Growth implants have not been widely used in heifer calves because of concern by herd managers about detrimental effects on subsequent reproductive performance of heifers kept as herd replacements. Past reviews of this subject have been quite thorough and generally concluded that one implant given at or after the heifer is 2 months of age has limited impact on future reproductive performance. Also, these reviews have concluded that implanted heifers have significantly greater pelvic area when measured at about 1 year of age, but these differences are very small at the time the heifer is delivering her first calf at or about 2 years of age. Consequently, the data on dystocia rate indicates that implanted heifers have no less calving difficulty than do non-implanted counterparts.
The possible effect of implanting on breeding season pregnancy rates is still the major concern for ranchers deciding whether to implant heifer calves. The available data is clear that implanting heifers at birth is detrimental to breeding season pregnancy rates. The tremendous variation in reproductive performance in trials when heifers are implanted during the suckling phase is partly due to the relatively small numbers of heifers represented in some treatment groups, but the variation is concerning to producers. When heifers are implanted once at weaning time, the risk of reduced pregnancy rates is very high. The summary of trials in which heifers were implanted more than once indicates that the risk of reproductive loss increases greatly as the number of multiple implants increase.
One strategy in using implants during the suckling is to not implant heifers that are born the first two-thirds of the calving season because that group of heifers are the ones from which potential replacement will come. With this strategy, you don’t risk the possible harm to reproductive performance.
With the wide variation in reproductive performance, there is risk in implanting heifers that are intended as replacements.
