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Resistance testing informs herbicide use plans

Matt and James Toscan have been growing cotton for seven years at Darlington Point in the Murrumbidgee. Speaking to their CottonInfo Regional Extension Officer, Matt Toscan outlined how they are managing the threat of resistant weeds.

The primary weeds encountered on their farm are barnyard grass, blackberry nightshade, sowthistle, fleabane, annual ryegrass and thistles.

Matt Toscan.

Matt and James have tested barnyard grass, sowthistle and ryegrass for resistance over recent years. The barnyard grass and sowthistle were found to be susceptible to glyphosate (Group M), while the ryegrass was found to be resistant to by glyphosate and Group A herbicides.

To counter the threat of further incursion and resistance, Matt and James have an integrated weed management strategy (IWM) that is developed over the full course of the year, in close consultation with a crop consultant. The summer crop program is decided in July–August, and the winter crop program in March–April.

“We recognise that weed resistance is a threat to the long-term cropping viability of intensive irrigation areas, which is why an integrated weed management approach is so important to us,” said Matt. “There are no new herbicides, so we need to preserve our chemical options by making use of non-chemical control tactics.”

“Weed control in cotton will never be as easy as it is right now, while we are a new area and can rely on glyphosate for summer weed control. Being so easy and effective is both a strength and a weakness in the current weed control system. We need to think long term and use multiple tactics.”

The Toscan’s take a whole farm approach to management of weeds, with particular attention paid to non-cropped areas, such as the sides of fields, channels, roadways, drains and bankless channels.

Their IWM strategy involves a range of control measures, including pre- and at-plant residuals, pre-emergent knock downs, post-emergent herbicide applications and chipping, particularly for ryegrass and milk thistle. The pre-emergents are used sparingly, due to the possible cold conditions at the start of the crop. Not all control measures have worked, so the strategy has evolved over time.

“We’ve learnt that spraying in less than ideal conditions can result in spray application failures – like spraying barnyard grass when it is too hot. Timing of applications is critical: there can be advantages re timing with aerial application,” James said.

Growers Matt and James Toscan
Cropping area Total hectares: 4200 ha. Cotton: 1100 ha irrigated
Soil type Majority Grey/brown self-mulching clay with some Transitional Red brown earth
Rotations Two-year rotation: cotton (summer); durum wheat (winter); fallow (summer and winter); cotton (summer)

Source: CottonInfo Weed Control case studies

Further information: CottonInfo weed management page

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