or increased as a result, hybrid vigour and other production benefits occurring in the F1 generation will be maintained or increased. Maintenance of heterozygosity is the key to maintaining the highest levels of hybrid vigour. This requires complex breeding programmes and high levels of management. Simplified crossbreeding programmes have been developed using hybrid or composite bulls, which was the motivation behind the development of Lim-Flex hybrids. The two major Limousin hybrids are Brahmousin (a cross between Brahman and Limousin cattle) and Lim-Flex (a cross between Angus and Limousin cattle), which were both developed before the significance of the F94L myostatin variant had been quantified. When Limousins homozygous for the F94L myostatin mutation are used in crossbreeding, only one of the mutations will be inherited (that is, progeny will be heterozygous for the mutation), and a high level of phenotypic uniformity and hybrid vigour would be expected in the progeny. However, breeding using heterozygous animals as parents, which could include purebred Limousins of low percentage Full French content, and Lim-Flex and Brahmousin hybrids that have not been bred to a uniform (homozygous) standard over several generations, would produce progeny with inconsistent carcase characteristics and production value depending upon whether or not the F94L mutation was inherited. The use of Lim-Flex and Brahmousin sires over a third breed of cow would benefit most from increased hybrid vigour, which should minimise any reduction in carcase value arising from the loss of the F94L mutation. According to research into the effects of the F94L mutation, live weights of progeny are unaffected by random inheritance of the mutation. Brahmousin Brahmousin cattle are a hybrid purebred breed of Brahman and Limousin first created in the US in the late 1970s. The goal was to blend the best of the Limousin and Brahman traits to create a breed that has reproductive efficiency, mothering ability, good muscling and growth traits, and adaptability to varying environmental conditions. Brahmousin are now bred in the US, Indonesia, El Salvador, and Australia. The first Brahmousin cattle were produced from a multiple embryo transfer from a French-imported Limousin dam. The resulting progeny were then crossed with Brahman cattle to achieve an F1 hybrid. Further crosses over a broader base led to the production of the 5/8 Limousin – 3/8 Brahman Brahmousin purebred, a mix which has been found to be the most widely accepted and most useful for the majority of the US.