The Effect of Progesterone on Ovulation Time in Dairy Heifers

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Abstract
Evidence for the existence of a nervous mechanism in the release of luteiniz- ing hormone and ovulation in the cow recently has been presented (7). In these experiments the parasympathetic blocking drug, atropine, administered to heifers at the beginning of estrus, was found to block ovulation. However, when atropine and chorionic gonadotrophin (as a source of luteinizing hormone) were administered concurrently to heifers at the beginning of estrus, ovulation oc- curred at the normal time or earlier. These results were interpreted to indicate the existence of a neural mechanism having a parasympathetic component in- volved in luteinizing hormone release and ovulation in the cow. As a result of these experiments, it was decided to study the effects of the ovarian hormones on ovulation time. The first of the ovarian hormones studied in this respect was progesterone. EXPERIMENTAL PROCEDURE The length of estrus and the time of ovulation were determined for 11 heifers in a control estrous period and ill an estrous period in which progesterone was administered as soon as possible after the beginning of estrus. The beginning and end of estrus usually were determined by checking the heifers with a teaser bull at 2-hr. intervals. In some cases, more frequent checks were made to deter- mine the beginning of estrus. The time of ovulation was determined by rectal palpation. In one case a double ovariectomy was performed in order to verify the findings of the rectal palpations. In some cases the control data were ob- tained prior to the treatment period, and in other cases they were obtained sub- sequently. In the latter event, one complete estrous cycle was allowed to elapse before the control data were taken. Some heifers were treated several times and data on several control periods also were taken in some instances. The proges- terone was dissolved in cottonseed oil at the rate of 5 rag. per milliliter and in- jected subcutaneously. The dosages ranged from 5 to 50 rag. but were 10 rag. or less in all but three cases. Heifers were considered to be in estrus when they would stand when mounted and out of estrus when they no longer stood. The heifers were grade Holsteins and Guernseys between 1 and 3 yr. of age. Data were collected during the first 5 too. in 1951. Additional data also have been collected in the same way on the effects of progesterone given prior to the begin- ning of estrus and later than the second hour of estrus. Sham injections were not made during the control estrous periods.