A genetic analysis of parasite resistance traits in a tropically adapted line of Bos taurus

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Abstract
For cattle raised in tropical and subtropical environments, production can be limited by the susceptibility of many breeds to parasites. Chemical control, coupled with the use of breeds with higher levels of resistance to parasites, allows beef production to be a viable industry, but at a cost. The Hereford Shorthorn (HS) line of beef cattle at Rockhampton has been used extensively to study genetic aspects of parasite resistance. The hypothesis that a gene with a large effect on resistance to cattle ticks is segregating in this line of cattle has been raised in the past, and is considered again here, with the benefit of data recorded up to the late 1990s. A model in which no major gene effect is fitted provides a satisfactory fit to the data, providing little evidence in support of the segregation of major genes for resistance to either ticks or worms in the HS line. This conclusion is supported by the lack of animals of extreme resistance in later generations, an unlikely result if key animals are assumed to be homozygous in earlier generations.