Effect of previous narasin exposure on performance and gastrointestinal morphometrics of finishing bulls supplemented with either sodium monensin or narasin

ABSTRACT The objective of this study was to investigate the potential carryover effect of narasin (NAR) supplementation during the backgrounding phase on subsequent feedlot performance and gastrointestinal morphometrics of finishing Nellore bulls. Ninety-six bulls were used in a 2 × 2 factorial design, evaluating conditional NAR supplementation on the backgrounding phase (control [CON] or NAR) and finishing supplementation (sodium monensin [MON] or NAR), resulting in four treatments as follows: 1) CON+MON, 2) CON+NAR, 3) NAR+MON, and 4) NAR+NAR. During the backgrounding phase, NAR-fed cattle showed reduced dry matter intake (DMI) expressed in kilograms and as a percentage (P≤0.03), however, it did not significantly affect final body weight (BW), average daily gain (ADG), or feed efficiency compared with the CON-fed cattle (P≥0.14). In the finishing phase, neither backgrounding nor finishing supplementation, nor their interaction, had significant effects on BW, ADG, DMI, feed efficiency, hot carcass weight, dressing percentage, or gastrointestinal morphometrics (P≥0.11). Relative to CON+NAR and NAR+MON, cattle fed NAR+NAR showed reduced fluctuation in DMI throughout the entire experiment (%; P≤0.02); however, fluctuations, even above 10% in some treatments, did not lead to reduced ADG or increased rumenitis incidence. We conclude that dietary supplementation with NAR did not enhance performance or gastrointestinal development in Nellore cattle during backgrounding or finishing, nor was a carryover effect observed between phases. Although NAR-fed cattle in the finishing phase performed similarly to those receiving monensin, the absence of a negative control limits interpretation, warranting further research on its role in stabilizing dry matter intake.