Impact of dry conditions on autumn weed control

After a record-breaking spring, dry soil conditions mean that patience is vital for controlling weeds between harvest and drilling.

In England, it was the driest spring in over 100 years; the warmest ever; and the second sunniest. Despite some rain in early summer, ground conditions remain very dry on most arable farms. 

Post-harvest plans

“After an early harvest, farmers are understandably keen to get moving with autumn workload.

“Unfortunately, if conditions remain hot and dry, it’s probably better to do as little as possible from a weed control standpoint,” comments Mr Oakley.

Even a light cultivation soon after harvest is unlikely to encourage germination because there is not enough moisture. It’s better to leave weed seeds on the soil surface to suffer the effects of UV light and predation.

Unless there are other pressing reasons for cultivation such as soil remediation or incorporating organic matter, it’s a case of being patient.

Stale seedbeds are one of the most effective integrated weed management tools, but they need moisture. Wait for significant rainfall to stimulate weed germination and improve conditions for cultivation if needed. 

“The biggest danger in dry weather is attempting to drill before spraying off a significant flush of grass weeds. The seeds are still there; they will germinate in the crop and be difficult to control. Added to that, residual chemistry tends to perform better in moist soils,” adds Mr Oakley.

“The minimum rate for Roundup to grass weeds at the seedling stage is 540g/ha. For bromes and Italian ryegrass up the rate to 720g/ha.”

Glyphosate stewardship

Mr Oakley emphasises the importance of glyphosate stewardship following the announcement of the first case of resistance earlier this year.

Most important is to avoid repeat applications of glyphosate to the same weeds, use cultivation or another method to control survivors. He also suggests looking out for any weeds which have survived in the stubbles from the previous season as these will be larger and may need a higher rate. 

Yorkshire AICC agronomist Patrick Stephenson anticipates less ploughing and more direct drilling this autumn as a result of the dry weather, which is likely to cause an uptick in brome problems.

Last autumn, heavy rain in September forced many farmers to drill much later than planned in October and November when conditions improved.

This was a win for cultural control and a win for herbicides as the efficacy of the products was very good in moist seedbeds. 

“Even after the success of late drilling, I still think many farmers will be minded to drill wheat earlier this autumn because of low prices and the limited break crop options available. For weed control, I think there’s much more discipline around timing of residual herbicides which is important,” says
Mr Stephenson.

“There is some good new chemistry for black-grass and ryegrass but they are not a miracle cure. In ideal conditions, the programme will give 90–95% control but it can be lower in less helpful conditions. If soils remain dry, splitting the doses to have a robust application at the second peri-em timing is a sensible approach.”

Mr Oakley agrees with this approach. “Proclus is less dependent on soil moisture for efficacy so fits in well at the pre-em timing. Using it in a mix with a metribuzin co-form like Alternator Met, Octavian Met or Cadou Met delivers four different modes of action.”

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