Fresh Weed Control Approach Advised To Ease Establishment Workloads

July 2006 - Ever-increasing workload pressures mean UK growers need to take a fresh approach to their establishment weed control strategies from this season, tackling any perennial weeds ahead of harvest and concentrating on a fast turnround stale seedbed or stubble regime for annuals.

This is the view of leading agronomists, Nick Myers of ProCam and Clare Bend of Masstock who see the new approach as the best way of keeping on top of the arable weed challenge at the least possible cost without compromising critical crop establishment schedules.

"The best and most economic time to deal with weeds is undoubtedly outside the actively growing crop," points out Nick Myers. "A well-timed dose of Roundup between harvest and drilling gives you a complete weed kill without the compromises inevitable with in-crop spraying. By eliminating a substantial proportion of the weed reservoir, it really takes the pressure off your post-em programme and helps keep your herbicide bill down. And, of course, it's vital if you're managing resistant weed populations.

"However, while most growers could count on a month or so between harvest one winter crop and drilling the next not to so long ago, the sheer scale of autumn workloads these days means many now have barely three weeks. This really doesn't give enough time to get sufficient perennial weed growth for effective spraying-off before drilling."

"In fact, with the autumn weather as unpredictable as it seems to have become, in many cases growers may even struggle to get in a good stale seedbed or stubble annual weed treatment if they aren't very careful," adds Masstock's Clare Bend. "Especially so if rainfall after harvest is negligible or they fail to follow best pre-planting weed control practice.

To cope with these pressures without compromising weed management, the agronomists suggest using pre-harvest treatment in both cereals and oilseed rape for controlling perennial weeds where necessary as well as managing the harvest. Like post-harvest treatment, this is outside the actively growing crop. Yet it takes perennial weed control conveniently out of the crop establishment phase, allowing pre-planting treatment to be focussed almost entirely on annuals.

"Pre-harvest Roundup Max is ideal for tackling perennial weeds," Clare Bend stresses. "Although hidden from sight at the base of the crop, they're often at their most susceptible growth stage at this time. The canopy also preserves moisture at or near the soil surface, ensuring the weeds are actively growing. This ensures the glyphosate works faster and more effectively.

"In timeliness terms, it's infinitely preferable to the post-harvest alternative of waiting 4-6 weeks for enough new growth to emerge for spraying to be effective followed by another five days to give the chemical the best chance of working before seedbed preparation.

"It also makes perfect sense with so many growers already appreciating the benefits of pre-harvest Roundup for oilseed rape desiccation and cereals harvest management," she notes. "The application rate will need to be increased to 2 kg/ha for perennial weed control, but that's only a slight increase on the normal OSR desiccation rate. And, of course, it's effectively transferring the extra chemical from the post-harvest treatment, since this can now be focussed almost exclusively on annual weed control at lower rates."

"Putting more emphasis on weed control pre-harvest will be particularly worthwhile this season," observes Nick Myers. "While last autumn's residuals generally worked well, too many people drilled early without leaving sufficient time for good pre-planting treatment. On top of this the cold, late season seriously compromised herbicide programmes this spring, leaving many grass weeds very well grown by the time they were sprayed.

"As a result, we're seeing larger populations of stunted but still fertile grass weeds than usual in the base of crops to interfere with harvesting and carry-over into next season. These, and the surprising amount of couch we're also finding in some places, will be well cleared-up if more attention is placed on pre-harvest control."

As well as saving critical establishment time, bringing perennial weed control forward with pre-harvest treatment, of course, is also likely to mean more effective annual and volunteer control, the agronomists explain. Primarily because post-harvest cultivations no longer need to be delayed to allow the greatest possible shoot emergence.

What is more, those still harvesting conventionally dried crops will find the dramatic increase in cereal and oilseed rape combining speeds possible following pre-harvest Roundup give them a hugely valuable extra time bonus to invest in establishment.

"Pre-harvest treatment can easily cut cereal combining time by a quarter, giving you far more leeway for your cultivations," continues Nick Myers. "And taking perennial weed control out of the post-harvest equation means you can focus uncompromisingly on rapid cultivation and consolidation after harvest, using the accumulated soil moisture to stimulate the quickest and most complete flush of annual weed germination.

"You should be able to get good greening-up in about two weeks - three if the ground is dry - and you can prepare the seedbed just six hours after spraying with Roundup Max, thanks to its faster uptake and more efficient translocation. This means you won't have to sacrifice good pre-planting weed control to ensure you get your following crop in on time."

"For the most effective and safe, pre-harvest treatment I'd always recommend a modern glyphosate like Roundup Max," Clare Bend advises. "In contrast to older formulations, it's rainfast in only an hour on annuals and works equally well in hot, dry conditions. This gives you altogether more flexibility and less risk in your post-harvest programme too.

"Don't use pre-harvest glyphosate on cereals or oilseeds that are going for seed, though, as it can adversely affect germination and establishment " she warns. "I wouldn't advise using it on cereals into which oilseed rape is to be autocast either, as there can be a transfer of chemical from the straw surface to the emerging rape seedling in certain circumstances. Also, be sure to leave at least seven days before harvesting for cereals and 14 days for oilseed rape to minimise the danger of grain residues.

"With these few exceptions, wherever perennial weed control is necessary I think it should be brought forward if at all possible. It makes so much sense every way you look at it in today's high pressure systems."