Changes in temperature and silo gas composition during ensiling, storage and feeding-out grass silage

An experiment is reported in which two bunker silos were filled with about 12 t of ryegrass; one was compacted with a surface pressure of 4·1 kPa and the other was not. Both were instrumented to record temperatures and gas compositions during fermentation and feed-out. Cores of silage (100 mm diameter) were removed from the bunkers and subjected to forced aeration in insulated cylinders. Simple mathematical models were used to simulate CO2 leakage from the silos and the associated loss of dry matter, and temperature changes during the forced aeration of silage cores. The leakage of CO2 from the silos, as shown by the fall in CO2 concentration with time, could be described empirically by an exponential equation and could be simulated with a simple mathematical model. Experimental evidence supported the hypothesis that permeation was the main method of gaseous exchange during fermentation and feed-out, but gas mixing by diffusion and/or convection also occurred within the silos during fermentation. Dry-matter losses by aerobic activity during storage of 120–150 d were conservatively estimated to be about 0·3% and 0·9% for the uncompacted and compacted bunkers respectively. These were considered to be lower than those that could be expected in farm silos, because the sealing was likely to be more thorough. The silages were also more stable, when subject to aeration, than others examined in this laboratory, probably because of high contents of acetic, propionic and butyric acids. Simple mathematical models were of value, but greater sophistication is needed (e.g. multi-compartmental models) to deal comprehensively with the heat and gas flows found in the complex biophysical systems of silage.