The induction of pregnancy in the anoestrous ewe by hormonal therapy I. Progesterone-pregnant mare's serum therapy during the breeding season

Extract

Four hundred and twenty-two ewes, of a number of different breeds and ages (flocks 1–9) were used in experiments with hormones in the normal breeding season. Two main trials were conducted, the first making use of 377 animals, the second involving 45.

Trial 1

Three hundred and seventy-seven ewes were observed, in eight farm flocks, in a study of response to different treatments involving the administration of progesterone (in oil solution) and p.m.s. The trial sought to determine the effect of various dosage levels of progesterone and intervals of injection on the normal breeding performance of the ewe. Hormone treatment was applied at such time in the breeding season as to make it most improbable that conception would be limited by adverse animal or environmental factors. The aim was to determine the effect of the hormone technique per se on both immediate and long-term reproductive performance. Injection procedures and methods of flock management were arranged so that they might later be repeated in applications during the period of anoestrus.

Two hundred and thirty-three ewes were injected intramuscularly with doses of progesterone in oil solution over a 7-day period. For eighty-nine ewes, four doses of 40 mg. were administered at 2-day intervals; for 144, 3 doses of 50 mg. were given at 3-day intervals. A single injection of 750 i.u. of p.m.s. was given subcutaneously to each ewe 2 days after the final dose of progesterone. One hundred and forty-four ewes remained untreated and served as a control.

Progesterone therapy was effective in inhibiting oestrus and ovulation in the majority of ewes. Most injected animals came in oestrus 1–5 days following the injection of p.m.s.; 10% showed evidence of ‘silent heat’. 70% of ewes receiving progesterone at 2-day intervals and 68·2% of those receiving it at 3-day intervals conceived at the ‘controlled’ oestrus. 75% of the controls conceived at first mating. Treatment did not adversely affect the conception percentage to any appreciable extent.

In animals conceiving at first oestrus, the average number of lambs born per ewe was 1·78 for injected and 1·44 for controls. In each flock there was an increase in lamb-crop, which would seem to be the result of the p.m.s. injection. Information on the effect of therapy on subsequent ewe fertility shows that 6·4% of ewes failed to conceive; this figure was the same for both treated and control animals. An attempt to use A.I. in conjunction with progesterone- p.m.s. therapy gave a conception percentage of 24, as compared with 68 in normally bred sheep.

The results are discussed in relation to factors such as breed and age of ewe, and according to the particular mating procedure employed. There is no evidence that the forms of progesterone-p.m.s. therapy employed in this work would constitute a limiting factor to conception in out-of-season breeding.

Trial 2

The forty-five mature Welsh Mountain ewes used were in one flock. Each animal was injected on the 13th day of the oestrous cycle with a single dose of either 50 or 100 mg. of a micro-crystal suspension of progesterone. Two forms of suspension were used, the difference between the two being in the size of the individual crystals. Ewes came in oestrus 7–13 days following injection, the range for 50 mg. dosage being 7–13, and for 100 mg. 10–13 days. 25% of animals showed ‘silent heat’. Results suggest that injection of suspension adversely affects the normal expression of oestrus.