The Impact of Dairy Herd Management on Nutrient Losses to Water Resources By Richard Kohn, Ph.D., University of Maryland

Introduction Current programs to reduce nutrient losses from farms have focused on soil and manure management. These practices by themselves are not adequate to reduce nutrient losses by 40% as needed to restore the Chesapeake Bay ecosystem.

The feeding and management of dairy cattle have a profound impact on reducing nutrient losses to water resources. With improved herd management, less manure is produced so fewer manure nutrients are left to runoff or be leached. In addition, productivity can be maintained with less feed, which means there is a lower requirement for crop production and fertilizer use.

The objectives of the current research are 1) to evaluate new technologies in herd management and feeding for their potential to reduce nitrogen and phosphorus excretion in manure, and their potential to reduce nutrient losses from the farm, 2) to estimate the cost-effectiveness of these technologies, and 3) to make recommendations for technology transfer, cost-share, tax credit, or other incentive programs to encourage implementation of desired programs.

Several new technologies were evaluated for their impact on changes in nutrient excretion to manure by summarizing data in the literature and developing mathematical models. Most animal research is conducted on individual animals and so the impact on the herd was calculated from the results of animal trials by aggregating according to expected herd distributions. The predicted change in nutrient losses from the farm that results from application of new technologies was calculated using a model adapted from previous efforts.

This research was supported by the US EPA Chesapeake Bay Program.

Results

• Management that increases production per cow can reduce nutrient losses to manure for the herd. Administration of bovine growth hormone to selected lactating cows, extending photoperiod with artificial lighting, and milking three times daily would each reduce nutrients in manure by 8, 5 and 7% respectively.
• A method was developed to fine tune dairy cow diets for protein feeding from analysis of milk composition. The amount of milk urea N and other variables can be used to predict N consumed in feeds and identify when cows are eating too much protein. Using this method to fine tune diets could reduce N output to manure by 6% initially, and lead to the discovery of other methods to improve N utilization in dairy cows.
• Current recommendations for phosphorus feeding assume that consumed phosphorus (P) is only 50% digestible. Research trials are needed to test the accuracy of this assumption. If P digestibility can be assumed to be 65% digestible, P in manure could be reduced by 35% and many farms that are currently accumulating P in soils will come into P balance.
• Most dairy cattle diets in the U.S. are balanced using the National Research Council (NRC) recommendations. A newer model called the Cornell Net Carbohydrate and Protein System (CNCPS) is often suggested to feed cattle more efficiently. We compared both models on two different large data sets. Using the CNCPS would have over fed or under fed dairy cows depending on the feeds used in the diets, and it is not recommended for routine formulation of diets for dairy cows. Further diet formulation research is needed.
• Theoretically, the use of protected amino acid supplements can reduce the total amount of protein needed in a ration and result in up to 20 to 40% less N in the manure produced by a dairy herd. Despite the theoretical benefits of using protected amino acids, in practice, further work is needed to improve our understanding of animal amino acid requirements. Research is needed to improve diet formulation models to balance for amino acid requirements.
• Dairy farmers typically feed all of the cows in a particular pen or barn the same diet. Each animal produces a different amount of milk, gains a different amount of weight, etc. and therefore each animal actually requires a different amount of energy, protein and minerals. Grouping cows affects nutrient balance in the herd. When feeding to meet the requirements of one cow in a group, a different cow may be overfed or underfed. When feeding all lactating cows together according to current recommendations, about 10% more N and P would end up in manure than when feeding each cow individually according to her requirements.
• Grazing is often considered an environmentally friendly method of animal production. Total N losses per acre were predicted to be 3.7 times greater for confinement systems compared to the grazing systems. However, milk production per acre was 4.3 times greater for the confinement systems. Grazing systems resulted in lower nutrient losses per acre but greater nutrient losses per unit of milk and meat produced.
• The potential to reduce nutrient losses by optimizing crop selection to meet annualized herd feed requirements with minimal nutrient losses from growing crops was investigated. Corn silage-based farms that import all grains would be able to comply with N-based nutrient management planning and need to purchase chemical N, while alfalfa-based farms that import grains would apply excess N. Nonetheless, the alfalfa-based farms would result in 3.3 units of N loss per unit of N in meat and milk while the corn-based farms would have resulted in 3.5 units of N loss per unit of N in meat and milk. The combination of alfalfa and corn silage was the best with only 2.9 units of N loss per unit of N in meat and milk.

Conclusions

Using multiple strategies to improve nutrient utilization in dairy cattle could reduce N and P feeding by more than 50%. About half the manure N will be lost from the farm before becoming available to crops in soils, and about half of the soil available N will be lost from the soil before being taken up and harvested in crops. With these assumptions, reducing feed N requirements by 50% without harming production could reduce the need for feed N by 50%, and reduce manure N output by 59%. In a typical dairy production system (including the production of imported feeds), improving N utilization in the animal by 50% would decrease total farm N losses by 55%.

Source: University of Maryland
Author: Richard Kohn, PhD