Fertility and prolificacy of crossbred ewes under two cycles of accelerated lambing.

The reproductive performance of 227 crossbred ewes representing five combinations of Finnsheep (F), Dorset (D) and Rambouillet (R) breeding was evaluated under two cycles of accelerated lambing, with lambing every 8 mo. The five breed combinations represented were: 1/2D 1/2R, 1/4D 3/4R, 1/4F 1/2D 1/4R, 1/4F 1/4D 1/2R and 1/4F 3/4R. The ewes were 4 and 5 yr old when put on the accelerated lambing program. Ewes were mated to yearling Hampshire, Suffolk, HxS or SxH crossbred rams each season. Each cycle was made up of three breeding seasons: winter (January-February), fall (September-October) and late spring (May-June). Reproductive performance averaged over the two cycles showed reduced fertility to May-June breeding (47.8%), compared to January-February breeding (91.8%) and September-October breeding (90.6%; P less than .05). During May-June breeding, 1/2D 1/2R ewes were more fertile (P less than .05) than 1/4D 3/4R or any 1/4F ewe groups. During the winter and fall breeding seasons, ewe fertility averaged 91.5% and was similar for ewes exposed to purebred and crossbred rams. In the late spring the use of crossbred rams resulted in 62.4% conception compared with 41.0% from the use of purebred rams (P less than .05). Prolificacy of the ewes averaged 1.67 lambs during winter, 1.80 lambs during fall and 1.35 lambs during later spring. The 1/4F ewe groups produced .10 and .16 more lambs/lambing (P less than .05) than 1/2D 1/2R and 1/4D and 3/4R ewes, respectively.