Induced LH release in spayed cows.

Abstract

Experiment I. LH levels rose from 3.1 ng/ml in 10 cows on day 1 following ovariectomy to 6.8 on day 28 (b=0.122, P< .01). Eighteen cows were injected IM with 10 mg estradiol-17β 25 days following ovariectomy. Nine were pretreated with 50 mg progesterone per day (IM) for 10 days from 14 to 25 days postovariectomy. Estrogen induced a significant LH release and estrus in all cows. Progesterone pretreatment had no effect on LH levels, acute LH release or expression of estrus. Experiment II. Eighteen ovariectomized cows were given one IM injection of 10 mg estradiol-17β. Six were given no other injections, six were injected with progesterone and six were injected with cortisol acetate. Twenty-five milligrams progesterone and 100 mg cortisol acetate were injected four times at 12-hr. intervals starting 12 hr. before the estrogen injection. Blood samples were taken by jugular venipuncture and estrus checked at −12 and 0 hr. from the estrogen injection and every 4 hr. from 12 to 40 hr. post-estrogen. All cows had an LH peak from 16 to 24 hr. post-estrogen injection. Progesterone or cortisol acetate treatment had no effect on interval to or height of LH peak. The proportion of cows detected in estrus was lower in progesterone-treated cows but the interval to estrus did not differ. Corticoid and estrogen levels were not affected by treatment but changed with time. Progestins decreased with time but progesterone-treated cows had elevated levels at 4 or 8 hr. after each progesterone injection.