1 Cows should be bred 45 to 80 days after they have calved, depending on age and condition. 2  The optimum bull-to-cow ratio is 1 bull per 10 to 30 cows, depending on the size of the pasture, and the bull's age, promiscuity/libido and experience. Some bulls, depending on the fertility rate of the cows and how many cows are in heat at one time, can breed over 50 cows in one season. 2  Body condition score your cows and heifers 30 days before breeding season.