Blue lupin control in the pipeline

Planted mainly to produce grain for sheep to forage, it was the first legume to tolerate these acidic soils and provided a much-needed nitrogen boost to the following wheat crops.

Its success prompted the development of the Australian Sweet Lupin, or narrow-leafed lupin, released by the Western Australian agriculture department in 1967. Since then, blue lupin has been generally considered a weed.

Although blue lupin (Lupinus cosentinii) is readily controlled in fallow and in cereal crops, it is like a wolf in sheep’s clothing in grain lupin crops. Being a close relative of the narrow-leafed lupin (Lupinus angustifolius), blue lupin has the same sensitivities to herbicides. Blue lupin has broader leaves and stands taller than narrow-leafed lupin, producing a distinct blue flower. In addition, blue lupin is highly competitive and very hard-seeded, with seeds remaining viable in the soil for at least 70 years.

WeedSmart western extension agronomist, Peter Newman, says that blue lupin is so competitive against narrow-leafed lupin crops that it only takes a few plants per square metre to warrant re-seeding the crop or risk considerable yield loss. In addition, contamination with blue lupin seed results in a considerable downgrade of the grain, as the bitter seed is impossible to grade out.

“The hard-seeded trait makes it near impossible to control blue lupins through seed bank management,” he says. “Growers can target blue lupin in cereal crops using mainly group 2, group 4 or group 18 herbicides to avoid adding to the seed bank, but there are no registered post-emergent control options for blue lupins in narrow-leafed lupin crops.”

Like all pulses, both blue lupin and narrow-leafed lupin are very sensitive to group 2 herbicides, which means growers must be aware of plant-back restrictions if these herbicides are used in other crops.

“Blue lupins mostly emerge with opening winter rains, so growers typically delay seeding the narrow-leafed lupin crop until after spraying out the blue lupin germination with a knockdown herbicide. This delay usually results in a crop yield penalty,” he says.

Peter says that the industry has been looking for a solution to this significant problem, and there have already been real wins in machine learning for detecting blue lupin in narrow-leafed crops.

“Realistically, though, growers need a herbicide solution,” he says. “After a GRDC-facilitated meeting with growers and agronomists from WA’s northern wheatbelt in 2021, it was determined that an imi-tolerant narrow-leafed lupin variety would be the best way to address the growing blue lupin weed concern. Since then, Australian Grain Technologies’ lupin breeding program has been working towards this goal.”

Leading this work is AGT’s lupin breeder, Matt Aubert. He says that, in response to grower demand, AGT developed a lupin population specifically aimed at producing an imi-tolerant lupin variety. To support this effort, AGT has partnered with the Department of Primary Industries and Regional Development (DPIRD), which conducted initial greenhouse screening of imidazolinone rates on both narrow-leafed lupin varieties and blue lupins.

“This screening work indicated that current narrow-leafed lupin varieties and blue lupins are sensitive to imidazolinone at the label rates similar to those used in cereal crops, but this will be confirmed through further field trials,” says Matt. “We also identified several narrow-leafed lupin lines from the lupin population that show varying levels of tolerance to imidazolinone.”

AGT are also partnering with grower groups to test the efficacy of different imidazolinone rates on blue lupins at the paddock scale in 2026. Matt says the initial experiments have yielded promising results, and the breeding program is pursuing the development of a narrow-leafed lupin variety with imidazolinone-tolerance.

“We are conducting thorough characterisation of the potential imi-tolerant lines and have engaged with the relevant herbicide companies, through GRDC, to ensure all herbicide labels are in place,” he says. “At this stage, there are no guarantees that the lines we have selected will have adequate tolerance or be suitable for lupin grain production and marketing.”

There is no firm timeline for the new variety’s release; however, Matt says AGT is fast-tracking the plant breeding program and will ensure the supply chain receives all the relevant information as early as possible.

The likely use pattern for the potential imi-tolerant narrow-leafed lupin would be similar to that of other imi-tolerant pulses, where the tolerant lupin could either be planted safely into imi residues from the previous season, or sprayed over-the-top (OTT) early in the crop development. The aim is to control blue lupin plants before they can compete with the crop.

“The focus of the development for this first imi-tolerant narrow-leafed lupin variety is on weed control efficacy,” says Matt. “If this proves successful in a wide range of field settings, breeding for subsequent imi-tolerant varieties will focus on achieving comparable yield and other desirable traits of the current narrow-leafed lupin varieties.”

Matt says they are also very aware of the herbicide resistance risks associated with imi-tolerance traits in crops. Blue lupin is susceptible to imi herbicide, and the resistance risk is very low; however, other weeds in the cropping system may have or evolve resistance to this mode of action. An effective stewardship program will be developed to support the sustainable use of this technology should it come to fruition.

“Imidazolinone is taken up into crop plants, and low-level residues can end up in the grain,” says Peter. “With so many imi-tolerant crops now available, there is considerable pressure on group 2 chemistry that could cause issues with herbicide resistance, and domestic and export market maximum residue limits must not be exceeded.”

The area sown to narrow-leafed lupin in Western Australia has fallen from around 1.2 million ha in the late 1990s to less than 0.5 million ha in 2023. This was due to a combination of weed and market price issues, causing growers to turn to alternative break crops, primarily canola.
Peter says many more growers would include lupin in their rotation if they had effective control options for blue lupin. He says that having a reliable legume back in the rotation would support more competitive crops, providing field-wide weed suppression.

In addition to the potential herbicide-tolerance technology, efforts have also been made to develop weed-detection algorithms for green-on-green spot spraying blue lupins in narrow-leafed lupin crops. Although humans can distinguish between the two species based on leaf shape at early growth stages, this remains a challenge for automated weed detection systems.

Peter says research at the Australian Herbicide Resistance Initiative (AHRI) demonstrated that it is possible to identify 80 per cent of the blue lupins in a narrow-leafed lupin crop when blue lupins occupied just 2 per cent of the field using high-resolution images captured early in the crop’s development stages.

“Spot-spraying offers great opportunities for growers to save on herbicide costs and to reduce herbicide damage to crops compared to broadacre sprays,” says Peter. “Blue lupin control is a good application for green-on-green weed detection technology. However, the early detection sensing is still improving, and the set-up costs are currently prohibitive to many growers.”

Bilberry’s ‘blue lupin in lupin’ application has two use patterns. The first is to identify blue lupin at flowering and to spot-spray these plants before seed set to avoid grain contamination. The second is to distinguish between the narrow-leafed plants and the ‘fatter’ leafed blue lupin from the 4 to 5 leaf stage onwards, when their oldest leaves are wide, open and visible.

Peter says the current use-pattern involves spot-spraying the blue lupins with a non-selective herbicide, which also kills the adjacent narrow-leafed lupins, leaving gaps in the crop, costing yield and allowing other weeds to establish.

If the imi-tolerant narrow-leafed lupin breeding is successful, it would be feasible for growers to combine the technologies to spot-spray blue lupins with imi chemistry, targeting herbicide to the weed and minimising crop damage.

Herbicide-tolerant seed technologies are an important tactic within the WeedSmart Big 6 ‘mix and rotate’ and ‘diverse crop rotations’ pillar strategies. Managing the risks associated with herbicide-tolerance traits requires commitment to stewardship programs and WeedSmart’s integrated weed management strategies that underpin sustainable use of the technology.

AGT has identified several narrow-leafed lupin lines from the lupin population that show varying levels of tolerance to imidazolinone.