Anhydrous ammonia and urea are used to improve aerobic stability and increase the nitrogen content of silages made from low protein forage. They are more often used with maize silage, but are also used with sorghum and whole crop cereal silages, and high moisture grain.
Thorough mixing is necessary to avoid variable silage quality and minimise the risk of stock poisoning. Urea is the preferred additive if the main goal is to raise the nitrogen content, as recovery of applied nitrogen is higher (see Table 7.20) and it has had a more consistent beneficial effect on animal production than ammonia.
However, rather than applying urea at the time of ensiling, it can just as easily be added at feed out, which may be more practical in some situations. In experiments where direct comparisons of the two times of application have been made, no difference in animal production has been observed.
When adding urea at the time of feeding, good mixing is important to ensure that all animals receive adequate, but not surplus, urea (and so avoid the risk of urea toxicity). Anhydrous ammonia is usually more effective than urea for control of aerobic spoilage.
However, there are safety issues to consider. Anhydrous ammonia is hazardous if it is inhaled or comes into contact with the eyes or skin. Both additives prolong the fermentation, because of their buffering effect, resulting in greater total acid production. However, in-silo losses are often increased, resulting in lower DM recovery.
The buffering effect of these additives can be a problem when ensiling forages with a low Water Soluble Carbohydrate (WSC) content and/or a high buffering capacity. Their use on such forages is not recommended.
The reduced DM recovery and inconsistent animal production responses are likely to limit the widespread adoption of these NPN additives, unless there are major problems with aerobic spoilage.
Site of application for aerobic spoilage inhibitors
Applying the additives at the time of ensiling is the best strategy for reducing aerobic spoilage losses, and inhibiting the growth of lactate fermenting yeasts, moulds and acetic acid bacteria. Maximum protection is achieved by treating all the forage being ensiled.
Depending on the type of silo and filling procedure, additive application may be restricted to the top layer, 0.5-1.0 m. This reduces the risk of aerobic spoilage of the upper, poorly compacted part of the silo, while the lower portion is protected by the better compaction at depth.
However, surface application of additives prior to sealing is not effective for silages prone to aerobic spoilage. It may reduce mould growth and spoilage on the surface, but will not protect silage immediately below the surface.
Spraying an additive on the silage face will not reduce aerobic spoilage. Air infiltration past this layer will result in heating of silage as far as 0.5-1.0 m behind the face of unstable silages.
Additives can be used to prevent subsequent heating of silage or total mixed rations in the feed bunk or on the feed pad. Although there has been some interest in additive application at the time of feeding, the efficacy of this strategy will depend on when the spoilage problem occurs.
If silage is heating in the bunker, significant losses of DM and quality have already occurred, and application of silage additives at feeding will have little benefit, other than to perhaps prevent further heating in the feed bunk.
Some silages that are stable in the bunker will heat soon after they are removed and exposed to air. This exposure occurs during the mixing and feedout process. Incorporating an additive at the time of feeding can reduce aerobic spoilage. This strategy can successfully reduce heating of the silage and total mixed ration in the feed bunk.