IN-VIVO AND IN-VITRO INVESTIGATIONS FOR N UTRIENT DIGESTIBILITY AND HEAT-PRODUCTION OF RUMINANTS UNDER HEAT-STRESS AND DIFFERENT NUTRITIONAL LEVELS .1. IN-VIVO INVESTIGATIONS - RESEARCH BACKGROUND, EXPERIMENTAL-DESIGN, FEED AND ENERGY INTAKES

In this article, the present state of knowledge concerning the influen ce of high ambient temperatures on nutrient digestibility and heat pro duction are discussed. The results reveal the complexity of possible f actors effecting nutrient digestibility and heat production under heat stress as well as a lack of systematic investigations concerning the number and choice of temperature levels and ranges, the types of ratio ns and feedstuffs examined. From this analysis an experimental design was developed in which nutrient digestibility and heat production of f our shorn wethers were measured in a combination of three levels of te mperature and nutrition. The ambient temperatures of 15-degrees or 20- degrees, 30-degrees and 35-degrees-C chosen for this study cover tempe rature levels below, at about and above a thermoneutral zone. The rati ons fed were: a straw ration supplemented with urea and wheat bran (ra tion 1: in dry matter 95% oat straw and 5%wheat bran), a concentrate r ation of medium nutritional level (ration 2: in dry matter 20% hay and 80% concentrate) and another concentrate ration of high nutritional l evel (ration 3: in dry matter 10% hay and 90% concentrate). With this feeding regime, the provision of energy in ration 1 was below (0.8), i n ration 2 almost equal to (1.2) and in ration 3 it was significantly above (2.2) the energy requirement for maintenance of 350 kJ/kg LM0.75 /d postulated in ARC (1980). As expected, feed intake decreased for al l rations with increasing ambient temperature, whereby the reduction w as higher with increasing proportion of structured carbohydrates in th e ration. When feeding ration 1 and 2 fibre intake decreased more than dry matter intake, which can be taken as indication for selective fee d intake. An increasing ambient temperature resulted in a higher metab olizability of feed energy (Q) in the straw ration (+ 19 %) and conseq uently in a higher level of metabolizable energy (+ 13 %) whereas both rations rich in concentrate showed a decrease in energy metabolizabil ity at 30-degrees-C ambient temperature.