Herbicides for control of clethodim-resistant annual ryegrass

Christopher Preston, Peter Boutsalis, Samuel Kleemann, Rupinder Saini and Gurjeet Gill,

School of Agriculture, Food & Wine, University of Adelaide.

^ɸExtra technical comment by Protech Consulting Pty Ltd

GRDC project code: UA00144, UCS00020

Keywords: herbicide resistance, canola, weed control.

Take home messages

• Annual ryegrass with resistance to clethodim is increasing across South Australia.
• Pre-emergent herbicides alone are insufficient to effectively manage annual ryegrass in break crops.
• Crop topping in canola offers an opportunity to reduce annual ryegrass seed set.

Herbicide resistant grass weeds in South Australia

Random surveys have shown that herbicide resistance in grass weeds is increasing in South Australia. For annual ryegrass, of particular concern is resistance to trifluralin and clethodim (Table 1). Replacement products for trifluralin have been identified; however, as yet there is no replacement for clethodim.

Table 1. Extent of resistance in annual ryegrass collected in random surveys in South Australia. Populations are considered resistant if there is more than 20% survival.

* Oust® at 20 g/ha used in 2013

** Select^TM at 500 mL/ha used in 2013

Resistance in brome grass to Group B herbicides is also increasing, particularly in the SA Mallee (Table 2). Resistance to the Group B herbicides is sporadic across South Australia and these herbicides will be useful for the time being. Imidazolinone herbicides are now being used widely to control brome. While resistance to Intervix® was not detected in the random surveys, this herbicide is starting to fail.

Table 2. Extent of resistance in brome grass collected in random surveys in South Australia. Populations are considered resistant if there is more than 20% survival.

* Targa® used in 2013

Resistance in wild oats remains at relatively low levels across South Australia, for both Group A and Group B herbicides (Table 3). There has been little increase in resistance to Group A herbicides; however, resistance to Group B herbicides is increasing.

Table 3. Extent of resistance in wild oats collected in random surveys in South Australia. Populations are considered resistant if there is more than 20% survival.

* Topik® used in 2013

^ɸWildcat is no longer registered

Managing clethodim resistance in annual ryegrass

Clethodim is the last Group A herbicide that provides effective control of herbicide resistant annual ryegrass. It has become an exceptionally important herbicide in annual ryegrass weed management strategies. The loss of clethodim to resistance will make annual ryegrass management more difficult. In order to address this problem, we have been looking at alternative herbicide strategies to clethodim in break crops.

In faba beans, sole reliance on pre-emergent herbicides failed to adequately manage annual ryegrass and large yield penalties occurred (Table 4). This trial was conducted in 2014 on a clethodim resistant population at Roseworthy. Clethodim at 500 mL/ha following a pre-emergent application of simazine suppressed the ryegrass and the dry spring in 2014 ensured the ryegrass plants had few heads. However, the effect of the early ryegrass competition on yield was obvious. Addition of Factor® to clethodim in 2014 reduced ryegrass numbers further and protected yield. This remains the most effective currently registered option for faba beans. Environmental conditions in 2014 of high rainfall early in the season likely favoured simazine activity.

Table 4. Ryegrass plant numbers, seed heads and faba bean grain yield at Roseworthy in 2014 following a variety of treatments to control clethodim resistant annual ryegrass.

Values with different letters within a column are significantly different (p = 0.05)

^{ɸ a} The rate on the Gesatop Granules label is 1.0-1.4 kg/ha and on the Gesatop Granules WG label is 1.1-1.4 kg/ha

^{ɸ b} Not registered for annual ryegrass in faba beans; but registered for wild oats in faba beans (and at this rate)

In TT canola in 2014, clethodim was not as effective at controlling annual ryegrass as it was in faba beans, probably due to lower early competitiveness of the TT canola (Table 5). Rustler® was the best of the stand-alone pre-emergent herbicide options examined, although weeds that emerged through this treatment were highly competitive and reduced yield. Addition of clethodim to Rustler® tended to stunt these weeds and reduce their competitiveness. The reduced rate of Factor® that can be used in canola, compared with pulse crops makes this product less effective on annual ryegrass that has low levels of resistance to this herbicide.

Table 5. Ryegrass plant numbers, seed production and TT Canola grain yield at Roseworthy in 2014 following a variety of treatments to control clethodim resistant annual ryegrass.

Values with different letters within a column are significantly different (p = 0.05)

^ɸ Pre emergence registered for suppression only (1.1 – 2.2 kg/ha).  Post emergence registered for control of 1-2 leaf stage only (0.5 – 1.1 kg/ha)

Crop topping canola

Late in 2014, Weedmaster® DST was registered for use as a crop-topping application in canola. This provides an opportunity to control grass weed seed set in canola crops. The registered uses are to apply over the top of the crop from 20% canola seed colour change or under the windrower. To use Weedmaster® DST effectively for crop topping the correct timing, rate of herbicide, water volume and environmental conditions need to be followed.

The main problem with over the top use is to get the herbicide through the canola canopy and onto the ryegrass growing underneath. Higher water rates and coarse spray quality will help achieve this. For under the windrow applications, water rates will always be low. In addition, windrowing may be done when environmental conditions are less favourable for glyphosate to work. Increasing the rate of Weedmaster® DST may help overcome these issues (Figure 1). Ryegrass maturity can also have an impact when crop topping canola. Seed set will not be controlled on annual ryegrass that has matured at the time of windrowing.

Figure 1. Germination of annual ryegrass seed following application of glyphosate at windrowing in canola. Average of two trials in mid-north of SA in 2010.

Contact details

Christopher Preston

School of Agriculture, Food & Wine, University of Adelaide PMB1 Glen Osmond SA 5064

(08) 8313 7237

christopher.preston@adelaide.edu.au