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Pituitary and Gonadal Hormones Associated with Fertile and Nonfertile Inseminations at Synchronized and Control Estrus¹

Michigan State University,³ East Lansing 48823

Abstract

Twenty-Four cows were fed 1.0 mg MGA daily for 18 days and 24 served as controls. HCG (2,500 IU) was injected intramuscularly at the beginning of estrus in 12 controls and in 12 MGA-cows, and all were inseminated 12 hr. after the onset of estrus. In a fifth treatment, another 11 cows were fed MGA for 18 days, injected with HCG 3 days later and inseminated at 12 and 24 hr. after HCG. Blood serum LH, prolactin, progesterone and estradiol were quantified before and after insemination. Neither serum LH nor prolactin was influenced significantly by MGA or HCG. From day 2 to 18 after insemination, prolactin in pregnant and non-pregnant cows did not differ significantly, but LH was slightly higher in nonpregnant cows. On the last day of MGA treatment, progesterone averaged 0.7±.3 ng/ml. Progesterone in MGA and control cows was similar; it increased from 0.3±.1 ng/ml at estrus to 6.9±.6 ng/ml by day 11. Progesterone concentration did not differ significantly in fertile and nonfertile cows during days 2 through 11 after insemination, but fell dramatically during the 4 days before return to estrus in nonfertile cows.

Estradiol on the last day of MGA averaged 30.1±19.6 pg/ml, about three-fold greater than in controls during proestrus and estrus. Most cows had clearly elevated estradiol for about 3 days beginning on the day of the last feeding of MGA. The results suggest that reduced fertility at estrus synchronized with progestogen may be associated with prolonged elevations of estrogen secretion preceding the synchronized estrus. However, serum estradiol in fertile and nonfertile cows did not differ during days 4 through 11 after insemination, and estradiol was relatively stable at 5.6±.4 pg/ml from day 18 through 75 of pregnancy with the exception of clear elevations between days 30 and 42 in half of the cows.

¹ Approved by The Director of the Experiment Station as Journal Article No. 5952. We thank Dr. L. E. Reichert (Emory Univ.) for the purified LH (LER-1072-2). NIH-LH-B5 and NIH-Prolactin-B-2 were supplied by The Endocrinology Study Section of NIH.

² NIH Predoctoral Fellow. Present address: Department of Animal Science and Industry, Oklahoma State University, Stillwater 74074.

³ Department of Dairy Science.