How herbal leys can benefit the whole system

From herbal leys to hemp crops, the Centre for High Carbon Capture Cropping is a four-year £5.9m multi partner project, led by Niab, which is exploring the carbon capture potential of input-efficient crops that can be widely grown across the UK.

See also: How an award-winning arable farm combines business and nature

And while many growers may want to wait until the carbon market is better regulated before considering these crops for their carbon-capture potential, they also offer a vast number of other agronomic benefits to both arable and mixed farmers, says Niab project lead, and head of science at FarmED, Dr Lydia Smith.

The research explores the use of rotational cover crops; fibre crops such as industrial hemp and flax; perennial food, feed and forage cropping including cereals and herbal leys; and perennial biomass crops like miscanthus, willow and poplar.

Lydia says: “We wanted to find a suite of exemplar crops and understand what they are doing, how they are capturing carbon and by how much.

“For farmers this could enable offsetting of emissions and identify new revenue sources in the carbon market, while supporting enhanced value chains for industries such as textiles and construction using bio-renewables.”

Herbal ley mixes

At the Cotswold-based trials site, FarmED, which is a regenerative farming demonstration farm near Chipping Norton, four different herbal ley mixes have been established to help get a better understanding of the contributions different components bring to a complex ley.

Lydia says: “As well as carbon capture, we want to know what these herbal ley mixes contribute to sustainable soil structure and health, animal health, and how they score as a resource for ecosystem services such as providing resources for pollinators.”

To maximise their benefits in an arable scenario, Niab’s research has shown that herbal leys should be left for at least two years.

Lydia says: “You get all the beneficial things you might expect from a cover crop, but you get much more and for much longer after a herbal ley.”

Work carried out by Niab and the Allerton Project found nitrogen rates in wheat crops following a herbal leys could also be significantly reduced.

Lydia says: “In addition to straight carbon capture, which is a bit of a blunt instrument, it’s the presence of legumes in the environment which gives so many other benefits. As those big chunky roots degrade they release nutrients but they do so quite slowly, so it remains in the root zone for your following crop.

“In the first year, farmers might want to put a little bit of nitrogen on, but in the second year it really speeds up and they can reduce the nitrogen requirement of your following crop.

“We cut our nitrogen requirements by half on the Niab site and all of our herbal ley treatments were yielding better than the control at full nitrogen by year two.”

They noticed some nutrient lock up in year one, where the treatments yielded just below the arable control, which was at full N-rate.

“Unfortunately, the project didn’t roll into the next year, but for the third year and possibly the fourth year there may well continue to be some legacy effect, as we have noticed on fields with a ley in the rotation.”

The team also noted that nutrient release was slower when the following crop was direct-drilled versus ploughed, following termination of the ley.

“It gave a real hit of nutrients with the plough, but then it started to reduce. Following the direct drill in the second year it was still giving dividends.

However, even if farmers ploughed, those beefy chunks of root from the ley continue to give structure to the soil and help ensure that they retain the connectivity of the pore space.

A soil structure that’s been significantly improved after a ley means that areas of compaction and plough pan are removed, so plants can access a much greater volume of soil and effectively have access to more nutrients.

“At our Niab trial site in Suffolk, which is very heavy clay, we saw a swathe of blackgrass going through the plots, but if you hold your nerve, once the dense very competitive biomass comes through it really knocks back your grassweed problems.”

For best establishment, herbal leys should be planted in late spring to early summer, or very late summer to early autumn, says Lydia.

“Soil needs to be minimum of 10C so legumes are happy, but not so late you’re competing with weeds. The establishment few months are critical – the last thing you want is difficult weeds like solanum, fat hen or dock jumping in there from the soil seed bank.”

About the plots

The trials plots are now well established, having been planted in April 2023, and mob grazed with livestock which are moved daily to prevent overgrazing and retain biodiversity.

Despite the Cotswold brash soils receiving very little of its average rainfall this season, most of the plots have fared surprisingly well.

Plot 1

The 17-way mix © Alice Dyer

The first trials plot is a complex herbal ley which includes 17 different species and with several varieties within some of those species.

Niab’s Lydia Smith says: “We want to see what a really complex ‘Rolls-Royce of mixes’ looks like against some alternative mixes that might be less complex.

“For SFI [Sustainable Farming Incentive] the key thing about a herbal ley is it needs a minimum of two legumes, two herbs and one grass species, but you might be quite ill-advised to have as few as that.”

By using such a variety of species, she is trying to maximise the chance that as many as possible of those three groups are going to survive that establishment period. Not all species will survive in any given place in any given year, she warns.

“On our Cambridge site, I grew this mix for five years and each season it never looked the same. Therefore, if you’ve only got the minimum requirement [for SFI], you might lose one and not get that complex multi-dimensional root structure that enables you to achieve additional biomass.”

The mix of flowering species including yarrow, birds foot trefoil, chicory, white and red clover, sainfoin and sweet clover means the ley also acts as an important nectar resource for pollinators throughout the summer, Lydia adds.

The inclusion of species like plantain also makes it incredibly drought tolerant thanks to its waxy leaf surface. “You can guarantee when grass is long dead, plantain will keep growing.”

The mix also offers a probiotic element to livestock.

Sainfoin, birdsfoot trefoil and salad burnet have complex tannins in their leaves, she explains.

“Sainfoin is queen of probiotics, acting as a natural anthelmintic. Unlike harsh synthetic medicines, which purges them of all living worms, the animal retains its natural resistance to worms because they remain in the gut, but at significantly lower numbers.”

The general microbiology of the rumen that is stimulated by their presence means that the amount of methane produced is also much reduced, she adds.

Plot 2

The eight-way mix versus the 17-way mix © Alice Dyer

The second plot is an eight-species mix with a higher percentage of grass, and some species removed that were less likely to establish or remain in the sward.

This makes it a little bit cheaper, and when it comes to going back into a cereal, it means potentially farmers may have a slightly easier outcome when terminating the ley.

“If you can get away with slightly lower seed costs with a similar benefit it would be great. However, from results so far, we can see that the carbon capture in the 17-way herbal ley is higher and the soil structure better.”

She explains that they wanted to try this for those who aren’t livestock farmers and want to focus on something that makes a major difference to soil structure or carbon capture, rather than for feed.

That farmer might either sell or mulch it, or even use in an anaerobic digestion plant.

“In that scenario, if you can get away with slightly simpler and cheaper mixture then why not? It’s not a massive amount of difference [in carbon capture], but overall, we are seeing a better outcome in the more diverse mix.”

The third mix, known as the lamb suckler mix crucially does not include any grass seed, and is aimed towards root crop growers.

“This is for those who are putting a ley in to improve soil structure, function and mineralisation, but who are concerned about their following crop. However, because it lacks grass, under SFI it would not class as a herbal ley.”

To qualify, she suggests adding 0.5% of something like festuca rubra which is a fine, very short non-competitive that’s not going to compete strongly and is less alarming with respect to providing resources for soil pests like wireworm.

The seed mix is the cheapest within the trial and this year appears the greenest looking plot of the four.

Lydia says: “The chicory in the mix is extremely deep rooting so is still pulling up water and is happy, and the same can be said for the legumes.

“We have left this for two years, which is perhaps longer than many farmers would leave it. It has very good biomass above and below ground.

“The first year wasn’t great because it was a little sparce with clear gaps between plants, but it was certainly good enough to do what a cover crop does, preventing erosion and keeping soil structure together.”

Plot 3

The final plot in the herbal ley trials was a two-way mix of grass and clover, but biomass remained poor.

Lydia says: “We have had four difficult years on the trot, so we have to admit this is more normal weather now and we need to find more resilient farming systems.”

Fibre crops

Hemp © Alice Dyer

Another crop with very good carbon-capture potential being grown as part of the project at FarmED is industrial hemp.

The crop has very low input requirements, with no current need for fungicides or insecticides, and no herbicides other than a clean seed-bed at establishment, says Lydia Smith.

“It’s not super hungry so requires less than half the N you would need for wheat. But the downside is you do need a Home Office licence.”

She says it has lots of really fibrous roots that just keep on tunnelling, making it very drought tolerant.

It’s also considered to have better carbon capture potential than a forest, because farmers have this fantastic harvest of 30t/ha in just 100 days.

Flax is another crop being grown as part of the project, which is “easier in many ways but harder in others”, says Lydia.

“If you are growing flax you can drill earlier before it can get weedy, but it doesn’t grow so tall or dense so it’s not so weed tolerant.

“Nothing will grow below the hemp because it’s so dense. A good fibre variety will reach 4m in height in the best conditions.”