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.
Keywords: Corpus luteum; F1; Follicle; Ovarian activity; Zebu.