INHERITANCE OF BIRTH-WEIGHT AND GROWTH TRAITS IN CROSSES BETWEEN THE BOOROOLA MERINO AND ASSAF SHEEP BREEDS

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.