The effects of accelerated growth rates and estrogen implants in prepubertal Holstein heifers on estimates of mammary development and subsequent reproduction and milk production

The objectives of this study were to determine the effects of accelerating growth rates and of imposing estrogen implants in prepubertal heifers on ma mmary development and subsequent reproduction and milk production. Sixty-ei ght Holstein heifers were assigned to 1 of 4 treatment groups by using a ra ndomized complete block design in a 2 x 2 factorial arrangement. The treatm ents were standard growth rate (700 g/d) or accelerated growth rate (1000 g /d) and estrogen implant or no estrogen implant. The treatments were impose d over 20 wk, beginning at 4.5 mo of age and 130 kg of body weight (BW). Du ring the treatment period, all heifers were fed individually and received t he same diet, but the dry matter intake of each heifer was adjusted weekly to achieve the designated growth rate. The estrogen implants were removed a t the end of the treatment period at 9.5 mo of age. After the treatment per iod, the heifers were group fed according to BW and age to allow the heifer s to have a similar BW and age at calving. The accelerated growth regimen d ecreased age at puberty by 32 d. Age, BW, and body condition scores at calv ing were not significantly different among treatments. The accelerated prep ubertal growth regimen decreased first lactation fat-corrected milk yield 7 .1%. Prepubertal heifers given estrogen implants produced 5.2% less fat-cor rected milk during first lactation than did heifers not implanted with estr ogen. Estrogen implants stimulated a large increase in teat length growth d uring the treatment period, but the advantage was lost posttreatment. Over both the treatment and posttreatment periods, the estrogen implants reduced teat length growth by 30%. Accelerated growth rates from 700 to 1000 g/d a nd estrogen implants in prepubertal heifers decreased first lactation milk production.