This research aimed to evaluate the monensin effect on protein degradation of soybean meal, wheat middlings and corn meal. The rumen fluid was taken from a fistulated steer fed a 40% concentrate diet. The incubations were done in an anaerobic environment at 39ºC. It was used 100 mg of each feedstuff, 14.7 mL of ruminal fluid and 0.3 mL of ethanol with or without dissolved monensin (5 µM was the final concentration). The soybean meal, compared with wheat middlings and corn meal, showed great values of ammonia production, microbial protein, final pH and specific activity of ammonia production (SAAP). The greatest ammonia production was due to higher protein content and high degradability of the soybean meal, associated with higher media pH, that stimulates deamination. The wheat middlings showed greater final pH and SAAP than corn meal, which was a feedstuff with lower degradability. The monensin decreased the SAAP and protein degradability, although it was more effective in reducing protein degradation in higher pH values. Monensin provides higher efficiency of protein utilization by ruminants in diets with high protein/carbohydrate fermentation rates and high ruminal pH, as in the case of young grasses pastures and forage legumes.
ammonia; fermentation; monensin; pH; protein; rumen
