Natural born killers lead new attack on weeds

By Alec Nicol

Wheat lines able to naturally control annual ryegrass, including strains resistant to Roundup®, have been identified. The trait is heritable and genetic markers for it have been established, allowing the rapid incorporation in future breeding programs.

Professor Jim Pratley, dean of the School of Science and Agriculture at Charles Sturt University, believes the discovery is an important step forward in weed control, with the potential to reduce the grains industry’s dependence on chemical herbicides.

“I have a view that we have only another five or 10 years left where we can depend entirely on chemicals to control weeds like ryegrass, so this natural allelopathic effect is something we need to investigate,” Professor Pratley says.

“The fact that we have something specific to annual ryegrass is particularly important, as this is the weed species showing the greatest ability to develop resistance to existing herbicides.”

Professor Pratley says that it has long been known that particular plants release chemical compounds to either prevent the germination or to reduce the vigour of a competitor: “These are complex compounds.

They are exuded through the roots and there is also some volatile discharge from other parts of the plant. They’re released at about the same level as the competitor’s seeds and the release is seemingly timed to coincide with germination. They don’t last long and are fully biodegradable.”

Professor Pratley says these natural herbicides are often very specific in their action: “Silver grass, for example, releases a natural herbicide particularly effective against lupins and wheat.

Now we’ve identified wheat lines that release a natural herbicide effective against annual ryegrass.” Professor Pratley led a team in which PhD student and post doctoral fellow Hanwen Wu investigated the allelopathetic effect of 453 wheat accessions from different countries. At one end of the scale, the most successful wheat strains were able to inhibit root elongation in annual ryegrass by more than 90 percent. Strains with low allelopathy inhibited elongation by only 25 percent.

“The work was done in the laboratory under sterile conditions, so we are sure that it was the allelopathic effect of the wheat responsible for the reduction in vigour in the ryegrass seedlings.

The next step will be to take this work into the field and we’ll need to be careful to separately identify the impact of competition and allelopathy,” says Professor Pratley.

Significantly for Australian farmers, strains with Janz as a parent line were high on the allelopathy scale, while those with Hartog as a parent were at the lower end of the scale. Continuing work has shown that the trait is heritable and two genetic markers associated with allelopathy have been identified.

Professor Pratley says that in addition to offering potential alternatives to synthetic chemicals, these natural compounds might also provide opportunities to synthesise new herbicides. “They are a complex of compounds, rather than a single active ingredient so that, while it’s possible that weeds would, in time, develop resistance to the allelopathetic effect, this would take longer to achieve and will be easier to control given a normal program of crop rotation.”

Professor Pratley says that natural allelopathy will never completely eliminate weeds but it may reduce a population to the point where it has little commercial impact. “We need to be asking, ‘do we need to eradicate weeds, reduce the population below a certain threshold, or inhibit their growth’? I don’t think anyone has that answer yet.”

For more information:
Professor Jim Pratley, 02 6933 2864,

GRDC Research code: PDF11, program 3