A much criticised feature of the Targeted Agricultural Modernisation Schemes (TAMS 3) is the likely exclusion of dribble bar slurry applicators, although trailing shoe type booms and vacuum tankers to which they are fitted will probably still be eligible.
To date, the Department of Agriculture, Food and Marine (DAFM) has not officially announced the decision, let alone give the reasoning behind it, despite the keen interest of the industry in an explanation.
Ammonia loss is major concern
There are two trains of thought which might have led to the lack of enthusiasm for the dribble bar and they both centre around the loss of ammonia from the slurry during the application process.
Ammonia (NH3) exists as a gas and is readily soluble in water. It is formed by the breakdown of nitrogenous compounds in the slurry such as urea and proteins.
However, it also has a tendency to evaporate from water and because of its pungent smell, it gives slurry its distinctive odour which is considered obnoxious by many.
Ammonia is also claimed to be a greenhouse gas (GHG) although its role in the atmosphere is complex and not fully understood, its half life is considered a rather unprecise few days.
The main accusation is that it forms smog creating particulates in combination with acidic compounds already present in the atmosphere.
It will also degrade to form nitrous oxide compounds which are considered to have a more direct effect, but the atmospheric concentration of these is just 324 parts per billion.
Plant nutrient
Ammonia as a pollutant is one major reason given for curtailing its escape into the atmosphere, the other is that it is a waste of a valuable plant nutrient, and this argument is more persuasive.
The role of nitrogen in plant growth is far better understood and appreciated, and there is little dissent from the view that it should be preserved and restored to the ground as a valuable asset.
It constitutes 60% of the total nitrogen in cattle slurry and up to 75% in pig slurry, the higher the concentration, the more that is likely to be lost to the atmosphere.
Returning it to the soil was, for a long time, a matter of squirting it over the fields with a vacuum tanker and splash plate, yet this would waste a good deal of the nitrogen as the greater the surface area exposed to the air, the greater the loss.
Various techniques were introduced in a bid to reduce this evaporation and these were collectively given the acronym LESS, which stands for Low Emission Slurry Spreading.
This was further divided into ground level application and injection, the former being the dribble bar and the trailing shoe and it is here where the contention lies.
Awaiting confirmation
As yet, it has not been officially confirmed that either will be eligible for the On-Farm Capital Investment Scheme (OFCIS) which is set to replace TAMS, and neither are mentioned in the list issued by DAFM on February 8 for TAMS 3.
The idea that the trailing shoe will be favoured over the the dribble bar is an understanding passed on to stakeholders by sources within the department, sources that may usually be taken as reliable, but the decision is not yet set in stone.
Assuming this turns out to be the case, it might be asked on what basis the decision was made, for the dribble bar is less expensive, is lighter, is more suited to retrofitting, is less prone to blocking, has a lower draught requirement, and can cope better with uneven ground.
All these factors make it more suitable for the smaller farmer who may not have the capital to step up to a trailing shoe model and all that it entails, which could well extend to a new tanker and a larger tractor to pull it.
A UK hand-me-down
No doubt the department will point to research that shows a significant difference in ammonia loss between the two types, indeed, Teagasc publications have quoted the following: “The trailing hose will deliver up to 30% reduction and the trailing shoe 60% reduction in ammonia losses.”
This may sound convincing but it fails to explain what they are being compared to, and what the average is. Simply quoting a major deviation from the mean can be highly misleading.
Where then is the raw data from which these figures are drawn? It appears that they have been borrowed from a UK government publication entitled – Code of Good Agricultural Practice (COGAP) for reducing ammonia emissions.
This, at least, has the grace to explain that given percentages are in comparison to a splash plate, and it gives a range for the figures rather than just a single number, although still no source for the data.
What becomes immediately clear is that the case for trailing shoes is nowhere near as clear cut as the Teagasc document would have us believe; the UK figures are 30-35% reduction in loss for dribble bar and 30-60% for trailing shoe.
However, even these figures are flawed, for it assumes that all the ammonia is lost with a splash plate, which is not the case at all. If 20% of the ammonia is still retained, the differences between the other methods becomes less significant.
The dribble bar in academia
It is all something of a statistical muddle, although the general trend is for trailing shoe to deliver more ammonia to the ground that a dribble bar, the problem arises in trying to quantify the difference.
If the DAFM points to research of unknown origin to try and prove a point, advocates of the dribble bar may turn to a paper by a Swiss research group (Hani et al, 2016) that found “applications with trailing hose and trailing shoe systems yielded an average reduction of 51% and 53%, respectively, relative to the reference technique”. A mere 2% difference.
Reviewing the academic literature, it becomes apparent that there are many other factors involved in determining the degree of ammonia loss – dry matter, pH, air temperature and soil conditions being four important drivers.
These may well make a far greater difference to nitrogen loss during slurry application than the method used to apply it.