MONENSIN AND NITROGEN UTILIZATION BY STEERS FED CONCENTRATE DIETS

Abstract

Monensin was fed at levels of 0 and 33 ppm in a series of experiments to determine its effect on nitrogen (N) utilization by steers fed diets high in sorghum grain (76%, steam-processed, flaked) and corn (90%, whole shelled). Total ruminal volatile fatty acid (VFA) concentrations measured 3 hr postprandially were not affected by monensin addition to either diet. Molar proportion of acetic acid decreased (P < .05) was observed with monensin addition to the sorghum grain-based diet. Monensin had no effect on apparent total tract digestibility of dry matter or energy in either diet, but consistently improved apparent total tract digestibility of crude protein (CP) in both diets. Improvement was greater (P < .05). Retention of N tended (P > .05) to improve in response to monensin with the sorghum grain-based diet (24 vs 20% of N intake; 41 vs 36% of N absorbed). Ruminal ammonia concentrations measured 3 hr post-prandially were decreased (P < .10) ruminal digestion of feed N (44 vs 51%) for steers fed monensin. Monensin decreased (P < .05) the contibution of bacterial N (42 vs 50%) and increased (P < .05) that of ruminally undegraded feed (bypass) N (58 vs 50%) to total N digested postruminally, resulting in less (P < .10) bacterial N (23 vs 28 g/day) and a tendency (P > .10) for more feed bypass N (32 vs 27 g/day) to be digested in the intestines of steers fed monensin with the corn-based diet. That monensin caused a greater proportion of feed OM and N to be digested and absorbed in the intestines than in the rumen (with possibly greater resultant metabolic efficiency) may account for some of the benefit of feeding this compound with high grain diets, as losses incurred in the ruminal transformation of feed nitrogen to bacterial nitrogen appear to be partially eliminated.