Effects of dietary protein levels and 2‐methylbutyrate on ruminal fermentation, nutrient degradability, bacterial populations and urinary purine derivatives in Simmental steers

Summary The objective of this study was to evaluate the effects of dietary crude protein ( CP ) levels and 2‐methylbutyrate ( MB ) supplementation on ruminal fermentation, bacterial populations, microbial enzyme activity and urinary excretion of purine derivatives ( PD ) in Simmental steers. Eight ruminally cannulated Simmental steers, averaging 18 months of age and 465 ± 8.6 kg of body weight ( BW ), were used in a replicated 4 × 4 Latin square design by a 2 × 2 factorial arrangement. Low protein (98.5 g CP /kg dry matter [ LP ] or high protein (128.7 g CP /kg dry matter [ HP ]) diets were fed with MB supplementation (0 g [ MB −] or 16.8 g steer −1 day −1 [ MB +]). Steers were fed a total mixed ration with dietary corn straw to concentrate ratio of 50:50 (dry matter [ DM ] basis). The CP × MB interaction was observed for ruminal total VFA , molar proportions of acetate and propionate, acetate to propionate ratio, ammonia‐N, effective degradability of neutral detergent fibre ( NDF ) and CP , microbial enzyme activity, bacterial populations and total PD excretion ( p < .05). Ruminal pH decreased ( p < .05), but ruminal total VFA concentration increased ( p < .05) with increasing dietary CP level or MB supplementation. Acetate molar proportion increased ( p = .043) with MB supplementation, but was not affected by dietary CP level. Propionate molar proportion decreased ( p < .05) with increasing dietary CP level or MB supplementation. Consequently, acetate‐to‐propionate ratio increased ( p = .001) with MB supplementation, but was not affected by dietary CP level. Ruminal ammonia‐N content increased ( p = .034) with increasing dietary CP level, but decreased ( p = .012) with MB supplementation. The effective degradability of NDF and CP increased ( p < .05) with increasing dietary CP level or MB supplementation. Microbial enzyme activity, bacterial populations and total PD excretion also increased ( p < .05) with increasing dietary CP level or MB supplementation. The results indicated that ruminal fermentation, nutrient degradability, microbial enzyme activity, ruminal bacterial populations and microbial protein synthesis improved with increasing dietary CP level or MB supplementation in steers.