Spraying-off patches of cereals badly-infested with grassweeds this season can be highly cost-effective in preventing disastrous levels of weed seed return, according to leading agronomy advisers. But only if it is done at the right time and with sufficient care.

In particular, they urge growers to set their own thresholds for patch spraying, plan ahead with as much flexibility as possible, avoid spraying too early, and employ the most effective glyphosate treatment and spray practice.

“Spraying-off part of your crop may be drastic, but it can save a lot on future herbicide bills as well as cultural controls in the rotation,” said ProCam head of crop production, Nick Myers. “It’s worth considering either where black-grass is just beginning to show its ugly face in fields or to tidy-up remaining patches of infestation in concerted control programmes. And it’s especially valuable where weed populations are known to be herbicide-resistant.

“If you can prevent the unbelievably large amounts of seed produced by even small patches of black-grass and brome getting back into the ground it’s surprising how much difference you can make. In one field I always remember passing, a patch of black-grass sprayed off in June was still clearly visible from a distance as a clean area of crop two years on in the rotation.”

“You can stop the return of up to 30,000 black-grass seeds/m2 by summer spraying,” pointed out Roundup technical specialist, Barrie Hunt.  What’s more, at current feed wheat prices you will be sacrificing as little as 10p/m2 in crop income to do it.  This makes it extremely cost-effective when compared with alternatives like higher future in-crop herbicide use, delayed winter wheat drilling or spring cropping.”

 Under these circumstances, it’s hardly surprising that fully 55% of the 380-plus growers involved in the most recent National Grassweed Control Study are currently employing the practice, with 42% undertaking hand-rogueing and 28% both these summer controls.

 Nick Myers and Barrie Hunt suggest growers set a threshold for patch-spraying based on the opportunities they have for effective cultural controls in the rotation and the resistance status of their grassweeds. The fewer the opportunities and the greater the resistance problems, the lower the level should be.

They then advise prioritising fields for treatment once the effectiveness – or lack of it  – of their last post-em herbicide spray has become apparent. Because the level of weed seed return depends as much on seed heads/plant as plants/m2, though, they recommend keeping sufficient flexibility to finalise spraying plans only following seed head emergence towards the end of May.

 “Glyphosate always works best once grassweed stem extension is over and seed heads are fully emerged,” stressed Barrie Hunt. “As a rule, you have a good three weeks between seed head emergence and the first weed seeds becoming viable. So you have everything to gain and nothing to lose by being patient and holding-off on spraying until mid-June in most cases.

“Apart from anything else, you’ll be far more accurate in targeting your weed patches if you wait for the bulk of the seed heads to be clearly visible above the crop.”

With efficacy and accuracy essential for the best result at the least crop cost, both advisers are adamant that growers cannot afford to employ anything less than the most effective treatment and the best practice in their patch spraying.

“This is the most precise glyphosate application you ever have to do,” Nick Myers insisted.

 “You must get enough highly-active glyphosate onto the weed leaves within your crop canopy under what can be very unsettled summer conditions – hot and dry one day with a sudden downpour the next.  At the same time, you need to minimise the amount of crop you destroy  beyond your weed patches.”

“This delicate balancing act is best achieved with 1080 g/ha of glyphosate in a modern formulation that brings both proven activity under challenging conditions and the greatest rainfastness,” advised  Barrie Hunt. “Formulations like Roundup Powermax with the added advantage of drift-minimising characteristics are of obvious value here too.

 “But it’s absolutely no good using a decent glyphosate if you don’t apply it carefully enough.  You want good canopy penetration but not at the expense of nozzle and pressure combinations that cause more than minimal drift. And you certainly want the stillest spray day you can find.”

“Whatever you do, don’t travel too fast either,” added Nick Myers. “Taking your time is vital if you are to use your spray boom shut-offs to target your weed patches as accurately as you need to. Just like spray timing, your patience will really be rewarded. 

“If you want more accuracy, of course, you could consider spraying with a knapsack or even hand-rogueing. Only for isolated patches of grassweeds or lower populations and where you have the time or can get reasonably-priced labour, though.  Otherwise, they’re unlikely to be financially viable.”