Effect of Bovine Somatotropin on Production and Reproduction in Prepubertal Friesian Heifers

Abstract

This experiment was conducted to evaluate the effect of bST on average daily gain, onset of puberty, first lactation milk yield, and reproductive efficiency in Friesian heifers. Heifers (n = 16 per treatment) were allocated to either: 1) control (1.5ml of vehicle) or 2) bST (15mg of bST in 1.5ml of vehicle) using a randomized complete block design. Subcutaneous injections of bST were administered once daily from 7 mo of age for 120 d or until heifers reached puberty. Heifers were weighed every 2 wk, and blood samples were collected twice weekly after heifers reached 200kg. Progesterone concentrations were used to determine onset of puberty. Heifers were bred between 16 and 18 mo of age and, following parturition, milk yield and composition were recorded twice weekly and once every 2 wk, respectively. Heifers assigned to bST treatment had an average daily gain (kg) of .8 compared with .7 in control heifers. Number of days from birth to onset of puberty for bST-treated heifers was 401 compared with 381 for control heifers. Treatment with bST had no effect on milk yield, milk composition, or reproductive efficiency during the first lactation. These data demonstrate that daily administration of bST to Friesian heifers from 7 to 11 mo of age does not affect average daily gain, onset of puberty, reproductive efficiency, or first lactation milk yield of heifers.