Evaluation of sheep genetic resources in North America: Ewe productivity of purebred, crossbred and synthetic populations. Can. J. Anim. Sci. 88: 399Á408. Ewe productivity from divergent genetic types of established purebreds e.g., Dorset (D), Lincoln (L), Rambouillet (Ra), Suffolk (Su) and Targhee (T); fecund-type breeds e.g., Finnsheep (F) and Romanov (Ro) breeds, and their reciprocal crosses; the Outaouais (O) and Rideau (R) Arcott breeds and their reciprocal crosses; and Synthetic I (½ F, ½ L), Synthetic II (½ D, ½ Ra) and Synthetic III (¼ F, ¼ L, ¼ D, ¼ Ra) populations were evaluated. Prolificacy, fecundity, ewe weight, grease fleece weight, wool grade, lamb survival and total lamb weights at birth, 30 d and 140 d per ewe lambing were considered jointly as a measure of ewe productivity. In general, fecund-type breed cross and Arcott breed cross were highest in productivity, the fecund-type breed, Arcott breed and synthetic populations were intermediate, in contrast the established breeds was lowest. Regardless of significantly heavier ewe and grease fleece weights, and superior wool grade of the established breeds, their poor performance can be attributed to lower prolificacy, fecundity and total lamb weights (P B0.05). Within established breeds, the D, L, Ra and T breeds were comparable in productivity to the Su breed, but lower than the F breed (PB0.05). The Ro breed surpassed the F breed because of significantly higher prolificacy and fecundity, and heavier ewe weight and total lamb weights, while grease fleece weight and wool grade were inconsistent. The F and Ro breeds were comparable to the Su breed despite significantly higher prolificacy and fecundity, in contrast to lighter ewe and grease fleece weights, lower wool grade, lamb survival, similar total lamb weights. Although the R breed produced significantly heavier grease fleece weight and total lamb weights than the O breed, both breeds were similar to the Su breed despite their significantly higher prolificacy and fecundity, in contrast to similar ewe weight, wool grade, lamb survival and total lamb weights, and lighter grease fleece weight. The O and R breeds were more productive than the F breed as a result of significantly heavier ewe weight, superior wool grade, higher lamb survival and heavier total lamb weights, despite their similar prolificacy, fecundity and grease fleece weight. Synthetic I and Synthetic III with Finnsheep lineage surpassed Synthetic II in productivity as a result of significantly higher prolificacy and fecundity, and heavier total lamb weights, despite similar ewe and grease fleece weights and inconsistent wool grade and lamb survival. At the same time, Synthetic I and Synthetic III were not only similar to the Su breed, but surpassed the F breed, whereas Synthetic II was similar to the Su breed and lower than the F breed. Finally, systematic crossbreeding and composite population of complementary fecund-type and established breeds achieved increased productivity from additive genetic variation and heterosis. . Pour mes...