Season and treatment effects on serum prolactin and milk yield during induced lactation

Nineteen nonpregnant, nonlactating dairy cows were allotted to three treatments to induce lactation during winter, 1976, or spring, 1977. All groups received 17 beta-estradiol (.1 mg/kg) days 1 to 7. Groups 2 and 3 also received progesterone (.25 mg/kg) days 1 to 7. Groups 1 and 2 were given reserpine (5 mg intramuscular) on days 8, 10, 12, and 14. Group 3 received reserpine (5 mg intramuscular) on days 2, 5, 8, 11, and 14. Blood samples were collected for prolactin analysis just prior to and 3 h after reserpine injection. Mean daily temperatures were 11.9 C for spring group and -6.5 C for winter group. Comparisons of spring with winter for basal prolactin concentrations, reserpine-stimulated prolactin concentrations, and 100-day milk yields were 44 with 10 ng/ml, 482 with 199 ng/ml, and 1991 with 862 kg. Differences in prolactin concentrations and milk yields among hormone and reserpine treatments could not be detected, but cows on treatment 3 in the spring gave the largest yield of milk. Prolactin concentrations were correlated with milk yields among cows and among cows within seasons. Seasonal differences demonstrate the critical role of prolactin in the treatment to induce lactation.