by Arable Farming October 2020 issue
Marginal areas of land including wildflower strips and field edges could be used as viable AD feedstock, benefiting native plant populations and freeing up land that could otherwise be used to produce food. Alice Dyer reports.
On the 1,200-hectare Scrivelsby Estate, Lincolnshire, grass has always been viewed as valuable anaerobic digester (AD) feedstock.
An early cut of whole fields is collected using a forage harvester prior to sheep grazing the grass for the remainder of the year. The silage then sits under plastic for a year before entering the AD plant.
Grower, Henry Dymoke, who took part in a study alongside Lincolnshire Wildlife Trust to harvest roadside verges in the county for AD, says the inclusion of grass in the digester makes sense financially and environmentally.
Using meadow silage has improved gas quality and reduced the amount of CO2 produced during the methane production process, says Mr Dymoke.
We have found the best [AD plant] production and efficiency is with grass of any kind, whether it be a grown crop or a meadow crop, and the maize crop together – they really complement each other.”
Alongside meadow silage, the estate’s AD plant uses a mixture of waste and grown crops including maize, triticale, chicken litter, veg waste from some of Boston’s freezing plants and sugar beet pulp from the Newark factory. The digester has hydrolysis tanks, meaning it can run at three different temperatures.
Mr Dymoke says: “Some sites just run one temperature and they will struggle a bit more to use different feedstocks, whereas because we run three different temperatures, are three different groups of bacteria which means the digester will break down a much wider variety of organic material. As long as it’s organic, one of the tanks will be able to break it down.
“Grass generally for most AD plants is good but some cannot cope with too much because it gets too thick in the tanks.”
from whole fields, harvesting much more marginal land for AD could be a worthwhile option for farmers looking for alternative feedstock, while helping to restore plant diversity and boost beneficial insect numbers.
This is according to Mark Schofield, conservation officer at Wildlife Trust, who says: “Insufficient cutting, cutting too frequently or cutting and leaving a mulch is leading to the loss of grassland wildlife on marginal land as coarser grass species, taller herbs and scrub take over.
“Managing permanent grassland margins or meadows with a diversity of native, locally occurring and locally sourced perennial vegetation, including native legumes, will also increase soil carbon and provide a continuity of pollen and nectar supply and larval food plants through the year for a broad variety of invertebrate life cycles. If managed on rotation with sanctuary strips, it will also provide vital structural refuge for a wide range of wildlife.”
With careful management, harvesting marginal land such as wildflower strips could benefit farmers by utilising scrubland and creating a more circular use of energy on-farm as well as reducing fossil carbon, says Mr Schofield.
“It also has the potential to be construed as a public good and it could be an income stream if Government funded it in the Environmental Land Management scheme.
“The basic principle is not to cut everything everywhere at once but to allow some cover to persist.
“Restricting cutting times to May and August would allow perennial wildflowers to regrow and re-flower and for invertebrates to recolonise. Although it does disrupt wildlife temporarily it sustains it in the long term.”
Cutting should also be limited to twice annually to ensure a biodiversity net gain, Mr Schofield adds. “The caveat is although this opens the door to economically viable, large-scale conservation through wildlife friendly habitat management, it is the lid to Pandora’s box if we are not careful. It could encourage people to maximise the productivity from what is currently semi-natural vegetation.
“Otherwise, this is a great way to deliver habitat management for biodiversity net gain and to deliver that in an economically sustainable way.”