Genetic variation in surface temperature measured using infrared thermography and genetic associations with production traits in grazing dairy cattle : a thesis presented in partial fulfilment of the requirements for the degree of Master of Animal Science at Massey University, Palmerston North, New Zealand

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Heat stress negatively impacts the production and reproduction of dairy cattle, and it is likely to worsen with increasing global temperatures due to climate change. Infrared thermography measured skin temperature could potentially be used as an indicator trait for heat stress in cows since it might correspond to the amount of heat dissipation. The aim of this research was to obtain infield skin temperature measurements for the eye, muzzle, and udder in dairy cows using infrared thermography in order to determine whether there is genetic variation in the skin temperature between cows in a dairy herd. Secondly, estimates of genetic correlations between skin temperature and economically important traits like milk yield were obtained. Thermal images and herd test records were obtained for 225 cows belonging to a once-a-day, spring calving dairy herd at Massey University’s dairy farm 1 in Palmerston North, New Zealand, during the 2022-23 production season. Breed, lactation number, and predicted milk yield were determined to significantly affect the average udder temperature. Holstein-Friesian × NZ Jersey crossbreds had the greatest average udder temperature as well as cows with higher lactation numbers and higher predicted milk yields. These results may be indicative of either higher producing cattle exhibiting greater udder temperatures and/or cows with greater udder temperatures exhibiting greater heat dissipation ability. The heritability estimates for infrared thermography measured eye, muzzle and udder temperature were low to moderate at 0.20, 0.24 and 0.39 respectively. The genetic correlations between the temperature and production traits were all positive and weak to moderately strong, with the exception of eye temperature with milk and protein yield. The estimates for the genetic parameters determined in this study indicate that increased skin surface temperature could be selected for without compromising production yield. These findings, if validated in a large-scale follow-up study would enable estimation of breeding values for udder temperature that could be incorporated into a selection index aimed at improving heat tolerance in dairy cattle.