LITTER SIZE AND UTERINE EFFICIENCY OF BOOROOLA MERINOS, CONTROL MERINOS AND THEIR CROSSES WITH BORDER LEICESTER AND DORSET

Genetic variation in embryonic success was examined by analysing litte r size (LS) resulting from naturally occurring multiple ovulations in Booroola Merinos (B), control Merinos (M) and their crosses with the B order Leicester (BL) and Dorset (D) breeds. Due to low incidence and h ence low numbers of higher order ovulations, uterine efficiency (UE) c ould be assessed for M and D X M ewes at ovulation rate (OR) = 2 only and was limited to OR = 2 and OR = 3 for BL X M and D X B ewes. B and BL X B ewes were assessed at OR = 2 through 4. M ewes exhibited lower LS (P < 0.05) than the other five genotypes at OR = 2 and BL X M ewes had lower LS (P < 0.05) than the other three genotypes examined at OR = 3. When compared at the upper limit of their ovulation range, M geno types exhibited lower UE than B genotypes both as purebreds (at OR = 2 ) and as crosses (compared at OR = 2 for D crosses and at OR = 3 for B L crosses). B purebreds and crosses exhibited similar UE at OR = 2, bu t B purebreds produced fewer lambs than either D X B or BL X B ewes at OR = 3 and OR = 4 (compared with BL cross ewes only). Repeatability o f litter size within ovulation rate ranged from 0.15 at OR = 3 to 0.29 at OR = 4 and none of the heritability estimates calculated within ov ulation rate class differed from zero.