LAUSR

Abstract This study was undertaken to investigate the effects of dietary crude protein (CP) levels and rumen-protected pantothenate (RPP) supplementation on ruminal digestion kinetics, fermentation, microbial enzymatic activity, and selected bacterial species relative abundance in beef steers. Eight ruminally cannulated crossbred beef steers, with 15 months of age and 462 ± 9 kg of body weight (BW), were used in a duplicated 4 × 4 Latin square design by a 2 × 2 factorial arrangement. Low protein (113.7 g/kg of CP [LP] or high protein (133.9 g/kg of CP [HP]) diets were fed with or without RPP supplementation (0 g [RP-] or 0.48 g per kg dietary dry matter (DM) [RP+]). Steers were fed a total mixed ration containing dietary concentrate to corn silage ratio of 50:50 on a DM basis. There were no RPP × CP interactions for ruminal digestion kinetics, fermentation, microbial enzymatic activity, and selected bacterial species relative abundance, except for Ruminobacter amylophilus . Mean ruminal pH decreased with RPP supplementation, but was unaffected by dietary CP levels. Ruminal total VFA concentration increased with increasing dietary CP levels or RPP supplementation. Ruminal acetate percentage was similar among treatments, whereas propionate percentage decreased with increasing dietary CP levels or RPP supplementation. Consequently, the acetate to propionate ratio increased with increasing dietary CP levels or RPP supplementation. Ruminal ammonia-N content was unchanged among treatments. Both in situ ruminal DM degradability of corn silage and CP degradability of concentrate increased with RPP supplementation and tended to increase with increasing dietary CP levels, while NDF degradability of corn silage and DM degradability of concentrate increased with increasing dietary CP levels or RPP supplementation. Activities of carboxymethyl-cellulase, cellobiase, xylanase, pectinase, α-amylase and protease, populations of Ruminococcus albus, Ruminococcus flavefaciens, Butyrivibrio fibrisolvens and Prevotella ruminicola, and urinary excretion of purine derivatives increased with increasing dietary CP levels or RPP supplementation, but populations of Fibrobacter succinogenes and R. amylophilus increased with RPP supplementation, and decreased with increasing dietary CP levels. The results showed that increasing dietary CP levels from 113.7 g to 133.9 g or with 0.48 g RPP supplementation improved ruminal fermentation and microbial protein synthesis by increasing microbial populations and enzymes activities.