Mature ewes of predominately Columbia type were used to determine the effects of two oral progestogens, 6-chloro-Δ⁶-17 acetoxy-progesterone (CAP) and 6-methyl-17-acetoxy-progesterone (MAP), and PMS on fertility in lactating and nonlactating (early weaning) ewes. The 184 lactating ewes were randomized into three treatment series. The two different oral progestogens were used in series 1 and 2. MAP was used in series 3. Series 1 received two short progestogen treatments (3 and 8 days) each followed by PMS 24 hr. later. Series 2 received a long progestogen treatment (14 days) followed by PMS 1 and 17 days later. Series 3 was like series 2, except that MAP was given again for 3 days before the second PMS. Forty-one ewes from the early-weaning group were also randomized into the series 1 and 2 treatments. Treatment started March 19 when the lambs were 30 to 90 days of age.

Only 4% of the treatment-1 series lactating ewes were marked by fertile rams following the first PMS as compared to 54% of the treatment-2 series lactating ewes (P<.01). Eighty-six percent in the treatment-1 series showed estrus following both PMS injections. Other estrous data were unreliable and excluded.

There was no significant effect of kind of progestogen or of lactation on percent of ewes lambing and of lambs at 2 weeks postpartum, but there was a significant effect of the length of the progestogen treatment before the first PMS on both variables. A comparison of the response of lactating ewes in treatment series 3 with treatment series 2 appears to indicate that there is a definite advantage in repeating the treatment with PMS irrespective of the occurrence of heat. A comparison of the treatment-1 and -2 series indicates that there is little or no detrimental effect of PMS on established pregnancies. The ewes lambing in the treatment-2 series had a higher percentage of lambs alive at about 14 days postpartum than the treatment-1 and treatment-3 series (128%, 97% and 100%, respectively).