Silage is by far the most widespread method of storing energy fodder, i.e. all cereals, with both summer and winter cycle, harvested at the waxing stage
Ensiling is a technique that can also be successfully applied to fibrous fodder, such as grassland and meadow fodder; among the latter, foxtail millet, ryegrass, or autumn-winter cereals are common, harvested, however, at the flowering stage. In this way, the early harvesting of these grasslands allows for the timely sowing of the main crop.
Silage preservation is achieved through the acidification of the water present and the consequent reduction of the pH. Therefore, the water content at harvest must be chosen according to the amount of simple, easily fermentable sugars present in the product to be harvested. Since in fibrous fodder the presence of sugars is reduced, there is a field phase during which, by evaporation, a certain amount of water is released. The most convenient harvest moisture for grass forage for silo storage is generally between 55 and 65%.
Partial drying of the forage in the field, up to moisture values of 60-65% for multi-grass meadow forages ensiled in horizontal silos, has many advantages over ensiling fresh grass. Indeed, the losses caused by the percolation of excess water are limited to the point of disappearing, and the bacterial development of clostridia is inhibited to a greater extent, as they are very sensitive to the amount of water available. Furthermore, the reduction in water content has a favorable effect on the palatability of the silage because dry matter ingestion tends to be higher when the organic acid content of the feed is lower. The optimal pH in the rumen of cows is between 6.2 and 6.8 and therefore an acid feed is well tolerated, whereas a strongly acid feed can upset the delicate microbial balance in the rumen if it does not itself contain those compounds that, through the activity of the microbial flora, can act as a buffer.
Harvesting is carried out with forage harvesters with pick-up reel (RTCs) that collect the forage from the windrow, chop it using a drum equipped with blades and eject it into the trailer proceeding alongside. More often, especially in the lowlands, self-propelled forage harvesters are used, machines used in the harvesting of silage and here modified in the header. These machines are capable of handling forages that are much tougher and more productive than grassland and even fibrous grass. However, the contractor likes to use them because they increase their annual use, and this allows for faster capital depreciation.
Horizontal Silo
The trench silo is the most common of the horizontal silos and consists essentially of a slab, two retaining side walls and generally a back wall. The silo floor, which is made at ground level, is made of concrete. The side walls may be slightly inclined outwards (a few degrees) to facilitate the compression of the mass.
The height of the silo is normally between 2 and 5 m; with greater heights, there are problems during the collection phase. Moreover, as the height increases, the danger increases considerably, especially during filling and compacting phase and during the covering with the tarpaulin, which requires hermetic sealing along the edges.
For safety reasons, when the height exceeds 2 m, a parapet must be installed along the edges. The silo must guarantee water drainage: to do this, the ensiled mass is given a ridge along the center line that is higher than the side walls by approximately 30-60 cm (depending on the width, which is normally between 6 and 10 m) and then get to the wall level. Greater slopes encourage unsafe situations to arise.
Another aspect concerns the presence of the back wall. When this is absent, loading and compressing operations are simplified by allowing the machines to always proceed in forward motion.
With regard to sizing, especially in large companies, it may be convenient to provide for one of the company silos to be of the movable-wall type; this solution allows the volume to be adapted to the specific needs of the year. The silo with movable walls uses space less efficiently because the mass rarely exceeds a height of 2 m; this is why it should be considered a structure that supports the conventional ones and not that replaces them. Furthermore, the preparation work involved in placing the self-supporting prefabricated elements is not simple.
The chances that the fermentation process will take place favorably increase when the mass to be ensiled is large, e.g. significantly higher than 100 t of fresh forage, but this implies, due to the need to complete the silo in a few days, the daily management of large areas and high quantities of forage. For example, a mass of 100 t of forage at 60 per cent moisture content is equivalent to the production of a cut of approximately 10-15 ha of permanent grassland.
The construction site must therefore be able to harvest over large areas, transport the product to the farm and provide for the formation of the silo in no more than two days. In fact, the golden rule is that the silo should be completed in the shortest time possible: hours or days depending on the quantity of mass ensiled (and the type of product).
The ensiling process
The ratio of water content and simple sugars is crucial to the success of the process. However, it is essential to create specific conditions of anaerobiosis (i.e. absence of oxygen) through chopping, compression and isolation of the mass in order to favor the activity of lactic acid bacteria and other categories of bacteria that are efficient in transforming sugar into acid. In this respect, it is useful to remember that there are micro-organisms that operate in conditions of complete anaerobiosis that produce two molecules of lactic acid for every molecule of glucose or fructose (and are therefore very efficient), while other types produce only one molecule of lactic acid releasing alcohols, acetic acid and carbon dioxide (they are therefore less efficient, but give the typical scent to the silage that attracts the cattle). If the forage mass is well compressed, the oxygen inside it is quickly consumed, creating anaerobic conditions. After that, it is necessary to avoid gas exchanges with the outside during storage. These are always serious because they allow the development of micro-organisms that oxidize the acids present and in doing so open the door to a ravenous and diverse troop of other micro-organisms, capable of degrading the forage to the point of rendering it unusable.
In a well-insulated silo, on the other hand, acidity increases, causing a decrease in the pH depending on the free water content of the ensiled mass. In fact, the lower the water content in the ensiled fodder, the lower the quantity of acidic substances required to produce a decrease in pH sufficient to guarantee preservation, inhibiting the development of butyric and proteolytic clostridia. In just a few days, from initial pH values of 6.5-7.0, limit values of 3.5 can be observed. However, a high dry matter content limits the development of both useful and harmful microorganisms, and therefore good preservation can also be achieved under lower acidity conditions, which with semi-withered fodder silage correspond to pH values between 4 and 5. The introduction of sugar by-products from the agri-food industry (waste from processing sugar beet, fruit, etc.) when forming the silo makes it easier to achieve acidity and pH values suitable for storage.
Lorenzo Benvenuti
