In order for a composite breed not to dissipate the initial advantage of increased heterozygosity by becoming inbred, it is essential that heterozygosity (heterosis) be retained by maintaining an effective population size (Ne) sufficiently large (Gregory et al., 1993). Also, Hill (2000) observed that for a trait with heritability 1/3, (for example weaning weight in beef cattle), a value of Ne=250 is required to maintain the additive variance at its initial value. The Brangus breed is the largest composite
population of beef cattle in the subtropics of Argentina. Since its creation in 1978, the Argentinean Brangus Breeders Association (AAB) has kept an open registry policy to maintain high levels of variability and to retain a high level of heterosis. The initial motivation for the research presented here was to quantify what is the current level of genetic variability, and to evaluate the pedigree structure of Brangus in Argentina.