Effect of ruminal degradability of crude protein and nonstructural carbohydrates on the efficiency of bacterial crude protein synthesis and amino acid flow to the abomasum of dairy cows.

Four lactating Israeli Holstein cows that were ruminally and abomasally cannulated were used in an experiment with a 4 x 4 Latin square experimental design to study the effects of different amounts of ruminally degradable crude protein (CP) and ruminally degradable nonstructural carbohydrates on ruminal fermentation and efficiency of bacterial CP synthesis. Four diets were formulated to contain the following percentages (percentage of respective fraction) of ruminally degradable protein (RDP) and ruminally degradable nonstructural carbohydrates, respectively: 1) 73.8 and 85.3%, 2) 72.4 and 75.3%, 3) 67.7 and 86.0%, and 4) 66.3 and 76.0%. The 2 x 2 factorial effects of high and low concentrations of RDP or nonstructural carbohydrates were examined. Intakes of DM and organic matter (OM) were similar among treatments, and apparent and true ruminal digestibilities of OM were also similar. Apparent digestibility of CP in the total tract was higher for diets containing high concentrations of ruminally degradable nonstructural carbohydrates. Efficiency of microbial CP synthesis was higher for diets supplemented with low concentrations of RDP and averaged 196 g of microbial CP/kg of OM truly digested in the rumen. Total and bacterial CP flows were higher for diets containing low concentrations of RDP. Therefore, greater amounts of amino acids (AA) of bacterial origin reached the abomasum. The abomasal flow of AA was higher for diets containing low concentrations of RDP. Most of the profiles for essential AA in the abomasum were influenced and balanced by profiles for bacteria. When diets contained a high concentration of RDP (73% of total dietary CP), the supplementation of a high concentration of ruminally degradable nonstructural carbohydrates had no positive influence on bacterial yield or efficiency of bacterial CP synthesis. Other factors, such as AA and peptides included in the RUP fraction, may be important to maximize the efficiency of bacterial CP synthesis.