Complexities of follicle deviation during selection of a dominant follicle in Bos taurus heifers

Abstract

Follicle deviation during a follicular wave is a continuation in growth rate of the dominant follicle (F1) and decreased growth rate of the largest subordinate follicle (F2). The reliability of using an F1 of 8.5 mm to represent the beginning of expected deviation for experimental purposes during waves 1 and 2 (n = 26 per wave) was studied daily in heifers. Each wave was subgrouped as follows: standard subgroup (F1 larger than F2 for 2 days preceding deviation and F2 > 7.0 mm on the day of deviation), undersized subgroup (F2 did not attain 7.0 mm by the day of deviation), and switched subgroup (F2 larger than F1 at least once on the 2 days before or on the day of deviation). For each wave, mean differences in diameter between F1 and F2 changed abruptly at expected deviation in the standard subgroup but began 1 day before expected deviation in the undersized and switched subgroups. Concentrations of FSH in the wave-stimulating FSH surge and an increase in LH centered on expected deviation did not differ among subgroups. Results for each wave indicated that (1) expected deviation (F1, 8.5 mm) was a reliable representation of actual deviation in the standard subgroup but not in the undersized and switched subgroups; (2) concentrations of the gonadotropins normalized to expected deviation were similar among the three subgroups, indicating that the day of deviation was related to diameter of F1 and not F2; and (3) defining an expected day of deviation for experimental use should consider both diameter of F1 and the characteristics of deviation.