A total number of 5949 birth records collected over a period of 9 years from a crossbreeding experiment at Sakha Animal Research Station were used in this study. The following breeding groups were included: Fleisch Merino (M), native Ossimi (0) and Barki (B) breeds, MO, MB, ¾M ¼ O, ¾ M ¾ B and the inter se mating groups of the backcrosses (’¾ M ¼O’ and ‘¾ M ¼B’). Productive traits studied were birth, 4-month and yearling weight, lamb liveability at 4 months of age; and first fleece weight (at 10 months of age).

Generally, the cross-bred lambs were heavier in body weights and sheared more wool than the pure breds, with the differences being almost significant. In liveability, native breeds showed significantly higher estimates than the cross-breds where the Merino breed was the poorest in this respect.

Merino × Ossimi first cross was the best breed group showing a high heterosis estimate of 0·9, 5·2, 10·3, 7·6, 0·9 and 0·7 kg in birth, 4-month, yearling males and females body weights and first fleece weight, respectively. They were followed by ¾ M ¼ O with a corresponding heterosis value of 0·8, 3·9, 7·4, 4·2, 0·5 and 0·5 kg respectively. Merino x Barki and their backcrosses to Merino are ranked second to the Ossimi crosses. Heterosis in the Barki backcross was 0·8, 2·9, 4·8, 2·1, 0·4 and 0·4 kg compared with 0·5, 2·5, 3·1, 2·7, 0·5, 0·4 kg for MB, on different traits, respectively. The interbreeding of the backcrosses caused a diminishing in hybrid vigour. Heterosis estimates in ‘¾ M ¼ O’ were 0·3, 0·9, 4·6, 2·3, 0·3 and 0·3 kg, whereas it was 0·1, 0·3, –0·1, –0·3, 0·0 and 0·1 kg for ‘¾ M ¼B’, in different traits respectively. All cross-breds showed negligible or/and negative heterosis in lamb liveability up to 4 months.

The effects of breed group, year of birth, age of dam, sex and type of birth were found to be highly significant on all traits except the effect of type of birth on lamb liveability.