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

¹ Department of Animal and Poultry Science, University of Guelph, Guelph, ON, Canada N1G 2W1
² American Cyanamid Company, Princeton, NJ

Thirty-seven primigravid Holstein heifers (600.7 ± 9.4 kg) 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 40 mg of recombinant bST. Four weeks before expected date of parturition, 8 heifers (3 receiving saline, 3 receiving 20 mg of bST, and 2 receiving 40 mg 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 20 mg of bST, but DMI was not affected by treatment. Average daily gain was increased in heifers treated with 20 mg of bST, but not in those treated with 40 mg of bST. Fat-uncorrected milk yield following prepartum bST treatment was 19% higher in heifers treated with 20 mg of bST, but the 40-mg dose of bST did not increase milk yield. Percentage of milk fat was lower in heifers treated with 20 mg 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 20 mg 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 20 mg of bST was due to enhanced mammogenesis. However, further research studying changes at the mammary tissue level is needed to confirm this hypothesis.

Key Words: lactation • heifer • somatotropin • prepartum treatment

Submitted on May 14, 1991
Accepted on September 3, 1991

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