Abstract
An experiment was conducted to evaluate the interaction between steroidal implantation and feeding ractopamine on nitrogen retention, blood metabolites, and messenger RNA (mRNA) expression. Six Holstein steers (initially weighing 509 lb) were implanted or not with Revalor-S (120 mg trenbolone acetate plus 24 mg estradiol-17β), and all were fed no ractopamine for the initial 28 days and then 2 grams per steer daily of Optaflexx (200 mg/day ractopamine-HCl) on days 29 through 56. Implantation increased nitrogen retention. Optaflexx increased nitrogen retention in nonimplanted steers, but did not significantly increase retained nitrogen in implanted steers. Implantation increased serum insulin-like growth factor (IGF)-I concentration. Optaflexx, however, numerically decreased serum IGF-I concentrations. Implantation numerically increased IGF-I mRNA in the longissimus muscle, but expression of IGF-I mRNA was significantly decreased when Optaflexx was fed. Both growth promotants increased nitrogen retention in steers, but, despite perceived differences in their mode of action, the combination yielded a less than additive response for nitrogen retention.
Highlights
In the feedlot industry, enhancing efficiency of growth of finishing cattle is a major objective
An experiment was conducted to evaluate the interaction between steroidal implantation and feeding ractopamine on nitrogen retention, blood metabolites, and messenger RNA expression
Total urinary nitrogen excretion was significantly decreased by Revalor-S and by feeding Optaflexx to control steers; it was not, affected by feeding Optaflexx to steers implanted with Revalor-S
Summary
In the feedlot industry, enhancing efficiency of growth of finishing cattle is a major objective. Improvements in growth have been achieved by use of steroidal implants. Steroidal implants such as Revalor-S that contain trenbolone acetate and estradiol-17β can improve daily gain and feed efficiency, which results, at least in part, from increased blood concentrations of IGF-I and local tissue production of IGF-I. Insulin-like growth factor-I affects postnatal muscle growth by increasing the number of satellite cells (which contain DNA), fusion of these satellite cells with existing muscle fibers, and muscle protein accretion. Addition of Optaflexx to the diet can repartition nutrients away from fat deposition and to muscle accretion, improving daily gain and feed efficiency. Little research has been conducted with steers that have been implanted with Revalor-S and fed Optaflexx
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