Trial results: Spreading digestate versus compound fertiliser

Grassland farmers could make substantial cost savings by growing grass with anaerobic digestate instead of compound fertiliser – and potentially gain more grazing days if modern methods of application are used, a study has found.

The research, led by Farming Connect and hosted by one of its Demonstration Farms, evaluated what benefits there might be in using digestate for grass production instead of bagged fertiliser.

The study demonstrated substantial cost savings and, if the digestate was injected close to the plant roots, it could grow more grass than compound fertiliser.

See also: Trials show better yields with one spring fertiliser application

When the nutrient levels in the digestate were analysed, the equivalent values as a compound fertiliser, based on this year’s prices, would cost farmers £112/ha (£45.40/acre), excluding the spreading cost, which works out at £15.20/ha (£6-£8/acre).

In comparison, the digestate applied in this trial and supplied by the 4R-Group, who worked closely with Farming Connect on this project, cost £2/t inclusive of spreading.

When applied at 22.5 t/ha (9.11t/acre), as was the case in this study, the cost to the farmer worked out at £45/ha (£18/acre).

Digestate more economical than compound fertiliser

Independent grassland and forage specialist Charlie Morgan, who monitored the study and analysed the findings, says the figures in this case show digestate is more economical than compound fertiliser.

And there were secondary benefits too, he says.

“Digestate also includes organic matter which is not present in synthetic fertilisers.

“The higher the organic matter in soil the more resilient it is to compaction and results in better rooting structure, healthier worm activity and improved water-holding capacity and nutrient uptake.”

The trial was carried out at Newton Farm, a Farming Connect Demonstration Farm near Brecon, run by sheep and beef farmers, Richard and Helen Roderick.

The Rodericks pay close attention to grassland management and soil performance on their 263ha holding, with a focus on maximising use of forage on a low-cost system.

They first used digestate at the end of August 2015 and were impressed with the resulting grass growth.

“Using the digestate instead of fertiliser should be lower cost as it is a by-product of an AD plant that generates energy from waste,” says Mr Roderick. “This should make it more sustainable than oil-based fertilisers.”

Newton Farm has grown from 90ha in 1989 to 260ha, with 75 suckler cows, over 1,000 ewes and 260 ewe lambs, plus 20ha of low-input cereals.

The use of alternative forage crops and alternative sources of nutrient inputs are priorities.

Making good forage is one of the most important things livestock producers can do, suggests Mr Roderick.

“The only thing we have control of is our costs, so we are going to have to do a lot more with less.”

Application methods and grass growth

How the digestate is applied influences how it performs.

The study at Newton Farm examined three application methods – disc injection, trailing shoe and splash plate – and compared all three with the performance of a plot sown with fertiliser and a control plot with no inputs.

Total grass growth measured over a six-week period demonstrated that disc injection resulted in 60 additional grazing days/ha – based on a suckler cow eating 10kgDM/day – than the plot where splash plate application was used.

This plot grew 4,295kg/DM/ha compared with 3,692kg/DM/ha in the plot where the digestate was applied with a splash plate (see graph below). 

The study was based on a 5.3ha (13-acre field) that had an eight-year-old grass ley which had mostly been grazed by cattle and sheep.

Application

An analysis of the food-based digestate showed that, with an NPK of 6:1.2:2.3kg/t, the advisory nitrogen limit of 250kg/ha (200 units/acre) would be reached at 41t/ha (16.5t/acre) therefore it was applied at 22t/ha (9.1t/acre) to achieve a nitrogen level of 137kg/ha (110 units/acre).

The digestate and fertiliser was applied on 26 July with grass growth measured weekly for six weeks, using a rising plate meter.

“It was clear the splash plate plot lacked growth compared with the other plots and the fertiliser plot overtook this early on.

“This supports the science, in terms of atmospheric nitrogen losses in warm dry conditions,” Mr Morgan says.

The level of digestate applied needs to be carefully considered, he adds.

“The level of inputs was higher than the normal recommendation for summer application because a focus of the trial was to compare it with the synthetic fertiliser equivalent.”

Mr Morgan stresses that farmers need to be aware of the nutrient content of digestate which varies according to its source.  

“Digestate in this trial was significantly more powerful than cattle slurry for available nitrogen, but application rates were agreed to highlight the synthetic fertiliser equivalent to stress the point and raise awareness.

“It is very important to know the soil analysis as N:P:K is applied using digestate and there may not be any requirement for the additional P and K.

Phosphate pollution is an increasing issue and these products need to be used responsibly. If in doubt use a Facts [Fertiliser Advisers Certification and Training Scheme] qualified adviser.”

Guidance from environmental bodies such as Natural Resources Wales advises farmers to use digestate sustainably by only applying what the crop needs and to avoid overuse to prevent soil indexes becoming too high.

Menna Williams, Farming Connect’s South Wales red meat technical officer, who supervised the study, says with a focus on increasing yields to feed a rapidly expanding population, soils would be under ever-increasing pressure to perform consistently.

With the number of anaerobic digestion plants increasing, farmers in areas where there is a high concentration of these can buy digestate at competitive prices, she adds.

“Fertiliser replacement products such as digestate may provide a suitable alternative to compound fertiliser.

“This alternative may also help to address commercial and environmental requirements by offering low environmental impact, with effective and sustainable solutions by nutrient cycling.”

Miss Williams says the use of digestate as a biofertiliser is known to benefit both farmers and the environment by naturally recycling nutrients, including scarce nutrients such as phosphorous.