How can I strengthen the pulse phase to combat weeds?

Pulses are increasingly important in farming systems around Australia, boosting profits and playing an essential role in improving soil health and managing weeds.

Dr Jason Brand, senior research agronomist with Agriculture Victoria Research, says that pulses can either break or make an effective crop rotation when it comes to weed management.

“In the Mallee, lentils now account for 20 to 30 per cent of the rotation. We see a similar percentage in other areas where different pulses, have a good fit,” Jason says. “The release of herbicide-tolerant pulses has played a pivotal role in expanding pulse production and widened the herbicide options available to growers.”

For pulses to be effective in a weed management program, they need as much attention as other major crops. Without attention to detail, the pulse phase can result in a weed blowout.

“For long-term weed control success with pulses, it is necessary to maximise above-ground and below-ground productivity,” he says. “And understand the sensitivities each crop and variety has to different herbicide actives. Many pulse varieties are very specific in their tolerance to different active ingredients, even from the same mode of action group.”

The WeedSmart Big 6 is built on robust and diverse crop rotations where each crop provides opportunities to drive down the weed seed bank. In the pulse phase, the main target is grass weeds while ensuring broadleaf weeds do not proliferate.   

What are the essential practices that underpin a competitive pulse crop?

In brief: Standing stubble, proper nutrition to support the crop and rhizobia, and optimal crop agronomy.

The details: Standing stubble aids moisture infiltration and provides trellising support for pulses – particularly lentil and field pea.

Inoculate the seed with rhizobia and support nodule development with adequate phosphorus, molybdenum and zinc. Feed the crop well with phosphorus, zinc, potassium and sulfur, and address any soil constraints that suppress crop growth or result in gaps within the crop. Manage crop diseases to avoid gaps opening up that allow weeds to grow unhindered and set seed.

Be aware of the suppressive effects of herbicide residues in the soil. Pulses are sensitive to residues of many standard Group 5 [C], Group 4 [I] and Group 2 [B] herbicides. Suppression in plant growth, reduced vigour and reduced yield may not be obvious as it is usually evenly spread across the field. Suppression is clearly seen in trials comparing nil herbicide residues, standard Group 5 [C] herbicide residues e.g. metribuzin and Group 2 [B] e.g. sulfonylureas (SU) residues.  

What are the benefits and risks associated with herbicide-tolerant pulse varieties?

In brief: Plant breeding in pulses utilising mutation techniques has developed novel tolerances to some herbicides.

The details: Always focus on the herbicide active ingredient, not the mode of action (MOA) group. Pulses are often quite specific in their tolerance to different actives within a MOA group. For example, imi-tolerant lentils are only safe with imazamox and imazapyr (registered product Intercept) and not with other Group 2 actives such as florasulam or chlorsulfuron.

The initial focus has been on breeding for imi-tolerance, and there are now imi-tolerant lentil, faba bean and field pea varieties, with other crops under development. Around 80 per cent of Australia’s lentil crop is the herbicide-tolerant varieties, despite grain yields often being slightly lower than conventional varieties.

The herbicide-tolerant varieties released recently have provided growers with new use patterns to control difficult weeds in pulse crops, particularly broadleaf weeds and grasses such as brome grass.

Whether the use is in-crop or to allow planting pulses into otherwise problematic herbicide residues, the new technology has underpinned the expansion of pulse crops in many areas. The risk is that an over-reliance on this technology can develop that can potentially allow herbicide resistant weeds to establish, rendering the tolerant varieties useless.

What is in the herbicide-tolerant pulse breeding pipeline?

In brief: The next phase of plant breeding will move onto sulfonylurea (SU), metribuzin and clopyralid tolerance, bringing varieties with combined tolerance traits in the next 5 to 10 years.

The details: Pulse breeders are now honing in on improved tolerance to a range of actives such as diflufenican (Group 12 [F]), various Group 5 [C] actives e.g. metribuzin and simazine, other Group 2 [B] actives such as the sulfonylureas (SU), clopyralid (Group 4 [I]) and isoxaflutole (Group 27 [H]) (in chickpea).

Grains Innovation Australia has released two multi-tolerant lentil varieties for 2023 – GIA Sire (imi plus clopyralid), which is safe to plant in soils with Lontrel^® residues, and GIA Metro (imi plus metribuzin), which potentially adds the option for in-crop metribuzin application against broadleaf weeds.

There is much more to learn about using these varieties to help manage weeds throughout the crop rotation. One area of current research is to assess the risks involved with applying multiple actives that may suppress pulse crop growth and nodule activity.

Once again, keep in mind that a herbicide-tolerant variety can be susceptible to other herbicides, even within the same MOA group. For example, terbuthylazine (Group 5 [C]) can cause severe crop damage and grain yield loss in GIA Metro even though this variety is tolerant to metribuzin, also a Group 5 herbicide.

Watch the full presentation at WeedSmart Week Mildura 2022