The UK metabolisable protein system (AFRC, 1992) requires the measurement of fermentable metabolisable energy (FME) to determine the potential yield of microbial crude protein in the rumen. FME is defined as:
The objective of this work was to measure the ME , ether extract and volatile fatty acid components of first cut grass silages to assess the range of FME and non-fermentable fractions and hence the range of fermentabilities (FME:ME).
Eighty-four clamp grass silages, predominantly first-cut rye grass, were analysed for modified acid detergent fibre (MADF), individual volatile fatty acids, oil (ether extract) and oven dry matter. ME was predicted from the MADF after correction for oven dry matter. The gross energy of the lactic, acetic, propanoic, butyric and valeric acid were taken to be 15.2, 14.6, 20.8, 24.9, and 28.0 MJ/kg DM, and for the Ether Extract fraction of grass 38.5 MJ/kg DM. The energy content of the volatile fatty acids in the silage were summed and FME was calculated as described above. The fermentability of the silage was the expressed as the proportion of FME to ME.