Cooking with Grass – Energy Production Message from SRUC

With despite uncertainty over future levels of subsidy for energy production there is still considerable interest in using grass as a feedstock for on-farm anaerobic digestors (AD).

Speaking at a seed trade open day near Aberdeen David Lawson, Grassland Specialist with Scotland’s Rural College, stressed that herbage grass is as productive as most other crops used for AD, with one tonne of grass dry matter capable of producing 270 cubic metres of methane gas.

The annual Grass seed Trade open day is an opportunity for commercial companies to discuss the progress of new grass varieties under trial before recommendation to the National List. The species routinely tested are:  Perennial, Italian and Hybrid Ryegrass, Timothy and Festulolium, along with white and red clovers.

SRUC contributes to a UK wide, four year programme in conjunction with SASA, the Scottish Government’s division offering Science & Advice for Scottish Agriculture. They evaluate new varieties under both conservation management and simulated grazing. However accepted varieties must then complete three further years of trials at SRUC sites in Ayrshire, Edinburgh and the NE before being approved as fit to thrive in Scottish conditions.

Addressing the meeting David Lawson explained the system has led to a year on year improvement in grassland productivity.

“For instance, the average dry matter output of the Scottish Recommended varieties of Perennial ryegrass has increased by 9% from 2010 to 2015. That’s a 9% increase achieved with no increase in inputs.”

He explained that for anaerobic digestion systems Italian ryegrass is the species most commonly used, because of its high dry matter productivity, although high yielding perennial ryegrasses or Timothy can also be used.

However, according to David Lawson, there are new sustainability criteria for AD which those planning grass based systems must take into account.

“One of them concerns the need to minimise the production of green-house gasses from any crop cultivation, including grass herbage,” he said. “So it is important to reduce the use of nitrogen fertiliser. Some, but not all, of the nitrogen needed in the process is supplied by the digestate material left after grass has been used to make gas. But to reduce the need for fertiliser nitrogen still further it is becoming very important for farmers to establish clover in their grass pastures and supply nitrogen naturally. The varieties of clover that will thrive in Scottish conditions can also be found in the Scottish Recommended List.