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201 Factors that limit out-of-season breeding success in ewes


 The American Sheep Industry recently stressed the importance of out-of-season breeding to meet year round demand for quality lamb. Because ewes naturally breed during short days, spring breeding is a challenge. The objective was to determine factors that reduce pregnancy rate in Katahdin ewes in Arkansas. In 2017, 95 ewes (1.6 - 9.4 yr of age; mean 2.6 ± 1.8) were bred between mid-April to mid-May. Between mid-August to mid-September, 82 ewes (1.0 - 8.2 yr of age; 2.6 ± 1.7) were bred. In spring, ewes grazed endophyte-infected tall fescue, but in fall, primarily bermudagrass was available. Body weight (BW) and body condition scores (BCS), and serum concentrations of anti-mullerian hormone (AMH), prolactin, and cytochrome P450 enzyme activity were determined before rams entered. Ewes were exposed to teasers 14 d before breeding. Data were analyzed by Proc GLM and CORR (SAS) with season of breeding in model and ewe age used as covariate. Pregnancy rate was greater in fall than spring bred ewes (89.3 ± 4.0 vs. 21.9 ± 3.3%, P < 0.001). Ewes that became pregnant compared with nonpregnant had a greater concentration of AMH regardless of season bred (3.3 ± 0.22 vs. 2.4 ± 0.41 ng/mL, P = 0.04; R = 0.22, P = 0.004), but tended to have a lower concentration of prolactin (20.2 ± 2.7 vs. 30.4 ± 4.8 ng/ mL, P = 0.06; R = -0.38, P < 0.001). The overall lower prolactin in spring suggests a reduction associated with fescue toxins, but further research is necessary. Cytochrome P450 activity was not significantly lower in ewes that became pregnant compared with nonpregnant (R = -0.28, P = 0.002), and was greater in younger ewes (R = -0.25, P = 0.03). Selecting ewes more tolerant to fescue toxins may improve success of out-of-season breeding in the Southeastern United States