Selected Parameters of Reproduction in Rambouillet and St. Croix Ewes

• Main Author: Evans, Ronald Cole
• Format: Others
• Published: DigitalCommons@USU 1987
• Subjects: Animal Sciences
• Online Access: https://digitalcommons.usu.edu/etd/4092; https://digitalcommons.usu.edu/cgi/viewcontent.cgi?article=5097&context=etd

Experiments were conducted to determine the genetic reproductive potential of Rambouillet and St. Croix ewes . Ovulation rate (monthly) and estrus (daily) were observed for two years on 10 ewes of each breed to establish seasonal patterns . Both breeds demonstrated seasonal anestrus from May through July. Ovulation rates reached 2.0 or greater for both breeds during September to November, and decreased to 1.25 by March. Breeding groups for St. Croix and Rambouillet ewes were bred each February and August for five years to establish lambing performance at six month intervals. Age at first lambing was earlier for St. Croix than Rambouillet (89% vs 0.0% lambing at 12 months of age). Both breed and age influenced lambing rate. Fifty-two of 108 (48%) lambings from mature St. Croix ewes occurred at six month intervals, compared to 1 of 85 for Rambouillet ( 1.2%}. No Rambouillet under 36 months lambed at 6 month interval, while 8 of 61, 12 month St. Croix ewes lambed at a 6 month interval (13.1%). Mature St. Croix ewes that lambed at six month intervals produced 3.44 lambs per year, compared to 2.03 for ewes that lambed once a year. Parturition interval was 262 days for mature and 302 days for 24 month and younger St. Croix ewes on a six month lambing schedule. Parturition interval for mature Rambouillets was 355 days. Blood samples were taken twice weekly and serum progesterone measured by RIA in 17 St. Croix and four Rambouillet ewes that lambed during the normal breeding season, to determine occurrence of postpartum ovulation. Average days to first postpartum ovulation was 35.8 and 35.6 for St. Croix and Rambouillet, respectively. Thirteen of seven teen (76.5%) St. Croix ewes showed estrus with first ovulation, while none of the Rambouillet showed estrus with first or second postpartum ovulation. The shortened postpartum period for St. Croix compared to Rambouillet (40 .2 vs > 70 days) was a result of estrus accompanying the first or second postpartum ovulation.