Effect of Sire Breed on Pregnancy Rate of Katahdin Ewes after Liquid Semen Vaginal Artificial Insemination

Previous work has suggested that sire breed (hair vs. wool) may have a significant effect on pregnancy outcome using liquid semen artificial insemination (AI) in hair sheep. In this experiment, we further evaluated the effect of sire breed on pregnancy rates using a flock of Katahdin ewes. Ewes (n = 88) managed under an annual breeding cycle were estrus synchronized by inserting CIDR devices for 12 d. Ewes were allocated to be bred to one of six Suffolk, Texel or Katahdin rams (n=2/breed) in equal numbers. Ewes were inseminated in October by simple vaginal timed AI at 48 h after CIDR removal, and again 6 h after initial AI with semen from the same collection. Semen was collected and processed on site using an artificial vagina, extended to 250 million sperm/ml in a simple UHT skim milk and egg yolk (5% v/v) extender, and packaged into 0.5 ml straws. For the initial insemination, straws were kept at processing temperatures (36°C) while straws for the second insemination were cooled to and stored at 4°C. Ewes were exposed to natural mating 5 d after AI to the same sire used for AI. Pregnancy was determined by trans-rectal ultrasonography 40 d after AI. Data were analyzed by Chi-square for the effect of sire breed, ram, and ewe age on pregnancy rates. Pregnancy rate to AI (15.9%) was lower than anticipated and following natural mating was 78.4%. There was no effect (P > 0.11) of sire breed on AI or natural mating pregnancy rate. However, differences between sires (P < 0.02) were observed following both AI (0%, 20%, 13%, 21%, 43%, and 0% for Kat1, Kat2, Suff1, Suff2, Tex1, and Tex2, respectively) and natural mating (100%, 73%, 47%, 86%, 78%, and 87% for Kat1, Kat2, Suff1, Suff2, Tex1, and Tex2, respectively). Pregnancy rates to AI ranged from 9% in yearling ewes to 22% in mature (2 - 4 yr), and 11% in older ewes (> 4 yr), but were not significantly different between groups (P = 0.1). Fetal numbers were not different in pregnancies resulting from AI or clean-up mating (71 and 77% multiples, respectively). Under the conditions of this study, sire breed did not influence the overall outcome of liquid semen AI even though there were differences observed among sires. Semen motility was affected by low ambient temperatures at time of collection and may have contributed to low AI pregnancy rates observed.