REPORT ON TRIAL WORK TO DATE FOR THE 2024/25 SEASON

By Tim Groom and Bree How

Ryegrass is a very productive plant and great for feeding animals, but is highly competitive in onions, and traditional controls are becoming less effective.

Resistance to group one herbicides such as fluazifop (Fusilade Forte) and clethodim (e.g Status) that selectively control ryegrass in onions is widespread and increasing. Resistance to glyphosate is less common, but has been recorded in several growing regions.

Ryegrass is escaping in rotational crops due to either effective herbicides not being used, or control failures, where inadequate control of ryegrass is being achieved. Ryegrass that goes to seed poses a major problem to following onion crops.

There is a very low tolerance for ryegrass in onions – not only is there an effect on yield but the grass blocks up lifting and harvesting equipment, leading to bulb damage and reduced harvest capacity. In south-eastern Australia, the long growing season also means that ryegrass germinates over an extended period and onions are a particularly poor competitor when small.

The Grains Research and Development Corporation publication Managing Annual Ryegrass in the High-Rainfall Zones of Victoria, South Australia and Tasmania captures what is required to manage ryegrass very succinctly:

“Effective pre-emergent herbicides followed by an in-crop tactic and a tactic to reduce seed return to the soil should be implemented in as many phases of the rotation as possible.”

This work has focused on one of the steps, to find effective pre-emergent herbicides in onions.

Trial work in the 2023/24 season demonstrated that whilst a number of herbicides were safe to the onions pre-emergent, ryegrass control was unsatisfactory. This included the registered herbicide Ramrod and the now de-registered herbicide Dacthal.

Following a Pub in the Paddock field day organised by AUSVEG in November 2023, collaboration with Dr Peter Boutsalis of Plant Science Consulting led to a change approach.

Peter screened 15 herbicides known to be active on ryegrass in pot trials when the onions were around the half true leaf stage. Of these, five were taken to a field trial, and the remaining deemed to be unsafe to the onions.

As part of the field trial, we were looking for any positive interaction between these pre-emergent herbicides applied early post-emergence, and the group one herbicide clethodim. Such a positive interaction has been found when mixing the herbicide Tenet (metazoclor) and clethodim in canola, and this use is now on the label.

Onions were sown on the property of David and Nadine Vertigan at Ironcliffe Road, near Penguin in northwest Tasmania, on red ferrosol soil on 15 September 2024. Ryegrass was present in the paddock and testing had determined that it was highly resistant to the ‘fops’ such as Fusilade and Verdict, and moderately resistant to the ‘dims’ such as clethodim.

Figure 1. Weeds germinating in stale seed bed

Figure 2. Onions at the time of spraying

Figure 3. Ryegrass at the time of spraying

Trial site planting history

• 2023/24 canola
• 2022/23 winter wheat
• 2021/22 poppies
• 2020/21 winter wheat
• 2019/20 potatoes
• 2018/19 onions

The last time the paddock grew onions in 2018 only a small amount of ryegrass escaped treatment, and this was confined to the edges of the paddock. The ryegrass population had increased since that time.

For a number of years David has employed a stale seed bed technique to encourage weed germination prior to planting, and this seed bed was very stale as continued wet weather in August had prevented planting.

The weeds were controlled by non-selective herbicides prior to crop emergence so that by the time of herbicide application in the trial on 22 October 2024, the onions were at the half to one true leaf stage and appeared in the Figures shown.

The trial plan

Results to date

• All pre-emergent treatments applied early post-emergence to the onions appeared safe, with plant numbers and stature not significantly affected.
• Clethodim interacted with several of the pre-emergent herbicides leading to some tip burn that the onions eventually grew out of.
• Natural variation in the ryegrass density across the trial has made assessment difficult, but the clethodim application reduced the ryegrass population across all the trial. Only Sakura has provided consistent control of ryegrass when applied alone, and the combination of Sakura and clethodim has provided fair to good control of ryegrass depending on the initial ryegrass pressure. Arcade plus clethodim provided some control, but was inconsistent.

Many thanks to Phil Frost at Arvensis Research for providing statistical support of trial data.

Before

CONTROL PLOT on 13 November, 22 days after treatment, when onions were at the 2-3 true leaf stage. The trial received a number of herbicides to control broadleaf weeds as part of the overall crop program.

SAKURA PLOT on 13 November, 22 days after treatment

After

CONTROL PLOT on 16 January 2025

SAKURA PLOT on 16 January 2025. Good control was achieved where initial ryegrass pressure was lower, demonstrating the importance of getting good ryegrass control in rotational crops.

Where to from here?

This was one trial in one location in one season. Further trial work over a range of locations, soil types and seasons are required to provide sufficient confidence min crop safety and efficacy on ryegrass.

Sakura appears to be the most promising herbicide. Due to the short-lived nature of some of the other herbicides trialled, they may suit a sequential low dose program to extend their activity in the onion crop until it reaches the 2-3 leaf stage. After this crop stage, a number of other herbicides could be employed to provide extended residual control.

Part of the trial work discussed in this article was conducted independently by TD and SM Groom Agricultural Consulting, and part conducted in collaboration with Dr Peter Boutsalis of the Plant Science Consulting with funding provided by the Accelerating the adoption of best management practices for the Australian onion industry project.