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 digesters.

According to Dr David Lawson, a grassland specialist with Scotland’s Rural College, speaking at a trade open day near Aberdeen, herbage grass is as productive as most other crops used for AD.

For instance, 1-tonne of grass DM was capable of producing 270m3 of methane, he told visitors.

Dr Lawson explained the Recommended List system for new grasses had 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 Dr Lawson, there are new sustainability criteria for AD which those planning grass-based systems must take into account.

“One of these concerns the need to minimise the production of green-house gasses from any crop cultivation, including grass herbage,” he told the meeting.

“So it's important to reduce the use of nitrogen. Some, but not all, of the N needed in the process is supplied by digestate left after grass has been used to make gas."

The meeting was also an opportunity to discuss the progress of new grass varieties under trial before going on to the National List.

SRUC contributes to a UK-wide, four-year programme and this system evaluates new varieties before they're fully approved.