Increased number of large non-atretic follicles and co-dominance effects account for high litter sizes in Bonga sheep.

Abstract

To understand the ovarian basis for prolificacy of Bonga sheep, a total of 31 ewes were selected based on litter size (LS) records and divided into two groups: High Prolificacy (HP) (n=20) with LS≥2 and Low Prolificacy (LP) (n=11) with LS=1. At a synchronized estrus, follicular dynamics were determined using transrectal ultrasonography. Plasma estradiol concentrations were also monitored. In total 27 ewes were observed in estrus being 9/11 LP (82%) and 18/20 HP (90%). On the day of estrus (day 0), the mean number of large follicles was higher (p<.05) in HP (1.78±0.19) than in LP (1.0±0.28) ewes. Prior to estrus, more (p<.05) medium follicles were visible for HP compared to LP ewes. Plasma estradiol concentrations were higher in HP compared to LP ewes (18.91±0.41 vs. 14.51±0.65pg/ml; p<.05) and similarly was ovulation number (2.3±0.15 vs. 1.28±0. 14; p<.05). Higher ovulation rates and litter size in Bonga sheep are evidenced by the previous presence of more large follicles and the existence of co-dominance effects as most likely medium follicles are selected to ovulate.