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

Four Friesian, four F1 Brahman x Friesian, and four Brahman heifers were fed on high and low quality diets in controlled environment rooms. Feed intake, growth, and nitrogen metabolism were assessed during four periods of 28 days when the animals were either subjected to heat stress (38°C) or maintained under cool ambient conditions (17°). Brahman x Friesian animals were the superior genotype only under heat stress. At 17°C the feed intake and growth rate of Friesians and Brahman x Friesians were similar. At 38° the Brahman x Friesians ate more and grew faster than the Friesians. Brahmans ate less than the Brahman x Friesians at both temperatures and grew more slowly than them at 38°. Water intake per kilogram dry matter intake was greater for the Friesians than for the other two genotypes at 17 and 38°C. Rectal temperatures and respiration rates increased between 17 and 38°C; the change was greatest for the Friesians and least for the Brahmans. These responses were not significantly correlated with feed intake or growth rate. On the high quality diet the digestibilities of dry matter and nitrogen were similar between genotypes. On the low quality diet they were higher for Friesians than for the other two genotypes, although this was confounded with differences in intake. The utilization of digested nitrogen differed between diets but was similar for the three genotypes within diets. The concentration of plasma creatinine was higher in Brahmans than in the other two genotypes. Creatinine excretion per unit of liveweight was greater at 38° than at 17°C for Friesians and Brahman x Friesians.