Dairy Cattle Rations Impact Manure N Chemistry and N Transformations in the Environment.

Dairy cows are fed a wide range of forages, grain, protein and mineral supplements that impact feed use efficiency (FUE, or percentage of feed nutrients consumed that have been transformed into milk), manure chemistry, and manure transformations in the environment.  For example, as feed N consumption increases, FUE declines, and excretions of urinary N increase. Ammonia emissions from dairy barns and from cropland after manure application have been positively related to urine N excretion and dairy ration components. Fecal N consists of (1) endogenous N from microorganisms and/or microbial products from the rumen, small intestine, and hind gut, and N originating from the digestive tract itself; and (2) undigested fiber N. After application to soil, fecal endogenous N mineralizes quickly, but fecal undigested fiber N degrades much more slowly. This presentation will provide a summary of interdisciplinary dairy nutrition-soil science research that discovered relationships between dairy cattle rations, manure N and P chemistry and environmental outcomes. The results of this research demonstrate that a range of dietary options are available that satisfy the nutritional requirements of healthy, high-producing dairy cows and at the same time produce excreta having differential effects on the environment, including soil N mineralization and plant N uptake.