Could lucerne provide the benefits of a living mulch system without some of the associated yield challenges? A Danish grower presented his innovative strategy at the recent BASE-UK conference, where CPM joined delegates.
“The idea is the living mulch remains in place for four seasons.” – FREDERIK LARSEN
By Mike Abram
Living mulches are an idea which may sound great in theory, but a concept some say are difficult to achieve in practice.
In the UK, growers have experimented with under-sowing cash crops with white clover as a multi-year living mulch to provide weed suppression and supply nitrogen. But yield penalties – around 30% on average according to Innovative Farmers field lab trials – plus problems with establishment or too vigorous growth, have so far limited adoption to a handful of mostly organic farmers.
Despite these challenges, Danish grower and agronomist, Frederik Larsen, wasn’t put off from developing an approach using lucerne as a living mulch on his 250ha arable farm in the Funen region of Denmark.
He suggests successfully utilising living mulches comes from understanding the three key concepts of intercropping: competition, complementarity, and compensation (see panel).
And it was considering these concepts which led him to choose lucerne rather than white clover for his living mulch. “White clover has very dense surface rooting so a lot of roots in the top layer of the soil, much like a cereal crop. That means there’s a large area of competition which reduces the main crop’s growth.
“Whereas with lucerne, we don’t have the same amount of root density in the topsoil layer because it grows more with a tap root which leaves room for the main cereal crop roots to explore the top surface. Hence, they complement each other.”
Below ground competition is an even bigger driver of yield loss than above-ground competition for light interception, he adds, arguing that using white clover for a living mulch because it doesn’t compete so much with the crop above ground is misguided.
Growing a legume, such as lucerne, and a cereal is also complementary, he adds. This is because during late summer as the cash crop matures and subsequently after harvest, lucerne captures untapped photosynthetic potential compared with a conventional system, making more efficient use of available resources.
Then in winter months, lucerne is mainly dormant in contrast to winter cereals or oilseed rape which continue to grow especially in early spring. That allows the crop to develop a canopy before the legume resumes growth in April.
But managing that mid-spring lucerne growth is crucial to success when growing winter cereals in the system, he highlights. “We have to manage lucerne’s biomass so it doesn’t become excessive in the critical yield-forming months of our winter cereal crop, but not kill the lucerne so we see regrowth during the late summer months.”
He’s now in year three of implementing his living mulch system commercially on a 25ha field on the farm, having started trials in 2018, with two other fields in year two and one, respectively. “The idea is the living mulch remains in place for four seasons,” he says.
UNDER-SOWING OSR
Year one starts with under-sowing a break crop with lucerne to get it established. His preference is winter OSR but he’s also trialled spring beans as an alternative. He uses a Cross Slot to drill the crops separately with the lucerne planted at an angle, but he says those with dual hopper drills could do it in one pass.
Frederik stresses that key to success is choosing a field free from thistles. “If you kill thistles, you’ll kill your living mulch.” A site also has to be reasonably free from grassweeds, although starting with OSR or even beans can provide options such as propyzamide (in OSR) for grassweed control, as it won’t kill the lucerne.
Post-emergence broadleaf weed control is limited, although Clearfield herbicide-tolerant OSR could be an option as lucerne tolerates some ALS chemistry. However, because that system isn’t approved in Denmark, pre-emergence herbicides mixed with glyphosate are important, highlights Frederik.
In OSR, managing the lucerne biomass in the spring isn’t necessary with the crop already 1.5m tall by the time the lucerne starts to grow. “But as the OSR opens up just before harvest it’s a race against time before the lucerne pulls the crop down.”
In the two years of commercial practice, Frederik’s OSR has yielded 3.8t/ha and 4.2t/ha. “It’s satisfactory – could be higher – but I don’t think we get a yield penalty from the lucerne because of the time complementarity of the two crops.”
Immediately following harvest, the lucerne is cut to no lower than 7-10cm height with a disc mower. “It’s important to not mow too low because you require new shoots from the crown – all of the growth points are above ground.”
One advantage of lucerne is it uses the energy stored in its tap root for regrowth, giving it a two-week head start compared with a freshly-sown cover crop, for example. “It also has high heat tolerance – around 32- 34°C – which makes it the perfect summer crop to grow vigorously.”
After around six weeks of growth, the lucerne is mowed again for forage, being careful to swath in one go to avoid mixing in OSR straw that’ll decrease digestibility for livestock. “We harvested around 8t/ha of lucerne silage from the field, corresponding to 3t/ha of dry matter and 90 units/ ha of nitrogen around a month earlier than if growing a cover crop.”
In Frederik’s system he’s sold the lucerne to both dairy and beef producers. “The deal with the dairy farm was they paid the cost of harvest and returned the nutrients as slurry in March.”
It’s now that the true living mulch management starts, he says. “Can we get a high winter cereal yield while keeping our productive lucerne at the bottom so it’s ready to grow after winter cereal harvest? That’s the key objective.”
Management starts with a low dose of glyphosate before any regrowth after mowing to remove grassweeds. Lucerne typically has a much higher tolerance of glyphosate compared with grassweeds so a low dose shouldn’t kill it, he points out.
Winter wheat is drilled in late September, with Frederik recommending 20-25% higher seed rates to compensate for lower tillering of the wheat when grown with the living mulch. Using pre-emergence herbicides such as prosulfocarb, diflufenican, aclonifen or pendimethalin, helps with broadleaf weed and grassweed control, he says, but he has no experience of using flufenacet or cinmethylin, which aren’t approved in Denmark.
“In the spring, you require a relatively high dose of early nitrogen to encourage growth and tillering of the wheat ahead of the lucerne regrowth – it’s very similar to grass clover forage crop production where early nitrogen encourages ryegrass yield at the expense of companion clovers for the first cut,” explains Frederik.
But the critical management step is around GS30-32 of the wheat as this is when the lucerne starts to regrow strongly. “We use what the Americans call chemical mowing – a sufficient dose of a cheap sulfonylurea herbicide or fluroxypyr to temporarily stunt the living mulch so it doesn’t rob too much yield.”
He says that’s enough to keep it from competing with the wheat during the critical month-long period around flowering that sets yield. Then depending on the year, a second chemical mowing might be required mixed with a T3 fungicide.
Frederik suggests an alternative to chemical mowing could be to use a row mower, but a £5-10/ha herbicide application is currently much better value for money.
Nitrogen applications for the wheat in his first year of commercial growing were fairly standard at 200kgN/ha, with Frederik suggesting the majority of the nutrient uptake by the wheat would be much earlier than anything fixed by the legume living mulch.
Foliar nitrogen applications might be useful, he suggests, allowing nutrition to be better targeted at the cash crop than benefiting the living mulch. “It means you might alter the competition between the two in favour of the cereal crop.”
Strong lucerne regrowth starts when the winter wheat begins to senesce, with growth by harvest filling empty spots in the crop and competing with weeds. Frederik admits this also makes combining a challenge. “It was a pretty fun day with the combine – we tweaked the settings to give plenty of air and wide concave settings. Afterwards there was some cleaning of the sheaves and drum.”
He believes a stripper header would be better, but cut as high as he could manage, Frederik’s wheat crop achieved 9t/ha at 16.5% moisture and decent uncleaned sample.
HARVEST OPTIONS
Another option for harvesting could be swathing and leaving for 3-5 days to ripen and dry green material and reduce the moisture content risk, he adds.
After harvest, the lucerne is mowed again to 10cm height to restart growth. “We didn’t bale it, but I think it could provide good bedding material. It would also remove the cereal straw to allow faster regrowth of the lucerne.”
As with the previous year, a cut of forage can then be taken after regrowth before replanting a new cash crop. In year two, dry matter production was slightly lower on Frederik’s farm – around 2t/ha equating to 65kgN/ha removed.
Now into the third year of commercial production on the first field, the crop for Harvest 2025 season is winter oats. Frederik expects management of the crop to be broadly similar, with glyphosate pre-planting or in with pre-emergence herbicides, early doses of nitrogen and chemical mowing.
“From a small trial undertaken in 2024, the oats are taller and stronger growing, so we shouldn’t require as much chemical weed mowing. In that trial the winter oats – a very uncommon crop for Denmark – yielded 9.5t/ha. If we can achieve even 8.5t/ ha then it would be very nice.”
It hasn’t all been plain sailing – establishment of the lucerne in the second field was poor after an ‘extreme’ slug year. “After OSR harvest it was very variable, so we didn’t have sufficient for a forage harvest. That was a failure.”
That’s led to a change for the third field with the living mulch now 75% lucerne, 15% red clover and 10% white clover. “The idea is that the white clover won’t be too competitive but will fill empty spots if slugs or other predators open up holes.”
The cost of establishing and managing the lucerne living mulch through three seasons has totalled around €300/ ha, with half of that being the initial seed. Frederik estimates the value of the fixed nitrogen in 2023 and 2024 at around €155/ha, while the value of the lucerne forage to a dairy farmer could be €805/ha in terms of feed units.
“There’s big potential on a mixed farm to capture more value where there’s livestock integration,” he suggests. That compares to around €125/ha/ year for establishing a cover crop.
“So over two years it more or less breaks even with the alternative, and if we can get three seasons it’ll start to be a positive effect compared with cover crops,” he concludes.
This article was taken from the latest issue of CPM. Read the article in full here.
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