IN-VIVO AND IN-VITRO INVESTIGATIONS FOR N UTRIENT DIGESTIBILITY AND HEAT-PRODUCTION OF RUMINANTS UNDER HEAT-STRESS AND DIFFERENT NUTRITIONAL LEVELS .4. IN-VITRO INVESTIGATIONS - RESEARCH BACKGROUND, EXPERIMENTAL-DESIGN, GAS-PRODUCTION IN RELATION TO PLANE OF NUTRITION AND AMBIENT-TEMPERATURE FOR DONORS AS WELL AS INCUBATION-TEMPERATURE - ENERGY CONTENT OF INCUBATED FEEDSTUFFS

IV. In vitro investigations - Research background, experimental design , gas production in relation to plane of nutrition and ambient tempera ture for donors as well as incubation temperature; energy content of i ncubated feedstuffs. Parallel to the in vivo investigations (details g iven in 1st - 3rd comm.), six in vitro trials were conducted according to the gas production test in order to examine the effect of increasi ng ambient temperatures on the energy metabolism in the rumen. For the simulation of in vivo conditions, the digesta of two fistulated wethe rs which were kept at ambient temperatures of 15-degrees, 30-degrees a nd 35-degrees-C and fed with a straw ration (Ration 1) and a ration ri ch in concentrate (Ration 3) was used to incubate the same feedstuffs that were fed in vivo. The energy content of incubated feed samples wa s determined by the gas production in relation to incubation temperatu re (39-degrees and 41-degrees-C) as well as plane of nutrition and amb ient temperature for the donor animals. As expected, the mean pH-value of digesta from animals fed with straw ration was higher (6.5) than t he respective values for the concentrate-rich ration (6.2). However, t he pH-value decreased for both rations with increasing ambient tempera ture for the donors, what might be a result of a higher concentration of fatty acids in the rumen of animals under heat stress. Differences in the gas production of rumen inoculum caused by different rations fo r the donors were apparent when incubating pure digesta in vitro. The influence of rising ambient temperatures for donors on the rate of gas production in vitro could only be observed for the inoculum of the co ncentrate-rich ration. The rate of gas production of all incubated fee dstuffs was positively influenced by an increase in the supply of nutr ients and ambient temperature imposed on the donors and also by increa sed incubation temperature. The energy content of the incubated feedst uffs estimated in vitro corresponded well with in vivo values, conside ring the prolonged mean retention time of fibre-rich feedstuffs in the rumen under heat stress.