Comparison of 2-pool and 3-pool digestion kinetics model predictions of neutral detergent fiber digestibility of forages from commercially available data

Sizes and rates of potentially digestible (B) and undegradable (C) pools of amylase-treated neutral detergent fiber (aNDF) are used to predict ruminal aNDF digestibility (aNDFD%) in widely used dairy cattle diet formulation programs. An exponential 3-pool model (3P) has been suggested for estimating digestion kinetic parameters for this purpose, however, the approach has not been compared with using a simpler exponential 2-pool model (2P), nor with using commercial laboratory (lab) data on which application would rely, nor on model impact on predictions of aNDFD% which is the aim of their application. Our objective was to determine whether 2P or 3P most accurately and efficiently characterizes aNDF digestion kinetics and if the models differed in predicted aNDFD%. Dry forages and silages (6 alfalfas, 6 species of grasses) were analyzed by 2 commercial labs which each performed 2 in vitro incubation runs with mixed ruminal microbes, with samples and blanks in duplicate at each of 11 time points; residual aNDF (Ut) was measured at each time point. Sampling hours (t) were 0, 3, 6, 12, 18, 24, 30, 48, 72, 120, and 240 h. Outlier Ut were removed. Pools as proportions of aNDF were B in 2P, B1 rapid and B2 slow pools in 3P, and C in both; B pools have digestion rates (kd, h^-1) and lag (L, h). Models were fit to data for each forage in each incubation with equations 2P: Ut = B'e^(-kd'z)+C, and 3P: Ut = B1'e^(-kdB1'z)+B2'e^(-kdB2'z)+C, where "z" = [-(L - t - |t - L|) / 2]. There were 48 curves for each model. Parameters were estimated with the "optim" function in base R. Akaike's Information Criterion (AIC) was used to select the model with the best fit for each forage in each incubation: 16 3P, and 32 2P curves were selected. Expressed as (difference between runs)/mean, average deviations between runs for lab1 and lab2, respectively, were: for 3P, B1 = 0.50, 0.17; B2 = 0.26, 0.33; C = 0.50, 0.06; kdB1 = 0.81, 0.32; kdB2 = 0.93, 0.54; for 2P B = 0.04, 0.01; C = 0.07, 0.01; kdB = 0.17, 0.08. Estimates of aNDFD% for 2P and 3P were calculated with no L at passage rates (kp) reported for forages of 0.02 through 0.07 h^-1. t-tests determined if differences were ≠ 0 for 2P - 3P for NDFD% at each kp for each feed evaluated. With 2P minus 3P differences in aNDFD% listed sequentially by 0.01 h^-1 from kp = 0.02 to 0.07 h^-1, for 16 AIC-selected 3P, differences were -0.29, -0.45, -0.65, -0.87, -1.04, and -1.20% and for 32 AIC-selected 2P, values were -0.15, -0.13, -0.14, -0.17, -0.19, and -0.22%. Some differences were significant, but all were quite small. With little difference between models, use of the more complex 3P conferred no advantage over 2P for prediction of aNDFD% in this data set.