By:  Benjamin T. Tibbitts,  Hazy R. Nielson,  K. C. Ramsay,  R. N. Funston

Stocker cattle programs often incorporate a growth implant. Revalor-G (which contains 40 mg trenbolone acetate plus 8 mg estradiol) is an implant approved for weaned pasture cattle, steers, and heifers. It is not intended for use in animals intended for subsequent breeding. Research reports indicate effects on reproduction varied in heifers implanted beyond weaning. Additional comprehensive research was conducted to evaluate effects.

Over three locations, 3,342 crossbred beef heifers of approximate 12 months of age, averaging 524 lb were either 1) implanted with Revalor-G or 2) not implanted as controls and both groups grazed together for 164 days. After 82 days grazing, all heifers were synchronized for estrus followed by artificial insemination and then clean-up bulls for 25 days. Pregnancy determination was made 45 days after bulls were removed.

At the end of the trial, implanted heifers were statistically significantly heavier, by 15 lb. However, pregnancy rate of implanted heifers was significantly lower at 46% versus 64% for controls. This effect continued with the next year’s breeding; implants had 93% pregnancy and controls 96%. Even this small difference was statistically significant, a reflection of the large number and uniformity of the animals in the study.

These effects on weight gain and reproduction should not be viewed alone. The authors note that increasing numbers of producers are developing more heifers than they need for replacements. In some cases this may involve developing an entire heifer crop and then retaining only early breeders. Those not retained could be marketed as bred heifers or feeder heifers or finished for slaughter by the producer. The authors concluded that “when pregnant heifer value exceeds feeder heifer value it is unlikely that additional weight gain in culled heifers will compensate for the decreased pregnancy rate. However, when pregnant heifer value is comparable to feeder heifer value, the additional weight gain from the implant increases the value and efficiency of stocker heifers”.