Effects of duration of zilpaterol hydrochloride feeding and days on the finishing diet on feedlot cattle performance and carcass traits1

Abstract

British and British x Continental steers (n = 560; initial BW = 339.4 +/- 1.76 kg) were used in a serial slaughter study with a completely random design to evaluate effects of zilpaterol hydrochloride (ZH; 8.33 mg/kg of dietary DM basis) on performance and carcass characteristics. Treatments were arranged in a 4 x 4 factorial (112 pens; 7 pens/treatment; 5 steers/pen) and included duration of ZH feeding (0, 20, 30, or 40 d before slaughter plus a 3-d ZH withdrawal period) and days on feed (DOF) before slaughter (136, 157, 177, and 198 d). No duration of ZH feeding x slaughter group interactions were detected for the performance measurements (P > 0.10). Final BW did not differ (P = 0.15) between the 0-d group and the average of the 3 ZH groups, but ADG was greater for the average of the 3 ZH groups during the period in which ZH diets were fed (P < 0.01) and for the overall feeding period (P = 0.05). As duration of ZH feeding increased, DMI decreased (P = 0.01) and G:F increased linearly (P < 0.01). With the exception of KPH (P = 0.022), no duration of ZH feeding x slaughter group interactions (P > 0.10) were detected for carcass characteristics. Regardless of the duration of ZH feeding, cattle fed ZH had greater HCW (P < 0.01), greater dressing percent (P < 0.01), less 12th-rib fat (P < 0.01), larger LM area (P < 0.01), less KPH (P = 0.03), and lower yield grade (P < 0.01) than the 0-d cattle. The 0-d group had greater marbling scores (P < 0.01) than cattle fed ZH diets, with a tendency for a linear decrease in marbling score (P = 0.10) as duration of ZH feeding was extended. A greater percentage of carcasses in the 0-d group graded USDA Choice or greater (P < 0.01) than in the 3 ZH groups, whereas the percentage of Select carcasses was greater (P = 0.01) for the 3 ZH groups. From d 0 to end (P = 0.04) and during the last 43 d on feed (P < 0.01), ADG responded quadratically to DOF before slaughter. No differences were detected among slaughter groups for DMI for the entire trial period; however, a quadratic response (P = 0.02) was observed for the final 43 d before slaughter. A quadratic response was also detected for the final 43 d before slaughter (P < 0.01) and from d 0 to end (P = 0.02) for G:F. Final BW, HCW, dressing percent, and 12th-rib fat increased linearly (P < 0.01) as DOF before slaughter increased. Our results indicate that no substantial effects on performance and carcass measurements were observed when ZH was fed for 30 or 40 d as opposed to 20 d, and that effects of ZH generally did not interact with DOF before slaughter.

Effects of duration of zilpaterol hydrochloride supplementation on growth performance, carcass traits and meat quality of grain-fed cull cows

“…The relatively larger differences between C and Z for carcass gain relative to live weight gain correspond with the increase (P , 0.05) in dressing percentage of up to 1.3 for Z30 and Z40. Similar improvements were reported by Neill et al (2009) for cull cows, whereas up to 2 increased dressing percentage were reported for young animals by Avendañ o-Reyes et al (2006), Vasconcelos et al (2008) and Montgomery et al (2009). No significant difference in carcass weight was recorded in this study, but this could have been due to large variation within cow groups combined with numerical differences in weight at the start of Neill et al (2009), zilpaterol had no significant effect on fat thickness measurements or chemical fat content of the three rib cut of cull cows that concur with fat measurements in this study.…”

“…In addition, limiting the zilpaterol to the last 30 days seemed to give the best results compared with 20 days and did not differ significantly from 40 days of supplementation. Improvement in rate of body weight gain (average daily gain (ADG); P.0.05; , 9%) and efficiency of gain (feed conversion ratio (FCR); P , 0.05; , 14%) of cull cows in this study was consistent with improvements in ADG (12.5% to 36%) and FCR (15.3% to 21%) of younger animals fed zilpaterol (Avendañ o- Reyes et al, 2006;Vasconcelos et al, 2008;Montgomery et al, 2009). When measured on a carcass basis, ADG and FCR of 18% and 23%, respectively, between C and Z30 were recorded.…”

“…; Vasconcelos et al, 2008;Montgomery et al, 2009) of zilpaterol compared with this study (6 p.p.m. ), which could have expressed a more pronounced effect with regard to growth, yield and meat quality (Dunshea et al, 2005) than in this study.…”

“…It seemed that this effect was optimized with 30 days (or 40 days) exposure rather than 20 days in this study. Vasconcelos et al (2008) did not find any effect of duration of zilpaterol supplementation on muscle-related traits such as rib-eye area. Robles-Estrada et al (2009) reported a positive trend (P 5 0.11) towards trimmed meat proportions in zilpaterol-supplemented young feedlot steers.…”

“…However, in this study, the higher proportion Z carcasses with fat score 2 (55% for Z30), compared with C carcasses (27%), suggest that zilpaterol may have had an effect on fat deposition in cull cows. Significant effects in this regard were reported by Vasconcelos et al (2008) and Robles-Estrada et al (2009) for young animals supplemented with zilpaterol, whereas Leheska et al (2009) found no differences in carcass fat percentage in treated and untreated steers and heifers. Numerically, lower values for trimmed fat (%) in the hindquarter of Z carcasses, compared with C carcasses, further suggested lower fat yields and were mirrored by significantly higher (P , 0.05) trimmed meat proportions in the hindquarter of Z carcasses.…”