SUPPORTING PRODUCTION OF MILK AND MILK COMPONENTS ONLOW PROTEIN DIETS

摘要

There is increasing interest in minimizing crude protein (CP) content of diets fed to dairy cows to reduce production costs and to improve environmental sustainability. Dietary CP not utilized for production is lost largely in the urine, the most polluting form of excretory N. Because microbial protein formed in the rumen has an AA pattern very similar to milk, optimizing its formation is an effective means of improving protein status in lactating cows. However, N in microbial nucleic acids and bacterial cell walls is of little value to the animal. Nonprotein N can replace only a portion of the dietary rumen-degraded protein. Reducing grain particle size and heat processing increase ruminai starch digestion and microbial protein formation, so long as ruminai pH is not depressed. Although predictions made by ration formulation models may be inaccurate, models can be used to improve metabolizable protein supply and essential AA pattern of absorbed protein. This will help minimize dietary CP without sacrificing production. A number of reversal-type feeding trials found that there was no increase in yield of milk or components on typical diets with more than about 16.5% CP. One such trial showed that feeding 15.'6% CP with added rumen-undegraded protein (RUP) from soybean meal (which is low in Met) did not give production equal to 16.6% CP, but milk and component yield on 16.6% CP was equal to that on 17.6% CP. A second study showed that cows fed 15.8% CP plus rumen-protected Met yielded as much fatand protein as cows fed 17.1% CP without rumen-protected Met. Other reversal trials have suggested that over-feeding CP may even reduce milk and component yield. There are substantial differences in the effectiveness of RUP sources due to differences inAA profile. Rumen-protected Lys should improve the value of the low Lys byproducts derived from ethanol production. Future research will explore ways to feed even lower dietary CP because of pressures to reduce N excretion.