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SummaryRumen fermentation characteristics were studied using sheep fitted with rumen cannulae. Diets of 100% hay and 20% hay:80% cooked flaked maize (concentrate diet) were fed at the maintenance level of energy intake as two equal portions per day. Both the gas and liquid phases of the rumen were continuously sampled over 33-h periods.Concentrations of O2 and N2 in rumen gas inoreased during feeding, whilst concentrations of CO2 and CH4 decreased. Thereafter the concentrations of both CO2 and CH4 rapidly increased. The CO2: CH4 ratio increased rapidly following feeding; it declined to baseline levels 2–4 h after feeding the hay diet, but with the concentrate diet the decline took longer. O2 concentration declined rapidly following feeding and was stable within the range 1–3% for long periods. At no stage was O2 absent from the rumen gas phase. H2 comprised 100–1500 μ1/1 and 100–6000 μ1/1 of the gas phase in sheep fed the hay or concentrate diets respectively and its concentration increased very rapidly with the onset of eating. CO concentration varied between 2 and 16 μ1/1 in the rumen gas of hay-fed animals and was not related to time after feeding. In concentrate-fed animals CO comprised 0—130 μ1/1 of rumen gas and increased very slowly after feeding.Eh and rH values ranged between —150 and —260 mV and 8·0 and 5·0 units respectively for rumen contents from animals fed the two diets. A diurnal cycle was evident, with the most oxidizing state being attained just before feeding, and the most reducing state just after feeding. The diurnal cycles were better denned by rH than by Eh. The magnitude of the decrease in rH (and pH) during feeding was greater for animals fed the concentrate than the hay diet. At no stage during feeding did Eh or rH change towards more oxidizing conditions. Changes in the concentration of H2 in rumen gas were related to changes in the rH of rumen contents.VFA molar proportions showed no changes during the 24-h cycle with hay-fed animals but showed erratic variation with concentrate-fed animals. When two sheep fed the concentrate diet were sampled daily for 21 days, it was shown that VFA molar proportions were not constant.It was concluded that the gas phase of the rumen was never completely O2-free, and that whilst a stable rumen fermentation existed in sheep fed the hay diet the fermentation appeared to be continuously changing in the concentrate-fed sheep, and on this last mentioned diet abnormal values for rumen gas composition and VFA proportions were produced from time to time.

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