In vivo degradation of protein in diets formulated for two degradabilities

In vivo protein degradability of two basal diets and bacterial protein synthesis were determined in four lactating dairy cows equipped with ruminal and duodenal cannulas. The diets contained corn silage, high moisture corn, and either soybean meal or a 60:40 mixture of soybean meal and corn gluten meal. Diets had calculated ruminal protein degradabilities of 69.3 and 62.3%, respectively. Both diets contained approximately 14% CP and 21% ADF. Duodenal flows of total N, total protein N, microbial N, and duodenal recovery of ingested N tended to be higher for the soybean meal and corn gluten meal diet; ruminally degraded CP was significantly lower than for the soybean meal diet. Ruminal ammonia and plasma urea concentrations tended to be higher for the soybean meal diet, as were molar percentages of butyrate and valerate. Ruminal and total tract apparent digestibilities of CP and OM were not significantly different between diets. Ruminal degradation of protein in the two diets differed by the amount predicted by the NRC system for lactating cows, although absolute values were lower than most previous estimates for similar diets.