Several growth implants are labeled for use in suckling calves. Some, but not all are labeled for use in heifers that are intended to keep as replacements, such Ralgro and Synovex-C.
An important question is to decide whether implants should be used in heifers that may become replacements. Many producers are concerned because of the possibility implants could negatively affect future reproductive performance. There have been a large number of studies and several thorough reviews of the overall results from all these studies. These reviews have generally concluded that if implants are given at 30 to 45 days of age, there is little impact on pregnancy rates as yearlings. There is a very slightly lower pregnancy rate (the average across many studies is less than 1%) for heifers implanted in this age period, and this is so small that it is more likely a result of chance than a true difference. However, among the large number of experiments, there have been a wide range of differences, all the way from large decreases in some studies to substantial increases in pregnancy rates in others. Level of nutrition during the suckling period may explain at least part of this variation. In general, heifers that had good nutrition throughout the suckling period were unlikely to show a detrimental reproductive response to the implant Heifers that had restricted nutrition (lower milking dams or poor quality forage) on the other hand, were more likely to show a decrease in pregnancy as yearlings.
Implanting at younger and older ages can have negative impacts. Implanting heifer calves at birth can decrease pregnancy as yearlings by as much as 39%; according to reviews of multiple implant studies. However, the label on these implants states that heifer calves should be at least 30 or 45 days of age (depending on the specific product) before implanting. Implanting heifers at or after weaning has also led to reductions in pregnancy rate as yearlings.
Breeding & Calving
Looking only at overall pregnancy rate does not tell the whole story. There is little information available about the influence of calfhood implants on when the heifer becomes pregnant. In other words, if a heifer becomes pregnant later in the breeding season because of a calfhood implant, then she will calve later, which means her calf will be younger and lighter at weaning, This heifer will also be later to return to estrous and rebreed as a two-year old. This setback could lead to delayed rebreeding and lighter calves throughout her life, or worse, could result in her failing to rebreed entirely, and thus become an open cow.
A second potential impact is that extra growth in heifers that have been implanted will lead to a larger pelvic opening and result in reduced calving difficulty in first-calf heifers. Reviews of past experiments indicate that the pelvic opening is indeed larger at weaning in heifers that received implants, but the difference disappeared by the time the first calf was delivered, so there was no difference in calving difficulty between heifers that did or did not receive an implant during the suckling period.
A producer should consider three options for using suckling-phase implants in heifer calves:
- Implant all heifer calves and retain extra replacement heifers as insurance against potential reduction or delay in pregnancy.
- Not to implant heifer calves. This would be the best option if raising the best possible replacements is the priority. This is especially true if a producer markets surplus heifers to other producers as replacement heifers rather than marketing them as feeders.
- Implant only late-born heifers. Being young, these are poor candidates for replacements and improved growth due to the implant will help them catch up in weaning weight.