Growth and Subsequent Lactation in Primigravid Holstein Heifers After Prepartum Bovine Somatotropin Treatment

Abstract

Thirty-seven primigravid Holstein heifers (600.7±9.4kg) were used to determine the effect of recombinant bST, administered during the last trimester of gestation, on growth and subsequent 270-d milk yield. Treatments consisted of daily subcutaneous injections of saline or of 20 or 40mg of recombinant bST. Four weeks before expected date of parturition, 8 heifers (3 receiving saline, 3 receiving 20mg of bST, and 2 receiving 40mg of bST) were fitted with jugular catheters, and blood plasma bST and insulin-like growth factor-I concentrations were determined in samples collected during a 9-h period. Feed efficiency (DMI/gain) was improved in heifers treated with 20mg of bST, but DMI was not affected by treatment. Average daily gain was increased in heifers treated with 20mg of bST, but not in those treated with 40mg of bST. Fat-uncorrected milk yield following prepartum bST treatment was 19% higher in heifers treated with 20mg of bST, but the 40-mg dose of bST did not increase milk yield. Percentage of milk fat was lower in heifers treated with 20mg of bST prepartum, but daily milk fat yields did not differ among groups. Solids-not-fat yield was highest in the group treated previously with 20mg of bST. The higher milk yield of these heifers became significant (P<.05) only after 90 d of lactation. Plasma bST and insulin-like growth factor-I concentrations were both elevated in bST-treated animals. There was indirect evidence that the increased milk in heifers previously treated with 20mg of bST was due to enhanced mammogenesis. However, further research studying changes at the mammary tissue level is needed to confirm this hypothesis.