Effect of the Feed Additives Chlortetracycline, Monensin and Lasalocid on Feedlot Performance of Finishing Cattle, Liver Lesions and Tissue Levels of Chlortetracycline

Charolais-sired, three way cross steers and heifers (n = 209) were fed ground high-forage diets for 56 d and then were finished on rolled grain based diets. Both diets were supplemented with chlortetracycline (CTC), monensin (MON) or lasalocid (LAS). Cattle fed the high-forage diets supplemented with MON or CTC gained faster (P < 0.05) than those fed an unsupplemented (CON) diet. Dry matter intake was also increased (P < 0.05) by CTC over animals fed the CON diet. Feed efficiency was not affected by CTC but was improved over CON by supplementation with MON or LAS. During the high-concentrate feeding period, inclusion of feed additives did not significantly improve average daily gain or dry matter intake when compared with CON. Of the carcass characteristics, only carcass weight was greater (P < 0.05) and only with CTC supplementation. Monensin and LAS inclusion significantly decreased the acetic to propionic acid ratio in rumen contents. There was no evidence that the incidence of liver lesions differed among the treatments. Analyses of liver, muscle and kidney samples from 61 animals fed CTC and 14 animals fed the CON diet produced no repeatably detectable traces of CTC. Averaged across sex and weight group, returns to labor per animal were $25.83, $61.46, $48.52 and $41.10 for the CON, CTC, MON and LAS treatments, respectively.