The nutritive value of spring-grown herbage produced on farms throughout England and wales over 4 years. III. The prediction of energy values from various laboratory measurements

Abstract

Relationships between gross energy (GE) and metabolisable energy (ME) content and a range of laboratory measurements have been developed for 173 samples of spring-grown herbage. The laboratory methods included three in vitro measures of digestible organic-matter content of the dry matter (DOMD), one using rumen fluid, one based on neutral detergent fibre (NDF)-cellulase and one using pepsin-cellulase, together with the measurement of several cell-wall fractions. Gross energy was most influenced by crude protein (CP) content and could be predicted from CP with a standard error of prediction (SEP) of 0.34 MJ kg −1 DM. All three in vitro digestibility methods produced prediction equations for ME superior to those of all cell-wall fractions, including the widely used modified acid detergent fibre (MADF). In addition, relationships based on the cellulase methods and NDF-cellulase in particular, were shown to be largely unaffected by factors such as year and county of harvest, predominant grass variety and age of ley, in contrast to the situation with relationships based on rumen fluid and MADF. The inclusion of CP and GE in bivariate regressions to predict ME significantly increased the accountable variance in all cases tested although the increase was relatively small. The three bivariate relationships of most practical use which accounted for most of the variability in ME ( Y, MJ kg −1 DM) were: Y=3.47+0.0102 X 1+0.0030 X 2 Y=5.85+0.0087 X 3+0.0042 X 2 Y=0.37+0.0142 X 4+0.0077 X 2 where X 1 X 3 and X 4 are DOMD (g kg −1) contents measured, respectively by NDF-cellulase, pepsin-cellulase and rumen fluid. X 2 is CP (g kg −1 DM). The minimum SEP of these equations were 0.64, 0.68 and 0.68 MJ kg −1 DM, respectively. The ME contents predicted by the relationships based on the three in vitro methods and MADF were significantly higher than those predicted by the MADF equation which is currently used for extension purposes in much of the U.K.