Effects of monensin on site and extent of whole corn digestion and bacterial protein synthesis in beef steers

Monensin was fed at 0 and 33 ppm in a crossover trial designed to determine the effect of this compound on ruminal and postruminal utilization of a corn-based diet and bacterial protein synthesis in abomasally fistulated steers. Monensin decreased (P greater than .10) ruminal true digestion of organic matter (OMc, corrected for bacterial cell synthesis) and apparent ruminal digestion of starch by 19%, but had not effect on apparent total tract digestion of OM or starch. Apparent ruminal and total tract digestibilities of crude protein (CP) were unchanged. Monensin decreased (P greater than .07) the contribution of bacterial N to total abomasal N (52 vs 58%), and increased (P greater than .06) the contribution of ruminally undegraded feed N (46 vs 40%), but had not effect on total N or amino acids recovered from the abomasum. Efficiency of bacterial protein synthesis (grams bacterial CP/100 g ruminally digested OMc) was unchanged. Monensin decreased (P greater than .05) the fraction of bacterial N to total N digested postruminally (42 vs 50%) and increased (P greater than .05) the contribution of ruminally undegraded feed N digested postruminally (58 vs 50%). Monensin caused a greater proportion of feed N and starch to be digested in the intestines than in the rumen (with possibly greater resultant metabolic efficiency), and this may account for some of the benefits obtained from feeding this compound with high grain diets.