Genetic variation in rectal temperature and its association with heat tolerance in Australian dairy cattle

Hot environments negatively affect dairy cattle in Australia both in regard to health issues including reproduction and also productive performance. Dairy cattle become stressed when they can no longer maintain their body temperature within the thermoneutral zone. This paper examines the effect of heat stress under Australian conditions on dairy cattle as well as estimating genetic parameters for important heat stress traits specifically rectal temperature and milk yield. Genetic correlations were estimated using multivariate analyses between Milk Yield and Rectal Temperature and also between the difference in Milk yield between hot and cool days and the difference in Rectal temperature. Rectal temperature data was collected from sixteen dairy operations in the Hunter Valley from October 2001 to March 2002. Individual animals had multiple rectal temperature and milk production records taken. The majority of the dairy cattle were pure Holstein Friesian. The data set included 2314 cows from 519 sires and 1538 dams from which 5559 rectal measurements were recorded from 59 individual herd-testing days. Heritability was estimated for Rectal temperature and difference in Milk yield between hot and cool THI days and difference in rectal temperature between hot and cool THI days with the estimates being 0.11, 0.46 and 0.17 respectively. The genetic correlation between Hot-Cool day difference in Milk yield and in Rectal temperature was 0.28 ± 0.33. Genetic gain for both production and heat tolerance could be achieved and would be especially useful for areas that have periods of high heat and humidity. However the additional benefit of measuring rectal temperature is limited. In contrast using difference in milk yield as a trait of heat tolerance could be implemented with no need for additional animal or production measurements.