Comparison of non-evaporative heat transfer in different cattle breeds

Abstract

Tissue conductance and non-evaporative heat loss from the skin were determined from measurements of body temperature, evaporative water loss, metabolic rate and heat storage in six steers in each of three breeds, Brahman (B), Brahman x Hereford-Shorthorn (BX) and Shorthorn (S). A group of six steers, two from each breed, remained in a climate room set at 25¦C overnight, and during the following day all were exposed for 1 h to sequential increases in air temperature (28, 32, 37, 41, 43, 45¦C). Each steer was measured at 25¦C and after a 30-min exposure to each temperature. Tissue conductance increased with air temperature (Ta), reaching maximum values at 41¦C, the rate of increase (W m-2 'C-I per degree rise in Ta) being for B 3.95, for BX 2.33 and for S 2,09. Between 41 and 45¦C, tissue conductance remained constant in B but declined in BX and S with a concurrent increase in heat storage. Mean tissue conductance (W m-2 ¦C-1) of B was 63.5; BX, 56.1; and S, 47.8, values that were significantly different (P < 0.01). Expressed in terms of metabolic weight, the breed means of tissue conductance (litres O2 h-1 W-0.75 ¦C-1) were also significantly different: B, 0.56; BX, 0.43; and S, 0.33 (P < 0.005), with the relative differences similar to those calculated per unit area. Breed differences in tissue conductance may be related to variations in ability to redirect blood from the core to the skin. Non-evaporative heat loss comprised 55-65% of the total heat loss from the skin in all breeds at Ta of 25¦C. The remaining heat was lost through sweating. As Ta increased and approached skin temperature, non-evaporative heat loss decreased but in B and BX remained 25% of the total heat loss from the skin. S steers, in contrast, sustained little non-evaporative heat loss as Ta increased because sweating rates increased 50% more than that required to dissipate the heat at the skin. The increase in absolute humidity of the chamber was associated with the excessive sweating in this breed.