Energy crops for anaerobic digestion

With good rotations, energy crops have several significant benefits to agriculture. Some energy crops such as maize are excellent break crops in the arable rotation and thus are popular with farmers. Varying the cycle of crops grown on agricultural land can significantly improve soil's physical characteristics and can help to ensure biodiversity and robust and resilient food crops. In addition, energy crops have respective supplementary benefits, including limiting the leaching of nitrates into waterways, reducing runoff into waterways (CLICK HERE to read more about leachate and runoff), controlling crop disease by breaking the life cycle of infection, and long-term financial viability and energy independence for many farmers when used in the Anaerobic Digestion process.

In the UK, feeding an Anaerobic Farm Digestion plant is not quite as simple as just collecting a crop and storing it to feed the AD plant over the coming year. Growing, harvesting and ensiling AD energy crops has a precise science that relies on technique and due care and is fundamental to the Anaerobic Digestion process to produce maximum efficiency. The correct dry matter and chop length are essential. In addition, a crop mix that allows biological flexibility protects against a single crop failure, and crop rotation is desirable.

We take three likely energy crops individually and explain their benefits and hazards:

Maize

Maize, the "go-to" AD crop, is sown in the spring, occasionally allowing another crop such as forage rye to be grown between crops in the same cropping year. Maize develops quickly and can reach 16' tall in a suitable environment. However, whilst an excellent crop for AD, its most significant hazard is the later harvest, potentially during a wet autumn. If you harvest maize too early to get it in the dry weather, the energy is depleted – harvest maize too late, and it will have lost its greenness and tonnage potential. In addition, wet weather can make getting this bulky crop off the field complex, with the risk of significant soil damage, mud on the road, and unreachable crop areas. However, ensiling is easy, and maize will store well for many years.

Whole crop wheat/barley/rye/oats

Whole crops are grown like a conventional arable crop, but instead of being harvested by a combine, they are harvested earlier by a forage harvester as an entire crop. Relatively easy to grow and a good performer in an AD plant, the most significant risk is the very tight harvest window – sometimes allowing only three days to get the crop at optimum moisture conditions. Too wet and the potential is lost; too dry and it will not ensile for storage. As per maize, ensiling is easy, and deterioration is slow.

Grass

Grass is probably the most challenging crop to digest but the easiest to grow. Whilst it can sometimes be cut three times a year, it is quite intensive in machinery cost per hectare as it needs mowing and rowing up pre the forage harvester. It is harvested throughout the summer and is an excellent recipient of digestate after each harvest. However, in a digester, the thin tubes of grass stalk are challenging to digest and readily form layers in tanks, requiring very good stirring at all times. Of the three crops discussed, the biogas yield for grass is the lowest. Grass doesn't store well over a year, so it needs to be used promptly.

Growing energy crops for Anaerobic Digestion purposes can provide many benefits. Using the arable rotation systems allows food crop growth alongside energy crops as break crops. This process alleviates pollution, controls crop disease, pests and weeds and improves soil quality whilst providing food.

If crops are managed well, harvested at the correct time, stored appropriately, and used for Anaerobic Digestion, this will provide energy and financial sustainability.