Many workers have studied on the induction of superovulation in the cow by application of gonadotropin. However, full information on controlling intervals between gonadotropin and induction of superovulation is still lacking. The main purpose of this study is to induce superovulation in the cow within certain limited period after gonadotorpin administration.
1. Induction of superovulation in the cow.
First gonoadotropin injection: Twelve cows in various stages of the estrous cycle were injected subcutaneously with 2, 500-3, 000 I. U. of PMS alone or 2, 000 I. U. of PMS combined with 2, 000 I. U. of HCG in order to stimulate follicular development. Second gonadotropin injection : The 1st injection was followed by intravenous injection of 1, 500-2, 000 I. U. of HCG in order to accelerate ovulation, 3 to 5 days after the ist injection when several developing follicles were palpated per rectum in all of the animals. Ovarian reactions in the treated animals were examined by rectal palpation every day :after the gonadotropin injections. Three of the treated animals were slaughtered 7-11 days after the 1st injection, for the purpose of examining ovarian reactions in detail. Results obtained are shown in Table 1. It was observed by rectal palpations that the great majority of developed follicles were in-duced to ovulation within 24-48 hours after the 2nd gonadotropin injection and few successive ovulations were followed within the next 24 hours. Multiple formations of corpora lutea were observed later. Eventually, 4-29 ovulations were induced in all of the treated animals. The ovarian reactions of cow No. 1 is shown schematically in Fig. 1. On the course of the experiments, ovum recovering tests by means of oviduct flushing were performed with success in 5 of 6 animals.
2. Recurrence of the estrous cycle in the treated animals
Further observations, regarding ovarian conditions and recurrence of estrous cycle after the superovulation inducing experiments, were continued for a long time in 7 of the treated animals. The results obtained were shown in Table 2. Multiple corpora lutea derived from superovulation began to regress about 18-46 days after begining of the experiment. And then, normal estrus recurred in all of them. Following estrous cycles were as normal as those before the experiment.
3. Ovarian reactions to repeat administrations of gonadotropin
In these experiments, cow No. 3-1 is the same animals as No. 3-2, and interval between the two.experiments is 128 days. Cow No. 1 was employed twice in the same kind of experiment, 50 days and 172 days after the 1st experiment respectively. Although superovulations were induced in the repeated experiments in cow No. 3, ovaries of cow No. 1 failed to ovulate in the repeated administrations of gonadotropin. It is possible that the ovarian refractoriness in the latter animal is due to ontihormone.
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