Changes in the total concentration of the volatile fatty acid (VFA) pool in the rumen were followed over a 24 hour period in 2 groups of sheep, 1 fed at 08h00 and the other twice daily at 08h00 and 20h00. Although similar maximum (143 and 147 meq/l) and average (100,3 and 102,1 meq/l) levels were found in the 12 and 24 h groups respectively, the twice daily feeding regimen resulted in a lower variation (S.D. = I7,0 meq/l and 28,9 meq/l respectively). It was concluded from changes in the percentage molar composition of the VFA pool over the same period that the order of VFA absorption from the rumen was propionate > acetate > butyrate for both groups, but that the differences were less marked for the twice daily fed sheep. Long term infusions of ¹⁴C labelled acetic, propionic and butyric acids into the rumen of sheep fed a total of I 600 g lucerne hay twice daily (08h00 and 20h00), gave an average net total VFA production rate of 4, 52 ± 1 ,01 moles/800 g/12 hours irrespective of the acid infused. The net individual turnover rates for acetic (2,81 moles/ 12 h), propionic acid (0,82 moles/12 h) and butyric acid (0 , 55 moles/12 h), derived by the subtraction of the inter-conversion factors from the gross production rates of the acids, and expressed as the percentage contribution of each acid to the total net VFA turnover (acetic= 62%, propionic= 18% and butyric= 12%) closely resembled the percentage molar composition of the VFA pool in the rumen (acetic= 60%, propionic=23% and butyric= 12%). The total net VFA production was found to be directly proportional to the total VFA concentration in the rumen (correlation coefficient = 0, 83), and the relationship can be described by the equation y = 0,034 x + 0,16 where y= VFA production in moles/ 12 hour and x = VFA concentration in meq/l. A specific VFA production rate of 0,85 moles per 100 g digestible organic matter was calculated from the average daily VFA production rate and the composition of the lucerne hay.
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