Rumen availabilities of nonstructural carbohydrate and protein estimated from in situ incubation of ingredients versus diets

Four diets formulated for high and low levels of rumen available nonstructural carbohydrate and protein were incubated in situ. Ingredients used to formulate these diets were incubated separately to determine if the degradation characteristics of the mixed diets could be predicted from the ingredients. A second objective was to determine if the level of rumen available nonstructural carbohydrate or protein affected degradation. Rumen availability of nonstructural carbohydrate was predicted within 4% and rumen available protein within 6% when estimates were made from ingredients versus the total diets. Diets that contained lower levels of rumen available protein had slower rates of degradation of dry matter, protein, and nonstructural carbohydrate than predicted from ingredients but rate estimate differences were not large enough to cause significant differences in actual versus predicted ruminal availabilities. Lower ruminal ammonia and(or) peptides may have inhibited degradation rate when cows were fed lower rumen available protein diets. Diets can be formulated for different ruminal availabilities of nonstructural carbohydrate and protein when these estimates are made from ingredients incubated in situ. Degradation rates of diets may be over-predicted if slowly degrading protein sources are used.