The breeding season does not start at a specific time; cows are reproductively active all year long. The best method to determine the start of the breeding season is to determine when you want your cows to calve. We generally think of "spring" time (February through May) as the ideal calving season (about 70 to 75% of all calves are born in this period, although it is not exactly "spring") because the forage is regrowing as sunlight and temperatures increase and can provide adequate nutrition to the cow and her calf (through her milk). Since the gestation length in cows is about 285 days (approximately 9 1/2 months, a few days longer in larger and Bos indicus breeds and a few days shorter in smaller and dairy breeds), then the "ideal" breeding season for spring-born calves is 9 1/2 months prior to that or beginning in April but for sure by May.