Do acids and organic acid salts have a role in silage fermentation?
These additives do have a role under poor wilting conditions. They are used in Europe for forages that are at risk of a poor fermentation. Direct acidification through an acid additive results in an immediate drop in pH, and the fermentation and growth of undesirable bacteria is restricted.
A wide range of chemicals have been used as silage additives. Their key properties vary considerably and factors such as cost, effectiveness, safety, volatility, corrosion of machinery and required application rate will affect the choice of additive.
Safety is a key consideration with the acids, as they are caustic to the skin and eyes. Formic acid is also volatile and, if inhaled, can damage the lungs and nasal passages. Always wear protective clothing when handling these acids and use a breathing mask when handling organic acids, such as formic, acetic and propionic acid.
Corrosion of silage-making equipment is another problem with the acid additives. The salts of the organic acids are much safer to handle and less corrosive. However, they need to be applied at higher rates to be effective.
The most commonly used and widely tested acid additive is formic acid (85% w/w solution). The application rate varies from 2 to 6 L/t fresh crop, depending on the crop’s Water Soluble Carbohydrate and DM content. The higher rates are used for low DM content legumes. At lower rates of application, a lactic acid fermentation develops after the initial fall in pH. Higher application rates result in a greater initial drop in pH and a more restricted lactic acid fermentation.
As is the case with most silage additives, the best results are obtained with forages that would produce a poor fermentation in the absence of additives. Some of the effects of formic acid addition are illustrated in Table 7.15. In this study with lucerne, increasing the rate of formic acid restricted the fermentation, as indicated by the increase in WSC content and decline in acid production.
Compared to the control, the additive favoured a lactic acid fermentation. In addition, formic acid reduced protein degradation in the silage, as indicated by the higher proportion of protein N and lower proportion of ammonia-N.
Formic acid treatment can significantly improve animal production from silage, particularly where the control silage produced without additive is poorly preserved.
Where silages are well fermented there is unlikely to be a response to formic acid. This is demonstrated in a study that summarised the results from a number of experiments with growing cattle