With anaerobic digestor plants appearing across the UK, there is more digestate available to farmers. But what is its potential for grassland farmers?
WITH an increasing amount of digestate available, farmers may think it looks like an economically efficient way to supply their land with some of the nutrients it needs.
However, independent grassland expert Charlie Morgan said while digestate was a good option in many cases, it was important to consider some key issues before taking the product.
Speaking at a Farming Connectorganised event in North Wales, Mr Morgan said: “Before you even think about applying digestate, you need to complete a soil nutrient plan on the land you intend to spread on. This will tell you the status of the fields and what the field wants in terms of nutrients.
“One of the most important things we need to be thinking about in the future is our soils. We have to improve soil health and make the soil work better.”
Once grassland farmers know what their nutrient status is, Mr Morgan said it was then important to get an analysis of the digestate, so calculations could be made to determine what would be the right amount of digestate for the land.
“There are huge variabilities in digestate, so it is important an analysis is done.
“Standards suggest digestate should supply no more than 50-60 per cent of the total nitrogen (N) requirement of the crop.
“The release of nutrients from the product can be variable so it is important to use manufactured fertiliser to supply the rest of the crop’s requirements.”
To establish the value of using digestate on grassland, Farming Connect has undertaken a trial at Llyn Rhys farm, Llandegla, Wrexham, which is farmed by Pearce Hughes and family.
Mr Hughes explained: “The trial compared five plots, with three plots applied with differing rates of digestate – 20cu.m/ha, 15cu.m/ha, and 10cu.m/ha – along with a plot which was given 125kg/ha of fertiliser [25:5:5] and a control plot which received nothing.”
The field was soil tested before the digestate was applied and then again after silage was cut.
The plots were measured for grass growth throughout June and July, and then the permanent pasture field was cut for big bale silage on July 13. Grass was analysed via a wet chemical analysis.
Mr Hughes said: “We took 140 bales off the 11 acres, which is the most bales we have had from this field, although it has been a good year for grass.”
In terms of grass growth, the plot with the largest digestate application performed the best after an initial dip while the other two digestate plots and the plot with the fertiliser applied grew at similar rates.
Mr Morgan said: “It has, however, been an exceptional year for grass growth and even the control plot did well this year.
“It is not just about yield though, and you need the quality to be there in the grass too.”
From the wet chemical analysis the metabolisable energy was slightly higher for all of the digestate plots and Mr Morgan said this was due to the shortfall in phosphate (P) and potash (K) requirement being more severe on the control plots rather than the N content of the digestate.
Digestibility was acceptable across all the plots at between 65-75 D-value but the control plot showed the lowest level and poorest quality.
Mr Morgan said: “In my opinion the best application rate in this instance would have been 10cu.m/ ha, bearing in mind that the balance of nutrients needs to then come from a manufactured fertiliser.”
Mr Morgan added it was also useful to look at costings. He said, as an example, he calculated it might cost about £90/ha (£36.43. acre) for the digesate, which includes the cost of the digestate and the spreading cost. And in comparison fertiliser would cost about £79.98/ha (£32.39/acre).
“The digestate, in this example, works out more expensive, but you are getting extra yield. So if you are producing 250kg DM/ha more, then for a 400kg steer eating 8kg a day, this works out as an extra 31 days of feed produced on that hectare.
“The costs do stack up, but it is a fine line and it is important to gauge the value of the digestate, and whether it is worth it for you.”
Mr Morgan added that the digesate was also valuable in terms of the organic matter it could bring to soils, which was superior to slurry although not as good as farmyard manure, and also suggested there was a positive message in terms of reducing carbon dioxide.
Mr Morgan said: “Using digestate can help reduce a farm’s carbon footprint by replacing the need to apply manufactured fertilisers. Replacing manufactured fertiliser with food-based digestate could reduce a farm’s carbon footprint by about 20kg CO2 equivalent/tonne of digestate applied.”