The timing of implanting suckling steer calves depends on the marketing arrangements of the farm or ranch. On-farm trials also demonstrate that the best performance enhancement occurs if calves are implanted when they are at least 45 days of age. This could be due to the fact that animals younger than this do not have the receptors necessary to utilize the hormones contained in the implants. Even though implants are profitable tools to use in steer calves, purebred producers and producers who raise their own heifers should recognize the situations when implanting suckling calves should be avoided. No implants are approved for use in bull calves because of the serious negative effects of these hormones on future fertility.
Heifers can also experience decreased fertility when implanted during the suckling phase.
Implants from two companies are approved for use in suckling heifers that are to be retained as replacements, but I do not recommend implanting calves identified at a young age as likely replacements. There are no benefits to implanting replacement heifers since producers do not benefit economically from maximum growth. Instead, economic benefits from replacement heifers occur due to early onset of puberty, high rates of fertility and a long productive life in the cow herd. Numerous studies have shown that heifers implanted with growth promotants at two to three months of age have a larger pelvic area as yearlings than heifers that were not implanted. There is also no improvement in the rate of dystocia in implanted heifers.