Elevation of Plasma FSH with a Low Level of FSH-P During the Early to Mid Follicular Phase Blocks the Loss of Greater Numbers of Medium Follicles in Control Line Gilts Compared to Gilts Selected for High Ovulation Rate
University of Nebraska 1995 Swine Day Report. The number of follicles ovulated (ovulation rate) at estrus is an important determinant of litter size in the pig because it sets the upper limit for litter size. Ovulation rate is a moderately heritable trait in pigs (h2 = 40%). Gilts selected for high ovulation rate (Relax Select, RS line) in the University of Nebraska Gene Pool population ovulated about 3.5 more follicles than randomly selected Control (C) line gilts after nine generations of selection. This difference continues to be maintained after many generations of random selection following the end of deliberate selection. Evaluation of the pattern of follicular development showed that RS gilts maintain a larger pool of 3 to 6.9 mm follicles than C gilts during the mid to late follicular phase of the estrous cycle. Also, a greater proportion of the 5 to 6.9 mm follicles were healthier in RS than C gilts during this period. Large preovulatory follicles were slower to develop in RS gilts and it was not until late in the follicular phase that RS gilts developed the number of large preovulatory follicles needed to achieve their advantage in ovulation rate. Other studies showed that RS gilts maintain elevated concentrations of folliclestimulating hormone (FSH), but not luteinizing hormone (LH), during the late luteal and early follicular phases of the estrous cycle (days 12 to 14). The elevated concentrations of FSH in RS gilts during the late luteal and early follicular phases may be involved with the maintenance of a larger pool of healthy 5 to 6.9 mm follicles from which preovulatory follicles are selected. Therefore, the present study was conducted to: 1) further characterize relationships between development and maturation of follicles and concentrations of gonadotropic and gonadal hormones in RS and C line gilts during the early to mid follicular phase of the estrous cycle; and 2) determine whether FSH therapy (treatment with FSH-P, a commercially available porcine FSH preparation) during the early to mid follicular phase of the estrous cycle will prevent the loss of medium follicles and reverse the pattern of development and/ or maintenance of fewer medium follicles in C than RS gilts during the mid follicular phase.