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In vitro gas production, measured by computer-interfaced pressure sensors, was used to follow the digestion of a crystalline processed cellulose, a bacterial cellulose, and mixtures of these substrates by mixed ruminal bacteria. A first-order, substrate limited model (simple exponential with lag) and two bacterial growth models (logistic, Gompertz) were tested to fit these data. No single pool model gave an optimal fit to all substrates, but dual pool versions of both the logistic and Gompertz models fitted the data extremely well. Derivations of these models in the context of gas production are presented. The dual pool version of the exponential model commonly used to analyze fiber digestion was not able to reproduce the slope variations seen with mixed substrates. A modified dual pool logistic equation, with a single lag value, was selected to model the in vitro digestion of these substrates. The model was able to predict adequately both the input composition and the kinetic parameters for a defined mixture and gave a good fit (r2> .995) to data from all the single and mixed substrates tested. This model may be useful for interpreting gas accumulation from natural feedstuffs.

The techniques reported in this paper were developed to facilitate the study of the kinetics of forage digestion in vitro by measuring gas production. Fiber disappearance as a measure of the reaction rate has been replaced by the use of computerized pressure sensors to monitor the gaseous products (CO2, CH4) of microbial metabolism. The recording system described requires a computer, pressure sensors, an interface card, and appropriate software to monitor gas production continuously. Several variables, including sample size, inoculum size, vessel size, and type of pressure sensor, have been investigated to determine ranges within which gas production can be measured accurately, and the reproducibility of the results has been established. Because this technique uses small (100-mg) samples, a modified NDF method has been introduced that allows determination of extent of digestion at the end of an incubation in which gas production has been monitored. A strong linear relationship existed between NDF disappearance and gas production.

The fermentation of the neutral detergent-soluble (NDS) fraction of two legumes (clover, alfalfa) and two grasses (timothy, guinea grass) was measured using a curve subtraction technique with in vitro gas production data from the whole forage and the isolated neutral detergent-extracted fiber. The NDF disappearance and VFA production also were measured. There were no significant differences between the VFA patterns from whole forage and NDF. There was a good linear correlation between the volume of gas produced and the mass of fiber digested in the NDF samples. Analysis of the gas curves with a dual-pool logistic model gave a lag value, digestion rates and pool sizes for the whole forage, the fiber component, and the NDS fraction. Rates for the NDS fraction ranged from .152 h-1 for clover to .191 h-1 for timothy. These rates are appreciably lower than values assumed in some models. Application of a triple-pool logistic model revealed the presence of faster-digesting material in the legumes. We discuss several different ways to measure the NDS pool size. The simplest method requires only a single gas measurement at the end of in vitro digestion of the whole forage coupled with an NDF disappearance measurement. The curve subtraction technique can provide information on the size and digestion kinetics of the NDS pool. This information is useful both for model studies and for agronomic research and may help us to understand the nutritional significance of this fast-digesting carbohydrate fraction.

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Samples of unfractionated forage and isolated NDF from six forages were fermented in vitro, and NDF disappearance and gas and VFA production were measured over time. Rates based on each of these data sets were calculated using a one-pool logistic model. The rates of NDF disappearance and gas and VFA production did not differ within each forage. Gas and VFA production were linearly related to NDF digestion. Gas yield was .35 mL/mg (r2 = .92) of NDF digested for the isolated NDF. The amount of total VFA produced per milligram of NDF digested was more variable than gas (r2 = .72), with a slope of .01 mmol VFA/mg of NDF digested. The relationship between gas and VFA production was linear (mean slope of 1.43 mmol gas/mmol VFA, r2 = .69). The ratios of end products (gas and VFA) to NDF digestion and the ratio of acetate:propionate were variable during the first 8 h of fermentation but changed little after this time. Changes in the acetate: propionate ratio explained 23% of the variation in gas produced per millimole of total VFA detected.

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Alfalfa and bromegrass, each harvested at five different stages of maturity, were separated into water-insoluble and -soluble fractions. The NDF concentrations ranged from 19 to 43% for alfalfa and from 42 to 58% for brome. The rates of digestion, by mixed ruminal microflora, of the unfractionated forage and of the water-insoluble and -soluble fractions were measured in vitro using pressure sensors to monitor gas production. Both forages showed the expected decline in fiber digestibility with increasing maturity. A dual-pool logistic model gave pool sizes, specific rates, and a single lag time for both the faster- and slower-digesting fractions. The main difference between alfalfa and brome was in the soluble pool. This pool produced approximately 40% of the total gas in alfalfa, 25% in brome. The specific digestion rates of the brome soluble pool were approximately 50% higher than those for alfalfa. Net VFA production showed a somewhat higher acetate: propionate ratio for brome (3.2) compared with alfalfa (2.2), but there was little change with increasing maturity within a given forage. Gas production curves for the unfractionated forage showed a 0 to 10% positive deviation from curves created by adding data from separate digestion of the insoluble and soluble forage fractions. Gas measurements offer a promising approach to the study of the water-soluble extracts of forages and the interaction of the soluble- and insoluble-fractions during fermentation.