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* Formerly National Institute for Research in Dairying, Shinfield, Reading, UK
Department of Agriculture, University of Reading, Reading RG6 6AR, UK
Institute for Grassland and Environmental Research, Plas Gogerddan, Aberystywth SY23 3EB, UK
Corresponding author: J. D. Sutton; e-mail: j.d.sutton{at}reading.ac.uk.
Five lactating dairy cows with a permanent cannula in the rumen were given (kg DM/d) a normal diet (7.8 concentrates, 5.1 hay) or a low-roughage (LR) diet (11.5 concentrates, 1.2 hay) in two meals daily in a two-period crossover design. Milk fat (g/kg) was severely reduced on diet LR. To measure rates of production of individual volatile fatty acids (VFA) in the rumen, 0.5 mCi 1-14C-acetic acid, 2-14C-propionic acid, or 1-14C-n-butyric acid were infused into the rumen for 22 h at intervals of 2 to 6 d; rumen samples were taken over the last 12 h. To measure rumen volume, we infused Cr-EDTA into the rumen continuously, and polyethylene glycol was injected 2 h before the morning feed. Results were very variable, so volumes measured by rumen emptying were used instead. Net production of propionic acid more than doubled on LR, but acetate and butyrate production was only numerically lower. Net production rates pooled across both diets were significantly related to concentrations for each VFA. Molar proportions of net production were only slightly higher than molar proportions of concentrations for acetate and propionate but were lower for butyrate. The net energy value (MJ/d) of production of the three VFA increased from 89.5 on normal to 109.1 on LR, equivalent to 55 and 64% of digestible energy, respectively. Fully interchanging, three-pool models of VFA C fluxes are presented. It is concluded that net production rates of VFA can be measured in non-steady states without the need to measure rumen volumes.
Key Words: dairy cow • volatile fatty acid • rumen fermentation • milk fat depression
Abbreviation key: ADER = energy apparently digested in the rumen, DDM = digestible DM, DE = digestible energy, LR = low roughage, PEG = polyethylene glycol
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