Extended dry spells, combined with sudden alternating wet and dry weather in the UK, can significantly alter the nutritional profile, physical structure, and fermentation behaviour of silage crops. Because water stress changes how plants allocate energy, drought impacts non-cereal crops (such as grass) differently than cereal crops (such as maize and whole-crop wheat or barley). While dry UK weather reduces overall fresh weight tonnage per hectare, it also fundamentally changes how these crops behave inside a biogas plant.
Non-Cereal Crops (Grass)
Dry weather can alter the growth rate and maturity timing of grass crops:
- Premature Heading: Drought triggers early flowering as a survival mechanism. If cutting is delayed waiting for bulk, the stem-to-leaf ratio rises rapidly, driving down digestibility.
- High Dry Matter Risks: Warm, dry ground can speed up field wilting. Grass can easily exceed 35–40% DM, making it stiff and difficult to consolidate in the clamp. This traps oxygen, which in turn can increase the risk of heating (aerobic spoilage) and mould growth at feed-out. Drought-stressed grass cut at high DM can in theory yield more biogas per wet tonne simply because there is less water. However, because of early stem heading and higher stem to leaf ratio, both biogas volumes and methane yield per tonne of volatile solids can drop due to the higher indigestible fibre fraction, e.g. ligneous or lignin-similar structures.
Cereal Crops (Maize, Whole-Crop Wheat, Barley, Triticale)
Drought can impact cereal crops primarily by disrupting grain set and potentially alter the stem chemistry:
- Fiber Lignification: Stalks become woodier and more fibrous. This can potentially reduce the availability of organic dry matter to the digester biology. Drought increases plant lignin content. Hydrolysis—the step where microbes break down complex polymers (predominantly the available organic dry matter) —becomes the main bottleneck. Lignin cannot be broken down anaerobically and shields celluloses and hemi-celluloses, reducing the overall conversion efficiency within standard Hydraulic Retention Times.
Rheology, Mixing, and Crust Formation
Dry, high-lignin stems do not absorb water or break down easily in the initial hydrolysis phase:
- Viscosity: High-DM feedstock increases digestate viscosity, requiring higher parasitic load (electricity) for mechanical agitators.
- Crusting: Stiff, dry grass and whole-crop fibres tend to float and trap gas bubbles, forming thick scum blankets or crusts that can blind gas off-takes or bind up mixers.
The solution
The use of enzymatic blends, especially feedstock-specific ones, like Viscomex, Axiase and Strawmex, can address the main challenges in digesting higher dry matter feedstocks. These blends have been engineered to address these difficulties in challenging feedstocks and aid in increasing the conversion of organic dry matter to volatile fatty acids, the intermediate materials for biogas and methane production.
The protein types contained in these products will catalyse the process of hydrolysis, hence accelerating the conversation rate of the available organics in the feedstock. Efficient hydrolysis will release water from the feedstocks and hence help reduce viscosity and the electrical load required to run mechanical moving parts in the process, e.g. pumps and agitators. Furthermore, the efficient breakdown of feedstocks in biogas plants ensures the minimisation or lack of crusting and increase the release of biogas from the digestate.