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.

Effects of high altitude and season on fasting heat production in the yak Bos grunniens or Poephagus grunniens.

Thirty growing yaks Bos grunniens or Poephagus grunniens, 1.0-3.5 years and 50-230 kg, from their native altitudes (3000-4000 m), were used to study the basal metabolism in this species and to evaluate the effects of high altitude and season on the energy metabolism. Fasting heat production (FHP) was measured at altitudes of 2260, 3250 and 4270 m on the Tibetan plateau in both the summer and the winter, after a 90 d adaptation period at each experimental site. Gas exchanges of the whole animals were determined continuously for 3 d (4-5 times per d, 10-12 min each time) after a 96 h starvation period, using closed-circuit respiratory masks. Increasing altitude at similar ambient temperature (Ta) did not affect (P>0.10) FHP in the summer, but decreased (P<0.05) it at different Ta in the winter. However, the decrease of FHP in the winter was mainly due to the decrease of Ta instead of the increase of altitude. In the summer, the respiratory rate, heart rate and body temperature were unaffected by altitude, except for a decrease (P<0.05) in body temperature at 4270 m; in the winter, they were decreased (P<0.05) by increasing altitude. In both seasons, the RER was decreased (P<0.05) by increasing altitude. At all altitudes for all groups, the daily FHP was higher (P<0.05) in the summer (Ta 6-24 degrees C) than in the winter (Ta 0 to -30 degrees C), and the Ta-corrected FHP averaged on 920 kJ/kg body weight(0.52) at Ta 8-14 degrees C and on 704 kJ/kg body weight(0.52) at Ta -15 degrees C respectively. We conclude that in the yak high altitude has no effect on the energy metabolism, whereas the cold ambient temperature has a significant depressing effect. The results confirm that the yak has an excellent adaptation to both high altitude and extremely cold environments.