Silage fermentation may be adversely affected if the ensiled grass has received large amounts of nitrogen as either fertiliser or slurry or if the grass has been unable to utilise all the nitrogen that was applied.
A general rule of thumb for calculating nitrogen fertiliser application rates is to allow 2 units of N per acre per day of growth (2.5kgN/ha/day), including any applied as FYM or slurry. Grass takes up N faster than it can use it for growth so nitrate may accumulate if this is exceeded or if conditions are such that growth is restricted, eg low light intensity, cool temperature. It is also a problem if there is a dry spell after application, when nitrate cannot be taken up by the roots, followed by a period of wet weather that results in luxury uptake. The plant cannot convert it to protein quickly enough so it accumulates in the plant.
High nitrates in grass influence fermentation
High nitrates in the grass ensiled can influence fermentation in several ways:
- It causes a reduction in crop sugars as they are used to provide energy for the increased rate of plant growth and for the manufacture of plant proteins.
- The increased growth rate leads to a lower crop DM.
- In the silo most nitrate is converted to ammonia. Ammonia production causes an increase in pH slowing down acidification.
- Nitrate itself, as well as many of its breakdown products, cause an increase in the buffering capacity of the crop which also counteracts the rapid acidification necessary for a good fermentation.
- When undesirable bacteria, eg enterobacteria from slurry and clostridia from soil, grow in the presence of excess nitrate they produce more acetic acid. This can change a good lactate fermentation to a less desirable acetate fermentation and also increases the buffering capacity.
It is the relationship between sugar content and free nitrate which is important
However, whilst the quality of silage is dependent on the levels of both grass sugars and free nitrate, there is not really a defined relationship between the two and care is needed interpreting analyses. There is not a defined relationship between grass sugars and nitrate. In fact, any more than really high nitrate concentrations can be synonymous with grass that will be difficult to preserve, there is German data showing that negligible nitrate concentrations can also be problematic for preservation (low/moderate concentrations of nitrate can be inhibitory to Clostridia)
In most cases once grass sugars are >3% of the juice (30 g/L) then preservation should be OK assuming anaerobic conditions are quickly achieved and thereafter maintained and that there is no contamination with soil, manure/faeces, etc. In some cases if buffering capacity is very high (e.g. autumn grass) this 3% threshold may need to be increased to 3.5 or even 4%. Similarly, if buffering capacity is very low (e.g. very stemmy grass) then 2.5 or even 2.0% sugar may be OK. Buffering capacity measures the resistance to a drop in pH.
Crops with high buffering capacity may have a poor initial fermentation. In the absence of knowing the buffering capacity it is usually OK to use the 3% threshold. As mentioned, main times for concern for this 3% value is if there is grass that has a very high buffering capacity due to high white clover, very high or late application of N fertiliser, or lush leafy grass. If sending a grass sample for sugar or nitrate analysis, it is extremely important to cool the sample quickly. If not the grass sample will remain respiring at ambient temperatures using up sugars.
Top tips for making late season silage
The silage expert Dr Dave Davies, of Silage Solutions has this advice.
Adjust cutting height depending on ground conditions
Dead material at the bottom of the sward, maybe as a consequence of the drought, can lead to higher risk of poor fermentation or aerobic spoilage (heating) at feed-out. Be careful when raking or picking up new leys to avoid soil contamination.
A rapid wilt, ideally to 30% DM
But do not wilt longer than 36 hours. Preferably 24hours. The longer the wilt the more sugar is respired and lost in the field, leading to an increased risk of secondary clostridial fermentation.
Consider using a chemical additive
If conditions are against you (crop, soil or weather), then they’ll inhibit microbial activity and you’ll more likely achieve a good preservation process.
Because of the drought it’s likely there is more soil nitrogen available than in a typical year, which could lead to higher levels of residual nitrate in the crop. However, nitrates are of secondary importance to sugar levels. Grass will ensile correctly with up to 800ppm nitrate provided sugars are adequate (>3%).
Instances of high nitrate levels in the crop, coupled with low sugars can result in poor fermentation and on rare occasions the production of nitrogen dioxide, a reddish-brown heavy gas, in the first few days of storage. This can pose a serious health risk to people and livestock, and is known as ‘silo gas’ poisoning.
Given the above issues it’s more important than ever to follow good silage making practices this autumn.