Growers face difficult decisions with post-emergence residuals in OSR
Weather conditions are not quite right for post-emergence residual herbicides in winter oilseed rape, but the risk of not getting them on is posing a few difficult questions for growers, says ProCam’s Agronomy Manager, Nick Myers.
’With the diminishing herbicide options for grass-weed control in cereals and the problem of increasing weed resistance, it is important that we take advantage of all opportunities to control grass weeds across the rotation,’ he says.
’Oilseed rape gives the opportunity to use herbicides with different active ingredients and different modes of action not available in cereals.’
But successful weed control with the residual-acting herbicides propyzamide and carbetamide relies on maximising their performance and this means that soil temperatures and soil moistures need to be right, he explains.
’Ideally soil temperatures at 30cm should be 10’C and falling, whilst soil moisture deficits should be 50mm or less. In many parts of the UK soil moistures are OK but soil temperatures need to get lower, which they appear to be doing now.
’The reason we need cold soil is that it slows down weed growth and so grass weeds are less likely to outgrow the herbicide treatment. Cold soils also reduce the rate of microbial breakdown of the residual herbicides, increasing their persistency so that weed flushes are controlled.’ more
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If soils are too warm, weeds continue to grow and the herbicide degrades too quickly leading to insufficient concentration in the rooting zone, Nick explains.
’Soil moisture levels are about right, which means that herbicides can be redistributed into the rooting zone of the weeds, where they are taken up more effectively. If soils get any wetter, we need to be aware of the need to avoid any risk of herbicides getting into water.
’Usually applications in November and December offer the best results, but this year it is more likely to be towards the end of November onwards. If you want to improve weed control consistency, mixing a contact graminicide with carbetamide or propyzamide such as Laser (cycloxydim)has been shown to improve performance, even when black-grass is showing levels of tolerance resistance to that contact graminicides.’
The use of an organo-silicone adjuvant can enhance the performance of propyzamide and carbetamide, adding around 10% more weed control, he adds.
’These wetters help to optimise distribution of the herbicide in the weeds’ rooting zone, creating a more uniform horizontal zone of product and spreading it around the surface of the soil particles.
’Both propyzamide and carbetamide are not recommended in cereals and offer important alternative herbicide groups to incorporate into an effective grass-weed control programme. They must be used carefully, particularly when it comes to risk of water contamination, so that they are retained for future use,’ Nick Myers concludes.
