Response of ewes primed with new CIDRs, previously used CIDRs, or previously used and autoclaved CIDRs to the ram effect during the non-breeding season

The ovulation induced in anestrous ewes by the sudden introduction of males is not accompanied by estrous behavior unless ewes are primed with progestagens. Although CIDR devices (which contain 0.3 g of progesterone; Pfizer, Auckland, New Zealand) may be used for more than one treatment, conception rates decrease when CIDRs previously used for long times are used again. In cows, if previously used CIDRs are autoclaved, the initial rises in plasma progesterone concentrations are not different to those obtained with a new CIDR. Our objective was to compare the estrous and pregnancy rates of ewes induced to ovulate with the ram effect during the non-breeding season to ewes primed with a new CIDR, a previously used CIDR, or a previously used autoclaved CIDR. Ninety-two ewes were randomly allocated to three groups and received a new CIDR (group NC, n = 30), a previously used CIDR (UC, n = 30), or a used-autoclaved CIDR (UAC, n = 32) for 8 days. Used CIDRs were used for 22 days; the CIDRs used in UAC ewes were previously autoclaved. At CIDR withdrawal ewes were joined with adult rams and 20 additional ewes with induced estrous (1 ram:13 ewes). Marked ewes were detected every 24 h for 4 days. Forty days after estrus, pregnancy was determined by transrectal ultrasound. Blood samples were collected from 8 ewes from each group 5 and 6 days after CIDR insertion, and at the moment of CIDR withdrawal, and progesterone was measured by radioimmunoassay. Estrous and pregnancy rates were greater in NC than in UC and UAC ewes (estrous: 56.7 vs. 26.7 and 15.6%; pregnancy rate: 50.0 vs. 13.3 and 15.6% for NC, UC, and UAC ewes, respectively; P < 0.05). Progesterone concentration was greater (P < 0.001) in NC than in UC and UAC ewes. No difference was found between UC and UAC groups for any parameter. Overall, we concluded that autoclaving the CIDRs previously used for 22 days had no positive effects on estrous and pregnancy rates when applied as primings for the ram effect during the non-breeding season. This may be explained by the fact that progesterone concentrations in UC and UAC ewes were below luteal levels during the last days of the treatment.