The by-product of anaerobic digestion, which is called digestate (also known as biofertiliser), is a good source of nitrogen (N) to apply to grassland. 80% of the total N in food-based digestate is readily available, compared with only 10-25% in farmyard manure.
Digestate comes in three forms – whole (looks similar to slurry), liquid (solids removed) or fibre (similar to compost), but the dominant form for agriculture is whole. The nutrient content of the product will vary between farms, so it is essential to obtain a recent nutrient analysis before deciding how to use it.
Due to its high ammonium content, there is great potential for N losses to the environment when spreading digestate, which reduces its nutritional value.
Follow good practice guidelines to minimise nutrient losses, such as applying in the spring and, where possible, using low emission spreading equipment such as injectors or a trailing hose.
There is little published research into the pathogen risk of using products derived from animal by-products, however a minimum of three weeks between spreading and grazing or harvesting is recommended to negate any risks. In addition, this type of digestate will have been pasteurised.
Digestate must be noted down in Nitrate Vulnerable Zone (NVZ) records and, where derived from livestock manures or slurries, must be accounted for in the farm average livestock N loading (170 kg/ha/year).
The Digestate and Compost in Agriculture (DC-Agri) project, funded by Defra and recycling organisation WRAP, has completed a series of scientific field experiments to determine crop available N supply, environmental impacts and the effects on soil and crop quality of applying digestate. More information can be found on the WRAP website.
Experiments on a grassland site in Wales showed that food-based digestate increased yields by 5% compared with manufactured NPK applications, as a result of the additional potash supplied by the digestate.
Even though recommendations from Defra‘s Fertiliser Manual (RB209) were followed, potash was still limiting grass growth in the manufactured fertiliser plots. So, providing the nutrients of the digestate are known, it can achieve similar results to manufactured fertiliser when applied to grassland.
Where to buy
The Biofertiliser Certification Scheme (BCS) provides assurance to consumers, farmers, food producers and retailers that biofertilisers are safe and of good quality. A map and contact details of biofertiliser producers are available online – a good place to start when looking for a digestate supplier.