The nutritive value of spring-grown herbage produced on farms throughout England and wales over 4 years. II. The prediction of apparent digestibility in vivo from various laboratory measurements

Abstract

Relationships between organic-matter (OM) digestibility and digestible organic-matter content of the dry matter (DOMD) in vivo and a range of laboratory measurements have been developed for 173 samples of spring-grown herbage. Laboratory measurements included three in vitro estimates of digestibility, based on the use of rumen fluid, on a neutral detergent fibre (NDF)-cellulase technique, and one using a pepsin-cellulase method. All three biological methods were superior in prediction power to cell-wall fractions. Lignin and hemicellulose were particularly poor as predictors. The three monovariate relationships accounting for most variability in OM digestibility ( Y) were: Y=0.313+0.584 NDF-cellulase OM digestibility Y=0.458+0.518 pepsin-cellulase OM digestibility Y=0.106+0.890 rumin fluid OM digestibility The minimum standard errors of prediction (SEP) for these equations were 0.028, 0.031 and 0.035, respectively. The three in vitro measures were significantly additive to one another in prediction power, although the additional variance accounted for was small. Relationships based on rumen fluid, modified acid detergent fibre (MADF) and crude fibre were significantly affected by year and county of harvest, predominant grass variety and age of ley. With the exception of predominant variety, the methods using cellulase, and NDF-cellulase in particular, were less affected by these factors. The DOMD contents predicted by the relationships based on the three in vitro methods, MADF and crude fibre were all shown to be significantly ( P<0.001) different from those predicted by the equation based on MADF and which is currently in use for extension work in much of the U.K. It is concluded that OM digestibility and DOMD of farm-grown spring herbage could be better predicted by methods based on cellulase, with the NDF-cellulase assay having some advantage over pepsin-cellulase.