The induction of pregnancy in the anoestrous ewe by hormonal therapy II. Progesterone-pregnant mare's serum applications in anoestrus
Authors: Gordon I. 
Published: March 2009

This paper deals with the application of various forms of progesterone-p.m.s. therapy to lactating and non-lactating ewes in the anoestrus of 1957. Twenty-one farm flocks, and a total of 753 sheep were used in three trials (3–5). In trials 3 and 4, the treatment employed techniques similar or identical to those employed in the previous breeding season (Part I of this series). Trial 5 was concerned with the use of micro-crystal suspensions of progesterone as a possible means of simplifying hormone administration procedure; again this was a follow-on to work in the 1956 breeding season. Applications of progesterone-p.m.s. therapy were made at two periods in anoestrus, April to May (deep anoestrus) and June to early August (late anoestrus). Observations on the occurrence of spontaneous breeding were made following hormone applications at these two periods, and studies on ram libido and fertility attempted. The main objective of the work was to determine whether low conception percentage is due to defective ram performance or to a failure in the supply of hormones essential for the maintenance of pregnancy. A preliminary attempt was made to induce previously injected ewes (lactating and non-lactating) to show a ‘repeat’ oestrus by extended treatments with progesterone and p.m.s.

Trial 3

Three hundred and twenty-one ewes, predominantly Chin Forests, were treated in April/May in twelve flocks. In five of these some proportion or all animals were lactating at time of injection. Six different combinations of progesterone and p.m.s. were employed. The majority allowed for pretreatment with progesterone over a 7-day period; doses of progesterone varied from 25 mg. (at daily intervals) to 50 mg. (at 3-day intervals). In each ewe, a dose of 750 i.u. freeze-dried p.m.s. was given 2 days after the final progesterone dose. One hundred and eighty-eight ewes mated (58·6%) 1–5 days after the end of treatment. The oestrous response was affected by various factors of treatment and animal. Progesterone treatment over 7 days gave rise to a higher response than did treatment applied over 5 days. Evidence of low libido in some rams was obtained, and possible methods for overcoming this problem are described.

Sixty-six of the ewes conceived and produced lambs, this being 20·6% of all animals injected and 34·4% of ewes which mated. An average of 1·39 lambs born per ewe conceiving was recorded. Nonlactating ewes conceived more readily (41·5%) than lactating (17·4%). Where ewes were subjected to extended hormonal treatment, to induce a ‘repeat’ oestrus, 26·9% conceived at first induced oestrus and 54·1% at the ‘repeat’ oestrus, a difference which was statistically significant (P < 0·05).

Trial 4

Three hundred and thirty-two non-lactating ewes, differing in age and breed, were used in studies conducted mainly in June/July. Eight different combinations of progesterone and P.M.S. were employed. For the majority, progesterone was administered over a 7-day period at 2-day (40 mg.) or 3-day (50 mg.) intervals. A single dose of 750 i.u. freeze-dried p.m.s. was given 1–4 days after final progesterone injection. Two hundred and eightyfive animals (85%) were served 1–5 days after the end of treatment. Ninety-two ewes conceived at the induced oestrus and produced lambs; as a percentage of those injected this is 27·7, and as a percentage of those that mated, 32·6. Average number of lambs born per ewe was 1·72. In the ewes which ‘repeated’ spontaneously after hormone treatment, 58·9% conceived. This was a significant increase over conception percentage at induced oestrus, but the average number of lambs born per ewe was lower (1·25).

Results on oestrus, conception and lambing in trials 3 and 4 are discussed in relation to treatment, ram fertility, breed and age of ewe, and several environmental factors. They indicate strongly that conception failure lies at time of implantation of the egg, possibly due to some inadequacy in the uterine environment. Conception may be facilitated where animals are either hormonally induced to show a ‘repeat’ oestrus or show it spontaneously. Factors of fundamental and practical interest in out-ofseason breeding are described, including length of gestation, incidence of lamb mortality and pregnancy toxaemia.

Trial 5

A total of 100 ewes, of several breeds and ages, and including sixty that were lactating received treatment with a micro-crystal suspension of progesterone, either in conjunction with p.m.s. or with doses of progesterone in oil and p.m.s. Treatments were applied in five individual groups during the period of deep anoestrus (April/May). The majority of ewes were given an initial injection of 50–200 mg. suspension, followed 7–8 days later by a single dose of 25 mg. progesterone in oil solutions; 750 i.u. of p.m.s. was administered 2 days after the final progesterone dose. 13% of animals mated after treatment and 2% conceived and produced lambs. The results for oestrus and conception were considerably poorer than those following the normal progesterone-p.m.s. technique.