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Ovarian Activity in Prepuberal Dairy Calves^1,2,

University of Massachusetts, Amherst

Abstract

Follicular growth followed by atresia occurs in young calves as early as 1 month of age. However, such activity as determined by the number and size of the follicles present on the ovaries upon post-mortem examination varies markedly among individual calves.

Observation of the ovaries of five calves by means of mid-ventral laparotomies at 14-day intervals over a 6-week period revealed a continual growth and regression of follicles with the largest follicles usually reaching diameters of 12 to 14 mm.

The administration of various combinations of 17-beta estradiol, progesterone, pregnant mare serum and human chorionic gonadotropin to 42 calves under 6 months of age in an attempt to achieve single ovulations without excessive follicular development yielded inconsistent results. Pregnant mare serum (PMS) and human chorionic gonadotropin (HCG) caused hyperfollicular development. The addition of progesterone to PMS and HCG reduced substantially the number of small follicles with a corresponding increase in medium and large follicles and in some animals caused ovulation. The addition of a fourth hormone, estradiol, in an attempt to simulate the endocrine pattern of the cyclic heifer, suppressed follicular growth.

Ovulation and luteinization were achieved in only 11 of 42 hormone-treated calves (26%). Six of the seven ovulations in the triple-hormone group and the only ovulation in the group which received the quadruple treatment were induced without excessive follicular development on either ovary. The presence of "smooth" ovaries in six animals which received the triple-hormone treatment cannot be explained since other calves of approximately the same age, receiving essentially the same dosages, exhibited medium or large follicles or ovulated and produced corpora lutea. While the treatments appeared to be more successful in the older calves, the influence of the age factor is not clear.

The rectal temperature of all calves, both treated and controls, showed substantial changes from day to day, as well as considerable differences among individual calves. There was no evidence in this investigation, however, of a thermal response to either exogenous or endogenous progesterone.

¹ Supported by the Northeastern Cooperative Regional Project NE-41 "Endocrine Factors Affecting Reproduction and Lactation" and by funds from the Animal Husbandry Research Branch, USDA, Beltsville, Maryland.

² Contribution No. 1312 of the University of Massachusetts, College of Agriculture Experiment Station, Amherst, Massachusetts.

³ This paper is part of a thesis submitted by the senior author to the Graduate Faculty of the University of Massachusetts in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Animal Science.