Abstract
Two commonly used growth promotants in the United States beef industry are β-agonists and anabolic steroid hormones. Each has been shown to increase lean muscle deposition in cattle provided treatments of each growth technology, but much is still unknown of how steroidal implants and β-agonists work in combination. It was our goal to determine the effect of implant strategy and β-agonist administration in beef feedlot heifers (n = 264). A 3 × 2 factorial randomized complete block design was used with 2 levels of OPT and 3 different durations of terminal implant (TI) windows for a total of 6 treatment groups with 9 replications. Terminal implants (20 mg estradiol/200 mg trenbolone acetate implant, Component TE-200) were provided to heifers 140 d from slaughter (TI140), 100 d from slaughter (TI100), or 60 d from slaughter (TI60). Animals receiving the later two TI being first implanted on day 0 (8 mg estradiol/80 mg trenbolone acetate implant, Component TE-IH). The second treatment of the cattle received was the orally active beta adrenergic agonist, ractopamine-hydrochloride (RH) in the form of Optaflexx®(OPT; 0 (NO) or 200 (YES) mg/hd·d-1) over the final 28 days of the trial. Thirty animals were subjected to longissimus muscle (LM) biopsies on d 0, 40, 80, 112, and at slaughter on d 140 to view mRNA levels of myogenic related genes and protein quantities of the β1-adrenergic receptor (β1 AR) and β2-adrenergic receptor (β2 AR). On the same days, blood samples were taken from 108 animals to assess changes in plasma blood urea nitrogen (BUN), non-esterified fatty acids (NEFA) and progesterone due to treatments. Relative mRNA levels of myosin heavy chain IIX (MHC IIX), AMPKα, and IGF-I were increased (P P = 0.09) due to OPT. An interaction between TI d and OPT administration caused an increase (P β2 AR was decreased (P β1 AR protein intensity tended to be lower (P P P β-adrenergic receptors, and blood metabolites in finishing beef heifers.
Highlights
Growth promotants are commonly used throughout the United States beef industry
Individual effects of OPT and terminal implant (TI) day on mRNA Real-time quantitative PCR was performed on the β1-adrenergic receptor (β1 AR), the β2-adrenergic receptor (β2 AR), myosin heavy chain I (MHC I), myosin heavy chain IIA (MHC IIA), myosin heavy chain IIX (MHC IIX), AMPKα, and IGF-I to determine if OPT would alter mRNA levels in longissimus muscle of heifers provided ractopamine HCl
It was discovered that OPT did not alter (P > 0.05) gene expression of any myogenic mRNA levels (Table 2)
Summary
Growth promotants are commonly used throughout the United States beef industry. These techniques include the use of a steroidal implants and the administration of β-adrenergic receptor agonists (β-agonists) in feed. Both of these approved methods result in lean tissue accretion, their modes of action are quite different. Two commonly used steroid hormones for muscle growth are estradiol-17β (E2, an estrogen) and trenbolone acetate (TBA, a synthetic form of testosterone). An increase in IGF-I mRNA [2], satellite cells in muscle tissue [3], and an increase in circulating IGF-I [1] has been documented in steers administered a steroidal implant. Animals provided combined TBA/E2 implants have been shown to increase both average daily gain and feed efficiency, 20% and 15% respectively [4] [5] compared with nonimplanted animals
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