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Weather challenges for post-em herbicide applications

Agronomy Issues
29.10.2020
Post emergence wheat timing

Adverse autumn weather conditions have now seriously disrupted remaining drilling, and the opportunity to apply pre- or peri-emergence herbicides.

Even where breaks in the weather enable planting to muddle on, soil conditions limiting further field work has often precluded spraying.

Georgina Wood

For many that will mean a switch to post-emergence application as and when opportunities arise. But that seriously limits growers’ options, especially for residual herbicides, warns Syngenta technical manager, Georgina Wood (above).

“With pre-emergence applications the crop is growing from seed drilled down below the herbicide treated zone,” she pointed out.

“Weeds germinating in that upper zone quickly absorb the herbicide as they germinate and can be very effectively controlled, whilst the crop is able to tolerate it as it emerges from beneath" Georgina explalined.

Wheat seedlings post emergence

“Once the crop is emerged, however, it no longer benefits from the positional selectivity away from the herbicide zone and is directly exposed to subsequent sprays.”

Herbicide options

That typically means growers have to avoid the highest rates of herbicide that can offer the best levels of  control at the pre-emergence timing, as well as resulting in the need to reduce the complexity of herbicide tank-mixes at the post-emergence timing.       

Extra care should be taken with application on small leaved crops in conditions that mean they are metabolising slowly, where soft growth has yet to be frost hardened or where frosts are imminent, Georgina advised.

“Post-emergence herbicide choice may need to be tailored on almost field-by-field basis," she advocated.

"For winter wheat when beyond the ‘green haze’ of peri-emergence and at 2-3 leaves (GS 12/13), good options remain with Defy at up to 3.0 l/ha in some mixes, which can still deliver effective contact and residual grass weed activity."

Wheat at post-emergence

“An agronomists’ understanding of different products’ actions and mixes will be important in tailoring decisions.”

Options for post emergence grass weeds in barley are much more limited, which will make crop competition from vigorous hybrid varieties especially useful this season, she added.

Application advice

Where a post-emergence treatment is possible, Syngenta application specialist, Harry Fordham, advocates the favoured 3D nozzle can optimise targeting contact with small grass weeds, as well as enhancing soil coverage with residual components.

“In practice, however, wind conditions can all too often compromise application in the autumn,” he reported.

“Switching to an appropriately sized Amistar nozzle would currently be the next best option, along with operating at less than 10 km/hr and maintaining boom height at 50cm.

3D90 nozzles

“Trials with a completely new Syngenta design of angled 90% drift reduction nozzle, however, could offer the next step in combining improved targeting of weeds with drift reducing capability,” he added. Further trials with the new nozzles are planned for Syngenta Innovation Centres this season.