THE INDUCTION OF BEHAVIOURAL OESTRUS IN PREGNANT CORRIEDALE EWES BY INJECTIONS OF PROGESTERONE AND OESTRADIOL

The experiment described in this paper initially aimed at developing a simple inexpensive method of pregnancy diagnosis for sheep. We considered that, if oestrus could be induced in the non-pregnant ewes in a flock (which may be in anoestrus) and this oestrus detected by the use of harnessed rams (Radford, Watson & Wood, 1960), a method of pregnancy diagnosis could be developed. Diamond & Young (1963) present evidence from a number of species that pregnancy prevents the virilizing effect of a range of exogenous steroid sub- stances. Lindsay & Francis (1969) and Fels (unpublished data) have shown that ovariectomized ewes do not respond to oestrogen while under the influence of exogenous progesterone. Thus, endogenous progesterone could be expected to prevent oestrous responses in pregnant ewes. One hundred and fifty-eight 18-month-old Corriedale ewes were mated with six Corriedale rams under paddock conditions for 8 weeks beginning 1st March. Seventy-eight days after the start of mating, the ewes were randomly divided into four treatment groups. Individual ewes were identified by numbered ear tags. The treatments were : Treatment A: 7 20-mg doses of progesterone at 2-day intervals over 14 days and 100 µ% of oestradiol 48 hr after the last dose of progesterone. Treatment : 4 20-mg doses of progesterone at 2-day intervals over 8 days and 100 ^g of oestradiol 48 hr after the last dose of progesterone. Treatment C: 1 20-mg dose of progesterone and 100 µg of oestradiol 48 hr later.