Variations in follicle-diameter deviation and a growth spurt in the dominant follicle at deviation in Bos taurus heifers

Follicle diameter deviation during a follicular wave in cattle is preceded by a common-growth phase and begins with continued growth rate of the future dominant follicle (DF or F1) and reduced growth rate of the future largest subordinate follicle (SF or F2). Follicle diameters were analyzed daily for wave 1 (n = 100) and wave 2 (n = 91). The beginning of expected deviation was designated day 0 and was defined as the day the largest follicle was closest to 8.5 mm. Conventional classification of deviation (F2 ≥ 7.0 mm at expected deviation) was more frequent (P < 0.0001) in wave 1 (59%) than in wave 2 (35%); deviation began abruptly. An F2-undersized classification (F2 < 7.0 mm at expected deviation) was greater (P < 0.0001) for wave 2 (40%) than for wave 1 (15%); separation of diameters of F1 and F2 occurred gradually during the last 2 days of the common-growth phase. The growth rate of F1 in conventional deviations of wave 1 was greater (P < 0.002) on days 0 to 1 (1.8 ± 0.1 mm/day) than on days −1 to 0 (1.5 ± 0.1 mm/day). Several other novel growth spurts of F1 were observed in association with deviation including during switching of diameter rank between F1 and F2 in each of waves 1 and 2. Hypotheses were supported that (1) conventional deviation is more common during wave 1 whereas F2-undersized deviation is more common during wave 2 and (2) a transient growth spurt occurs in F1 of wave 1 at the beginning of diameter deviation.