Bovine somatotropin and rumen-undegradable protein effects on skeletal growth in prepubertal dairy heifers

The objectives of this study were to determine the effects of dietary rumen-undegradable protein (RUP) and bovine somatotropin (bST) during the period from weaning until puberty on body weight (BW) and skeletal growth rates and age at puberty. Fifty-one Holstein heifers at 90 d of age were randomly assigned to 4 treatment groups consisting of 0.1 mg/kg BW per day of bST and 2% added dietary RUP (dry matter basis) applied in a 2 x 2 factorial design (n = 13 per group, except bST with no RUP group, n = 12). From 90 to 314 d, bST increased average daily gain (ADG) by 0.07 kg/d and BW by 16.2 kg, while added RUP increased ADG by 0.10 kg/d and BW by 21.4 kg. Both bST and added RUP effects on BW and ADG were additive. Skeletal growth rates, as measured by withers height (WH) and hip height (HH) were increased by both bST and added RUP. Somatotropin and RUP increased WH by 1.8 and 2.7 cm and hip height by 2.5 and 4.0 cm, respectively, at 314 d of age. Growth curves showed that added RUP effects on rates of BW, WH, and HH growth were greatest from 90 to 150 d age and diminished thereafter, suggesting that protein was limiting during this time period. Conversely, bST effects tended to be greater as the heifers approached puberty, but only in the presence of added RUP. Age at puberty was not affected by treatment, averaging 314 d of age across treatments. From 314 to 644 d of age, rates of BW, WH, and HH growth were similar among treatment groups. However, treatment differences present at 314 d of age persisted through 644 d of age, more than 10 mo after treatments ceased. These results suggest that protein during the early postweaning period and bST during the 200 to 300 d of age period just prior to puberty could be used to accelerate simultaneous increases in both BW and skeletal growth rates in dairy heifers without reducing age at puberty.