An Illustrative Analysis of Atypical Gas Production Profiles Obtained from In Vitro Digestibility Studies Using Fecal Inoculum

SIMPLE SUMMARY: The gas production method is a laboratory technique that measures the amount of fermentation gases produced from incubating animal feedstuffs with microbes from ruminal fluid or fecal samples. It is faster and cheaper than evaluating feedstuffs using animal trials. Models may be applied to the gas production profiles generated in order to rank feedstuffs or to determine the extent of feedstuff digestion either in the rumen or in the hindgut. Typical gas production profiles show a monotonically increasing monophasic pattern. However, atypical gas production profiles exist whereby at least two consecutive phases of gas production or additional extraneous features which distort the typical profile are present. Such profiles are more likely to occur with the use of a fecal inoculum and are much less well described. This article illustrates the analysis of these atypical gas production profiles and explores the methodology of numerical modeling to construct equivalent typical growth-like trends. ABSTRACT: Gas production profiles typically show a monotonically increasing monophasic pattern. However, atypical gas production profiles exist whereby at least two consecutive phases of gas production or additional extraneous features that distort the typical profile are present. Such profiles are more likely to occur with the use of a fecal inoculum and are much less well described. The presence of multiple phases or non-descript extraneous features makes it difficult to apply directly recommended modeling approaches such as standard response functions or classical growth functions. To overcome such difficulties, extensions of the Mitscherlich equation and a numerical modeling option also based on the Mitscherlich are explored. The numerical modeling option uses an estimate of relative rate obtained from the smoothed data profile and an estimate of maximum gas produced together with any lag time information drawn from the raw data to construct a simple Mitscherlich equation. In summary, this article illustrates the analysis of atypical gas production profiles obtained using a fecal inoculum and explores the methodology of numerical modeling to reconstruct equivalent typical growth-like trends.