“Pre-ems work best when applied to moist seedbeds within 48 hours of drilling,” says Darren Adkins, commercial technical manager at Bayer CropScience.
“Clearly this isn’t always possible; a lack of rainfall in autumn has prevented ideal conditions a number of times in recent years and many farms do not have the capacity to cover their whole cereals acreage within 48 hours when rain does arrive.”
Where growers are forced to compromise do not delay herbicide applications - timing and growth stage are more important than moisture availability, says Mr Adkins.
“Applying a pre-em to bone dry soil is likely to result in less-than-optimum efficacy,” he admits. “But missing the boat and applying residuals to grass-weeds that have 1-2 leaves and are actively growing away will result in much poorer levels of control.
“Until the recent band of rain moved across the UK, soils were very dry,” says Jock Willmott of Strutt and Parker. “And the rain didn’t reach everywhere, Essex for example didn’t get much at all and conditions will still be very dry indeed.
“Even where rain did fall, it generally wasn’t heavy and where soils are subsequently cultivated, there won’t be much moisture in the top two inches. Many soils are still too dry for drilling and crops won’t chit until we get more precipitation,” he says.
“But once you’ve got moisture, use it; get the crop in and seal it with pre-emergence herbicides.”
Darren agrees: “An alternative to holding back with pre-ems is to delay drilling until rainfall is forecast and follow up with the sprayer soon after.
“Pre-ems need to be applied within 48 hours of grass-weed seed germination in order to maximise control but seeing as most growers are far too busy to be digging about in freshly drilled fields at this time of the year, the safest option is to apply residual herbicides straight after drilling.
“Looking at the forecast we’ve showers predicted for the coming weeks - with reasonable temperatures, low dormancy in seed, a restricted window for stale seedbeds and in many cases, high seed return - we’ve ideal conditions for black-grass germination in-crop.
“Make the most of the opportunity the showers bring - hit grass weeds hard and sensitise those that are difficult to kill for an autumn application of Atlantis WG (iodosulfuron + mesosulfuron).”
Cambridge, 12th, September, 2013