Thinking of growing grass for AD? Here’s what you need to know

Farmers involved in Northern Ireland’s booming anaerobic digestion sector could improve performance by concentrating on raising their grass yields and quality, according to industry experts.

High sugar and high digestibility grasses make the best fuel for anaerobic digestion plants, farmers at a recent AD conference were told.

The conference – organised by agricultural merchant Morton’s – offered the opportunity for attendees to hear about the latest technical and policy developments and to share experiences on how to maximise revenues through raising operational performance.

Increasing yields

Opening the event, John Burgess from seed supplier KWS explained that AD operators could improve performance by moving to hybrid rye varieties carrying more grains per ear.

Burgess advised growers to focus on increasing grains per ear, rather than plant height to increase wholecrop yields.

He said: “There is a balance to be had between plant height and grains per ear when seeking to maximise wholecrop yields.

“But with the ear contributing roughly 50% of the final yield variety choice and diligent use of plant growth regulators are essential.

Variety suitability

Jonathan Dunn from Morton’s explained the impact of different grass varieties and their suitability AD.

He pointed out that dry matter yield and quality were paramount to the production process.

“Yield is probably the most important factor,” he said. “Hybrid ryegrass is approximately 20% higher yielding than perennial ryegrass but has a life span of only four years compared to the longer term perennial ryegrass.

“Equally, in perennial there is a difference in yield of varieties. AberGain tetraploid perennial is 18% higher yielding in a two-cut system when compared to the lowest yielding variety on the Recommended List.

Biogas production has a strong correlation to digestibility so varieties with good digestible yield should feature in the decision making process.

“Sugar is a fast source of energy for bacteria; Aber High Sugar Grass varieties have been bred to have elevated sugar levels and should in turn produce higher biogas yields.”

Fermentation

But as well as the variety of grass grown, the method used to preserve it can also have a big impact on its productivity.

John Bax from Biotal added that for an AD plant, silage must be stable across the year and of high quality.

“It’s crucial to drop pH of ensiled crop as quickly as possible and exclude oxygen,” he said.

“Otherwise, there is a greater risk of mycotoxins, spoilage – which can be as high as 15% – and reduced gas yield.”