Spring weed control needs extra care in stressed crops – South East Farmer
Heavy rain and flooding throughout the autumn and winter have created some real challenges for spring weed control ...
Heavy rain and flooding throughout the autumn and winter have created some real
challenges for spring weed control, says Hutchinsons agronomist Lizzie Batt, based out
of the Canterbury office. She recommends carefully assessing crop condition and weed
pressure before deciding on the most suitable strategy.
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.
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 the weather has already done.
Address crop stress first
Where crops are stressed, due to waterlogging or, potentially, in cases where pre-em has been washed into the rooting zone, 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 is 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
Decisions must be made on a fi eld-by-fi eld basis. 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 fi eld, 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 or brome.
In terms of residuals, 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 important to understand the issues faced.
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. There is likely to be more reliance 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.
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.
Correct rates and timing will be more important than ever; otherwise this could compromise control and increase resistance risks. Avoid using products if it’s too frosty, as they aren’t going to work.
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