Effect of season and breed group on the follicular population and cyclicity of heifers under tropical conditions

The aim was to determine the effect of season and breed group on follicular population, and presence and size of CL of heifers under tropical conditions. The seasons were hot-dry (March–June), hot-humid (July–October), and fresh-humid (November–February). Thirty Zebu (Brahman) and 38 F1 (Simmental?×?Brahman) heifers were used. Five evaluations were made in each season, at intervals of 7 days, to assess ovarian activity by ultrasound. Follicles were classified as small (≤4 mm), middle (4.1–8 mm), and large (≥8.1 mm) sizes, and also the size of CL, when present, was measured. Data were analyzed using analysis of variance and logistic regression procedures. Mean number of small follicles was 11.6?±?2.3 with no effect of season, breed group, or their interaction (P?>?0.05). Mean number of middle follicles was influenced by season and breed group; the highest average was found in the fresh-humid season (4.0?±?0.2) and in F1 heifers (3.6?±?0.2; P?<?0.05). The highest mean number of large follicles was in the hot-humid season (1.4?±?0.1; P?<?0.05). The highest maximum follicle diameter (MFD) mean was registered in the hot-humid season (1.3 mm; P?<?0.05) and the lowest proportion of heifers with CL occurred in fresh-humid season (33.3%; P?<?0.05). No effect of season, breed group, and interaction on the maximum diameter of the CL was found. In conclusion, season was a very important source of variation. Heifers in the hot-humid season had the largest follicles and MFD, and better cyclicity.