REDUCED CRUDE PROTEIN RATIONS FOR HIGH PRODUCING COWS: PRODUCTION AND ENVIRONMENTAL EFFECTS

摘要

Ruminants make efficient use of diets that are poor in true protein content because microbes in the rumen are able to synthesize a large proportion of the animal's required protein. The amino acid (AA) pattern of this protein is of better quality thannearly all of the dietary ingredients commonly fed to domestic ruminants (Broderick, 1994; Schwab, 1996). In addition, utilization of ammonia by ruminal microbes allows the feeding of nonprotein N (NPN) compounds, such as urea, as well as the capture ofrecycled urea N that would otherwise be excreted in the urine. Many studies have shown that lactating dairy cows use feed crude protein (CP) more efficiently than other ruminant livestock. However, dairy cows still excrete 2 to 3 times more N in manurethan they secrete in milk, even under conditions of optimal nutrition and management. Inefficient N utilization necessitates feeding supplemental protein, increasing production costs and contributing to environmental pollution. A cow producing about 8000kg of milk per lactation also excretes about 20 tonnes of wet manure containing about 120 kg of N (Van Horn et al., 1996). The 15 million dairy cows and replacement heifers in the U.S. produce over 1 million tonnes of manure N every year (Kellogg et al., 2000). It is estimated that about 25% of dairy manure N is lost as ammonia under current U.S. practices (NRC, 2003). Dairy farmers are increasing in size, importing more feed, and feeding more protein, further contributing to nutrient accumulation on land in dairy regions and to greater impacts on the environment (Bundy and Sturgul, 2001).