The fasting metabolism of Brahman, Africander and Hereford×Shorthorn cattle

Abstract

1. The fasting metabolism of twenty-seven animals, nine Brahmans, nine Africanders and nine Hereford × Shorthorns was measured after fasts of 96 h duration. Urine was collected over successive 24 h intervals and gas exchange was determined 96–103 h after the last meal.2. Urinary urea decreased and creatinine increased throughout the fasting period. There was no difference between breeds in the urinary excretion of nitrogen, urea and creatinine on the 4th day of fasting (72–96 h). Excretion of nitrogen, urea and creatinine increased by 259, 197 and 45 mg/kg increase in fasted weight and by 8.9, 7.6 and 0.91 mg/kcal increase in fasting metabolism respectively. Protein oxidation accounted for 25.6, 21.7 and 22.7% of the fasting heat production of Brahman, Africander and Hereford × Shorthorn cattle respectively, values which were not significantly different.3. The breeds differed in fasting metabolism per kg fasted weight ( P < 0.05) and in fasting metabolism adjusted for fasted weight (0.05 < P < 0.10), mainly owing to lower values for the Brahmans. The respective values for the Brahmans, Africanders and Hereford × Short-horns were 20.7, 25.2 and 24.1 kcal/kg (86.6, 105.4 and 100.8 kJ/kg) and 5.856, 6.947 and 6.600 Mca1/24 h (24.50, 29.07 and 27.61 MJ/24 h) at a fasted weight of 277 kg. The variation in fasting metabolism between animals within breeds, expressed as a coefficient of variation, was 13.7%.4. The results are discussed in relation to published estimates of fasting metabolism, possible reasons for the breed difference and the relation of fasting metabolism to heat tolerance.