The effect of heat stress on growth and nitrogen metabolism in Friesian and F1 Brahman × Friesian heifers

Abstract

Friesian and Brahman × Friesian (F1) heifers were kept in controlled environment rooms and given a high quality diet. Feed intake, growth and nitrogen metabolism were assessed during three periods of 21 days when the animals were maintained at 20, 30 or 38°C (68, 52 and 46% relative humidity respectively). The F1 animals were superior only under heat stress. Feed intakes and growth rates of Friesians and F1 animals were similar at 20°C. With each successive increase in temperature the feed intake and liveweight gain of Friesians were significantly reduced. With F1 animals the reductions were much smaller. Respiration rates and rectal temperatures of Friesians were higher than those for F1 animals at 30 and 38°C. Although the water intakes of Friesians were much higher than those of F1 animals under heat stress, the calculated evaporative water losses were similar. The efficiency of digestion in the two genotypes was similar at all temperatures. Friesians responded to increasing heat stress by increasing the nitrogen lost in urine and by decreasing nitrogen retention in relation to nitrogen intake. Similar trends for the F1 animals were much smaller. The reduction in nitrogen retention in Friesians was associated with a decrease in RNA concentration in muscle tissue and an increase in the excretion rate of creatinine.