Effect of season on follicular population, quality and nuclear maturation of bovine oocytes under tropical conditions

•Season influenced follicular population, quality and maturation of oocytes.•The major follicular population was found in the hot-humid season.•More oocytes of better quality were found in the hot-humid season.•The major nuclear maturation of oocytes was found in the hot-humid season.•The presence of a CL decreased follicular population and oocytes viability. The aim was to determine the effect of season of the year and the presence of a corpus luteum (CL) on follicular population (FP) and the quality of the oocytes, and of season on nuclear maturation of the bovine oocytes under tropical conditions. Three seasons were evaluated: hot-dry (March–June), hot-humid (July–October) and fresh-humid (November–February). In a first study, 1112 bovine ovaries were obtained from a local slaughterhouse. Follicles were classified as small (≤4mm), middle (4.1–8mm) and large (≥8.1mm); and the maximum diameter of the follicle (MDF) and CL (MDCL) were also recorded. The oocytes were collected by aspiration and classified as viable (grade I and II) and damaged (grade III and IV). In the second study, 2261 viable oocytes were matured in vitro, and then fixed and stained with Lacmoid to classify the stage of development as mature (metaphase II), immature or degenerate. Data were analyzed using analysis of variance and chi-square procedures. The largest FP of large follicles (0.67), MDF (1.18mm), MDCL (1.87mm), and the highest proportion of viable oocytes (34.19%) were obtained during the hot-humid season (P