Rumen fermentation studies on two contrasting diets. 2. Comparison of the performance of anin vitrocontinuous-culture fermentation within vivofermentation

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Abstract
Summary: An impermeable continuous-culture artificial rumen was constructed and rumen microbial populations were culturedin vitrofor periods of up to 23 days. Diets of hay and 20:80 hay:cooked flaked maize (concentrate diet) were fed to thein vitrocultures and also to sheep, and the performance of thein vitrofermentation compared with that of thein vivofermentation.With the hay diet, changing the initial inoculum from strained rumen liquor to unmodified rumen contents (containing solid ingesta) reduced the time required to attain a steady-state fermentationin vitrofrom 13 to 4 days. VFA molar proportions and the CO2: CH4ratio indicated that the type of fermentation taking placein vitrowas remarkably close toin vivofermentation. However, rates of cellulose digestion, CH4production and number of bacteria/unit volume (as judged by DNA concentration) were much lessin vitrothanin vivo, showing that the rate of fermentation was reduced in the artificial rumen. Total VFA concentration and the osmotic pressure of fermentor liquor were slightly higher than the maximum values foundin vivo, and it was concluded that these were responsible for the reduced rate of fermentation.Regardless of the duration of culture, a satisfactory steady-state fermentation was never attained when the concentrate diet was administered. DNA concentration in fermentor liquor declined to very low levels after 4 days; CH4production rapidly declined and was replaced by H2production after 5 days. VFA molar proportions bore little resemblance to those foundin vivo. Both the osmotic pressure of fermentor contents and total VFA concentration rapidly increased with time, and by day 3 were approximately 2·5 times the values foundin vivo. It was concluded that high VFA concentration, and the associated increase in osmotic pressure, were the main factors responsible for the unrealisticin vitrofermentation, and that this problem was much greater than encountered with the hay diet due to starch being digested at a faster rate than cellulose and hemicellulose.Methods of regulating VFA concentration in continuous culturein vitrorumen systems are discussed and some improvements suggested.