Gonadotropin concentrations associated with variations in diameter deviation during follicle selection in Holstein heifers

Diameter deviation or selection of the future dominant follicle (F1) from the future largest subordinate follicle (F2) during a follicular wave occurs when F1 is 8.5 mm (expected deviation, day 0). Deviation has been classified as conventional (F2 ≥ 7.0 mm), F2-undersized (F2 < 7.0 mm), and F1,F2-switched (F2 larger than F1 on day −1 or 0). Concentrations of gonadotropins were compared within and among deviation classifications in waves 1 and 2 in 48 heifers. A three-way (wave 1 compared with 2, classification, day) analysis indicated no effect of wave 1 compared with 2 on F2 or FSH. An interaction of classification by day for F2 diameter (P < 0.001) and FSH concentration (P < 0.005) was primarily from differences on day −1. Rankings on day −1 from greatest to least for F2 diameter were switched, conventional, and undersized and for FSH concentration were undersized, conventional, and switched. Lower FSH concentration in conventional compared with undersized deviations during the decline in the FSH surge was presumed to represent greater output of FSH inhibitors by larger follicles. The incline in FSH surge 2 began significantly later for undersized than for conventional deviation. Switched deviation was associated significantly with emergence of F2 before F1, lower FSH concentration during the decline in the FSH surge, and earlier occurrence of the post-surge FSH nadir. Results supported the hypothesis that diameter differences among deviation classifications are temporally associated with differences in FSH concentration within each classification. These novel findings may complicate studies on the mechanisms of follicle selection.