The fermentation of soluble carbohydrates in rumen contents of cows fed diets containing a large proportion of hay

I.Studies were made of the fermentation of D-glucose, D-fructose, D-galactose, D-xylose, L-arabinose and sucrose by rumen contents from two cows given 70 yo hay and 3 0 "/o dairy cubes once daily.2. In in vivo experiments the monosaccharides only were infused into the rumen for 8 h at zoo g/h.Glucose and fructose were almost completely fermented but the other carbohydrates were fermented more slowly.Changes in the proportions of volatile fatty acids (VFA) in the rumen indicated marked differences among the carbohydrates in the proportions of VFA produced. 3.In in vitro experiments all the carbohydrates were incubated for z h with 150 g mixed rumen contents.The carbohydrates were added at 10 min intervals at a rate equivalent to that used in vivo.Between 97 % and 100 yo of the sucrose, glucose and fructose were fermented, but only 42-56 % of the other carbohydrates.Acetic acid was the predominant acid produced, especially from galactose and the pentoses.Propionic acid constituted less than 20 % of the VFA produced from galactose but about 3 0 yo from the other carbohydrates.n-Butyric acid formed about 20 yo of the VFA from the hexoses and sucrose but only 10 % from the pcntoses.Appreciable amounts of lactic acid were produced from glucose and fructose only.4. Net recovery of carbon from fermented carbohydrate in VFA plus lactic acid was about 35 %; if results were not corrected for VFA produced in control flasks, the resulting gross recovery was 5-63 %.The relative validity of the two terms is discussed. 5.There was good qualitative agreement between results from in vivo and in vitro experiments and this was taken as evidence of the general validity of the in vitro technique.Only the development of accurate methods for determining rates of fermentation in vivo would allow evaluation of the technique in absolute terms.Such a comparison would be of great value. 6.The relevance of the results to the fermentation of the carbohydrates of feeds is considered.The differences found between the fermentation products of sucrose and those of glucose and fructose demonstrate that it is not valid to extrapolate results obtained with monosaccharides to their related polysaccharides without experimental evidence.