EVALUATION OF PROTEIN FLOW TO THE DUODENUM IN DAIRY-CATTLE BY THE IN SACCO METHOD

In situ evaluation of the amount and composition of duodenal crude pro tein flow was examined in a 4 x 4 Latin square study using four lactat ing Holstein cows fitted with ruminal and duodenal cannulas. Dietary s upplemental CP sources, making up 40% of the dietary CP, were: soybean meal, cottonseed meal, com gluten meal or urea. Duodenal flow of orga nic matter (OM) and CP was determined in vivo by means of constant inf usion of Yb and Cr, and was predicted in situ from rumen degradation. Purines were used as microbial markers. The two methods of estimating ruminally undegraded protein ranked the diets similarly. Microbial CP (1.9 to 2.1 kg/d) was estimated to make up 57% to 68% of the duodenal CP flow in the experimental diets. Efficiency of microbial protein syn thesis were 201 (SEM 16) g CP/kg OM truly digested in the rumen, and 2 24 (SEM 18) g CP/kg OM disappearing in the rumen. Data are interpreted to suggest that the in situ method enables a prediction of total prot ein flow to the duodenum which is comparable to that obtained using th e in vivo method. The added practicality of the former method renders it advantageous.