Lengthened superstimulatory treatment in cattle: Evidence for rescue of follicles within a wave rather than continuous recruitment of new follicles.

A study was designed to compare the effects of a conventional (4 days) versus a lengthened (7 days) superstimulation protocol on follicle dynamics and to test the hypothesis that superstimulatory treatment only rescues small follicles within the wave. Nonlactating beef cows received a progesterone-releasing intravaginal device [PRID] and were superstimulated with 400-mg FSH on the day of follicle ablation-induced wave emergence (Day 0). The control group (n = 5) received FSH over 4 days, whereas the long group (n = 5) received FSH over 7 days. PGF was administered twice on Day 2 (control group) or 5 (long group), and PRIDs were removed 24 hours after the first PGF. Cows received 25-mg LH 24 hours after PRID removal. The cows chosen for the present study represented a subset from a larger group of 24 cows in which superovulation results were obtained and published. Cows in the present study were those with the lowest antral follicle counts at the time of wave emergence in order to facilitate tracking of individual follicles. Daily ultrasonographic examinations monitored follicle diameters and numbers. A reduction (P < 0.01) in the number of 1- to 2-mm follicles from Day 0 to the end of the superstimulatory treatment was associated with a progressive shift to the next size category in both groups. On the day of LH treatment, the number of follicles in any size category did not differ between groups. The number of follicles of diameter 5 mm or less decreased during superstimulation (i.e., no continuous recruitment of small follicles; P < 0.001), and the number of follicles of diameter 1 mm or greater at the end of superstimulation did not differ from that at the beginning of superstimulation (P = 0.739). However, the total number of follicles of diameter 3 mm or greater at the end was greater than the number of follicles of diameter 3 mm or greater at the beginning of superstimulation because of growth of the 1- to 2-mm population during treatment (P < 0.001). Follicle growth was slower in the long group than in the control group. Results support the hypothesis that both 4-day and 7-day superstimulatory protocols rescue small antral follicles present at the time of wave emergence; there was no evidence for continuous recruitment of new follicles. Results also provide rationale for the hypothesis that a lengthened treatment protocol is associated with greater follicle maturation and capacity to ovulate.