During ensilage readily fermentable organic matter (OM) is fermented to lactic acid and short chain volatile fatty acids (VFA). These acids provide little energy as ATP for rumen microbial growth and are essentially absorbed intact. The UK metabolisable protein system defined the energy available for microbial growth, termed fermentable metabolisable energy (FME), as the proportion of metabolisable energy (ME) in a diet/feed less the ME in oil and fermentation acids. The aim was to establish if fermentation acids yield direct gas production resulting from microbial fermentation. Grass silage was simulated using grass hay (GH; containing no fermentation acids) with additions of individual fermentation acids in solution.

GH (~2 kg) was oven-dried overnight at 100°C, nulled (1 mm screen) and then sieved (25 μm screen). 0.5 g GH was weighed into 250 ml Duran bottles according to the treatments; 1) GH + anaerobic incubation mixture (AIM, 85/15 v/v anaerobic medium and strained rumen fluid) + ~5 ml distilled water; 2) GH + AIM + ~5 ml fermentation acid solution (100 mg DL-lactic acid (LA)/10 ml, 30 mg acetic acid (AA)/10 ml or 20 mg n-butyric acid (BA)/10 ml) (equivalent to 100, 30 and 20 mg/g GH dry matter (DM) for LA, AA and BA respectively); 3) anaerobic medium (85 ml) + 15 ml clarified rumen fluid + ~5 ml fermentation acid (as treatment (2)).