Validation of genetic parameters for breeding Bos indicus cross cattle in the dry tropics

Summary

Bos indicus cattle and, their crosses have been increasingly used in the tropical regions of northern Australia to the extent that they now form 78% of the cattle population in Queensland (Anon. 1989). The~, main productive advantages of the Bos indicus genotypes in the dry tropical environment are their greater drought and heat tolerance and increased resistance to internal and external parasitism, this being manifested primarily in a faster growth rate and lower mortality (Turner, 1975). In contrast, their reproductive performance has been described as sub-optimal (McClure, 1973), with the bull being recognised as a major contributor to this reduced herd reproductive performance (Entwistle, 1983).

Selection of genetically superior breeding stock is an important component of beef production. Livestock 'improvement depends on accurate and relevant estimates of the heritability of production traits as well as genetic and phenotypic correlations between these traits. Responses to selection may, depend on the environment (Frisch and Vercoe, 1982; Baker and Morris, 1984). Therefore, estimates of genetic parameters derived from other environmental regions may not be applicable to all regions of northern Australia. There is a paucity of estimates for northern Australia so that it is important to use available data to improve and validate these estimates.

Direct selection based on estimates, of genetic parameters for female fertility is inefficient because this is a sex limited trait. While selection among females is phenotypically effective, it does not allow for significant genetic improvement due to low heritability of most reproductive traits in females, low selection differentials and low replacement rates of breeding cows (Entwistle, 1983; Hammond and Graser, 1987). The majority of genetic improvement can be expected to come from use of superior sires since they transmit one half of their genetic composition to their progeny, and are responsible for the overall reproductive performance of the cows to which they are joined (Coulter and Foote, 1979).

There are two possible strategies for sire selection for improving fertility in cows. The first is by early, identification of superior sires through progeny testing, followed by direct selection. Since progeny testing of beef bulls for daughter fertility is impractical given the extensive nature of production in northern Australia, an alternative strategy is necessary. This involves indirect selection, where male traits are identified which are highly heritable and genetically correlated with other male traits and with fertility in females. Thus, reproductive and productive measures in males that will improve reproductive efficiency in both males and their female offspring need to be identified and described.