Date of Award:

1990

Document Type:

Thesis

Degree Name:

Master of Science (MS)

Department:

Animal, Dairy, and Veterinary Sciences

Advisor/Chair:

David P. Marcinowski

Abstract

Sixteen lactating regularly cycling dairy cows (14 primiparous, 2 multiparous) were used to determine the effects of somatotropin administration on basal, peak, and pulse luteinizing hormone concentrations during the breeding period. In addition, the effects of somatotropin administration on the pituitary response to a gonadotropinreleasing hormone challenge was studied. The experimental group received daily somatotropin treatments (25mg) for 24 days. The control group received a daily placebo. All animals were treated with three injections of prostaglandin F2-alpha (25 mgjtreatment) for estrus synchronization. Somatotropin and placebo treatments began with the second prostaglandin injection. Sixty hours following the final prostaglandin injection, all cows were catheterized via a jugular vein. Blood samples were collected at 15-minute intervals for 24 hours. The estimated mean time to the preovulatory luteinizing hormone peak was 62.5 +/- 1.8 (SE) hours among 6 of 16 animals responding. Mean plasma peak luteinizing hormone concentration (regardless of treatment) was 13.49 +/- 4 .18 (SE) ng/ml.

On Day 10 of the subsequent estrous cycle, blood samples were collected at 20-minute intervals for 6 hours to determine basal levels of luteinizing hormone. Immediately fo llowing this collection period, a gonadotropin-releasing hormone challenge (100 ug) was administered intravenously. Blood samples were collected at 15-minute intervals for an additional 8 hours. BST did not affect basal, peak, or pulse luteinizing hormone concentrations . There were no differences in basal luteinizing hormone concentration, pulse amplitude, pulse interval, or pulse frequency. BST did not affect the pituitary response to a gonadotropin-releasing hormone challenge. Time to peak, time to return to basal, peak concentration, peak width, and peak luteinizing hormone amplitude were not different among treatment groups following the gonadotropin-releasing hormone challenge. Milk production of BST-treated cows was increased 7% (2.1 kg/d) over controls.

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