Self-weeding crops produce own herbicides

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Certain canola varieties can slow ryegrass growth.
Credit: Carolyn Hebbard / getty images

A crop that does its own weeding may sound like a dream to any farmer.

But two varieties of canola can do just that, a team of Australian researchers discovered. These varieties stop weeds from growing by releasing chemicals into the soil, a finding which could revolutionise the development of crops.

For farmers, weeds are not just irritating – they’re a costly problem. Australian grain growers alone lose an estimated $3.3 billion and 2.76 million tonnes of harvest to weeds every year.

And while weeds are still largely controlled with synthetic herbicides, many species, including the most problematic annual ryegrass, are evolving resistance.

“There are 15 modes of action in herbicides for ryegrass control – we have resistance to 11 of those,” says Jim Pratley, agricultural scientist at Charles Sturt University. With farmers starting to run out of options, he says, “there is groundswell to look around for other approaches”.

Pratley came across an alternative by accident nearly 30 years ago. He discovered a weed called silver grass actively reduced the growth of wheat and lupin crops by secreting 25 different herbicidal chemicals from its roots, giving itself a better chance to thrive.

Pratley describes this natural competition between plants, or “allelopathy”, as “chemical warfare” where the victor is most tolerant of the other’s cocktail.

“We thought, if weeds have these chemicals, crops may have them too,” Pratley recalls. Problem is, he says, “we’ve bred out any allelopathic capability in crops because we haven’t been challenging them with weeds during their development as varieties”.

So his team sought to reverse that exercise and turned to canola. In lab experiments, the team screened 70 different canola varieties for how they affected the growth of ryegrass plants around them. Two varieties, Av-opal and Pak85388-502, reduced the growth of the weed’s roots by at least 70%.

The scientists confirmed their lab results in a field trial. Not only did these two canola varieties significantly slow down ryegrass growth, they also stopped a number of other weeds in the trial’s second year. “That was a bonus,” says Pratley.

The researchers returned to the lab and identified three chemicals common to the roots of both allelopathic canola varieties – sinapyl alcohol, p-hydroxybenzoic acid and 3,5,6,7,8-pentahydroxy flavones – which they believe are at least partly responsible for inhibiting weed growth.

Pratley hopes these allelopathic chemicals are the beginning of a list of biomarkers which scientists could screen and select for in early breeding programs. Selecting crops for weed-destruction during crop development could reduce the need for synthetic herbicides, he says.

With further field trials of Av-opal and Pak85388-502 underway, the team will continue the search for more allelopathic canola varieties. Even though ryegrass will likely evolve resistance to allelopathic chemicals eventually, Pratley says, rotating crops that secrete different sets of chemicals will help farmers ensure weeds remain challenged.

“The allelopathy agenda is not going to be the panacea,” Pratley says. “But it is going to be a tool that will reduce our dependence on synthetic herbicides and prolong the efficacy of our existing herbicides.”

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