Environmental Engineering Reference
In-Depth Information
4.2.1 Mechanism
A variety of microorganisms are present on harvested crops, including some
that can be harmful such as clostridia, coliforms and mycotoxin-producing
fungi. These can grow on inappropriately stored crops and cause loss of
substrate as well as disease in livestock and farm workers. LAB are also
present on harvested crops and produce acids (mainly lactic acid) from
sugars available in the plants. In contrast to hay production, in which the
growth of all microorganisms is prevented by removing water, ensiling aims
to promote the growth of these LAB and inhibit the growth of harmful
microorganisms. This is done by storing the moist crops in a closed airtight
system, where the oxygen is quickly used up by the plant's respiratory
enzymes. The resulting absence of oxygen inhibits the growth of aerobic
microorganisms including aerobic fungi. LAB are very tolerant to low water
conditions so, by using a relatively dry crop, such as maize or wilted grass,
or by using additives, LAB can grow faster than other, undesirable
anaerobic bacteria. The acids produced by LAB cause a drop in pH and
most undesirable microbial growth is
inhibited (Wilkinson, 2005;
McDonald et al., 1991).
4.2.2 Production
A range of equipment can be used for producing silage, from expensive and
very effective to cheaper and less reliable. Concrete tower silos are a more
expensive option, but the vessel can be well sealed from oxygen (McDonald
et al., 1991). Walled bunker silos are less expensive but also a good option
for keeping oxygen out. Considerably cheaper options include holes in the
ground and silage heaps, which can be sealed with sand, biogas digestate or
polyethylene sheets weighed down with, for example, rubber tyres. A large
proportion (probably 25%) of silage in Europe is made using big bales,
facilitated by the introduction of baler-wrapper machines (Wilkinson,
2005). Baled silage is particularly good for grasses and legume crops due to
the method of harvest and the shape and structure of the harvested crop.
The three most important crop parameters in ensiling are: dry matter
(DM) content, the amount of fermentable sugar available (water-soluble
carbohydrate, WSC) and buffering capacity (Wilkinson, 2005). The right
DM content is essential, as water is necessary for the growth of LAB, but
too much moisture allows the growth of undesirable bacteria before the
LAB have reduced the pH. Maize has a high DM content, which makes it
particularly easy to ensile. Although grass has a lower DM content, it is also
commonly ensiled. The ensiling of grass can be aided by cutting the grass on
a dry day and leaving on the field after cutting, allowing water to evaporate
from the plant and thereby increasing the DM content. This is known as
Search WWH ::
Custom Search