ARS | Publication request: HOW DAIRY DIET AFFECTS MANURE NITROGEN EXCRETION AND CYCLING IN SOILS

Submitted to: Meeting Abstract
Publication Type: Proceedings
Publication Acceptance Date: January 21, 2003
Publication Date: January 21, 2003
Citation: POWELL, J.M. HOW DAIRY DIET AFFECTS MANURE NITROGEN EXCRETION AND CYCLING IN SOILS. MEETING ABSTRACT. 2003.

Technical Abstract: The next generation of environmental policy for agriculture will focus more strongly on nitrogen (N), particularly the reduction of ammonia volatilization from animal feeding operations. Ammonia loss from animal agriculture can have major detrimental impacts on air quality at regional, national and global scales. Most efforts to improve nutrient management on dairy farms neglect the effects of feeding practices on overall nutrient utilization and loss on a whole-farm basis. This paper provides an overview of N cycling on dairy farms and shows how diets can be manipulated to support high levels of milk production, and at the same time, produce excreta that is less susceptible to nutrient losses. Dairy cows typically convert 25 to 30% of feed N to milk N. Current ration formulation on many farms, however, results in much lower N conversion rates. Nitrogen not secreted in milk is excreted as 1) urinary N, 2) fecal endogenous N of microbial and gut origin and 3) fecal undigested feed N. Urea N (approx. 90-95% of total urine N) is rapidly hydrolyzed by the enzyme urease found in the feces, and is liable to be lost as ammonia to the atmosphere. Fecal endogenous N (60 to 80% of total fecal N) mineralizes quickly in soils to ammonium and nitrate, and is readily available to plants. Undigested feed N in feces, however, mineralizes at a slower rate and is consequently more stable in soils. A reexamination of the response curve for milk production, protein yield and N excretions as a function of diet would provide quantitative information about production losses (if any) to achieve alternative targets, such as reduced manure N excretion or diversion of greater proportions of excreta N to more stable fecal versus urinary forms.