IN-VIVO AND IN-VITRO INVESTIGATIONS FOR N UTRIENT DIGESTIBILITY AND HEAT-PRODUCTION OF RUMINANTS UNDER HEAT-STRESS AND DIFFERENT NUTRITIONAL LEVELS .3. IN-VIVO INVESTIGATIONS - NITROGEN-BALANCE, ENERGY-BALANCEAND PHYSIOLOGICAL-PARAMETERS OF THERMOREGULATIONS

A total of nine digestibility trials, each with four adult wethers wer e conducted to investigate the effect of feeding a straw ration (Ratio n 1), a medium level concentrate ration (Ration 2) and a high level co ncentrate ration (Ration 3) at 15-degrees-C or 20-degrees-C, 30-degree s-C and 35-degrees-C ambient temperatures on nitrogen-balance, heat pr oduction as basis for calculations of the energy-balance and the physi ological parameters of thermoregulation. (The details are given in 1st and 2nd communication). While calculating the nitrogen and energy-bal ance for animals under heat stress, important discrepancies between th e calculated values for retention and the measured liveweight changes were observed. The differences between the values increased with risin g nutritional levels. The discrepancies for the nitrogen-balance may b e partly explained by a systematic error in the experimental design. H owever, the differences in the calculations of the energy-balance can only be explained by changes in the energy metabolism of animals under heat stress. The physiological parameters of thermoregulation are app arently not important for the indication of the thermoregulative statu s of heat stressed ruminants. Therefore, the ambient temperature at wh ich the physiological parameters for thermoregulation reached its basi c values were not identical with the temperature at which digestibilit y values were highest or the mean rate of heat production was lowest. In general, the effects of ambient temperature on the saturation of fr ee fatty acids in the blood plasma of heat stressed animals were not l arge enough to support the hypothesis of a significant additional ener gy requirement as consequence of increased reaction enthalpies for the synthesis of fat under heat stress.