The time of ovulation and efficiency of fertilization following progesterone and pregnant mare serum treatment in the cyclic ewe

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Abstract
A factorial experiment designed to determine the time of ovulation relative to oestrus and to the cessation of progesterone treatment in cyclic maiden ewes, and subsequent fertilization of ova, is described. Seventy-two ewes were used. Factors were: (a) type of service (natural v. A.I with fresh undiluted semen v. A.I. with semen diluted 1:1 with heated cows' milk); (b) frequency of progesterone (daily v. every 2 days); (c) post progesterone withdrawal treatment (500 i.u. PMS v. no PMS); (d) week of treatment (3 successive weeks); and (e) days within weeks (2 successive days). Insemination was carried out on the third day after cessation of progesterone without reference to the results of teasing with vasectomized rams.Progesterone (20 mg.) administered every 2 days was as effective as 10 mg. administered daily for suppression of ovulation. PMS resulted in a highly significant increase (P < 0·001) in the numbers of ewes which ovulated and which exhibited oestrus after progesterone, and in the number of fertile ewes (P < 0·05).There was a considerable spread of time of onset of oestrus and of ovulation. This was unaffected by any treatment. Most ewes were served 24–96 hr. (peak 48–84 hr.) and ovulated 48–108 hr. (peak 72–108) after cessation of progesterone. The time relationships of oestrus, ovulation and ovum transport appeared normal.

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