Comparison of forage digestibility in vitro with enzymic solubility

Abstract

Two methods of determining the enzymatic solubility of organic matter were compared with digestibility evaluated in vitro (IVOMD). The first method (OMSA) used pepsin and Trichoderma viride cellulase, whereas the other (OMSB) entailed boiling with HCl prior to enzymatic attack by low-cost Aspergillus niger cellulase. The relationship between digestibility in vitro and acid detergent fibre (ADF) content was also considered. Determinations were made in 6 successive runs on 19 samples of grasses and 13 of legumes. On the basis of 3 standard samples (whose digestibility in vivo was known and which were included in each batch), significant differences among runs were found with the in vitro method but not with the enzymatic methods. Repeatability, expressed as a standard deviation, was 1.29 units of digestibility for IVOMD, 0.87 for OMSA and 0.80 for OMSB. The IVOMD was more closely correlated with OMSA ( r = 0.96∗∗∗) than with OMSB ( r = 0.91∗∗∗) or with ADF content ( r = −0.88∗∗∗). The 2 enzymatic methods were also shown to be closely correlated; ADF content correlated best with OMSB. Reference samples with known digestibility in vivo indicate that OMSA and IVOMD underestimate organic matter digestibility, especially for less digestible samples, and OMSB overestimates digestibility more or less constantly. The OMSA of legumes was greater than that of grasses for the same digestibility in vitro. No differences in solubility between botanical families were revealed by OMSB.