The volatility of components of grass silage on oven drying and the inter-relationship between dry-matter content estimated by different analytical methods

This study reviews the volatility coefficients used to convert the oven dry-matter (DM) content of grass silage to an accepted true DM base, volatile-corrected oven dry matter (VCODM). The revised coefficients quoted for DM determination at 60 degrees C, 85 degrees C and 100 degrees C are based on 18 grass silages with DM contents in the range 153-365 g kg(-1). The volatility coefficients for drying at 60 degrees C, 85 degrees C and 100 degrees C were 0.090, 0.224 and 0.375 for lactic acid and 0.554, 0.716 and 0.892 for total volatile fatty acids respectively. The volatilities of ammonia and total alcohols remained unchanged from previous work and showed no temperature dependences in the range 60 degrees C to 100 degrees C. These revised coefficients were validated using 36 grass silages from three harvests in 1996 and 1997, and no significant differences were found among absolute dry matter (GCDM), alcohol-corrected toluene dry matter (ATDM) and VCODM contents based on the three drying temperatures (VCODM100, VCODM85 and VCODM60). A series of regression equations relating absolute DM content to oven DM content determined at different temperatures gave coefficients of 1.024, 1.013 and 1.000 and constants of 12.67, 11.43 and 11.16 for oven drying at 60 degrees C, 85 degrees C and 100 degrees C respectively. Mathematical manipulation of these equations enables interconversion of DM contents at the three drying temperatures. A new method is described for the analysis of volatile fatty acid, lactic acid and alcohol concentrations in grass silage by gas-liquid chromatography using a single injection in an automated procedure that makes the routine estimation of VCODM a practical proposition to satisfy routine high-volume requirements. Finally, in a separate study over 4 years using 2381 grass silages from research and commercial farms throughout Ireland, a simple regression is described, which, for advisory purposes, allows true silage DM content to be estimated from oven dry matter content (ODM) for silages in which ODM is >200 g kg(-1).