Ovarian activity during sexual maturation and following introduction of the ram to ewe lambs

The aim of this study was to assess the applicability of transrectal ovarian ultrasonography in quantifying ovarian antral follicles during the sexual development of ewe lambs, from 4 to 31 weeks of age. Also to record how the introduction of a ram at 25 and 29 weeks after birth affected ovarian function in peripubertal ewe lambs. Suffolk x Western White Face ewe lambs born in April underwent ultrasonography every 2 weeks (from 4 to 24 weeks of age; n = 27) or weekly (from 26 to 30 weeks of age; n = 14). When the ewes were 25 weeks old, a vasectomised ram was placed in the pen next to the ewes for 1 week, and at 29 weeks of age, the ram was placed with the ewes for another week. One week after second contact with the ram, estrus was synchronised by a 12-day treatment with intravaginal sponges containing medroxyprogesterone acetate (MAP). Ovarian ultrasonography was done every 6 h, from 0 to 96 h after sponge removal, and then daily for three more days, to determine the number of ovulations and the time at which they occurred. The mean number and total diameter of all follicles greater than or equal to 3 mm increased from 14 to 16 weeks, declined between 16 and 18 weeks, and then increased between 22 and 24 weeks after birth. All these changes in total follicular number and diameter were significant. Maximum follicular diameter increased from 8 to 16 weeks of age (P < 0.05). The first fence-line contact with a ram resulted in increases (P < 0.05) in follicular diameters (from 25 to 26 weeks of age), the number of luteal structures per ewe, the percentage of ewe lambs with luteal structures, and mean serum progesterone (P4) concentrations (from 1 to 2 weeks after the contact began). The second, full ram contact increased (P < 0.05) the number of luteal structures per ewe, the proportion of ewe lambs with luteal structures, and serum P4 concentrations, from 29 to 30 weeks of age. Subsequent treatment with MAP sponges was followed by estrus, ovulations and full-length luteal phases in all ewe lambs studied. In summary, there were two phases of increased antral follicle development during the post-natal period in ewe lambs, with peak follicle numbers at 16 and 24 weeks of age. Fence-line contact with the ram at 25 weeks of age resulted in increased follicle size and stimulated the formation of some luteal structures, and subsequent full ram contact at 29 weeks of age markedly enhanced the number of luteal structures. Transrectal ovarian ultrasonography has proven to be a valuable tool in the study of ovarian function in ewe lambs from 4 weeks of age onwards.