Following preliminary work conducted in Part II of this series, an attempt was made in the anoestrus of 1958 to provide further information on the conception percentage which follows hormonal induction of a ‘repeat’ oestrus. Two hundred and twenty-one ewes, seventy-six of which were lactating (lambs 8 weeks or older), were available in seven farm flocks. Ewes were of several different breeds and ages but excluded yearling or maiden animals. The trial was conducted during anoestrus over the period starting 26 April and ending 2 August. Treatment consisted of the repeated application of the following injection sequence: three doses of 50 nig. progesterone in oil solution, given at intervals of 3 days, with a dose of 750 i.u. p.m.s. administered 2 days after the final progesterone dose. This therapy was repeated after an interval of 7 days; a 15-day-period therefore separated the two administrations of p.m.s., i.e. approximating to one ovarian cycle.

Rams were joined at the cessation of therapy; in most flocks, one ram was allowed for every ten ewes injected. Two hundred and six of the ewes came in oestrus (93·2%) over the period 2–8 days after the final p.m.s. dose. The oestrous response varied little among flocks, the percentage in oestrus rangingfrom 85 to 100%. One hundred and forty-one conceived at the induced ‘repeat’ oestrus (63·8%) and produced a total of 207 lambs (1·47 per ewe). As a percentage of ewes mating, 68·4 conceived. In none of the flocks did the conception percentage fall below 50; lactating ewes conceived as readily as ‘dry’ sheep. The period during which autumn lambings occurred varied from 4 to 8 days among the flocks.

The results strongly suggest that a much higher conception percentage is likely to follow this extended treatment than follows normal progesterone-p.m.s. treatment. From the practical viewpoint, the technique has the disadvantage that eight injections are necessary and that treatment is prolonged to a 24-day period.