Reproductive performance is measured by several parameters. Those most commonly used are the proportion of ewes exposed or bred to the ram that lamb (the measure of fertility), the proportion of lambs born (alive and dead) per ewe lambing (also called drop rate and a measure of prolificacy), and age at first lambing. Also often measured are lambs tail-docked per ewe lambing (lambs marked); lambing percentage (number of lambs born per ewe exposed to the ram); and weaning percentage (percentage of lambs weaned per ewe exposed to the ram), although the latter is more of an economic measure. To measure lamb losses, it is preferable to calculate stillbirth rates and preweaning mortality rates as a proportion of lambs born and lambs born alive, respectively.
Goals for reproductive performance vary tremendously between different sheep-raising systems and must be matched with the management system and constraints. Extensive range flocks that produce primarily wool (eg, the Australian Merino) may seek only flock replacement so that lambing rates of <90%, 1 lamb born/ewe lambing, and <0.8 lambs weaned/ewe lambing are considered acceptable. Ewes are traditionally not expected to lamb until 2 years of age. Conversely, flocks that produce primarily meat under conditions that require the feeding of stored feeds (eg, where growing seasons are short) and/or housing for a significant proportion of the year must have much better fecundity and lamb weaning percentages to be economically viable. Age at first lambing should be 12–14 months, and a ewe should raise at least 1.8 or more likely 2 lambs/year. Fertility rates during the ovulatory season should be 95%–100% in ewes and 90% in ewe lambs.
Producers seeking to further increase ewe production and to reach the more lucrative lamb meat markets also practice accelerated lambing in which ewes lamb every 8 months (ie, three lambings in 2 years) or every 7.2 months (the Cornell STAR© system, see Breeding Programs of Sheep). Both of these systems require that ewes breed during the anovulatory and transition season, as well as the ovulatory season. However, in most of the world, fall breeding with spring lambing is the predominant reproduction management system, which coincides with optimal grass production for the lambs and lactating ewes, regardless of the desired prolificacy of the management system.