have shown that exposure of human new borns to cow’s milk increases the risk of IDDM approximately 1.5-fold. The Committee considered whether exposure of new borns to milk from BST-treated cows might further increase this risk. It concluded that, because of its unchanged composition, the milk of BST-treated cows does not represent an additional risk to the development of IDDM.

JECFA, which met at FAO in Rome from 17 to 26 February 1998 to evaluate certain residues of veterinary drugs in food, had originally stated in 1992 that BST-treated animals and animal products do not pose any risks to humans. JECFA is an independent scientific committee whose recommendations to FAO and WHO are relied upon by governments and international organizations on scientific matters such as food additive safety and usage, tolerable levels of contaminants, or residue levels of veterinary drugs in foods.

The Committee’s report will now be considered by the Codex Alimentarius Commission. Codex works to harmonize standards used in international trade and to prevent food that is unfit for human consumption from entering commercial channels. It has developed 237 food commodity standards and has established over 40 guidelines and codes for food production and processing.

* * * * * * * * * * *

The Joint FAO/WHO Expert Committee on Food Additives (JECFA) Summary and Conclusions may be read on FAO’s Website at this address: www.fao.org\WAICENT\FAOINFO\ECONOMIC\ESN\jecfa\jecfa.htm

Source: Food and Drug Administration

Joint FAO/WHO Expert Committee on Food Additives
Fiftieth meeting
Rome, 17-26 February 1998

SUMMARY AND CONCLUSIONS

A meeting of the Joint FAO/WHO Expert Committee on Food Additives (JECFA) was held in Rome, Italy, from 17 to 26 February 1998. The purpose of the meeting was to evaluate certain residues of veterinary drugs in food.

Dr J. Boisseau, Director, National Agency for Veterinary Medicine, Foug?res, France,served as chairman and Professor J.G. McLean, South Melbourne, Victoria, Australia, served as vice-chairman.

Dr J. Paakkanen, Food Quality and Standards Service, Food and Nutrition Division, Food and Agriculture Organization of the United Nations, served as FAO Joint Secretary. Dr J.L. Herrman, International Programme on Chemical Safety, World Health Organization, served as WHO Joint Secretary. Dr M. Miller, Center for Veterinary Medicine, Food and Drug Administration, Rockville, Maryland, USA, and Dr R. L. Ellis, Food Safety and Inspection Service, Department of Agriculture, Washington, DC, USA, served as Joint Rapporteurs.

The present meeting was the fiftieth in a series of such meetings and was the eleventh JECFA meeting convened to deal xclusively with residues of veterinary drugs in food. The primary tasks before the Committee were to further elaborate principles for evaluating the safety of residues of veterinary drugs in food and for establishing acceptable daily intakes (ADIs) and maximum residue limits (MRLs) for certain drugs when they are dministered in food-producing animals in accordance with good practice in the use of veterinary drugs.

The report of the meeting will appear in the WHO Technical Report Series (TRS). Its presentation will be similar to that of previous reports, namely, general considerations, specific

comments on substances on the agenda, and recommendations. The report will include an annex containing a detailed table (similar to Table 1 in this report) summarizing the conclusions reached by the Committee after its evaluations of the substances on the agenda.

Toxicological monographs summarizing the data that were considered by the Committee in assessing the safety of the substances on the agenda will be published in WHO Food dditives

Series No. 41. Residues monographs summarizing the data that were considered by the Committee in establishing MRLs will be published in FAO Food and Nutrition Paper series No. 41/11.

NOTE This document has been distributed prior to publication of the full report of the fiftieth meeting of the Joint FAO/WHO Expert Committee on Food Additives (JECFA) to ensure the fast dissemination of information, in particular to the Codex Alimentarius Commission, for which JECFA is the scientific advisory body on matters relating to residues of veterinary drugs in food.The FAO and WHO Joint Secretaries of JECFA request that further inquiries regarding the compounds evaluated at the fiftieth meeting be made only after the full official report has been published and distributed by WHO in the name of both sponsoring organizations, FAO and WHO. Your cooperation is much appreciated.

TABLE 1. RECOMMENDATIONS ON COMPOUNDS ON THE AGENDA

Anthelminthic agents

Eprinomectin

Acceptable daily intake: 0-10 µg/kg bw
Residue definition: Eprinomectin B_1a

Recommended maximum residue limits (MRL)
Species	Muscle (µg/kg)	Liver (µg/kg)	Kidney (µg/kg)	Fat (µg/kg)	Milk (µg/litre)
Cattle	100	2000	300	250	20

Febantel, fenbendazole, and oxfendazole

Acceptable daily intake: 0-7 µg/kg bw (group ADI for febantel, fenbendazole, and oxfendazole)
Residue definition: Determined as the sum of fenbendazole, oxfendazole, and oxfendazole sulfone, expressed as oxfendazole sulfone equivalents

Recommended maximum residue limits (MRLs)1
Species	Muscle (µg/kg)	Liver (µg/kg)	Kidney (µg/kg)	Fat (µg/kg)	Milk (µg/litre)
Cattle	100	500	100	100	100
Horses	100	500	100	100
Pigs	100	500	100	100
Goats	100	500	100	100
Sheep	100	500	100	100	100

Moxidectin

Acceptable daily intake: 0-2 µg/kg bw (established at the forty-fifth meeting of the Committee (WHO TRS 864, 1996))

Residue definition: Moxidectin

Recommended maximum residue limits (MRLs)
Species	Muscle (µg/kg)	Liver (µg/kg)	Kidney (µg/kg)	Fat (µg/kg)	Milk (µg/litre)
Cattle1	202	100	50	500
Sheep1	50	100	50	500
Deer	20	100	50	500

¹Recommended at the forty-fifth meeting of the Committee (WHO TRS 864, 1996), except for sheep muscle, which was recommended at the forty-seventh meeting (WHO TRS 976, 1998)
²At the forty-fifth meeting (WHO TRS 864, 1996), the Committee noted the very high concentration and great variation in the level of residues at the injection site in cattle over a 49-day period after dosing.

Antimicrobial agents

Gentamicin

Acceptable daily intake: 0-20 µg/kg bw

Residue definition: Gentamicin

Recommended maximum residue limits (MRL)
Species	Muscle (µg/kg)	Liver (µg/kg)	Kidney (µg/kg)	Fat (µg/kg)	Milk (µg/litre)
Cattle	100	2000	5000	100	200
Pigs	100	2000	5000	100	200

Procaine benzylpenicillin

Acceptable intake: Residues of benzylpenicillin and procaine benzylpenicillin should be kept below 30 µg of penicillin per person per day.

Residue definition: Benzylpenicillin

Recommended maximum residue limits (MRL)
Species	Muscle (µg/kg)	Liver (µg/kg)	Kidney (µg/kg)	Fat (µg/kg)	Milk (µg/litre)
Cattle	50	50	50		4
Pigs	50	50	50
Chickens	50	50	50

¹Procaine benzylpenicillin is also used in horses, sheep, turkeys, rabbits, quail, and pheasants. Due to the lack of information, MRLs could not be established for those species.

Sarafloxacin

Acceptable daily intake: 0-0.3 µg/kg bw

Residue definition: Sarafloxacin

Recommended maximum residue limits (MRL)
Species	Muscle (µg/kg)	Liver (µg/kg)	Kidney (µg/kg)	Fat (µg/kg)	Milk (µg/litre)
Chickens	10	80	80	20
Turkeys	10	80	80	20

Spectinomycin

Acceptable daily intake: 0-40 µg/kg bw (established at the forty-second meeting of the Committee (WHO TRS 851, 1995))

Residue definition: Spectinomycin

Recommended maximum residue limits (MRLs)
Species	Muscle (µg/kg)	Liver (µg/kg)	Kidney (µg/kg)	Fat (µg/kg)	Milk (µg/litre)	Eggs (µg/kg)
Cattle	500	2000	5000	2000	200
Pigs	500	2000	5000	2000
Sheep	500	2000	5000	2000
Chickens	500	2000	5000	2000		2000

Chlortetracycline, oxytetracycline, and tetracycline

Acceptable daily intake: 0-30 µg/kg bw (group ADI for oxytetracycline, chlortetracycline, and (tetracycline)

Residue definition: Parent drug

Recommended maximum residue limits (MRLs)1
Species	Muscle (µg/kg)	Liver (µg/kg)	Kidney (µg/kg)	Fat (µg/kg)	Milk (µg/litre)	Eggs (µg/kg)
Cattle	200	600	1200		100
Pigs	200	600	1200
Sheep	200	600	1200		100
Poultry	200	600	1200			400
Fish2,3	200
Giant prawn2 (Penaeus monodon)	200

¹Singly or in combination
²Applies only to oxytetracycline
³Temporary pending evaluation of use pattern of oxytetracycline in aquacultureAntiprotozoal agents

Diclazuril

Acceptable daily intake: 0-30 µg/kg bw

Residue definition: Diclazuril

Recommended maximum residue limits (MRLs)
Species	Muscle (µg/kg)	Liver (µg/kg)	Kidney (µg/kg)	Fat (µg/kg)	Milk (µg/litre)
Sheep	500	3000	2000	1000
Poultry	500	3000	2000	10001
Rabbits	500	3000	2000	1000

¹Skin/fat

Imidocarb

Acceptable daily intake: 0-10 µg/kg bw

Residue definition: Imidocarb

Recommended maximum residue limits (MRL)1
Species	Muscle (µg/kg)	Liver (µg/kg)	Kidney (µg/kg)	Fat (µg/kg)	Milk (µg/litre)
Cattle	300	2000	1500	50	50

¹Temporary. Residue depletion studies in lactating and non-lactating cattle using recommended subcutaneous doses of unlabelled imidocarb and analyzing samples using the proposed regulatory method with enzymatic digestion are required for evaluation in 2001. If MRLs are to be recommended for sheep, a residue depletion study using the recommended dose and route of administration would be required.

Nicarbazin

Acceptable daily intake: 0-400 µg/kg bw

Residue definition: N,N’-bis-(4-nitrophenyl)urea

Recommended maximum residue limits (MRL)
Species	Muscle (µg/kg)	Liver (µg/kg)	Kidney (µg/kg)	Fat (µg/kg)	Milk (µg/litre)
Chickens (Broilers)	200	200	200	200

Glucocorticosteroid

Dexamethasone

Acceptable daily intake: 0.015 µg/kg bw (established at the forty-second meeting of the Committee (WHO TRS 851, 1995))

Maximum residue limits: The forty-second and forty-third meetings of the Committee recommended temporary MRLs of 0.5 mg/kg in muscle, 0.5 mg/kg in kidney and 2.5 mg/kg in liver of cattle, horses and pigs and 0.3 mg/L in cattle milk based on an

ADI of 0-0.015 mg/kg body weight. The MRLs were temporary because there was no adequate method to determine compliance with the MRLs. The Committee requested performance data on the analytical method for evaluation at its forty-eight meeting but no data were received. The Committee decided to withdraw the temporary MRLs for dexamethasone due to lack of an adequate analytical method for enforcement of the MRLs. The present Committee reviewed the documentation for an HPLC-MS method for measuring dexamethasone residues in tissues and milk. The Committee concluded that the proposed method does not meet the required performance characteristics for idntification and quantification of residues in incurred tissues. MRLs could not be recommended because a suitable method for residue analysis was not available.

Production aid

Recombinant bovine somatotropins (rbSTs)

Acceptable daily intake: ADI “not specified”1 (applies to somagrebove, sometribove, somavubove, and somidobove)

Maximum residue limits: MRLs “not specified” in cattle milk and edible tissues2 (applies to somagrebove, sometribove, somavubove, and somidobove)

¹See Annex 1. ADI “not specified” means that available data on the toxicity and intake of the veterinary drug indicate a large margin of safety for consumption of residues in food when the drug is used according to good practice in the use of veterinary drugs. For that reason, and for the reasons stated in the individual evaluation, the Committee concluded that use of the veterinary drug does not represent a hazard to human health and that there is no need to specify a numerical ADI.
²See Annex 1. MRL “not specified” means that available data on the identity and concentration of residues of the veterinary drug in animal tissues indicate a wide margin of safety for consumption of residues in food when the drug is used according to good practice in the use of veterinary drugs. For that reason, and for the reasons stated in the individual evaluation, the Committee concluded that the presence of drug residues in the named animal product does not present a health concern and that there is no need to specify a numerical MRL.

Tranquilizing agent

Azaperone

Acceptable daily intake: 0-6 µg/kg bw

Residue definition: Sum of concentrations of azaperone and azaperol.

Recommended maximum residue limits (MRL)
Species	Muscle (µg/kg)	Liver (µg/kg)	Kidney (µg/kg)	Fat (µg/kg)	Milk (µg/litre)
Pigs	60	100	100	60

The information provided in this annex is the draft report item summarizing the assessment of rbSTs at the fiftieth meeting of JECFA, and is included to provide quick dissemination of information. It is subject to extensive editing, and should not be quoted or referenced until publication of the report.

Annex 1. Recombinant bovine somatotropins (rbSTs)

The four analogues of bovine somatotropins (rbSTs) somagrebove, sometribove, somavubove and somidobove that are produced by recombinant DNA techniques were previously evaluated by the Committee at its fortieth meeting (WHO TRS 832, 1993). At that time the Committee established an ADI and MRLs “not specified” for these four rbSTs. The term “not specified” was used because of the lack of oral activity of rbSTs and of insulin-like growth factor I (IGF-I) and the low levels and non-toxic nature of the residues of these compounds, resulting in an extremely large margin of safety for humans consuming meat and dairy products from rbST-treated cows.When considering adoption of these recommended MRLs at its Twenty-second Session in 1997,the Codex Alimentarius Commission postponed a decision pending a reevaluation of the rbSTs by the Joint FAO/WHO Expert Committee on Food Additives (JECFA) to consider scientific information that has become available since its previous evaluation.

Information was submitted by organizations and individuals relating to the following concerns:

the increased use of antibiotics with a higher rate of violative drug residues in milk due to a possible increased incidence of mastitis in rbST-treated cows,

the possibility that increased levels of IGF-I in milk of rbST-treated cows might lead to increased cell division and growth of tumours in humans,

the potential effect of rbST on the expression of certain viruses in cattle, particularly the retroviruses,

the possibility that the incubation period of bovine spongiform encephalopathy (BSE) is shortened due to an IGF-I-induced increase of the production of pathogenic prion proteins, and

the possibility that early exposure of human neonates to milk from rbST-treated cows increases the risk for developing insulin-dependent diabetes mellitus.

Use of antibiotics

After reviewing available information, the Committee considered the risk of mastitis induced by rbST as an issue of animal health that is not within the terms of reference of the Committee.However, the possible increased use of antibiotics was considered.

A post approval monitoring program (PAMP) has been established in the United States to address the following areas:

the incidence of mastitis and responses related to herd health (not within the terms of reference of the Committee),

the treatment with any medications in a 28-herd study with rbST-treated cows (not within the terms of reference of the Committee),

the incidence of milk discard due to violative drug residues in key dairy states representing at least 50 % of U.S. milk production.

In New York State the percentage of milk discard resulting from antibiotic residue testing was not significantly changed after introduction of rbST. In other states a small, but statistically significant, increase was observed in 1995 which coincided with a change to a more sensitive testing method. The Committee concluded that the use of rbST will not result in a higher risk to human health due to the use of antibiotics to treat mastitis and that the increased potential for 9 drug residues in milk could be managed by practices currently in use by the dairy industry and by following label directions for use.

IGF-I levels in milk and tissues

IGF-I is a normal component of milk and is found in abundance in variety of body fluids (see Table 1).

Table 1: IGF-I in milk and body fluids

			[ng/ml]		[ng/ml]
Milk				Gastrointestinal secretions (human)
	human		5 - 10	Saliva	6.8
		colostrum	8 - 28	Gastric juice	26
	bovine (bulk milk)			Pancreatic juice	27
		untreated	1 - 9	Bile	6.8
		rbST-treated	1 - 13	Jejunal chyme	180
Plasma
	child		17 - 250
	adolescent		180 - 780
	adult		120 - 460
Daily production			[ng/day]
	adult humans		107

The presence and concentrations of IGF-I were at the center of much of the scientific discussion in the original scientific review of bST undertaken at the fortieth meeting of the Committee and in submissions to the present meeting. Information that was reviewed is summarized in FAO Food and Nutrition Paper No. 41/5 (1993). IGF-I concentrations in milk are variable and have been shown to depend on state of lactation, nutritional state and age.

Methods for assaying IGF-I were considered by the Committee. Although incomplete removal of IGF-binding proteins or variation of standard source and extraction methods might influence reported values, these factors were not perceived to materially alter any conclusions. Relatively high values previously reported in milk were considered to reflect inadequate extraction procedures.

Since the previous evaluation, very little additional data on residues have appeared in the literature or in reports provided by interested parties. However, the manufacturer of sometribove submitted additional information on levels of IGF-I in retail milk after the approval of rbST in the United States. The results showed no difference in the IGF-I concentrations between labeled (certified to be derived from cows not treated with rbST) and unlabeled milk. However, the percentage of milk derived from cows receiving rbST was not specified for the unlabeled milk.

Concerns have been expressed that any rbST-induced increase of IGF-I in milk contribute to the endogenous levels of IGF-I in the gastrointestinal tract and in serum if not biodegraded and if absorbed. A recent study in rats confirms that IGF-I is rapidly degraded in the gastrointestinal tract. However, in these studies a protective effect of casein on IGF-I was demonstrated. It was postulated that the retarded degradation leads to increased serum levels of IGF-I (as has been shown in one study in rats) as well as to prolonged exposure in the gut as well as to increased serum levels of IGF-I. The Committee also noted that seven days of oral ministration of high doses of IGF-I in milk replacer did not increase circulating concentrations of IGF-I in newborn calves and piglets, indicating that significant absorption of IGF-I is unlikely to occur under physiological circumstances.

Considering the decreased rate of degradation observed in the small intestine of the rat in the presence of casein, levels of the growth factor would likely deplete to less than 5% of their initial values within two hours, indicating that milk-borne IGF-I would not be expected to contribute to levels of IGF-I in the large intestine.

Assuming the ingestion of 1.5 litres of milk per day, the average ingested amount of IGF-I will be 6000 ng in milk from untreated cows containing an assumed IGF-I concentration of 4 ng/ml and 9000 ng in milk from rbST-treated cows with an approximate average concentration of 6 ng/ml. It has been calculated that IGF-I in gastrointestinal secretions amounts to about 380,000 ng/day. Therefore the additional amount of IGF-I in 1.5. l of milk from rbST-treated cows as compared with milk from untreated cows is only about 0.8 % of gastrointestinal secretion of IGF-1.

The total amount of IGF-I in serum has been calculated to range from approximately 50 000 to 1 220 000 ng depending on age. The total daily IGF-I production in adult humans has been estimated to be 107 ng. Therefore, the daily amount of IGF-I ingested with milk from rbST-treated cows compared with its daily production will be less than 0.09% for adults. Even if the total amount of milk-borne IGF-I were absorbed the additional amount would be negligible.

The Committee concluded that any increase of IGF-I in milk from rbST-treated cows is orders of magnitude lower than the physiological amounts produced in the gastrointestinal tract as well as in other parts of the body. Thus, the Committee concluded that the intake of IGF-I will not increase either locally in the gut or systemically. Consequently, the potential for IGF-I to promote tumour growth will not increase when milk from rbST-treated cows is consumed, resulting in no appreciable risk for consumers.

Recent studies have been performed in which sustained-release rbST was administered to cattle once every two weeks for a total of 20 weeks. Tissue levels of rbST and IGF-I were measured two weeks after the final administration of rbST. No significant increases in rbST and IGF-I levels were observed.

Expression of retroviruses

Concerns that rbST treatment of cattle would increase the expression of retroviruses, including Bovine Leukemia Virus (BLV), were addressed by experiments in a goat model that used caprine arthritis encephalitis virus. Infectivity was not increased when measured by numbers of infected cells and there was no evidence of increased reverse transcriptase activity. These studies provided no evidence that rbST affects the expression of BLV, a lentivirus in cattle. Furthermore it has been shown that BLV will be destroyed by simulated pasteurization conditions by heating milk to 60oC for 30 seconds. Inaddition, there is no evidence of human susceptibility orresponses to ruminant retroviruses.

Expression of prion proteins

Concerns have been expressed that rbST treatment could shorten the incubation period for BSE. This hypothesis is based on in vitro results in a neuronal cell line indicating an increased formation of mRNA of prion proteins (PrP) in response to IGF-I. Furthermore, in transgenic mice harbouring multiple copies of PrP gene, an increased formation of PrP shortened the incubation period of Scrapie. However, no data were available that directly address whether rbST or IGF-I increases the formation of normal PrP or its pathogenic protease-resistant mutant in the brain of cattle. The Committee considered that the possibility of a link between rbST-treatment and BSE to be highly speculative.

Source: FAO/WHO

Risk of insulin-dependent diabetes mellitus (IDDM)

It has been shown that exposure of neonates to cow’s milk increases the risk of IDDM 11 approximately 1.5-fold. The Committee considered whether exposure of human neonates to milk from rbST-treated cows further increases this risk and concluded that, because of its unchanged composition, the milk of rbST-treated cows is not expected to represent an additional risk to the development of IDDM.

On the basis of the following:

insignificant changes in the quantities of milk discarded due to antibiotic residue testing after introduction of rbST into commercial use;

low residue levels of rbST and IGF-I in milk;

the degradation of IGF-I in the gut and its abundance in gut secretions;

the extremely low levels of ingested IGF-I when compared to endogenous production;

the lack of evidence that rbST stimulates expression of retroviruses;

lack of information directly linking rbST-treatment and BSE;

and the absence of significant changes in composition of milk from rbST-treated cows which may contribute to the additional risk of development of IDDM

the Committee concluded that rbST can be used without any appreciable health risk to consumers. The Committee reaffirmed its previous ADI and MRLs “not specified” for somagrebove, sometribove, somavubove and somidobove”.

Nutrient Management Strategies

J. M. HART, E. S. MARX, N. W. CHRISTENSEN,
and J. A. MOORE
Department of Crop and Soil Science,
Oregon State University,
3017 Agricultural and Life Sciences Building,
Corvallis 97331-7306

ABSTRACT

Large dairies on relatively small land bases are rich in nutrients, which sometimes accumulate in excess of crop use. Twenty-six silage corn fields in the Willamette Valley, Oregon were sampled over a 2-yr period. Soils at approximately 40% of the sites contained NO3 N in excess of crop need. Excess amounts of P and K were measured from the same fields. Unused plant nutrients can be potential pollutants or animal health risks.

Nutrient accumulation on dairy farms can be reduced by eliminating or reducing commercial fertilizer and reducing the net accumulation of on-farm nutrients from purchased feed. Each dairy should produce as much of its feed as possible to reduce the importation of nutrients in feed and the cost of purchased feed. Dairy producers must match plant nutrient need and uptake with crop growth and distribute nutrients on land as needed. A program of monitoring nutrient need and sufficiency through analysis of manure, soil, and plant tissue allows nutrient distribution to be matched to crop need. ( Key words: nutrient loading, soil testing, soil nitrate nitrogen, monitoring system)

Abbreviation key: PSNT = pre-sidedress soil nitrate test.

INTRODUCTION

In the five decades since the end of World War II, agricultural production in the US has become specialized. One result of specialization is large livestock units on a small land base. In addition, the production of feed grain and animals is concentrated in disparate areas. One factor that has allowed the segregation of production to occur has been an inexpensive and reliable source of commercial N fertilizer (6).

Commercial fertilizer, primarily N, is fixed ortransformed from atmospheric N2 gas to a form thatis available to plants via the Haber-Bosch process(10). Nitrogen is the nutrient that is most likely tolimit feed grain production. When an alternative Nsupply was made available, feed grain productioncould be uncoupled from the traditional N source,animal manure, but this specialized production concentratednutrients on animal production sites withno mechanism for the return of the unused nutrientsin manure to the site of feed grain production (6).

The central components of Figure 1 illustrate thesituation before World War II. A cow consumedforage, utilized some of the nutrients in forages, andexcreted the remaining nutrients in manure. The manurewas applied to the soil, which in turn producedadditional forage. Small amounts of nutrients left thefarm in milk and meat. After the introduction ofcommercial fertilizers, additional nutrients were utilizedin production on the dairy and brought to thefarm as feed. As more nutrients were added, the soilacted as a buffer, holding the nutrients in place. Onmany dairies, enough nutrients have been added sothat the soil can no longer hold them.

Examples of N, P, and K loading from dairies inwestern Oregon are shown in Figures 2 to 4 (8).Figure 2 compares soil NO3 N in fields from a smalldairy (800 cows). In addition, the smalldairy used a clear water source for irrigation, and thelarge dairy used a wastewater lagoon as an irrigationwater source. The soil profile from both dairies containedapproximately the same amount of NO3 N atplanting. The primary difference between the twosites is the amount of N remaining at the end of thegrowing season. After harvest, the soil profile at thesmall dairy contained 193 kg/ha (172 lb/a) NO3 N,which is approximately 65 kg/ha (60 lb/a) less thanat planting, compared with approximately 400 kg/ha(350 lb/a) more at harvest than at planting for thelarge dairy. Approximately 40% of 26 fields sampledover a 2-yr period contained similar amounts of NO3 N in their profile as the large dairy used in thisexample (8).

At planting, the soil profile from the large dairyshows an increase in NO3 N concentration beginningat 90 cm (3 ft). Lower NO3 N concentrations in thesurface 60 cm of soil are most likely from leaching of NO3 N from the surface to <90 cm (3 ft) in the soilprofile during rainy winter months. Christensen andBrett (1), and Kjelgren ( 5 ) showed that the N from aspring fertilizer application that remained in the soilprofile at the end of the summer growing season waslost by January or February and presumed to accumulatein groundwater.

The accumulation of extractable P as a result ofcontinual application of manure and commercial fertilizeris illustrated in Figure 3. Two adjacent fields ofthe same soil type are compared. Extractable P concentrationsare higher in the surface 30 cm (12 in)than the remainder of the soil profile, indicating littlemobility for P in soil. Subsurface concentrations of Pare similar in both fields, which provides additionalevidence that P has not moved appreciably in thissoil. The comparable amounts of P measured at plantingand harvest indicate that the application rate isthe same or greater than crop utilization or that thesoil-buffering capacity produces an equilibrium betweencrop use and P application. Lower utilization ofP by crops than the application rate is another reasonthat the surface P concentration in a field spread withmanure is more than three times the P concentrationin a field without manure. Most dairy producers andagronomists have been applying manure to meet cropN demands. In doing so with dry stacked manure, anequal amount of N and P2O5 are applied. Corn silagewill use most of the N but only < 25% of the applied P.The high concentration of extractable P in the surfaceof the soil is an indication of an excess application ofmanure for many years.

Figure 1. Nutrient flow, cycling, and sources on dairies after World War II.

Soil profile K characteristics, which are similar to those for P, of the same two fields are presented in Figure 4. Surface soil concentrations of K are higher than subsurface measurements. Subsurface K concentrations are similar <90 cm (3 ft). The amount of K in the soil at planting and at harvest is the same, and the field that was spread with manure contains 10-fold the K in the surface as does the field on which no manure was spread.

The accumulated nutrients have economic and environmental implications. The excess nutrients in the soils that have been spread with manure can be discharged into the environment as pollutants or cycled through forage at elevated concentrations. Elevated amounts of each nutrient pose different risks. Examples of concern about elevated nutrient concentrations in groundwater, surface water, and animal nutrition follow.

The standards of the Environmental Protection Agency (14) require nitrate concentrations in drinking water to be < 10 mg of NO3 N/L. Drinking water containing NO3 in excess of this amount is associated with methemoglobinemia, a restriction of oxygen renewal in the bloodstream. Infants are especially sensitive to NO3 in drinking water, which causes “blue baby syndrome” (2). The Oregon Department of Environmental Quality rates agricultural activities fourth in a ranking of 12 major sources of groundwater contamination. Nitrate N and some pesticides are the potential agricultural contaminants of greatest concern (11).

Phosphate contamination of surface water is a problem in areas where a large livestock population is located. Phosphates move to surface waters by overland flow and erosion. The P concentration in surface water is one of two nutrients controlling growth of freshwater phytoplankton and is often considered to be the leading indicator of water quality in regard to eutrophication (13).

In contrast to NO3 and PO4, K poses little or no contamination threat to ground and surface water but presents the dairy producer with a concern. Animal nutritionists recommend that dietary K not exceed 3% of a dairy cow ration. Dry cows are especially susceptible to problems induced by excessive K, which include milk fever, hypocalcemia, downer cow syndrome, and, in severe cases, death. Hypocalcemia affects the smooth muscle contraction of the rumen, abomasum, and uterus, increasing the risk of retained placenta and displaced abomasum (17). Perennial grasses accumulate K in excess of growth requirements, especially on soil to which manure has been added. Adequately fertilized orchardgrass forage consumed by animals on pasture or as silage rarely contains < 2.5% K and generally contains ?3% K (4). In an effort to utilize the maximum amounts of manure N from large herds on an inadequate land area, dairy producers grow perennial grasses and fertilize with 560 to 675 kg of N/ha per yr (500 to 600 lb N/a per yr). Manure from a reception tank or storage lagoon contains equal amounts of N and K. Less than one-half of the K is needed by the plant for growth, but excess K is taken up, producing high K forage. An increase in forage K concentrations was documented in British Columbia where grass forage K increased from 2.7% in 1983 to 3.6% in 1992 (12).

Excess nutrients and the continued increase in nutrient concentration in soil and plants can be reduced by planning nutrient flow and crop utilization, monitoring nutrient flow on a dairy, and reducing nutrient inputs.

TABLE 1. Nutrient content of dry matter for selected forages.

MANAGEMENT THEORY

Plan

Feed and forage needs should be planned for the next 5 to 10 yr, and needs should be examined to determine whether feed can be produced on the farm to replace purchased feed. A plan should be developed for each field that maximizes the use of nutrients in manure while producing feed for the dairy. The goal is to reduce nutrient input from sources off the farm such as commercial fertilizer and purchased feed while distributing manure based on crop requirements. Each field should have a nutrient management plan through several rotations that can be adjusted using soil test results. The goal should be to match manure nutrients with crop need. Manure storage needs should be computed based on nutrient needs and appropriate timing of manure applications.

Matching manure application to cropping conditions requires an understanding of nutrient removal by crops. Table 1 shows that the relative amounts of N and K are approximately equal in the three forages listed, but the amount of N removed per ton of dry matter in silage corn is approximately half the amount of N contained in alfalfa or orchardgrass chopped for silage. In the Pacific Northwest, where 18 to 22 Mg/ha (8 to 10 t/a) of dry matter from grass can be produced annually, substantially more N can be removed with grass than in a silage corn crop of 16 Mg/ha (7 t/a) of dry matter [67 Mg/ha (30 t/a)].

Fields with a history of manure application typically have high soil test P and K. These fields could be planted to alfalfa to use the P and K. No manure would be needed because alfalfa will fix N. Soil test K can be lowered by several years of alfalfa production, but the same cropping plan would not be expected to have a measurable impact on soil test P.

Matching crop need and manure application is termed application at an agronomic rate. Agronomic rates are amounts of a nutrient that produce optimum growth or yield without detrimental economic or environmental consequences. An agronomic nutrient rate is a combination of soil science, plant growth, philosophy, and politics.

One approach to determining the agronomic rate is to apply the amount of nutrient that has been removed by a crop. This approach assumes that the soil does not supply nutrients to the crop and that 100% of the added material is taken up by the crop. Neither assumption is valid. The soil can supply a range of nutrients, from very little to the entire amount needed by a crop. Plant absorption of added nutrients under the best of growing conditions may be as low as 5 to 20% for P and as much as 50 to 60% for N.

Most agronomists contend that manure or commercial fertilizer applications are site specific. Recommendations for manure or any nutrient application are based on general guidelines. A nutrient application from manure or commercial fertilizer, based on the general guidelines, is an appropriate starting point. After the application of commercial fertilizer or manure, soil and tissue levels should be monitored to determine whether an appropriate amount of nutrients has been applied.

In contrast, water quality regulators from state and federal agencies would like a plan that would fit all situations and does not vary yearly.

Monitor

Soil should be monitored in relation to manure nutrient content, forage nutrient content, and end of season corn stalk and soil NO3 N. Manure application rates should be altered, or distribution of your manure nutrient should be revised, based on results of nutrient monitoring. The manure application rate can be reduced by increasing the farm acreage base or by hauling manure to neighboring land. Standard soil tests for pH, P, and K should be used.

Soil testing differs from mineral nutrient analyses of animal feeds, expressed on a total elemental basis. In contrast, P and K analyses of soil samples extract a portion of each nutrient that is correlated with plant growth. A parallel analysis performed by animal scientists is analysis of feed for digestible nutrients or a fraction of the feed instead of a total carbohydrate analysis.

Soil test results are generally expressed in parts per million of extractable PO4 or K and can be considered an index of nutrient availability. For example, a P soil test of 100 ppm is considered to be very high and indicates that no fertilizer P is needed. Simple conversion of the soil test for P to a rate (pounds per acre) is not an appropriate method for making a fertilizer recommendation because soil test P is correlated with plant growth rather than being a measure of P amount in the soil.

Fertilizer concentration of P and K are traditionally expressed as P2O5 or K2O. A fertilizer with a grade of 10-20-5 contains 10% N, 20% P2O5, and 5% K2O. Local extension offices provide information on analyses and interpretation of a standard soil test.

Reduce

Excess nutrients added to cropland should be reduced by reducing or eliminating the purchase and use of commercial fertilizer, which will reduce nutrient loading and save money. For example, amounts and nutrient contents of manure produced by a cow vary. For our purposes, a high figure will be used to ensure that sufficient manure storage and land are available for manure application. Following this logic, each year a 640-kg (1400-lb) lactating cow producing 32 kg (70 lb) of milk/d excretes the following nutrients in manure: 110 to 135 kg (250 to 300 lb) of N, 20 kg (45 lb) of P, and 75 kg (165 lb) of K (15). The manure from one cow is sufficient to supply all of the N needed for 0.6 ha (1.5 a) of silage corn or 0.25 ha (0.66 a) of grass forage cut five to six times. Using the nutrient excretion information for a cow, a ratio of 3.75 to 5 cows/ha (1.5 to 2.0 cows/a) provides a sufficient amount of nutrients to grow most crops with no addition of commercial fertilizer. If the dairy producer has sufficient amounts of nutrients in manure and is purchasing commercial fertilizer, use of manure only would result in a savings of $150 to $250/ha ($60 to $100/a) for the production of corn for silage.

IMPLEMENTATION

The remainder of this paper presents methods for monitoring N on dairies with the production goal of agronomic, environmental, and economic crops. The examples used monitor N in corn production for silage.

Nutrient management can be addressed by answering three questions:

How much N should be applied?

When should N be applied?

What source of N should be used?

Application Rate

To determine how much N should be applied, the N requirement of the corn plant and the N that is available for those needs must be determined. A 65-Mg/ha (30-t/a) silage crop requires approximately 220 kg/ha (195 lb/a) of N (8). When corn is at the five- to six-leaf stage and is 30 cm (12 in) tall, the crop has taken up approximately 5% of the total requirement, or 11 kg/ha (10 lb/a) of N. From the sixleaf stage until silking, about two-thirds of the total N is taken up by the corn plant. This period is characterized by rapid growth and N demand (Figure 5a).

An adequate supply of N from the soil after the sixleaf stage is critical for optimal yield of silage corn. The interpretation of available soil N, primarily as NO3 N, has been relatively unsuccessful in humid climates until the last decade. The reason for the inability of soil tests to predict N availability in humid climates is illustrated in Figure 5b. Nitrogen mineralization, the conversion of organic to available inorganic N, is a biological process governed by organic N supply, temperature, soil pH, and soil moisture. Mineralization proceeds at various rates, some of which supply sufficient N for the crop (Figure 5b, A or B), or a rate (C) that is insufficient to meet crop demand. Soil sampling and analyses early in the season could not differentiate among the mineralization rates (Figure 5b), or adequately predict the amount or sufficiency of NO3 N for a corn crop.

Pre-