Follicular Selection and Atresia in Gilts Selected for an Index of High Ovulation Rate and High Prenatal Survival

1999 University of Nebraska-Lincoln Extension Swine Report. Previously, we reported (See Yen et al., Nebraska Swine Report 1998) White Line gilts selected for an index of high ovulation rate and high prenatal survival (White Line-2, WL-2) maintained a larger pool of medium follicles (3 to 6.9 mm) during the early- to mid-follicular phase than randomly selected controls (White Line- 1, WL-1). The present study evaluated the health status of the medium follicles to determine whether WL-2 gilts maintain a healthier pool of medium follicles and are able to continue selection of ovulatory follicles later in the follicular phase to achieve their ovulation rate advantage (6.6 ova). Ovaries were recovered on days zero, two three, four and five after induced luteolysis with PGF2a on day 13 (day zero) of the estrous cycle. Numbers of follicles (F) equal or greater than 3 mm in diameter were categorized by size and recorded as follows: medium-1 (M1F, 3 to 4.9 mm), medium-2 (M2F, 5 to 6.9 mm) and large (LF > 7 mm). Estradiol (E) concentration in follicular fluid was used to classic individual M2F and LF as healthy (> 100 ngE/mL) or atretic (< 100 ngE/mL). M1F were not estrogen- active (<60 ngE/mL) and could not be evaluated for atresia with this method. Mean E concentrations in M2F increased linearly from day two to day five in WL-2 gilts while E concentrations increased rapidly between day two and day three and then plateaued in WL-1 gilts. All LF were estrogen active (> 100 ngE/mL) and classified as healthy in both genetic lines. The percentage of healthy M2F increased rapidly in WL-2 gilts between day three and day five whereas percent of healthy M2F remained unchanged in WL-1 gilts during this period. Mean numbers of healthy M2F increased rapidly in WL-2 gilts between day two and day four and then declined to day five. Numbers of healthy M2F in WL-1 gilts increased between day two and day three and then declined to day four and day five. The greatest difference occurred on day four. WL-2 gilts maintain a larger pool of healthy M2F to day four of the follicular phase and rapidly select and mature these follicles into ovulatory follicles to achieve their ovulation rate advantage. Both genetic lines need to select about six additional ovulatory follicles from the M2F pool after day five to achieve final ovulation rates. Greater understanding of the biological basis of the improvement in follicular dynamics in the WL-2 population may prove useful in developing more efficient methods for improving ovulation rate and enhancing litter size in swine.