The choice of organic matrix to be used in a DA process, more or less combined, is conducted following different management approaches including:
The use of corporate waste
The use of dedicated biomasses
The cost of biomass
The production of biogas from biomasses
The use of by-products
Corn silage is the most widely used biomass as it meets requirements related to biogas productivity, market availability (direct cultivation), and biomass costs. Biomass has always been used to feed ruminant animals and the anaerobic digestion process replicates the process of digestion in the rumen on an industrial scale. Conservation using the ensiling process is a very ancient technique, (historical documents dating back to 1500 BC attribute paternity to the Egyptians) which allows for the preservation of moist fodder through natural acidification via the fermentation of forage sugars using lactic bacteria in the absence of oxygen (anaerobiosis). Lactic bacteria mainly produce lactic acid and acetic acid. Microorganisms that can cause silage to deteriorate are inhibited by the synergistic effect of organic acids that are produced (and therefore a low pH) and the simultaneous absence of oxygen. The main factor influencing the efficiency of silage is the degree of anaerobiosis achieved when the silo is completed, sealed and retained during storage.
There are four ensiling stages:
Initial stage: respiration - characterised by poor acidification due to oxygen being naturally present in the mass. Aerobic microorganisms are the most active (yeasts, enterobacteria and acetic bacteria). Microbial activity at this stage leads to the degradation of valuable nutrients and it's therefore essential to minimise it.
Main fermentation stage: characterised by anaerobic microorganism activity consisting mainly of lactobacilli, which ferment the soluble sugars in lactic acid, having a strong acidifying power and inhibiting unwanted microorganisms. It’s important to create fast and adequate lactic acid production for stable silage, bringing pH levels to 3.8 - 4 for corn silage
Anaerobic stabilisation stage: at this stage, if proper acidification has occurred, silage will be stable. It is important to ensure that the covering does not suffer any damage and that no oxygen is introduced into the mass
Consumption stage: opening of the silos and use of silage in implantation; immediate exposure of new fermentations to air mainly carried out by yeasts with a consequential rise in pH and temperature
An optimal ensiling process needs:
Good agronomic practices
A mowing period
Optimum earth compaction
Proper trench management with fast and effective closure
Reduction of the aerobic phase
Reduction of the pH as quickly as possible in aerobic and fermentation stages
Reduction of the presence of contaminating flora
Reduction of deteriorating phenomena, both in the ensiling and post-opening phases
During storage, part of the energy value of the silage product is inevitably lost through metabolic processes of various origins. Losses are usually attributable to percolation, heating, mould and yeast production.
Table: Percentage of loss of dry substances in silage