Farm Ireland
Independent.ie

Thursday 30 March 2017

Slurry biodigestion produces a win-win situation for farmers and is a process worth pursuing

John Shirley

Pig slurry is applied to winter barley at the Teagasc Oakpark demonstration, using a boom-type dribble bar fed from a tanker on the
headlands through an umbilical pipe
Pig slurry is applied to winter barley at the Teagasc Oakpark demonstration, using a boom-type dribble bar fed from a tanker on the headlands through an umbilical pipe

Is slurry an asset or a liability? Do you have a farm fertiliser plan that can be viewed in the event of a Single Farm Payment (SFP) inspection? Depending on whether you are in the glass-half-empty, or, the glass-half-full category, slurry/dung/animal waste can be viewed as a smelly pollutant or as a rich plant nutrient and energy source.

Lately there's much talk of slurry biodigesters. On RTE Duncan Stewart reckoned that we could light up Ireland with the energy released from our pig and cattle slurry if only we had biodigestion up and working. We badly lag the rest of Europe in this activity.

The other half of slurry's story is the fertiliser content. At a Teagasc farm demo last week in Oakpark, Carlow farmers were told that the recent jump in bag fertiliser price could be offset for cereal growers by a switch to slurry, especially pig slurry.

Such a move has also been facilitated by a change in the Nitrates regulations. This change allows cereal growers to now continuously apply organic fertiliser on the same field subject to an overall nitrogen limit.

The Oakpark event was about bringing precision to slurry use. Precision in terms of knowing the nutrient content of the slurry you are handling. Precision in terms of uniform application of slurry to your crops. And precision in terms of working to an overall fertiliser plan for your farm.

Such an overall farm fertiliser plan is part of cross compliance for the single farm payment but the Oakpark speakers accepted that, to date, few farmers have drawn up such a plan. However, they urged farmers to start now and draw up a farm fertiliser schedule for the coming season.

This should start with soil testing and an inventory of all fertiliser purchased on the farm and a record of fertilisers applied. This is more red tape but efficient use of fertiliser will also help achieve bottom-line profit.

One of the issues with slurry is the variability. Slurry fertiliser potential can vary according to the diet and species of animal providing the dung, the level of water dilution, the method and timing of application.


Most of the variation arises in the nitrogen capture. P and K uptakes are fairly constant. Grassland can tolerate some inaccuracy of application but wrong amounts on cereals can lead to lodging.

In cereals, slurry gives most benefit when applied in spring. The nitrogen component is wasted when applied in the autumn on winter sown crops. Often on farms the slurry is applied before the ground is ploughed for maize. But whether for maize or cereals, the strong Oakpark advice is that the slurry should be ploughed in as fast as possible -- they talked of ploughing within two hours of spreading. Otherwise most of the nitrogen is lost to the air.

Spring application of slurry onto growing winter barley was also demonstrated.

This job was done via a massive 12m boom spreader dribble bar which was connected by an umbilical pipe to a tanker in the corner of the field.

This unit was applying 3,000gal/ac of pig slurry. Given a continuous supply of slurry the unit could apply up to 35,000 gallons per hour. On cereals it is vital that slurry is uniformly applied.

A similar boom type dribble bar fitted to an Abbey 3,100ga slurry tanker was present at Oakpark but was shown working. Speakers stressed the importance of large low pressure tyres on slurry equipment, especially so when driving heavy loads over the growing crop.

At the Oakpark event guideline value figures for slurry based on today's fertiliser prices were:

  • 1,000ga of pig slurry, 4pc dry matter, equated one bag of 19-7-20 or about €28.
  • 1,000ga of cattle slurry, 5pc dry matter, equated to one bag of 5-4-28 or about €21.

But these figures can be doubled, or even halved, depending mainly on the dry matter content of the slurry.

Teagasc specialist Mark Plunkett showed how a hydrometer (developed by his Johnstown Castle colleague Hugh Tunney) could be used to give a read on the dry matter of freshly agitated pig and cattle slurry.

Adding to the general confusion over slurry is that the descriptive language can be in gallons per acre, cubic metres per hectare, or even tonnes. One ratio given to us at the demonstration was cubic metres per hectare x 89 = gallons per acre.

Farmers believe that best grass response comes when they apply cattle slurry in spring. This has been confirmed by a Teagasc time and method of application trial in Johnstown Castle. This compared April versus June, and, splashplate versus trailing shoe, applications. There was no difference in P and K uptake. Nitrogen recovery varied widely.

Looking at the slurry in terms of the value of 1,000ga in replacing a bag of N-P-K fertiliser the results were:

  • June application by splashplate = 3.2 -- 5.4 -- 39
  • June x trailing shoe = 6.5 -- 5.4 -- 39
  • April x splashplate = 6.5 -- 5.4 -- 39.
  • April x trailing show = 9.7 -- 5.4 -- 39.

As well as trapping more of the nitrogen the trailing shoe application should reduce the bad odours and allow slurry into longer grass.

At Oakpark they demonstrated splashplate, trailing shoe, dribble bar, soil injecting, plus a Bergmann unit spreading mushroom compost on a 24m band width.

Potential savings from slurry quickly dissipate in transport and spreading costs.

Getting back to biodigestion; this process removes the energy and greenhouse gases, takes away much of the odour but leaves behind the fertiliser value. A win-win situation that should be worth chasing.

Indo Farming