A study was made of the effect of increased rectal temperature on digestibility, nitrogen balance and water metabolism in Brahman x British (BX), Africander x British (AX) and Shorthorn x Hereford (SH) steers fed a fixed amount of lucerne hay.
An increase in rectal temperature of c. 1·4 °C caused a decrease in food intake and nitrogen balance and was associated with an increase in dry-matter digestibility in all breeds. Both dry-matter and nitrogen digestibilities were consistently higher in the Zebu crossbreds (BX and AX). Water intakes and ‘water balance’ (water intake – faecal and urinary water) were higher in all breeds when rectal temperatures were elevated. Water intakes were highest in the BX at both normal and elevated temperatures and increased most in the AX with an increase in temperature. ‘Water balances’ were similar in all breeds at normal body temperatures (4–5 1/day) but were lowest in the SH at elevated temperatures. This could partly explain the lower heat tolerance of the SH steers observed under these experimental conditions.