| The objective of this study was to evaluate the kinetics of digestion and flow of plant tissue residues in cattle. Two rumen-cannulated cows were utilized to evaluate the fiber degradation of four feedstuffs. Fourteen mathematical models were fitted to the data of undegraded fiber remaining at each incubation time point. The model, I + G4/E, was selected to evaluate the digestion kinetics of fiber. Indigestibility (I), mean digestion lifetime (MDL), and mean digestion rate (MDR), were statistically compared among feeds and entities. The effect of feed was statistically significant for all the variables. The flow of plant tissue through the gastrointestinal tract was studied in eight yearling steers fitted with ruminal and duodenal cannulae. Steers were fed a total mixed ration (TMR), (16% CP) in a first trial, and ryegrass hay (12% CP) in a second trial. Trials were divided in five and two periods, respectively. Samples of feeds were labeled with rare earth elements and offered to the animals at the beginning of each collection period. Samples from duodenum and feces were taken from 0 to 108 hours, each 6-h the first day, and each 12-h the other 4 days. Fifteen non-linear models were fitted to data of markers concentration in duodenal and fecal samples. The model G3→E→0 was selected as the preferred model to evaluate flow. The parameters, mean resident lifetime and mean escape rate, were estimated by using the selected model. Information obtained from the digestion and flow studies was integrated in order to estimate the extent of digestion of potentially digestible fiber (PDFD). In vivo extent of digestion was calculated and compared to the expected PDFD obtained by several different approaches. The estimation of PDFD, when assuming the existence of two simultaneous digesting sub-entities of PDF and two sequential flow pools, yielded estimates of PDFD (5% larger than observed) that most biologically resembled that observed in vivo. |
