Resistance risk to eliminate herbicides on irrigated farms

This can be a high-risk practice unless survivors are removed after every spray application, as there is no crop competition to restrict weed growth, resulting in the production of large volumes of seed.

Eric Koetz, NSW DPIRD weeds research agronomist, says that having limited options for managing weeds along irrigation infrastructure and other non-crop areas is a problem, and is putting additional pressure on knock-down herbicides in irrigated systems.

“Recent surveys of cotton fields in Queensland and NSW showed that cotton paddocks were generally relatively weed-free, however, the incidence of resistance to glyphosate is quite high in fleabane and windmill grass samples collected mainly from non-crop areas on cotton farms,” he said.

“Preliminary results from the samples collected across cotton farming systems in 2020–21 show 95 per cent of fleabane samples, 80 per cent of feathertop Rhodes grass samples, 70–80 per cent of windmill grass samples and 20 per cent of sowthistle samples tested as resistant to glyphosate.”

“Keep in mind that flaxleaf fleabane, feathertop Rhodes grass and windmill grass are not registered for control with Roundup Ready Herbicide with PLANTSHIELD as control of susceptible plants is generally poor, so further selection for resistance only exacerbates the problem,” he said. “Although still lower, the level of resistance emerging in sowthistle is very concerning, given that this species is listed on the herbicide label. Recent surveys by Crop Consultants Australia (CCA) found that growers also identify annual ryegrass and barnyard grass as showing signs of glyphosate resistance in the field.”

Eric says the lack of diversity in herbicide use in many cotton systems is likely to contribute to the increased incidence of herbicide resistance.

“The attitudinal study by CCA indicates that growers are increasing their use of pre-emergent herbicides to add diversity; layering weed control tactics such as a post-emergent or lay-by herbicides in addition to their applications of glyphosate in cotton,” he said. “The label for Roundup Ready Herbicide with PLANTSHIELD states that this product must not be the only form of weed control used in Roundup Ready Flex cotton varieties.”

The updated Cotton Herbicide Resistance Management Strategy (HRMS) has shifted the focus from glyphosate alone to include more weed control tactics in the crop and fallow periods. The old HRMS, which served the cotton industry well, focused on 2 non-glyphosate tactics in fallow + 2 non-glyphosate tactics in-crop and 0 survivors. The new HRMS now centres on adding as many as six different weed control tactics during the fallow and cropping phase of the farming system.

“In the next 5 to 10 years, there will need to be a shift towards non-herbicide controls such as robotic cultivation and microwave technologies, which are well-suited to summer cropping on rows or beds,” he said. “Until then, optical spray technology is a good option for growers to keep weed numbers low in the fallow.”

Spray drift of Group 4 [I] herbicides (e.g. 2,4-D products) late in the summer fallow to control large fleabane is causing considerable damage to cotton crops. Eric recommends growers change their fallow weed management program to target small plants earlier in the spring, before cotton emergence, using a double-knock of glyphosate followed by cultivation or paraquat, plus a residual herbicide as the second knock.

Related links

• Protecting knock-down herbicide options
• Watch ‘Minimising glyphosate resistance’ video on YouTube

First published in March 2017, this article has been reviewed and updated.