Silage additives - what does the science say?

Published 24 April 15

The task of explaining this was given to Dr Pádraig O’Kiely of Teagasc, Ireland, at the AHDB Dairy and AHDB Beef and Lamb silage workshop held in Coventry this week. Soil conditions and management of the crop during growing, harvest, clamping and feed out must be considered before silage additives, according to Dr O’Kiely. Once these are optimised, the use of sugars, enzymes, acids, lactic acid bacteria (LAB) and anti-microbials may have a role to play.

Silage additives operate on two principles; either through reducing in-clamp losses or by improving the nutritive value of the forage. Sugars are added to the forage to provide more substrate for fermentation, aiding lactic acid production (Table 1). However, added sugar is only beneficial where the initial plant sugar content is low. As it takes 10kg of additional sugar (20kg molasses) per tonne to raise the total sugar content by 1%, this strategy is likely to be uneconomical for most livestock farmers, unless improvements in DMI are realised. Other strategies such as mowing the forage later in the day and allowing time for wilting may be more cost-effective.

Table 1. The effect of various silage additives on fermentation and aerobic stability relative to untreated silage, where untreated silage is assigned a value of zero (adopted from O’Kiely, 2015)

 Silage additives table

Fibrolytic enzymes work by reducing cellulose and hemicellulose into simple sugars that can be used for fermentation. However, these sugars are often released too slowly to ensure the rapid reduction in pH required. Dr O’Kiely’s research showed no effect of these enzymes on reducing ensilage losses but there were reductions in ammonia and butyric acid production suggesting the enzymes reduced protein breakdown in the clamp.

Formic acid causes a rapid decline in the pH to a level which promotes LAB (pH 4.5) and is most beneficial in poorer weather when the sugar content is low or where N content is high. Farmers using these products should always ensure that correct precautions are taken to reduce the risk associated with acids.

The use of LAB gave results in literature that largely depended on the amount of sugars available, according to Dr O’Kiely. For forages where the sugar content was marginal or high, there was likely to be a beneficial effect on fermentation losses and animal performance, respectively. However, for low sugar forages there was likely to be little benefit. Lastly, Dr O’Kiely reported on the use of antimicrobials, in this case sodium benzoate, sodium propionate and sodium nitrite. These have shown potential to reduce the extent of ensilage losses most likely due to the promotion of LAB (lactic acid increased by 105g/kg DM).

Where farmers are considering the use of these silage additives, it is important to know what the quality of the crop is at the time of ensilage. In particular, the sugar content must be known, as when there are insufficient sugars the efficacy of many of these products will be reduced. Any anticipated reduction in forage losses or improvements in DMI must be realised in improved animal performance on the farm for these strategies to be cost-effective. This will require some quick thinking on behalf of the farmer at the time of ensilage but the Silage Additives handbook produced by AHDB Dairy can help in this decision-making process.