When to put bulls in pasture when you want spring calving?

Are you asking about when to put the bulls with the cows in order to get cows calving in the spring? If so, then the best time to put the bulls with the cows is in late summer. For instance, if you're wanting cows to calve early May, then you put the bulls in in mid July. Estimate 9 months plus 2 weeks (285 day) gestation period to perform your calculations.