Comparison of mathematical models to describe disappearance curves obtained using the polyester bag technique for incubating feeds in the rumen

Different nonlinear models were evaluated as candidates to describe ruminal degradation kinetics of forages from data obtained by the incubation of th e feeds in the rumen using polyester bags. Nine models were used: segmented model with three straight lines (Mod0); simple Mitscherlich or exponential (Mod1); inverse polynomial (Mod2); compartmental model with two exponentia l terms (Mod3); generalized Mitscherlich (Mod4); generalized Michaelis-Ment en (Mod5); logistic (Mod6); Gompertz (Mod7); and generalized Von Bertalanff y (Mod8). All these models can be represented in the general form D = W + S -0 x Phi(t), where D is in situ disappearance at incubation time t, W and S -0 are positive scalars, and Phi is a positive monotonically increasing fun ction unique to each of the models studied. Based on first principles, a ge neral formula for calculating the extent of degradation of feeds in the rum en has been derived that is applicable to all the models. The disappearance curves of different feed components (DM, N, and NDF) of 87 Mediterranean f orages (i.e., a total of 261 curves) were fitted to all the models. A compa rative study was carried out based on the mathematical, statistical, and bi ological characteristics of the models. Flexible models that can accommodat e both diminishing returns and sigmoidal behavior were more appropriate in describing the curves. A discrete-lag parameter was introduced into Mod0, M od1, and Mod2 to describe the initial stage of the disappearance curve, and this parameter considerably improved the fit of experimental data. Based o n statistical criteria, models Mod1, Mod4, Mod5, and Mod8 were better than the others for most statistical tests and disappearance curves, but differe nces among these four models were not consistent. The estimates of degradat ion parameters to quantify the rate (half-life, fractional degradation rate ), and extent (undegradable fraction, effective degradability) of ruminal d egradation of feeds were also used as a means to discriminate between model s, although in most cases all of the models gave similar values of the degr adation parameters. In particular, when the extent of degradation was calcu lated for each forage and feed component, differences between the estimates obtained with the different models were of little nutritional significance for the animal.