A comparison of the follicular dynamics in heifers of the Czech Fleckvieh and Holstein breeds

n = 23) heifers were investigated. Follicular dynamics was evaluated by daily sonographic scanning during three interovulatory intervals. The mean duration of the interovulatory interval was 20.66 ± 0.32 days, no differences between breeds were observed. The proportion of the non- alterna-ting pattern was nearly the same as that of the alternating pattern (54% and 46%, respectively). The majority of IOIs ≤ 21 days were of the 2-wave pattern (71%) whereas only 29% of them were of the 3-wave pattern. Conversely, the majority of IOIs ≥ 22 days were of the 3-wave pattern (84%), whereas only 16% were of the 2-wave pattern. Differences could be observed in the Czech Fleckvieh heifers. Comparing 2- and 3-wave interovulatory intervals, 44.2% of the heifers exhibited 3 waves and 55.8% of the heifers exhibited 2 waves of follicular growth. The ratio of 3- to 2-wave heifers was about the same in the Holstein breed; in Czech Fleckvieh 2-wave cycles slightly dominated (11/12, 8/12; respectively). In Holstein heifers, the first follicular wave occurred 0.92 ± 0.15 days after ovulation in 2-wave interovulatory intervals, and the emergence of the first wave in 2-wave Czech Fleckvieh heifers appeared later ( P < 0.05), 1.83 ± 0.3 days after ovulation. The maximal size reached by the dominant follicles in all animals and in the Czech Fleckvieh differed in the first and in the second wave of 2-wave cycles ( P < 0.05). In 3-wave interovulatory intervals the dominant follicles in the second wave differed ( P < 0.05) from the mean diameters of the first and the third wave in the Czech Fleckvieh. The ovulatory follicles were significantly ( P < 0.05) smaller in 2-wave than in 3-wave interovulatory intervals among all animals and between the Holstein and Czech Fleckvieh heifers. In conclusion, we found a similar pattern of ovarian follicular dynamics in Czech Fleckvieh and Holstein heifers kept under identical nutritional and environmental conditions. Whether the significant difference in the emergence of the 1 st fol- licular wave in 2-wave IOIs between C and H heifers is of real biological significance is ambiguous.