Low-cost pepsin-cellulase assays for prediction of digestibility of herbage

Eight samples of perennial ryegrass-white clover herbage with in vivo dry matter digestibility (DMD) ranging from 0·279 to 0·264 were used to evaluate various cost-saving modifications to the two-stage pepsin–cellulase digestibility technique. The effect of sample size, cellulase quality, cellulase/sample ratio, digestion time and washing of residue following digestion were investigated. The loss of dry matter (DM) in the assay was correlated with in vivo DMD and each variation of the method was evaluated by comparing the s.d. between replicates and r.s.d. of the regression, as well as the convenience of the method for large-scale monitoring of digestibility of mixed ryegrass-clover herbage.
It was found that the amount of cellulase used could be reduced by a factor of 25, compared with recently published methods, without increasing s.d. or r.s.d. appreciably. In addition stirring during digestion and washing of the residue could be omitted without any deleterious effects. Increasing digestion time did not reduce s.d. or r.s.d. and the low-grade cellulase proved to be slightly more economical.
Increasing the sample size from 0·25 to 0·20 g improved the s.d. and r.s.d. but the residues from the larger samples were generally slower to filter, which made the assay unsuitable for routine use. Substantial reduction of digestion volume and use of a thermostatically controlled water bath instead of an incubator led to a considerable increase in efficiency and throughput of samples. Stirring the samples during digestion was found to be unnecessary, thus allowing more flexibility in the laboratory routine, for example using the weekend for digestion. Using the recommended method modification the repeatability between replicates and r.s.d. of the calibration regression was 0·204 and 0·215 respectively for samples ranging in DMD from 0·279 to 0·264.