The prolonged period of dry weather followed by early May rainfall is likely to affect herbicide strategies for potato growers this season.
The bulk of weed control in the sector is carried out using residual chemistry applied to ridges shortly after planting, however, April’s average rainfall dropped below 15mm and meant that weeds did not have the moisture to germinate. Further to this, some growers have seen ridges degrade.
Corteva Agriscience’s field technical manager, Craig Chisholm, says this scenario means recent rainfall – or irrigation – will stimulate both surface weeds and those coming from depth.
He said: “The dry, cold weather means potato growers are in a different position this year when it comes to weed control and strategies will have to be adapted to 2021’s conditions.
“Dry ridges will lose soil from their sides, causing disruption of the seal required for residual herbicides to be effective, and will allow weeds to germinate and grow away unchecked once stimulated by the arrival of moisture.”
Unchecked weeds can impact yield and slow down harvesting machinery, however, a greater threat may come from the increasing risk of blight through sheltering leaves or a more favourable microclimate.
Mr Chisholm added: “The aim is always to start with a clean field then tackle any late germination but, with such limited rainfall, a more effective course of action would be to apply a herbicide when weeds begin to actively grow.”
He recommends Titus, containing the active ingredient rimsulfuron, which targets the key problem weeds and could be a valuable tool in a dry season when pre-emergence activity has been affected. It can be used in a tank mix with metribuzin, where varieties permit, or alone with a wetter to provide post-emergence control across all varieties, excluding seed crops.
Mr Chisholm said the key weeds to target are charlock, cleavers, chickweed, hemp-nettle, small nettle and volunteer oilseed rape. Polygonums and couch grass should also be taken out.
He said: “We have seen rain since the Bank Holiday weekend and in many areas, this will stimulate germination after the damage was done in terms of a pre-emergence herbicide.
“We would advise a mixture of Titus plus metribuzin and a wetter to broaden the weed spectrum, but care should be taken to check that the variety is tolerant of metribuzin before adding to the mixture.
“Being a sulfonylurea herbicide Titus will work most effectively against small actively growing weeds, so application should target weeds at cotyledon four-leaf stage.”
Application timing should be from the rosette stage until 25cm, at which point hitting the target weed may be impaired.
For further details go to www.corteva.com