Spring weed control needs extra care in stressed crops

As Hutchinsons northern technical manager, Cam Murray, explains, at one end of the scale, there are those crops sown relatively early last September, that established nicely in good conditions and received a timely pre-emergence spray that, in many situations, worked well in moist soils.

Conversely, there are later-sown wheats drilled after the weather broke, perhaps into sub-optimal conditions and without any pre-em herbicide. There are also crops that, regardless of drilling date, have struggled to establish, or suffered rooting damage due to severe flooding or waterlogging.

On the worst-affected areas, tough decisions may be needed as to whether any re-drilling is required, but with a shortage of spring cereal seed this year, options are limited.

Mr Murray warns that where crops have experienced prolonged periods of waterlogging and anaerobic conditions, compromised root development is likely to increase crop stress, so great care is needed when planning spring herbicide applications.

“Even if autumn weed control was compromised and you’re keen to get on with a spring herbicide once conditions allow, don’t apply anything if crops are showing stress or yellowing; it’ll just make a bad situation worse.

“Accurate spray timing is going to be paramount this spring and key to not doing any more damage than has already been done by the weather.”

Address crop stress first

Before doing anything, Mr Murray advises growers to carefully assess crop condition and weed pressure, then consider the most suitable strategy.

Where crops are stressed, due to either waterlogging, or, potentially where the pre-em has been washed into the rooting zone, he says it may be better to delay herbicide application, especially of stronger contact grassweed chemistry, and prioritise early nitrogen to stimulate growth and improve resilience to any potential herbicide effects.

“If it looks stressed, sometimes allowing an extra week or two to get the crop into a stronger position could be worthwhile.

“Phosphites can help stimulate roots to get the nutrition pumping around the plant system. Also consider nitrogen timing. Soil nitrogen reserves are very low, so a ‘little and often’ approach could be the way to go in terms of availability to the plant.”

Consider a residual top-up

Yorkshire-based agronomist Sam Hugill of Hutchinsons, estimates around 30–40% of winter wheat drilled in his area last autumn did not receive a residual herbicide due to the weather, putting more emphasis on spring controls.

“In an ideal world, this would involve contact herbicides to treat weeds already present, plus residual chemistry to catch later emerging weeds, and hopefully avoid the need to come back with more expensive contact chemistry later. But, with the weather-related problems we’ve seen and ongoing pressure on margins and costs, a perfect world may not exist.”

Decisions must be made on a field-by-field basis, he adds. “If, for example, it looks as though conditions will not allow travel until March to apply residual chemistry, there could be more sizeable weeds by that time, so the focus may be better spent on contact chemistry rather than residuals. There needs to be an ongoing discussion between grower and agronomist about what’s best for each field though.”

Even where crops received a pre-em, heavy rainfall will have dramatically reduced the persistence of residual chemistry, so a spring top-up may be required to control any flush of problematic grass weeds like black-grass, ryegrass and brome, says Mr Murray.

In terms of residuals, he says flufenacet-based chemistry, with diflufenican, picolinafen or pendimethalin, are the go-to options for black-grass control. But, with evidence showing wider resistance levels to flufenacet in ryegrass species, care is required as to its use, and testing is paramount to understand the issues faced. “If you are seeing levels of control dropping in your own circumstances, it probably points to an underlying issue with resistance.

“However, there’s been a lot of work done on three-way mixes of chlorotoluron, diflufenican and pendimethalin at the peri-emergence stage, which seems to show some uptick in the levels of control for ryegrass and bromes in particular, which looks quite positive. If you can get on at that stage, then use contact chemistry later, you can probably get decent control, although it’s all down to timing and the weather.”

For crops drilled last autumn that did not receive any pre-em, immediate weed pressure could potentially be greater, especially where black-grass emerged late. More reliance is likely to be on using stronger contact chemistry, such as iodosulfuron-methyl-sodium + mesosulfuron-methyl mixes, but ideally this should be accompanied by a residual partner to control later emerging weeds, Mr Murray says.

“That should tidy up a plethora of meadow grasses, grass and broadleaf weeds, and provide a level of residual control. But remember, in terms of black-grass and ryegrass in particular, a lot depends on the growth stage of the weed. Be realistic about the control you can expect to achieve, especially given the levels of ALS resistance out there generally. There’s no silver bullet.”

Mr Hugill also expects more use of contact products this spring, so reiterates the importance of correct rates and timing. “If rates are cut and optimum timings aren’t adhered to, that will compromise control and increase resistance risks.”

He says florasulam and pyroxsulam also offer alternative herbicide options, although weed size and temperature greatly affect control. “Avoid using products if it’s too frosty as they aren’t going to work.”

Spring weed control advice

• Assess weed pressure on a field-by-field basis, matching herbicide to target weed spectrum
• Avoid applying herbicides to stressed crops – address nutrition first
• Ideally use contact products for weeds already present, plus a residual for later-emerging weeds
• Beware of resistance issues – use appropriate products, rates, and timing
• Generally best to apply herbicides to small, actively-growing weeds, before tillering
• Good application is key to maximising efficacy.

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