Effect of feedlot management system on response to ractopamine-HCl in yearling steers1

Abstract

Two experiments evaluated the effects of conventional and natural feedlot management systems (MS) on ractopamine-HCl (RAC) response in yearling steers. Feedlot performance, carcass characteristics, skeletal muscle gene expression, and circulating IGF-I concentrations were measured. The conventional system included a combined trenbolone acetate and estradiol implant, Revalor-S (IMP), as well as monensin-tylosin feed additives (IA). Treatments were arranged in a 2 x 2 factorial and included: 1) natural (NAT): no IMP-no IA, no RAC; 2) natural plus (NAT+): no IMP-no IA, RAC; 3) conventional (CON): IMP-IA, no RAC; and 4) conventional plus (CON+): IMP-IA, RAC. In Exp. 1, one hundred twenty crossbred steers (initial BW = 400 +/- 26 kg) were allotted randomly to treatment in a randomized complete block design (BW was blocking criteria); pen was the experimental unit. In Exp. 2, twenty-four individually fed crossbred steers (initial BW = 452 +/- 25 kg) were used in a randomized complete block design (BW was blocking criteria) and assigned to the same treatments as Exp. 1, with 6 steers/treatment. In Exp. 2, serum was harvested on d 0 and 31 and within the 28-d RAC feeding period, at d 0, 14, and 28. Longissimus biopsy samples were taken on d 0, 14, and 28 of the RAC feeding period for mRNA analysis of beta-adrenergic receptors and steady-state IGF-I mRNA. In Exp. 1, ADG, G:F, final BW, and HCW were greatest for CON+ (P < 0.01). During the final 37 d, RAC increased ADG (P = 0.05) and increased overall G:F (P = 0.02). Marbling score was reduced (P = 0.02), and yield grade was improved with RAC (P = 0.02), but RAC did not affect dressing percentage (P = 0.96) or HCW (P = 0.31). In Exp. 2, MS x RAC interactions were detected in ADG and G:F the last 28 d, overall ADG and overall G:F, final BW, and HCW (P < 0.01). Dressing percentage, yield grade, and marbling score were not altered by MS or RAC (P > 0.10). Circulating IGF-I concentration was increased on d 31 by the conventional MS, and concentration was greater throughout the study than NAT steers (P < 0.01). Circulating IGF-I concentrations were not changed by RAC (P = 0.49). Abundance of beta(1)-AR mRNA tended to increase (P = 0.09) with RAC, but RAC did not affect beta(2)-AR, beta(3)-AR, or IGF-I mRNA (P > 0.40). Management system did not affect beta(1)-AR, beta(2)-AR, beta(3)-AR, or IGF-I mRNA (P > 0.18), yet a trend (P = 0.06) for MS x RAC for beta(2)-AR mRNA was detected. These results indicate that response to RAC is affected by feedlot management practices.

Effects of ractopamine hydrochloride are not confined to mammalian tissue: Evidence for direct effects of ractopamine hydrochloride supplementation on fermentation by ruminal microorganisms1

“…This agrees with observations of Quinn et al (2008), who observed no difference in ADG between heifers supplemented with RAC for 28 d and control heifers fed 0 mg RAC (P > 0.10). These researchers observed only a tendency for RAC to improve G:F (P = 0.06).…”

“…These results coincide with observations of Quinn et al (2008), who reported that supplementation of RAC to heifers did not impact HCW or LM area (P > 0.10). In contrast, and observed improvements in HCW and LM area in steers supplemented with RAC (P > 0.05).…”

Increased body condition score through increased lean muscle, but not fat deposition, is associated with reduced reproductive response to oestrus induction in beef cows

“…Hence, the high level of supplementation in the present study could have overridden the effect of the β-agonist on insulin concentrations. However, the better BCS of the cattle in this study also may have influenced the lack of effect, as neither zilpaterol nor ractopamin have been shown to affect serum concentrations of insulin in fed steers in good BCS (Winterholler et al, 2008;Carroll et al, 2014). Nevertheless, similar to previous findings (Guzmán et al, 2012), insulin concentrations was not associated with the response to oestrus stimulation, suggesting that relative to IGF-I and leptin, insulin may have less of an effect on the control of reproduction in cattle.…”

“…Muscle mass influences BCS only at the lower range of the body condition scale (⩽4) (Apple et al, 1999;BIF, 2010). However, the use of β-AA provoked a change in body composition by increasing muscle accretion (Winterholler et al, 2008;Holland et al, 2010). Moreover, to avoid the direct effect of nutrient intake and/or β-AA treatment on follicular development and gonadotrophin secretion both treatment and feed supplementation were suspended prior to the initiation of the oestrus induction treatment.…”

“…Beta-adrenergic receptor agonists cause redistribution of energy and improve daily weight gain and muscle accretion in steers (Winterholler et al, 2008). However, in a lactating cow, this energy redistribution could redirect nutrients towards milk production, changing the energy balance of the cows.…”