Maintaining the best options with herbicides

Christopher Preston, Peter Boutsalis, Jenna Malone, Patricia Adu-Yeboah, Sam Kleeman, Rupinder K Saini and Gurjeet Gill.

School of Agriculture, Food & Wine, University of Adelaide

GRDC project code: UA00113, UA00121, UA00124

Keywords: herbicides, annual ryegrass, resistance

Take home messages

• Many annual ryegrass populations are resistant to Intervix® as well as other herbicides
• Frost in the few days after clethodim application reduces activity of the herbicide on annual ryegrass
• The best management for clethodim resistant annual ryegrass in break crops is a pre-emergent herbicide followed by a mixture of clethodim plus butroxydim on small ryegrass plants

Herbicide resistance in annual ryegrass

Surveys across South Australia and Victoria from 2005 show an increase in incidence of resistance to herbicides in annual ryegrass populations (Table 1).  We have recently started testing Intervix® and have found that many annual ryegrass populations are resistant to this herbicide. 

Table 1. Percentage of paddocks with herbicide-resistant annual ryegrass in cropping regions of South Australia and Victoria

Resistance to clethodim and its management

Resistance to clethodim is increasing in southern Australia.  This is a problem because there are no highly effective alternatives to clethodim for controlling annual ryegrass post emergent. This means control of annual ryegrass in break crops will be dependent on pre-emergent herbicides and crop-topping practices.  We conducted a trial to examine the utility of various treatments for the control of clethodim-resistant annual ryegrass in canola and faba beans at Roseworthy in 2012 (Table 2). 

Table 2.  Control of clethodim-resistant annual ryegrass in canola and faba beans at Roseworthy in 2012. POST herbicides were applied 7 weeks after sowing.

Relying on pre-emergent herbicides alone will be insufficient to control annual ryegrass in canola or faba beans. A few experimental treatments provided some good early control of annual ryegrass, but all failed to control annual ryegrass seed set.  Some treatments also caused damage to the crop, which reduced competition against weeds.

While this population had resistance to clethodim, the application of Select® + Factor at 7 weeks after sowing provided the best reduction in annual ryegrass seed heads and highest yields. A pre-emergent herbicide followed by Select® + Factor on small weeds appears to be the best approach to management at the moment.

Growers are reporting that control of annual ryegrass with clethodim can be variable.  In part this is caused by resistance.  Clethodim resistance can range from low to very high and the varying levels of clethodim resistance coupled with a range of rates of clethodim used make for variable results.  In addition, a lot of clethodim is used in late winter when conditions are cold and this reduces its performance. 

We conducted a trial under controlled conditions where annual ryegrass seedlings were exposed to 3 consecutive nights of 4^oC prior to or immediately after clethodim treatment to mimic the effects of a light frost.  Cold treatment prior to clethodim application had no effect on clethodim activity.  However, cold treatment after clethodim application reduced clethodim activity (Figure 1A).  The amount of clethodim required to reduce biomass by 50% doubled with frost treatment after application.  A similar effect was seen with a clethodim resistant population (Figure 1B).

Figure 1.  Response of susceptible (A) and resistant (B) annual ryegrass populations to clethodim with 3 days of simulated light frost prior to or post application of clethodim.  Annual ryegrass seedlings were treated at the 3-leaf stage.

This reduction of clethodim activity from frost after herbicide application could be a factor in the variable response of clethodim in late winter.  It will reduce activity further when clethodim is applied to large annual ryegrass seedlings where resistance to clethodim is present.  In the presence of resistance to clethodim, applying the product to smaller plants and under warmer conditions will improve control.

Current state of glyphosate resistance

The amount of glyphosate resistance continues to increase in Australia. At present there are 6 weeds species with confirmed glyphosate resistant populations (Table 3).  Glyphosate resistant populations are present in all states of Australia.  Chemical fallows and non-crop areas still make up a large number of the situations with glyphosate-resistant weeds.  However, growers are seeing more small patches of glyphosate resistant weeds in cropping paddocks after application of knockdown herbicides.

Table 3. Weed species with resistance to glyphosate in Australia

Contact details
Christopher Preston,
School of Agriculture, Food & Wine, University of Adelaide,
christopher.preston@adelaide.edu.au