Nutritional Strategies for Optimizing Nitrogen Utilization by Dairy Cows

266 p.Thesis (Ph.D.)--University of Illinois at Urbana-Champaign, 2004.A meta-analytic review and three experiments with multiparous Holstein cows were conducted to test the hypothesis that the crude protein (CP) content of the diet can be decreased to improve the efficiency of N utilization for milk production without compromising the supply of metabolizable protein and the lactational performance of dairy cows. Results from the meta-analysis showed that large variation exits in the responses of dairy cows to the amount and source of dietary CP. A large proportion of this variation is explained by the source of CP in the control diets, the proportion and source of rumen undegradable protein in the experimental diets, and the CP percentage of the diet. Experiment 1 was designed as a 6 x 6 Latin square to examine the effects of the percentage and source of CP and the amount of starch in the diet of dairy cows on ruminal fermentation, passage of nutrients to the small intestine, and nutrient digestibility. Two sources of CP (soybean meal [SBM] and a mixture of SBM and a blend of animal-marine protein supplements plus rumen-protected Met) and three percentages of dietary protein (14, 16, and 18) were combined into six treatments. In experiment 2, the effects of the same treatments on the lactational performance and efficiency of N utilization for milk production were evaluated in a 210-day lactation trial that involved 60 cows. In experiment 3, four cows were used in a 4 x 4 Latin square design to evaluate the replacement of SBM with expeller SBM, heat-xylose treated SBM, or whole roasted soybeans. Results indicate that the CP percentage of the diet of lactating cows that consume large amounts of feed can be decreased to 16 to 17% to improve the efficiency of N utilization for milk production without compromising the supply of metabolizable protein and lactational performance of dairy cows if the source and amount of dietary CP and carbohydrate are properly matched. In addition, it appears that the escape of rumen undegradable protein in dairy cows that consume large quantities of corn-SBM-based diets might be underestimated by current feeding standards.U of I OnlyRestricted to the U of I community idenfinitely during batch ingest of legacy ETD