Data on birthweight, pre and post weaning growth rates and 150 day weight of 1058 Assaf (As), Booroola Merino (B) X As (F1), F1 X As (3/4As), As X F1 (3/4As'), As X 3/4As (7/8As) and B X F1 (3/4B) lambs were analyzed. Genotype, sex of lamb, genotype by sex interaction, crop, type of birth and sire within genotype had significantly (P< 0.05 ) affected the different traits. Assaf lambs had the highest and 3/4B lambs had the lowest birthweights. A negative relationship was found between the birthweight and the proportion of the Booroola blood in the lamb and its mother. As, F1, 3/4As, 3/4As' and 7/8As ram lambs did not differ significantly (P > 0.05) in their post weaning growth rate and in their 150 day weight. Similar situation was found among ewe lambs except for F1 ewe lambs which were significantly (P < 0.05) smaller than Assaf ewe lambs at 150 days indicating sex linked effect on growth in the Booroola-Assaf crosses. The Assaf transmitted effects were positive and significant (P < 0.05) for birthweight, post weaning growth and 150 day weight, and the Assaf maternal effect was positive and significant (P < 0.001) for birthweight. Significant (P < 0.001) heterosis was found for post weaning growth rate. Sex linked and recombination effects were significant (P < 0.01) for pre weaning growth.
