A new chapter opens in weed control
GroundCover™ Issue: 101 | Author: Dr Gio Braidotti
The discovery of naturally occurring herbicide resistance in the germplasm of several major pulses may remove one of the Australian industry’s main production constraints.
The discovery comes from a pulse herbicide tolerance project looking for natural resistance, rather than utilising gene modification.
A painstaking search of seed collections began in 2010. With the development of a rapid test to detect herbicide tolerance in seedlings, researchers soon found chickpea, lentil, field pea and faba bean plants with levels of innate herbicide tolerance high enough to be able to develop further through breeding.
Given the increasing importance of pulses in crop rotations, the discovery stands to expand growers’ weed control options, including more effective post-emergent spraying and use of a wider range of chemicals.
Researchers say that if their work continues at the current rate of progress, herbicide-tolerant commercial varieties could be available to growers within five to 10 years.
The GRDC-supported research at the South Australian Research and Development Institute (SARDI) and the University of Adelaide was initiated just two years ago. Its aim was to boost the rotational and cash-crop benefits of pulses by finding a weed-management option within the plants’ natural genetic diversity.
The herbicide-tolerance traits that have been found have the potential to help the industry realise its ambition of at least doubling production. Australian pulse production is worth more than $1 billion a year and accounts for seven to nine per cent of the total area cropped in Australia.
The project’s leader, SARDI’s Larn McMurray, says that the project was conceived during industry consultations in which growers, Pulse Breeding Australia (PBA) and researchers agreed that available weed management options for pulses currently carried too much risk. “Too often the advantages of pulses in rotations are reduced or lost through the inability to control weeds,” he says.
“There is also a limited availability of suitable and safe herbicides, particularly for post-emergent applications, and furthermore, many registered herbicides only have a narrow safety margin between toxicity to the weed and toxicity to the crop.”
Pulse varieties with in-built, genetic-based tolerance to key herbicides would radically change the odds in favour of the crop. So over the past two years, the research team has developed a way to efficiently test and select seedlings for herbicide tolerance.
“While we still need to field-validate a lot of the material we selected, we have some plants that look really exciting – lines that we think have improved tolerance to not only tried and tested herbicides but also to new herbicides,” Mr McMurray says.
Natural herbicide tolerance to open new chapter in weed control
- Pre-breeders in South Australia are discovering pulse germplasm with improved tolerance to key herbicides
- Breeding with the most advanced imazapyr tolerance trait is underway in faba beans
- The new traits raise hopes for improved weed management protocols within 10 years for both northern and southern pulse-growing regions
When it came to screening seed collections of lentils, field peas and faba beans, a major priority was to screen for tolerance to the Group C herbicide metribuzin. However, the project is also trialling other combinations including isoxaflutole in chickpeas, diflufenican in lentils and field peas, and simazine in chickpeas and faba beans.
Promising traits so far discovered by screening seed collections include:
- four faba bean lines with significantly improved metribuzin tolerance over AFO3109, the most tolerant known line in the Pulse Breeding Australia (PBA) breeding program;
- 10 field pea lines with significantly improved metribuzin tolerance over Yarrum and Kaspa; and
- two lentil lines with some degree of improved metribuzin tolerance, with work still ongoing.
Underlying this success was the development by South Australian Research and Development Institute (SARDI) research officer Dili Mao of a fast, cost-effective, non-destructive way to screen hundreds of genetically unique plants and reliably assess their level of tolerance to targeted herbicides.
“We developed a hydroponic technique to test for herbicide tolerance,” Ms Mao says.
“We might get 200 lines from a germplasm collection and we sow them in our growth room where we apply the herbicide and select for different levels of tolerance. The tolerant plants, however, only return limited seed, so further seed multiplication over the next year is required to bulk up sufficiently to validate the tolerance trait in a paddock environment.”
Genetic diversity boost
The same project has also boosted the level of genetic variation at researchers’ disposal by introducing changes to seed DNA using the same mutation technique that breeders in Canada used to produce Clearfield® canola, which tolerates the herbicide imazapyr.
The mutation strategy firstly enhanced the genetic diversity of Nura faba beans and PBA Flash lentils using the chemical ethylmethanesulfonate (EMS). This involves treating genetically uniform seed with a specific dose of EMS that causes mutations that increase genetic diversity. The mutation process was carried out by SARDI’s Dr Klaus Oldach, who heads the gene function group.
The resulting ‘mutant population’ is screened in the field to detect any mutations that confer herbicide tolerance.
Among the priorities for EMS-treated faba beans is tolerance to the Group B herbicide imazapyr. In lentils, the priorities are tolerance to metribuzin and the Group G herbicide carfentrazone-ethyl.
“What we tend to do is choose one herbicide where we anticipate good success rates – herbicides like imazapyr for which tolerant varieties have proven possible in many crops including wheat, lentils, barley, canola, corn and sunflowers,” Larn McMurray, leader of the project at SARDI, says. “So for faba beans we tried for imazapyr tolerance.”
That approach worked, with imazapyr-tolerant faba beans among the project’s most advanced germplasm. The trait, however, requires additional validation and selection. Mr McMurray expects it will take five to 10 years before grower-ready cultivars are released.
“The mutation process often results in deleterious genes being selected along with the gene of choice, which can slow down the process of getting the new technology to market,” he explains.
But he stresses that the availability of a post-emergent herbicide option for faba beans is of particular significance since there is no in-crop control option available. However, this could result in an over-reliance on Group B chemistries, so the team is also looking for tolerance to new herbicide groups that are not registered for use in pulses or which currently kill the pulse crop.
“Through this mutagenesis process we hope to get tolerance to a whole new group of herbicides so that the weed-management options available to growers can be expanded in ways that make a big difference to the industry,” Mr McMurray says.
Mutations that confer the sought-after tolerance trait are, of course, rare. This means that success is dependent on screening vast numbers of seed – far more than can be handled by the researchers and their facilities. In response, SA growers such as Mark Schilling have jumped in, using their paddocks, machinery and expertise to scale up the project.
During 2012, that amounted to Mr Schilling sowing 45 hectares to EMS-mutated PBA Flash lentil seed. Mr McMurray says that targeting new herbicides simply would not be possible without the growers’ active participation.
Noteworthy progress with EMS mutant germplasm includes:
- 23ha of faba beans being screened in the field to an alternative herbicide group during 2012;
- 45ha of lentils being screened in the field for tolerance to three alternative herbicides;
- three lentil lines with a high level of tolerance to metribuzin (but none yet to carfentrazone-ethyl, although work is still ongoing);
- three distinct mutations have been identified that confer high-level imazapyr tolerance in faba beans; and
- tolerant faba bean and lentil seed is being multiplied over winter for further work, including genetic studies, dose response and field trials in preparation for a handover to PBA breeding programs.
“The project has been working closely with lentil and faba bean breeders at PBA to allow for the rapid incorporation of herbicide tolerance traits into grower-ready cultivars,” Mr McMurray says. “This means opportunistic crossings have occurred between PBA’s advanced lines and plants that tested well for herbicide tolerance in 2011.”
GRDC Project Code DAS00107