Virginiamycin Increases Performance and Carcass Weight of Feedlot Cattle Under Mexican Conditions

The objective of this analysis was to evaluate the growth performance and hot carcass weight (HCW) of bulls and heifers fed monensin (MN) alone (400 mg/hd/d) or in combination with virginiamycin (VM) (200 to 250 mg /hd/d) from five Mexican feedlots (trials). There were 224 pens representing 16,197 animals of which 5,072 were heifers (n = 66 pens) and 11,125 were bulls (n = 158 pens). Within each feedlot, animals were blocked by initial BW (< 230, 230 to 300, or > 300 kg) and sex (bulls or heifers), but diet composition and feeding strategies were different among trials. The statistical model for growth performance had block within trial, sex within trial, and trials as random effect variables, and treatments (MN or VM+MN) as a fixed effect variable. The number of pen animals was used as a weight variable. The ADG was adjusted to a common dressing percentage (DP) of 62.1% based on measured hot carcass weight. The initial BW was used as a covariate for the HCW analysis. Overall, animals were fed from 37 to 175 days, the initial and final BW varied from 234 to 351 and 294 to 572 kg, respectively, the ADG varied from 1.026 to 2.150 kg/d, and the DMI ranged from 5.46 to 11.53 kg/d. The MN+VM treatment increased the ADG (P = 0.0002) by 4% compared to MN treatment (1.544 x 1.484 kg/d, respectively). The DMI for the MN+VM treatment was not different (P = 0.797) from MN treatment (9.01 x 9.03 kg/d, respectively), but the feed conversion ratio (DMI÷ADG) was 3.9% less (P = 0.0003) for MN+VM than MN treatments (5.88 x 6.12, respectively). The DP was greater (P = 0.0002) for the MN+VM treatment than MN alone (60.86 x 60.46%, respectively). Similarly, the unadjusted HCW was also greater (P = 0.0024) for the MN+VM treatment than MN alone (306.7 x 302.4 kg, respectively), and the initial BW was a significant covariate (P = 0.036). In conclusion, animal performance (ADG, F:G) and HCW were improved when animals were supplemented with the combination of monensin (400 mg/an/d) and virginiamycin (200 to 250 mg/an/d) compared to when the animals were supplemented with monensin alone.