Monensin, Protein Source and Protein Levels for Growing Steers

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Abstract
Two cattle growth trials were conducted to evaluate the performance response to monensin when supplementing diets with various sources and levels of protein. Growth trial I determined performance of growing steers fed two sources of supplemental protein [brewers dried grains (BDG) vs urea] two levels of diet crude protein (CP) (10.5% vs 12.5%) and two levels of monensin (0 vs 200 mg/head/day). Results indicated appreciable monensin response in feed/gain when fed in combination with BDG and amounted to a 16.3% improvement on the 10.5% protein treatment and an 8.7% improvement for the 12.5% protein treatment. Monensin addition with either level of urea supplementation had a tendency to decrease rate of gain and feed efficiency. This may indicate microbial protein synthesis is inhibited by the addition of monensin. Monensin was effective in altering volatile fatty acid (VFA) concentrations in favor of propionate (P<.05) on both BDG and urea supplemented diets. Rumen NH3 -N concentrations were reduced by monensin. Steer growth trial II involved two protein levels, supplied by soybean meal (SBM), (either 11.1% or 13.1%) and two levels of monensin (0 or 30 g/908 kg of feed). Monensin addition supported an 8.1% improvement in feed/gain on the 11.1% CP rations, whereas only a 3.2% improvement occurred on the 13.1% protein diets. The largest response in feed/gain occurred on the lower protein diets (10.5% CP trial I and 11.5% CP trial II). Monensin additions did not appear to increase the protein requirement (as a percentage of the diet) for growing steers. Copyright © 1979. American Society of Animal Science . Copyright 1979 by American Society of Animal Science.