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The Levels of Ovarian Hormones Required to Induce Heat and Other Reactions in the Ovariectomized Cow

Cornell University¹

Abstract

The average level of estradiol benzoate required to bring ovariectomized heifers into heat is 600 rat units daily for 3 days. The duration of heat is usually less than one day even though the injections are continued. This low threshold is probably reached early in the development of the Graffian follicle in the normal cow. ‘Estrous block,’ apparently in the central nervous system, then sets in, so that the cow is out of heat before ovulation.

Mucin is produced in the vagina if the dose of estrogen is below 5,000 r.u. but not if it is above this level.

Bleeding is not produced by estrogen injection and does not follow its withdrawal.

The mean dose of stilbestrol needed to bring the ovariectomized heifer in heat is 0.255 mg.

Uterine muscle of ovariectomized heifers is relatively inert, both spontaneously and to pituitrin, epinephrin, lentin and arecolin. Uterine muscle of heifers estrogenized after ovariectomy has the same reactions as that of heifers in heat.

Progesterone in doses of 35 rabbit units or more given for 6 days produces the same reactions of the uterine muscle as those found in heifers during the diestrum. Progesterone in doses of 18 rabbit units given for 6 days produces a diphasic response to epinephrin, first a contraction, followed by relaxation, similar to that found during proestrum.

The average length of the uterine muscle cells is greater in the estrogenized heifers than in the ovariectomized controls. Progesterone, at the levels given, does not inhibit this action of estrogen.

In domestic animals the level of FSH in the pituitary, the length of the heat period, and the estrogen threshold and excretion during heat are strikingly parallel; if one is high, the others are high, etc. The following descending order is, in general shown: man, horse, hog, sheep, cow.

The connection of the low hormone threshold in the cow with the free-martin problem is discussed.

¹ Laboratory of Animal Nutrition, Agricultural Experiment Station, and College of Veterinary Medicine, Ithaca, N. Y.