With pressure on input costs, soil health and environmental performance, anaerobic digestion operators are increasingly looking at how alternative crops could help build a more resilient feedstock supply.
Maize has long been a cornerstone of UK anaerobic digestion (AD), delivering high energy yields and reliable performance. However, its cultivation comes with challenges. Early establishment can leave soils exposed, while late harvesting often results in bare, trafficked ground heading into winter, increasing the risks of erosion, runoff and nutrient loss.
With greater scrutiny of soil management, alongside rising input costs and increasingly unpredictable weather, there is growing interest in how a wider mix of crops could complement maize rather than replace it altogether.
Wholecrop cereals such as rye, triticale and wheat are established alternatives, offering earlier harvesting opportunities and flexibility within cropping rotations. Their establishment costs can be lower than maize, although yields and energy output may be more variable, particularly where harvest timing is compromised.
Grass-based systems also play an important role, especially in wetter and cooler regions. Multi-cut ryegrass leys can provide a steady feedstock supply across the growing season, helping spread the harvest workload while supporting soil structure, organic matter and ground cover.
However, grass generally delivers lower methane yields per tonne than maize, and requires careful management of fertiliser inputs and cutting schedules.
Multi-species and herbal leys offer further opportunities, particularly where soil health and biodiversity are priorities. Deep-rooting species can improve soil resilience and water management, while legumes contribute to nitrogen fixation. The trade-off is potentially greater variation in feedstock composition, which can introduce challenges for digester management.
For AD operators, the question is increasingly not which single crop performs best, but how different crops can work together within a more balanced feedstock strategy.
One crop attracting increasing attention is Silphium perfoliatum, commonly known as cup plant.
Unlike annual crops, Silphium is a perennial broadleaf species that can remain productive for 10–15 years once established. This fundamentally changes the economics and practical management of the crop.
Its extensive root system and year-round ground cover help protect soil from erosion, improve water infiltration and reduce the need for repeated cultivation. These characteristics could make it particularly valuable on sloping, erosion-prone or more marginal land.
Once established, input requirements are generally lower than maize. Digestate can supply a significant proportion of its ongoing nutrient requirements, helping recycle nutrients within the AD system and potentially reducing dependence on purchased fertilisers.
Silphium may also offer opportunities for mixed farming enterprises, where it can be considered as a forage crop alongside its use in anaerobic digestion.
From a methane production perspective, maize remains difficult to match.
Under favourable growing conditions, maize can typically deliver around 6,000–8,000 m³ of methane per hectare, depending on crop yield and dry matter content.
Reported Silphium yields are commonly in the region of 4,000–6,000 m³ CH₄/ha once fully established, although performance varies considerably with site conditions, crop maturity and management.
While these figures generally favour maize, methane yield per hectare is only one part of the economic picture.
Silphium's longer productive lifespan means establishment costs can be spread across many years. Reduced cultivation, potentially lower fertiliser requirements and improved soil protection may also contribute to the overall value of the crop.
Establishment is an important consideration, however. Silphium generally requires careful management in its first year, and full productivity may not be achieved until subsequent growing seasons.
The comparison therefore needs to consider the whole cropping system and the longer-term cost of production, rather than headline methane yields alone.
Changing the feedstock mix also creates opportunities and challenges within the digester itself.
More fibrous crops and variations in feedstock composition can affect the rate and efficiency of digestion. Understanding those differences is important when introducing alternative materials into an existing AD system.
Targeted trace element supplementation can help support the microbial populations responsible for anaerobic digestion, while appropriate enzymes or biological additives may assist the breakdown of more fibrous material.
However, these approaches need to be considered alongside the characteristics of the feedstock and the operating conditions of the plant.
This is an important part of Realistic Agri's approach to biogas performance: looking beyond the individual feedstock to understand how crop selection, nutrient balance and digester management work together to support stable, efficient gas production.
Maize is likely to remain an important component of UK anaerobic digestion, but it need not be the only answer.
Combining maize with wholecrop cereals, grass leys and potentially longer-term perennial crops such as Silphium could help operators spread risk, improve soil management and reduce reliance on a single growing or harvesting window.
The right approach will depend on local conditions, available land, establishment costs and the requirements of the AD plant.
Ultimately, the opportunity lies in optimising the whole system, from field to digester, rather than simply maximising the yield of any one crop.
Further reading
Realistic Agri's Sean Quinn also contributed to a recent feature in CPM Magazine, which explores the potential of Silphium for UK anaerobic digestion, including practical growing considerations, methane production and experiences from Germany.
Read the full CPM Magazine feature – Cup plant takes AD feedstock beyond maize