Date of this Version
Two accelerated lambing systems, Morlam using Morlam sheep (USDA, Beltsville 1966 to 1975) and Carnal using Dorset ewes (Cornell 1978 to 1981), were evaluated for first lambing ages, interlambing intervals and conception probabilities. Morlam ewes were continuously exposed to rams over the year, while Carnal Dorset ewes were exposed every other month. Morlam lambs were mated as early as 367 d of age and Carnal Dorset lambs as early as 340 d. Early lambing was associated with higher rates of perinatal mortality (P>.05) and smaller litter size (P<.O1). Lambing years among Morlam ewes and season of birth of Carnal Dorset ewes influenced (P<.01) their first lambing ages. Lambing intervals averaged 293 and 303 d among Morlam and Carnal Dorset ewes, respectively. Age at first lambing and season in which the previous lambing occurred with influential factors (P<.01) on lambing intervals of Morlam ewes; longer intervals resulted when ewe lambs were mated at early ages (<12 mo), and when the previous lambing occurred in winter. Estimates of conditional probabilities of conception by month given the occurrence of estrus, reflected seasonal changes in both systems. The overall probability of conception for the Morlain system (P=.16) was relatively higher than that for the Carnal Dorset system (P=.14); numbers of lambings per ewe per yr were 1.28 and 1.21, respectively. Estimates of heritability for age at first lambing, lambing interval and conception probability were. 31, .06 and .30, respectively.