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SummaryThe effects of coat colours on thermal balance, behaviour and weight gain were studied. Measurements of thermal balance were made over 36 days in strong sunshine on highly contrasting coat colours of three white and three dark steers in each of two breeds, Brahman (B) and Shorthorn (S), and three light- and three medium-red steers in Brahman Hereford-Shorthorn cross breed (BX) where there was less contrast in coat colour. Each steer was measured for 30 min on six occasions randomly distributed among 6 h between 08.00 and 13.00 h, and on six occasions between 11.00 and 16.00 h, all on different days. For B, the difference between colours in mean rate of environmental heat gain at the skin was 64 ± 22·5 W/m2 and for S, 44 ± 20·5 W/m2, while the difference in mean rate of evaporative heat loss, between colours for B and S respectively was 44 ± 15·1 and 42 ± 14·3 W/m2. However, only within S was rectal temperature significantly higher in steers which were dark red and the reasons for this are discussed. Within BX the range of colour shades diminished during the experiment so thermal effects of colour were small. Colour had significant effects on growth; these effects were greatest in S where white steers gained 0·13 kg more per day than dark red ones. There were also differential effects within colour groups on activities at pasture; light steers spent more time in the sun and grazing than dark ones. In addition to colour, deep or woolly type coats negatively affected growth (P < 0·05), time spent in the sun (P < 0·05), and grazing time (P < 0·05); however, between animals within breeds the magnitude of the adverse effect of coat type on growth and behaviour was greater in dark- than light-coloured steers. These results provide evidence that coat colour is a trait which, interacting with coat type, exerts an influence on performance of cattle under heat stress.

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