Brahman cross calves exhibit unusual inheritance of birth weight: Brahman-sired crossbreds out of Bos taurus females are heavier with greater difference between sexes than calves of the reciprocal cross. The objective of this work was to investigate genetic effects that may influence differences in Brahman (B) × Simmental (S) crosses. 5,102 calves produced by embryo transfer (ET) (n = 2,914) and natural service/artificial insemination (n = 2,188) were born from 1983 – 1991 . Data were analyzed using an animal model. Fixed effects investigated included contemporary group (n = 87; combinations of ET status, birth year, month, and location), sex, recipient breed, and linear covariates of direct breed effects, maternal breed effects, expected breed heterozygosity of the calf (HET) and the dam, mitochondrial origin, Y chromosome effects, X chromosome effects (XCHR), genomic imprinting (GI), non-random X inactivation by breed of origin, and non-random X inactivation by parent of origin (NONRP). ET calves were constrained to 0 for maternal breed and breed heterozygosity effects. Random effects included direct and maternal additive genetic. The regression coefficient for HET (4.09 ± 0.96 kg) (P < 0.001) The regression coefficient for GI was –4.24 ± 0.95 kg (P < 0.001). XCHR has a probable role but was antagonized by the presence of NONRP in the model. This is likely due to insufficient data and structure available to partition the effects. Direct and maternal breed and breed heterozygosity effects estimated from models not including GI and XCHR are not straightforward to interpret in Bos indicus-Bos taurus crosses.
