Short-term Pain for Long-term Gain: Controlling Wild Radish in Wheat

By BCG Staff and Contributors
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2012 Report

Wild radish populations and their level of herbicide resistance are increasing across the Wimmera and Mallee cropping regions. In the low and medium rainfall zones of Western Australia (WA), the problem is much advanced. In WA, wild radish populations have exploded and widespread resistance to Group B, C, F and I herbicides already exists. The practice of following tight wheat-lupin-wheat-lupin rotations and the limited available alternatives, have significantly contributed to, and exacerbated, the herbicide resistance problem in the west.

While there is greater crop diversity within Victorian cropping regions, growers have commonly selected herbicides according to their ability to control grasses such as ryegrass or brome grass. As a consequence, growers have placed less emphasis on rotating their broadleaf herbicide groups, thereby increasing the potential for resistance to develop in weeds such as wild radish, Indian hedge mustard and turnip.

Thanks to improved growing conditions over the past three years, farmers have had the option to use canola and pulse crops to reduce grass numbers. Triazine and imidazolinone based herbicides that can be sprayed on Triazine Tolerant (TT) and Clearfield canola varieties also provide an obvious break for broadleaf weeds. However, the overuse of imi-based herbicides is a big concern for the industry, particularly since growers are taking advantage of improved Clearfield cereal varieties (as well as canola) which allows further use of imidazoline-based herbicides. The fear is that resistance will develop in weeds such as wild radish because, to reduce herbicide costs and plant-back restrictions, the rates at which these products are being used are often low, especially in the Mallee. Additionally, with limited broadleaf herbicides available for use in crops such as lentils and field peas, if resistance continues to develop, crop options in future years will be restricted. This, in turn, would place greater pressure on diflufenican (active ingredient in Brodal®). Already in Victoria, there are confirmed cases of diflufenican and phenoxy (MCPA LVE) resistant wild radish populations. During the cereal phase (wheat and barley) of a cropping rotation, there are more herbicide options (with different modes of action) available to control wild radish. Rotating herbicide groups (using herbicides of different modes of action) will help delay the onset of resistance.

In 2012, several trials were established to compare the various options available to growers to control radish in-crop.

Take Home Messages

• Flight, Velocity, Jaguar and the Agritone plus Affinity mix were the herbicides that
provided the most effective wild radish control in wheat.
• A two-spray strategy is required to control later germinations of wild radish.
• Short-term pain for long-term gain: including new herbicides (with different modes of
action) into the cereal phase will delay the onset of resistance.

Aim

To compare the efficacy of broadleaf selective herbicides to control radish in wheat.

Method

The replicated field trial was established on a typical sandy loam soil with a dense wild radish
population. Treatments were applied using BCG’s Goldacres custom-built 12-line sprayer.

Location:                                 Corack
Replicates:                              3
Crop:                                       Lincoln wheat
Crop growth stage:                 5-leaf to 1st node
Weed growth stage:                Four-to-six leaf
Application date:                     23 July
Seeding equipment:               Knife point, press wheels on 30cm row spacing

The trial was not harvested and was terminated after the final assessment had been taken.

2012 Short term pain radish Tab 1 and 22012 Short term pain radish Tab 3

Results and interpretation

At the time of spraying, wild radish plants ranged between two and six leaves, as a result of staggered germination. There was a distinctive header trail effect across the paddock and the trial was subsequently designed so that each treatment ran perpendicular to the header trail. This enabled each treatment to be compared under high and low densities. Weed counts were taken from high density areas as well as outside the header trail (low density). Table 4 shows the reduction in radish plants of each treatment 50 days after herbicide application.

2012 Short term pain radish Tab 4

Four days after herbicide application, the plots sprayed with Agritone 750 (MCPA) plus Affinity DF (carfentrazone-ethyl – Group G) showed the most visual symptoms of herbicide activity and within 21 days of application, the majority of radish plants had been successfully controlled.

Some crop damage (foliage only) was observed where Velocity® and Affinity had been applied. However, the damage was only minor and the crop showed little sign of any damage at 28 and 50 DAA.

Velocity and Flight® (720ml/ha rate) were the most effective treatments. These two treatments, along with Jaguar® and the Agritone plus Affinity mix achieved greater than 85 per cent reduction in plant numbers. Precept and Tigrex did not perform well. This was unexpected as Precept, in particular, has been found to be very effective on wild radish populations in previous studies. Given that Velocity and Precept both contain pyrasulfatole, it appears that the bromoxynil component in the Velocity contributed to the improved result. Other more effective products, such as Flight and Jaguar, also contain bromoxynil. By itself, bromoxynil is not very effective on radish, but the results from this trial would suggest that, when mixed with another product it can improve efficacy. Mixtures containing bromoxynil improved control, performing as well as the Agritone plus Affinity treatment.

Despite having little effect on the number of wild radish plants in the high density area, Conclude® stunted the plants and dried the leaves out. It was uncertain as to what the seed viability and production would have been had the trial gone through to maturity.

2012 Short term pain radish Fig 1

The findings from this trial indicate that there could be some hormone resistance occurring, which may have led to some of the unusual results. The addition of MCPA LVE to Velocity usually improves the control of wild radish but, in this trial, control was weakened. This could perhaps be attributed to the higher water rate used here. Due to the particular set up of BCG’s spray rig, a water rate of 150L/ha was used as opposed to 75L/ha which is the usual water rate used with a commercial spray set-up. It should also be noted that this investigation looked at wild radish control from a one spray perspective. Keeping in mind that wild radish is known to have a long germination period, it is possible that a secondary germination occurred within the 50 day period.

Seed was collected around the trial area at harvest to check the resistance level and also from the control plots to quantify the seed bank level of wild radish. These results were not available at the time this report was written. They will be published at a later date and may provide the answers to some of the questions raised about the unexpected performance of some of the treatments.

Commercial practice

From the results of this trial it was concluded that the best wild radish control was achieved by products that contained bromoxynil. However, despite this result, anecdotal evidence and the results of other research trials would suggest that most of the herbicides tested in this trial do work effectively in our region. The particular population tested in this trial may have some hormone resistance not commonly found in the region. The resistance test currently underway should reveal if this was the case.

Farmers should take this as a warning. Reports of hormone resistant wild radish populations are increasing. Wild radish has been an on-going weed issue in the Corack area, motivating regular use of phenoxy-based products, especially during the past decade. This trial reveals some of the issues that may arise if hormone resistance develops in radish populations.

To delay the onset of resistance, farmers should rotate herbicides with different modes of action and/or herbicide sub-groups. While this may increase costs in the short term, it will reduce the longer-term costs.

Acknowledgments

The project was funded by GRDC (UA00134).

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