Know the enemy: data on weeds frontline
GroundCover™ Issue: 121 | Author: Toni Somes
Herbicide resistance is here and “it is bad” is the confronting message to northern grain growers from the chief executive officer of the GRDC-funded grower solution group Grain Orana Alliance (GOA), Maurie Street.
The agronomist from Dubbo, New South Wales, says two years of survey data show herbicide resistance is increasing at an alarming rate across the central western NSW cropping belt and growers need to change tactics urgently to develop effective, long-term weed-management strategies.
Critically, he says, the data identifies a sharp rise in the number of glyphosate-resistant samples tested, from six per cent in 2013 to 57 per cent in 2014.
While Mr Street says the survey represents a “worst-case scenario”, with many samples taken from populations with suspected resistance, he warns the data reflects the future reality for the northern grains sector.
He says for far too long many of the region’s growers have clung to the false belief that herbicide resistance is “someone else’s issue” or “not too bad”.
However, results from the two extensive surveys coordinated by GOA through the 2013 and 2014 harvests to quantify the extent and type of resistance in problem weeds, such as annual ryegrass and wild or black oats, revealed multiple resistance in the vast majority of samples.
“In the GOA surveys both ryegrass and black oats demonstrated a high incidence of resistance, with 100 per cent of the ryegrass and 86 per cent of black oats showing resistance to at least one herbicide,” Mr Street says.
“Alarmingly, 94 per cent of ryegrass and 81 per cent of black oats also showed resistance to two or more alternate herbicide options.
“In several cases the incidence of multiple resistance was such that there would only be a few potentially effective herbicide options left that might control those weeds.
“This means there are populations in the central west of NSW that rival and possibly surpass the severity of Western Australian herbicide resistance.
“This survey work was invaluable for identifying which herbicide groups are most challenged in terms of effectiveness and it serves as a warning for growers that herbicides many have long considered ‘safe and effective’ options – such as Select®, atrazine and trifluralin along with glyphosate – have clear signs of resistance developing.”
Herbicides play a pivotal role in the northern region’s minimum-till and zero-till farming systems and in many cases no alternate methods or modes of weed control are employed.
Mr Street says the loss of effectiveness of glyphosate in particular could seriously challenge the sustainability of the central western NSW’s profitable cropping systems.
“Glyphosate is invaluable in the control of weeds in our fallow systems, which are essential to conserve out-of-season rainfall to achieve profitable crop yields,” he says.
“It is also important for managing pre-planting flushes of weeds, potentially the largest germination of winter weed.
“So the loss of the effectiveness of glyphosate will seriously challenge our sustainability.”
The GOA survey highlights significant levels of glyphosate resistance in three-leaf ryegrass, with 57 per cent of populations showing resistance at one litre per hectare, 18 per cent at 1.5L/ha and eight per cent at 2L/ha.
The data also shows there are generally lower levels of resistance in populations that have received fewer than eight applications, while higher levels were recorded where more than 100 applications were reported.
“A total of 97 per cent of ryegrass populations that demonstrated resistance at the lower rate of glyphosate also showed resistance to multiple alternatives,” Mr Street says.
“Worryingly, 66 per cent of them demonstrated resistance to five or more alternate herbicide groups.
“The positive – albeit it short term – is using a higher rate of glyphosate as per label rate on these samples is still an alternative, with resistance levels dropping from 57 per cent to eight per cent when the rate was doubled.”
In the immediate to longer term he stresses northern growers will have to be prepared to change tactics and incorporate chemical and non-chemical control options in their weed-management programs.
Know the enemy
However, before growers and agronomists decide on a course of action, Mr Street says it is critical they start testing survivor weed samples so they can map the resistance status of individual farms and paddocks as part of the process to develop effective long-term strategy for the management of hard-to-kill weeds.
“Unless growers understand the resistance status of weeds in their paddocks they cannot have confidence that existing control measures will work into the future,” he says.
“These surveys prove we need to employ new tactics to control weeds.
“The first step for growers is to determine the extent and nature of herbicide resistance and, once that is clear, develop an effective program of chemical and non-chemical control options.
“There has been interest in harvest weed-seed control measures, such as narrow windrow burning, to start the campaign against resistant weeds and I expect wide adoption of these non-herbicide tactics over the next few years.”
More information:Maurie Street,
0400 066 201,
Report on the 2013 GOA herbicide resistance survey
‘Time to open the herbicide resistance toolkit'
GRDC Project Code GOA00001
Region North, South