Manipulation of the Dietary N-Fractions to Improve Ruminal Microbial Synthesis and Yield

Ruminants are efficient users of diets poor in amount and quality of crude protein (CP) because ruminal microbes synthesize a large proportion of the animal’s needs for metabolizable protein and essential amino acids (EAA). In addition, microbial utilization of ammonia allows the feeding of non-protein N (NPN) compounds, such as urea, and enables ruminants to utilize urea-N that is recycled to the rumen (Lapierre & Lobley, 2001). This latter mechanism might be employed to capture some urea N that would otherwise be excreted in the urine. High secretion rates for milk protein are why lactating dairy cows have the highest EAA requirements among domestic ruminants. Although dairy cattle can utilize diets with relatively poor quality CP, they typically excrete 2 to 3 times more N in manure than they secrete in milk. This N inefficiency necessitates feeding large amounts of supplemental protein, which increases production costs and contributes to environmental pollution. Because of stagnant milk prices, U.S. dairy farms are increasing in size to capture the economies of scale and these larger operations are importing more feed and increasing animal units per hectare Greater animal densities result in increased nutrient accumulation on the farmland and in greater environmental impact from dairying.