Effects of altitude, ambient temperature and solar radiation on fasting heat production in yellow cattle (Bos taurus).

Growing yellow cattle (Bos taurus, n 30, 1.0-3.5 years old and 75-240 kg) from their native altitude (2000-2800 m) were used to evaluate the effects of altitude, ambient temperature (Ta) and solar radiation on the basal energy metabolism in this large mammal. Fasting heat production (FHP) was measured at altitudes of 2260, 3250 and 4270 m on the Tibetan plateau both in the summer and winter respectively, after a 90 d adaptation period at each experimental site. The gas exchanges of the whole animal were determined continuously for 3 (2260 and 3250 m) or 2 (4270 m) d after a 96 (2260 and 3250 m) or 48 (4270 m) h starvation period, using closed-circuit respiratory masks. Increasing altitude from 2260 to 3250 m at similar Ta in the summer significantly elevated FHP for all animals (P<0.01), and from 3250 to 4270 m for young cattle (P<0.05); increasing altitude from 2260 to 3250 m in the winter also significantly elevated FHP (P<0.05), but the increase was mainly due to the decrease of Ta and the increase in wind speed. No results were obtained at 4270 m in the winter, due to the problems of the animals, adapting to the altitude. The magnitude of FHP elevation caused by increasing altitude was greater with summer sunshine or winter wind than without them. Increase of Ta from 10.0 to 22.0 degrees C, in the presence of solar radiation, slightly (2260 m) or significantly (3250 and 4270 m, P<0.01) elevated FHP, but slightly reduced it in the absence of solar radiation; decrease of Ta from 0.0 to -30.0 degrees C linearly increased FHP. At 3250 and 4270 m, FHP at the same Ta was higher with summer sunshine or winter wind (3250 m) than without them, but this did not occur at 2260 m. In conclusion, high altitude elevates FHP in yellow cattle in the warm season, and the summer solar radiation and winter wind at high altitude significantly increase metabolic rate. It may be also concluded that the effects of solar radiation on metabolic rate depend on the altitude and the environmental temperature.