Polled gene marker refinement: Refinement of the CSAFG29 Microsatellite Marker Test for Polled

Summary

Dehorning is routinely practiced in beef cattle, as horns are an important cause of bruising, hide damage and other injuries, particularly in yards, feedlots and during transport. Although it is advisable to dehorn at a young age, as a result of the mustering practices and especially in northern Australia, dehorning is frequently carried out in older calves between 3 and 10 months of age. Dehorning in older calves is labour intensive and causes more pain to the animal. The wound takes longer to heal, is prone to secondary infection and leads to mortality in some cases. 
The difficulty of distinguishing between homozygous polled animals and heterozygous carriers of the allele responsible for horns is an impediment to breeding for polled. In an earlier project, a gene test based on the CSAFG29 microsatellite was developed. The test is useful, but not perfect. For many animals it provides a clear result, distinguishing between homozygous and heterozygous animals. However, for other animals the test is unable to provide a clear result. The frequency with which this occurs depends on breed, and is particularly high in Brangus and Limousin, where for as many as 50% of polled animals the test cannot distinguish between homozygous polled and a carrier of the alleles responsible for horns. 
The objective of this project was to improve the CSAFG29 test. A number of approaches were tried, and the most successful was the addition of other microsatellites markers to the test, markers that are closely linked to CSAFG29. Coupled with a haplotype based analysis method, in a discovery population these markers improved the test beyond our wildest expectations. Associations between haplotypes and polled appear to be robust across breeds, and therefore should have utility in crossbred and composite herds. The validation of the improved test in a larger population should now be a priority, and if validated, the release of the test to industry.