Significant expansion of the UK biogas sector is creating exciting opportunities for farmers supplying energy crops and with more plants in the pipeline demand looks set to grow further.
There are currently 175 anaerobic digestion plants operating solely within the UK agriculture sector, most of which use some proportion of crop-based feedstock, such as maize silage, energy beet and rye.
Figures from industry body the Anaerobic Digestion and Bioresources Association (ADBA) show, of this total, 122 use farm wastes (mainly manures) and crops; 31 are solely crop-based and the remaining 22 use only farm waste.
This has already created strong long-term demand for energy crops (many plants should last at least 20 years), but with a further 246 agricultural plants granted planning permission, there is potential for significant future expansion of UK biogas.
ADBA estimates total cropbased feedstock demand could reach three million tonnes by 2020, requiring about 0.3% of agricultural land – current demand requires 0.2% – although growth will depend on policy support and how many projects get built.
Security of feedstock supply is one of the biggest concerns for any AD plant operator and while many developers source crops ‘in-house’, others will be keen to secure some or all supplies on long-term (often 10 or 20-year) supply contracts, so creating more opportunities for nearby growers.
While the type of anaerobic digester technology and/or feedstock contract is likely to determine the type of crop, the increasing range of purposebred energy varieties on the market offers growers significant choice within each sector.
New varieties are much better at combining optimum energy yields with the agronomic traits needed for crops to successfully fit into any rotation.
Energy maize (see opposite) remains the cornerstone feedstock for many AD plants, but demand for hybrid rye is increasing fast in the UK, especially among growers looking to overcome the challenges of late maize harvesting on heavy land,
says Heather Ayre, energy crop manager at Elsoms.
She says: “Rye is an easy crop to grow and will perform well on any soil type providing it is established correctly and not sown deeper than 2cm.”
It is already widely grown for AD elsewhere in Europe (notably Germany and Holland) where it has proven effective at increasing biogas yield when used with other feedstocks, she says. The crop also has a notably shorter retention time than maize at about 20 days to breakdown in the digester compared with 80 days.
Harvesting rye below 40% dry matter is essential for crops going into AD.
But when it comes to maximising gas yields, freshweight yield remains key, she says. Modern ‘turbo hybrid’ varieties from European breeder Saaten Union have been developed without the pollen enhancing gene Iran 9, which has been linked to a yield drag.
Turbo hybrids instead have a small amount of a conventional variety (Dukato) added to the commercial hybrid seed to increase natural pollen production and produce more vigorous growth. This results in clean varieties which establish quickly and stand well, she says.
Yields vary depending on farm conditions, soil and climate, but typically range from 42t/ha freshweight (37% dry matter) around the south coast to nearer 53t/ha further north in eastern England and Lincolnshire, says Ms Ayre.
Jonathan Baxendale, Elsoms agricultural seed sales specialist says: “Rye is generally harvested as wholecrop at the milky ripe stage (about 37% DM) towards the end of June/early July, allowing a good entry for the following crop, or for black-grass control measures such as stale seedbeds.
“Rye is sometimes pedalled as a silver bullet for black-grass control. That’s not the case, but it is a valuable tool in the armoury.”
Harvesting rye below 40% dry matter is essential for crops going into AD, so growers may need to start cutting slightly earlier where large areas have to be covered.
Saaten Union is currently developing early maturing hybrid rye to give growers the option of extending harvest windows. “Trials in the Baltics have shown it to be around three days earlier than existing varieties, which doesn’t sound much, but can make a huge difference on-farm.”
Very high dry matter yields and stable methane content mean maize remains a firm favourite for cropbased AD, according to Matthias Schmauch of Saaten-Union.
Breeders have responded to this demand and used maize’s genetic variability by developing a range of varieties specifically for biogas production, or as ‘multipleuse’, which can either be forage/biogas or grain/biogas, he says.
“The major difference is the ear content, which gives double purpose hybrids a higher energy content but regularly less total dry matter yield. There are no significant differences in methane yield as this is produced by starch as well as leaves.”
Generally 32-36% DM content produces the best silage for biogas, and harvesting can be scheduled slightly later than forage maize due to a slightly different DM requirement, he notes.
Problems harvesting late in the season on heavy soils are a common criticism of maize, but new early-maturing hybrids suited to early sowing are quickly overcoming such issues, he says.
Saaten Union’s portfolio for example, now includes very early hybrids bred for UK conditions with FAO maturity ratings from 150, while other high dry matter yield varieties offer FAOs of 200-220.
Given this variability among hybrids, Mr Schmauch says small farm trials are an invaluable way of evaluating which perform best on individual farms.
“Farmers should react to adverse growing conditions such as cool, wet soils, by planting very early hybrids with a slightly reduced plant density,” he adds. “They should also look for a very early planting opportunity.
“Hybrid rye seems to be very complementary to maize as it can be grown as a second crop in the same crop year,” he says.