RELATIONSHIPS AMONG MILK-YIELD, METABOLISM, AND REPRODUCTIVE-PERFORMANCE OF PRIMIPAROUS HOLSTEIN COWS TREATED WITH SOMATOTROPIN

Thirty-two primiparous Holsteins were assigned to receive 0, 5.15, 10.3, or 16.5 mg/d recombinant bST. Treatment began between 28 and 35 d postpartum and continued until 60 d prior to next expected calving or 400 d postpartum. Metabolic hormones and metabolites were measured in blood weekly during the first 10 wk of treatment, and progesterone was measured in milk twice weekly to assess ovarian activity until conception. Milk production (305 d, actual) was 8350 +/- 562, 8348 +/- 515, 9571 +/- 515, and 9070 +/- 515 for cows given 0, 5.15, 10.3, or 16.5 mg/d and did not differ statistically. Insulin, glucose, NEFA and blood urea nitrogen were not influenced by dose of bST. However, insulin-like growth factor-I increased linearly with dose of bST. Days from parturition to first detected estrus, days open, and services per conception did not differ among groups; however, days from parturition to first service increased with dose of bST. Rate of detection of estrus decreased with dose of bST, resulting in a longer interval to first insemination. The lower rate of detection of estrus in bST-treated cows may have been associated with reduced expression of estrus. Additional statistical analyses were conducted to determine the relationship between metabolism, milk production, and reproduction over all doses of bST. There was a negative correlation between 305-d milk yield and glucose (r = -.44) and insulin (r = -.46) concentrations during 30 to 100 d postpartum. In stepwise regression analyses, insulin accounted for 21% of the variance in 305-d milk yield. Glucose and NEFA accounted for 18% of the variance in days open, and there was a negative correlation (r = -.32) between glucose and days open. Days open and milk production were not correlated.