Fifty-nine mature "Western" (Suffolk x Rambouillet) and 71 mature ¼ - Finn (Suffolk x Finn-Rambouillet) ewes were bred to Suffolk-Hampshire rams to lamb in either spring (March) or fall (September). Effects of ewe breed type and lambing season on ewe fertility (ewes lambing/ewes exposed), prolificacy (lambs born/ewe lambing) and lambing date, and on lamb birth weight, 30-d survival, and average daily gain (ADG) to 55 d and to weaning were evaluated. Fertility was higher in spring than in fall (94 vs 64%, P<.01). Use of %-Finn ewes tended to increase fertility in fall lambing (69 vs 59%, P<.25) but not in spring (94 vs 93%). Use of ¼ -Finn increased prolificacy by .36 lambs; fall lambing reduced prolificacy by .49 lambs. Lambs of Western ewes were born 5 d later (P<.01) and .6 Ib heavier (P<.001) than lambs of ¼ -Finn ewes. Lamb survival and ADG to 55 d and to weaning were not affected by ewe breed. Season affected ADG: fallborn lambs grew faster to 55 d (P< .05) and to weaning (P< .001) than spring-born lambs, attributable to differences in environmental factors. Effect of weaning on growth of spring-born lambs grazing summer pasture was analyzed. Spring lambs weaned at 90 d of age to graze independently grew more slowly and were older at market weight than lambs remaining with their dams on native perennial pastures (.24 vs .42 lb/d, P<.001; 201 vs 186 d of age, P<.001). Use of millet (Pennisetum americanum [L.] Leeke) as annual mid-summer forage and turnip (Brassica rapa L.) as annual late-fall forage were investigated. Dry ewes grazing millet lost weight in one yr, but averaged .23 Ib/d gain in a second yr; weaned lambs grazing pearl millet averaged .32 lb/d gain over 2 yr. Lactating and dry ewes grazing turnip had average weight changes of -.37 and +.11 lb/d, respectively. Budgets for production systems were constructed. Variable revenues and expenses for all combinations of ewe breed type, lambing season, weaning treatment, and annual forage use were integrated into an annual production system. The most profitable production systems were those that exhibited the greatest reproductive efficiency, required the least drylot time to finish lambs, and maximized use of perennial pastures. / Master of Science