A blackbelly ewe comes into estrous (is in heat; will stand to be mated) every 17 days. The duration of estrus is 24 to 36 hours. Blackbelly sheep are non-seasonal breeders, meaning that they will breed anytime during the year, unlike wooled sheep who generally will breed only in the fall for a spring lambing.
Year-round breeding allows you to time your lambing to your convenience. You can choose to lamb after the seriously cold weather ends in April. You can choose to lamb to meet the needs of your market. For instance, if you sell to an ethnic market who purchases lambs for for religious holidays, you can breed specifically to have your lamb born and raised to the desired weight by that holiday.
This chart shows how you can maximize the Blackbelly's nonseasonal breeding capabilities and achieve three lambings in 18 months. It also allows you to choose the best months for lambing by showing you the month you need to breed. For example, if you want to lamb in nice, warm August, then follow the arrow backwards and see that you should breed in March. (Reprinted with permission from http://www.critterhaven.biz)