The induction of pregnancy in the anoestrous ewe by hormonal therapy V. Progesterone-pregnant mare's serum therapy on Clun forest maiden sheep and general conclusions

Abstract

This trial concerns the induction of pregnancy in two flocks of Clun maiden yearling ewes and involves seventy-six animals. Previous observations suggested that treatment of this particular category usually gives rise to a higher conception percentage than is found with dry post-partum or lactating sheep. The trial sought to elucidate some factors of animal and environment which may be associated with this response.In flock 45, during the period 10 February to 29 March, observations were made on the incidence of spontaneous breeding activity in forty-seven ewes; hormonal therapy was first applied at the latter date. Treatment for all ewes consisted of three doses of 50 mg. progesterone (in oil) given at 3-day intervals; a single dose of 750 i.u. p.m.s. was administered 2 days after final progesterone.Nineteen of the forty-seven ewes (40%) in flock 45 showed spontaneous oestrus in February or March. Three (16%) of those ewes previously mated came in oestrus after treatment; forty-nine of the fiftyseven unmated ewes (86%) showed oestrus 3–5 days after the end of injection. Of the nineteen sheep that mated spontaneously, ten (53%) produced autumn lambs. Twenty-six of the fifty-two sheep (50%) served in response to the progesterone-p.m.s. therapy conceived. 1·25 lambs were born per ewe that lambed. Some evidence for total embryonic or foetal loss in the ewes during pregnancy was obtained. Results indicate that conception in these maiden ewes may have been facilitated by the presence of corpora lutea or recent luteal activity, which maintained the oestrogen-progesterone balance necessary for providing an adequate uterine environment. For this reason, maiden ewes of breeds with short breeding seasons (e.g. Scotch Half-Breds) may not give such satisfactory response when injected at times comparable to long season breeds, such as the Clun.