For best results, use grass

“There’s a reason why farms were always ‘mixed’,” says Mhairi Dawson, R&D manager for grass and soil sustainability experts Barenbrug UK. “There are many advantages in having a longer rotation where grass leys feature.

“We’re increasingly aware that farming practices affect climate change, carbon emissions and biodiversity,” Mhairi continues, “but we really need to focus on soil health. It’s there we can make a difference by including grass and forage within arable rotations.”

Mhairi points to three main groups of factors affecting soil health. First, chemical make-up and constituents: nitrogen, micro and macro nutrient content and pH value. Then physical characteristics – the soil type, its condition (degree of compaction and aeration) and its risk of erosion, waterlogging, etc. Finally, biological status: organic matter content, its ability to sequester carbon, and the ecosystem of invertebrates, bacteria, fungi, insects and mites that contribute to nutrient cycling.

“Remember the intrinsic link between these three groups. Think of them as the three legs of a milking stool: lose one, or even weaken it, and the whole structure collapses.

“Grass and forage serve to reinforce each of those three groups, creating a robust soil system and improving soil health,” says Mhairi.

It’s a sentiment that’s gaining traction throughout the industry, says Mhairi, citing April’s publication of a report from Natural England that suggests grasslands will play a major role in ensuring the UK – not just the agricultural industry – reaches its 2050 net zero carbon targets.

“Depending on location, reintroduction of grass can even stimulate new habitats, allowing absent animal or plant species to return, or increase,” she adds.

But what’s so special about a grass or forage crop that delivers these benefits? Mhairi says it’s all about depth.

“The deep-rooting species typically found in a good quality, well-composed herbal leys are the key to good soil structure,” she explains. “A healthy soil has good ‘aggregation’ and the soil pore space between these soil particles is important. Essential for air and water movement, it’s this space that influences nutrient availability.

“Both drought tolerance and water infiltration – valuable soil properties for other crops within the rotation – can be boosted with improved soil structure.”

Mhairi suggests Barenbrug’s modern, soft-leaved cocksfoot and tall fescue varieties, or red clovers and lucerne, as examples of deep-rooting species proven to improve and stabilise soil structure.

“Barenbrug’s breeding programme has long identified deep-rooting characteristics as a favourable trait,” Mhairi notes, “improving drought tolerance, and nutrient-use efficiency through mobilising minerals deep in the soil profile. The roots of Barenbrug’s tall-fescue varieties Bardoux and Barelite, for example, can reach a depth of more than 100cm, more than three times deeper than standard perennial ryegrass.”

Deeper roots also offer stable carbon sequestrations. Carbon sequestered in the soil’s top 20cm is much less stable than that found at greater depths.

In composing a suitable ley, Mhairi advises mixing deep rooters with other species that display diverse rooting habits – shallow roots, alongside those with wide, extensive root systems.

“A more extensive root system optimises nutrient and water utilisation, and soil stability, guaranteeing better ecosystem support,” she notes, “particularly as root exudates and dead root material will themselves feed soil microbes.”

“Healthy soil – plenty of air and space – promotes this higher microbial population, in turn increasing activity and speeding breakdown of organic matter like dung or dead vegetation. Nutrient cycling – getting them back to growing plants – is more efficient, so plants enjoy better health.”

“Healthier plants also deliver higher photosynthetic rates. More photosynthesis improves carbon sequestration and produces healthier, more efficient livestock as well as higher yields from the crop.”

Perhaps most attractive to the arable farmer is the ability of certain forage species – legumes – to fix atmospheric nitrogen in the soil. Crops not only feed themselves but leave residual soil nitrogen available to subsequent crops after incorporation, reducing requirements for ‘chemical’ nitrogen – one of agriculture’s biggest contributions to greenhouse gases (GHGs).

“A good red clover can fix nitrogen at up to 200kg/ha/annum in favourable soil and growing conditions, minimising the risks of leaching, run-off and volatilisation associated with applications of artificial nitrogen.”

For arable enterprises, the herbal ley is invaluable in soil conservation. “Used as an overwinter ground cover, leys can reduce or eliminate erosion from wind or water,” Mhairi points out.

“But it’s not just about keeping your soil in the field; it also prevents watercourse contamination, and an active crop can use and ‘reserve’ the residual soil nutrients so that they’re not lost to the following crop.”

In considering how to include a herbal ley or even simple grass leys in the rotation, Mhairi suggests initial consultation with an agronomist. “Those same three groups of factors influence the species selection that’s right for your farm. A mixture will almost always outperform the best monoculture and, if you’re considering using or offering the ley for grazing, it won’t have a deleterious effect on sward quality.”