Heterogeneity of the digestible insoluble fiber of selected forages in situ

Long-term in situ incubations were performed to verify the likelihood of the heterogeneity concept of the potentially digestible fraction of the insoluble fiber (NDFom) by fitting both heterogeneous and homogeneous potentially digestible NDFom versions of a generalized compartmental model of digestion (GCMD). Corn silage and eleven tropical grasses and alfalfa hay were studied. Data were gathered from a study in which forage samples in nylon bags were incubated in rumen cannulated steers so that three profiles per forage were generated. The incubation endpoint was used to form sets of time profiles. The original set consisted of profiles ending at 1440 h, and the other two were formed by using 96 and 240 h as the incubation endpoints, respectively. The indigestible residue was estimated using nonlinear least squares or by assuming it to be 2.4 times lignin determined by the sulphuric acid method (Lignin (sa)). Therefore, eight different models were evaluated by combining end points of digestion, and the homogeneous and heterogeneous versions of GCMD with the two ways of estimating the indigestible residue. The likelihood of the models was assessed by computing Akaike information criteria. The effects of forage, model, and their interaction were analyzed by taking model as a repeated measurement. Heterogeneity of the potentially degradable fraction for NDFom was detected with long-term incubation trials (up to 1440 h) for some forages, and the introduction of the 2.4×Lignin (sa) as a direct measure of the indigestible residue improved the likelihood of the heterogeneous version of GCMD. The forage by model interaction was significant for many comparable parameter estimates, which means that specific and inconsistent results for models within forages were produced depending on the definition of the incubation end-point. The indigestible residue was overestimated with short-term incubation profiles, but the overestimation was lower for the profiles ending at 240 h whether compared to profiles ending at 96 h. Given the likelihood of the heterogeneous version of GCMD fitted to profiles ending at 1440 h and at 240 h for some forages, the heterogeneity concept should be investigated whenever the research interest relies on estimating the kinetic attributes of the degradation profiles of the NDFom in situ.