Feeding to Minimize Nitrogen Excretion by Dairy Cows

Dairy cows utilize feed crude protein (CP; N x 6.25) with much greater efficiency than other ruminant livestock but still excrete about 2-3 times more N in manure than in milk. This contributes to increased costs of milk production and to environmental N pollution. The function of dietary CP is to supply the cow with metabolizable protein (MP) as absorbed amino acids (AA) but any extra dietary CP that does not contribute absorbed AA that are used in production will be lost in the urine. Urinary N is the most polluting form of excretory N because much is lost as atmospheric ammonia or into surface and ground water. We conducted a number of trials testing various levels of CP in diets formulated from typical Midwest feeds. Generally, there were no increases in yields of milk, fat-corrected milk or protein with more than 16.5% dietary CP. In one trial, reducing CP to 15.6%, but adding rumen undegraded protein (RUP) as heated soybean meal (SBM), did not give production equal to 16.6% CP. However, fish meal, especially low soluble fish meal, and canola meal were found to be more effective sources of RUP than SBM or cottonseed meal. Supplementing rumen-protected methionine has also been shown to be effective for allowing some reduction in dietary CP without losing milk yield. Frequent sampling and analysis of feed ingredients is very important for tracking the CP contents of the actual diet fed. Monitoring milk urea can also be used to assess both dietary CP and urinary N 1 Mention of any trademark or proprietary product in this paper does not constitute a guarantee or warranty of the product by the USDA or the Agricultural Research Service and does not imply its approval to the exclusion of other products that also may be suitable. excretion in lactating cows. The NRC (2001) protein feeding model is useful for predicting production responses to alterations in dietary protein and should be used regularly. Hay-crop silages are the most degradable source of dietary CP. Where possible, replacing alfalfa silage with alfalfa hay will improve CP efficiency and reduce N excretion. Reducing grain particle size increases ruminal starch digestion and increases microbial protein formation, so long as ruminal pH is not depressed. The NRC (2001) model can also be used to match rumen-degraded protein with carbohydrate fermentation. Future research developments will allow even lower dietary CP levels to be fed, thus reducing N excretion, without loss of animal productivity.