The accuracy of the pepsin-cellulase technique for estimating the dry matter digestibility in vivo of grasses and legumes

A study was made of the accuracy of predicting dry matter digestibility (DMD) in vivo of 32 legume samples and 50 grass samples from the dry matter disappearance (DMS) of the samples in pepsin (48 h) followed by Onozuka cellulase (48 h). The effects of grinder screen size, incubation temperature and sample size on the accuracy of predicting dry matter digestibility were examined. The DMD in vivo of legumes and grasses was significantly correlated with DMS at 39° C (r = 0.91 and 0.94) and increasing the incubation temperature to 50° C had no effect on the correlations, although DMS was greater. There was no significant difference between the regression equations for grasses and legumes. The regression equation for the combined 82 samples had an RSD of ± 3.0 digestibility units. The corresponding RSD for regressions based on fermentation with rumen fluid in vitro was ± 2.6 digestibility units. Grinding the grass samples through a 0.4 mm screen prior to analysis resulted in regressions with a larger RSD (± 3.3) than when a 1.0 mm screen was used to prepare the samples (P < 0.001). It was concluded that the digestibility in vivo of both legumes and grasses could be predicted by the pepsin-cellulase method by using the same equation, but to eliminate biased results, samples of known digestibility in vivo similar to those being tested should be included as standards in each study. The analytical time could be reduced by 48 h if shorter incubation periods (24 h) were used, but only at the expense of slightly larger errors in predicting DMD in vivo (P < 0.01).

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