Abstract
Data representing 1928 lambing records were analysed to study the effects of inbreeding and cross-breeding on lamb survival to the ages of 7, 30, 120 and 180 days. The sheep flock consisted of two native breeds, Barki (B) and Rahmani (R); and the imported Fleisch Merino (M) as well as the crosses M × B and M × MB. Analysis was made by fitting a least-squares linear model that included the main effects of year and season of birth, sex, age of dam, type of birth, inbreeding of the lamb (Fd) and the dam (Fd) and breed group and the interactions season × type of birth, season × Fo and season × breed group. Lambs were classified into six groups according to the magnitude of the inbreeding coefficient: F ≤ 0·05, 0·05 < F ≤S 0·10, 0·10 < F ≤ 0·15, 0·15 < F ≤ 0·20, 0·20 < F ≤ 0·25 and F > 0·25. The average inbreeding coefficient in the flock was 0·12 and 0·08 For the lambs and their dams, respectively. Sex of lamb and age of dam had no significant effect on lamb survival at any age while single-born lambs had a higher (P < 0·01 or P < 0·05) survival than twins at all ages except 7 days. Inbreeding of the lambs showed significant effect at 7 and 120 days. Increase in inbreeding of the dams was associated with higher mortality at all ages but the trend was not significant. Differences among breed groups were not significant at 7 and 30 days, but were highly significant at later ages. The contribution of breed group to the total variance in survival increased with age. Hybrid vigour was highly significant in both crosses at 120 and 180 days.