To evaluate the effects of season on semen, scrotal circumference and testosterone concentration in Corriente and European-breed bulls, 758 samples were analyzed in four years. Comparisons were made between bulls of the same breed in different seasons and between bulls of different breeds in the same season. Scrotal circumference of both breeds was higher in the summer and autumn (P < 0.01). Sperm concentration, individual motility and live cells were lower in summer and autumn in European-breed bulls (P < 0.01). All percentages of sperm defects were higher, and testosterone was lower in European-breed bulls during summer and autumn (P < 0.01). Gross motility tended to be lower (P = 0.09) in European-based bulls during winter. Scrotal circumference was higher in Europeanbreed bulls than Corriente bulls in all seasons (P < 0.01). Ejaculate volume and sperm concentration were similar in Corriente and European-breed bulls in all seasons (P > 0.1). Individual motility was lower in Europeanbreed than in Corriente bulls in summer and autumn (P < 0.01 and <0.01). Live cells count was lower in European-breed than in Corriente bulls during the summer and autumn (P < 0.01 and <0.01). Major sperm defects were higher for Corriente than European-breed bulls during winter (P < 0.01). Minor cell defects were higher in summer and autumn (P < 0.01) in Europeanbreed than in Corriente bulls. Total defects were lower in Corriente than in European-breed bulls during summer and autumn (P < 0.01), and higher during winter (P < 0.01). Testosterone was higher in Corriente bulls during summer (P < 0.01) and autumn (P < 0.01) than in European-breed bulls. Gross motility was lower (P < 0.05) in European-breed than in Corriente bulls in winter. Season had no effect on most semen characteristics in Corriente bulls, while European-breed bulls had lower semen quality during the hotter seasons. This indicates a better adaptation of Corriente cattle, suggesting that these bulls can be used for mating all year in hot environments.

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