EFFECTS OF BOOROOLA MERINO BREEDING AND THE FEC(B) GENE ON PERFORMANCE OF CROSSES WITH LONGWOOL BREEDS .2. EFFECTS ON REPRODUCTIVE-PERFORMANCE AND WEIGHT OF LAMB WEANED BY YOUNG EWES

Numerous reproductive and lamb production traits were measured for Romney, Perendale and Booroola Merino crossbred ewes from Romney and Perendale dams to assess independently the effects of Merino breeding and the Fec(B) (''F'') gene on net lamb production. Crossbred ewes were designated as being heterozygous (F+) or non-carriers (++) based on their ovulation rates and known genotype of their Booroola sires. Ewes were born over three years, mated over one to three years to either Booroola rams or to purebred or F1 rams of their own longwool breed and given the opportunity to rear all lambs born. Relative to ++ ewes, F+ ewes produced 1.1 more lambs at birth but weaned only 0.4 more lambs per ewe lambing. Among F+ ewes which lambed, fewer weaned lambs and their lambs were lighter at weaning even when compared within weaning rank. F+ ewes giving birth to triplets weaned only 0.25 more lambs than those giving birth to twins and those producing more than three lambs at birth weaned fewer lambs than ewes giving birth to triplets. Weight of lamb weaned per ewe weaning lambs was higher for F+ ewes than for ++ ewes, but weight of lamb weaned per ewe present was not significantly different. Crossbred ewes weaned lighter lambs than purebred ewes but weaned more weight of lamb per ewe present, largely due to lower barrenness. Perendale ewes produced more weight of lamb than Romney ewes by weaning both more and heavier lambs per ewe present. Estimates of heterosis for weaning weight averaged 5% while both additive and maternal effects favored the longwool breeds.