The GasFit System to fit nonlinear functions to in vitro gas production profiles of ruminant feeds: History, science, and user guide

This document covers the history, science, and user guide for GasFit (GasFit), version 4.2, a Windows program that fits nonlinear models to in vitro gas production profiles of ruminant feeds. Instead of fitting a single model, GasFit fits sixteen models, from the exponential, logistic, and Gompertz to their two-pool forms and the generalized gamma compartmental models. It then selects the best-fitting profile for each one by SSE, AIC, or BIC, and derives from the selected curve the pool sizes, rates, inflection points, equivalent exponential rates, and effective degradability. The modeling is done in R, behind a graphical interface for users who are not versatile in R, and every run writes a report and a page that documents how each result is computed. The first chapter traces the program from Cornell University's fermentation chambers to the chamber built at Texas A&M University. It continues through the first R script of 2008 and its Windows interface, and the papers presented at the 4th Workshop on Modeling in Ruminant Nutrition in Juiz de Fora, Brazil, in 2008. It ends with GasFit 3 and GasFit 4 and the second version of the chambers in 2025–2026. The second chapter summarizes the science: the data and their adjustment for the blank bottles; the models; starting values and fitting; goodness of fit and the choice of model; two pools; rates and inflection points; the equivalent exponential rate and effective degradability; and the comparison of treatments and of profiles. The third chapter revisits the first evaluation of the models on the chamber's standard alfalfa hay. The fourth is a user guide to every window of the program, the treatment key, the results, and the Excel template. The fifth works through the sample dataset and an essential-oil example of comparing treatments.