Effect of dietary soybean meal and fish meal on protein digesta flow in Holstein cows during early and midlactation.

Six lactating cows were fitted with ruminal and duodenal cannula to measure protein digesta flow to the duodenum during early and midlactation. Diets were composed of corn grain, corn silage, and orchardgrass hay plus supplemental fish meal or soybean meal. Diets contained 15.5% CP and 20.7 ADF. Cobalt-EDTA and Yb were used as liquid and particulate digesta markers and cytosine was used as microbial marker. Corrected organic matter digestibilities in the stomachs were 48.4, 49.8, 44.9, and 53.2% for fish meal and soybean meal diets and early and midlactation, respectively. Preduodenal degradabilities were 47.2, 65.8, 56.7, and 56.2% for fish meal and soybean meal diets and early and midlactation, respectively. Nitrogen recoveries at the duodenum were 93.2 and 84.3% for fish meal and soybean meal diets. Intake of amino acids was greater when cows were fed the soybean meal diet, but total flows of amino acids to the duodenum were similar for both diets. Greater quantity of protein escaping ruminal degradation in cows fed fish meal compared with soybean meal was counterbalanced by less microbial synthesis in the rumen.