The latest slurry application equipment and GPS control systems are finding a niche for spreading digestate from the growing number of anaerobic digesters, reports Jane Carley.
The NFU estimates there will be 1,000 anaerobic digesters (AD) plants on farms by 2020, and the developing industry also offers other opportunities for farmers and contractors, both as suppliers of feedstocks and users of the digestate by-product.
Digestate offers valuable nutrients and can be used to cut fertiliser costs and improve soil condition. However, it requires accurate application to get the best out of it, and may require investment in more sophisticated equipment than has traditionally been used to apply farm slurries.
Tramspread managing director Terry Baker has seen increasing demand for equipment to apply digestate from AD to farmland, either as part of a contracting operation, or where farmers wish to make use of the nutrients on their land.
The company manufactures umbilical and tanker slurry applicators, hose reels and pump units, as well as being distributors and importers of the leading brands. Equipment is tried and tested by the company’s contracting operation, Tramspread Contracting, which applies thousands of tonnes of slurry and digestate each year throughout East Anglia and beyond.
Mr Baker says: “The nutrient value in digestate is higher than typical farm livestock slurries, therefore inaccuracy when applying will soon show in the crop.
“Most farmers and contractors are using 12-metre to 36m dribble bar applicators, with macerator distributors on both slurry tankers and umbilical systems.
“To achieve the desired amount of nitrogen, phosphate and potassium applied per hectare the digestate should be analysed and this is achieved by sending a sample to a laboratory or using an on-site slurry test kit. Most modern dribble bars applicators are fitted with a flow meter. Tramspread offers IsoBus slurry monitoring and other manufacturers such as Vogelsang also combine this with boom section shut-off.”
The latest technology can help achieve even application and monitoring of digestate and livestock slurries.
For optimum accuracy Tramspread offers a GPS control terminal from RDS Technology for the applicator, which works with the flow meter to give the required application rate in cubic metres per hectare. Autosteer refines the process further, giving accurate bout widths and application mapping, and some dribble bar applicators can be specified with GPS controlled boom sectional shut-off.
An alternative to a separate control terminal, on suitably-equipped tractors, is an IsoBus slurry monitor, which can be used with the flow meter supplying flow data via the applicator ECU to the in-cab monitor screen.
Mr Baker explains: “The IsoBus terminal is set up with the applicator width and the display will then show field area, quantity applied to the field and total area.”
He adds: “John Deere is currently working on a slurry monitoring system which reads the slurry nutrient while in flow (see AF Sept/Oct 2014)) and feeds this information via IsoBus to the company’s 2630 GreenStar monitor.
This allows the operator to see in an instant how much N, P and K is being applied per hectare, with the data displayed on a field map. The operator can also programme the system to apply a set number of kilograms of nitrogen per hectare and the system will control the tractor forward speed to achieve this.”
Estate manager Edward Vipond says: “We apply 45-50,000cu.m per year to grass and cereal crops including wheat, barley, oilseed rape and linseed, working within Nitrate Vulnerable Zone regulations.”
Soils in each field are tested for their nutrient status and digestate is analysed at the lagoon and as it comes out of the dribble bar. The product from the Northwick Park AD provides 3.2kg of nitrogen/cu.m, along with 1.4kg/cu.m of phosphorous and 1.8kg/cu.m of potash, plus magnesium and sulphur, with N availability of 70%.
Applications of 50-60cu.m/ha are made to wheat and 40-50cu.m/ha to barley and oilseed rape.
The oolitic limestone soils in the Cotswolds give the advantage of a long application season – spreading starts in late-January and goes into April, conditions permitting.
Northwick Estate is also in a Level 2 Source Protection Zone, so a pollution management plan and site-specific risk assessment is an essential part of the strategy, although Mr Vipond points out this would be the same for any fertiliser.
Applications to 500ha of wheat, 350ha of oilseed rape and 225ha of winter barley are made on 30m tramlines using a 15m dribble bar supplied by Tramspread and connected to one or two pumps depending on the terrain. Digestate is pumped from a 30,000cu.m lagoon sited in an old quarry up to 7.5km across the estate and a neighbouring farm using a series of above and below ground pipework. Outputs are 15-28ha/day.
Tanker and nurse tank systems were considered, but Mr Vipond suggests an umbilical system offers twice the output, as well as avoiding the need for tanker movements on the estate roads.
“We have 2km of pipework below ground and 5.5km above so it’s not that visible, which is important to the estate. Plus we can pump 1,000cu.m per day.
“We apply in two splits on wheat and winter and spring barley – there is a small amount of crop damage on the wheelings of the application tractor, but it soon recovers. Only one application is possible on oilseed rape because it reaches stem extension so quickly and there is a risk of damage after this. It’s also important to keep an eye on soil conditions as the Cotswold brash can smear.
“We achieve 9.5t/ha yields and only need to use 40kg/ha of artificial N per year, plus the digestate adds organic matter, and often much-needed water. There is visible difference in crops which have had it applied.”
Applications are made at 100cu.m/hour, so any variations in pump pressure need to be quickly spotted – data from the two pumps is displayed in the tractor cab so the operator can remedy any problems.
“We use an assigned telemetry network to avoid interference from any other radio traffic,” says Mr Vipond. “Each field is also individually mapped to give the correct set up for spreading according to the pipe layout, and the application is controlled by RTK using autosteer.
“We have GPS monitoring and recording from the flow meter via the RDS control unit supplied with the applicator which was funded by a Farming and Forestry Incentive Scheme grant. It was supplied with an extended antenna due to the topography of the estate – we can’t risk the signal dropping out and ending up with a pool of digestate.”
Applications are recorded on a field-by-field basis and uploaded to Gatekeeper; Mr Vipond has explored variable rate and believes this may be an option in the future.
“Using digestate is now fundamental to our cropping. Developing the plant was a significant investment, and upkeep of the application equipment means the investment is ongoing, but we are seeing a continual reduction in our fertiliser costs.”