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Making The Most Of Every Out-of-Crop Weed Control Window

Making the most of every window of weed control opportunity between crops is now an agronomic essential rather than the valuable ‘weather-permitting’ bonus it used to be.

Particular windows need to be identified ahead of time, extended wherever possible and exploited with the right cultural controls if increasingly challenging grass weed and volunteer problems are to be tackled in an affordable way.

This was the firm advice to agronomists and growers across the country from a recent grass weed management forum at ADAS Boxworth.

“To deal with the sort of grass weed problems so many fields are suffering these days, we simply have to do things differently,” stressed ADAS weed scientist and business manager, James Clarke. “Maximising weed control ahead of drilling with the most appropriate combination of cultivation and glyphosate is essential to take the pressure off hard-pressed
in-crop chemistry.”

“Far more emphasis needs to be placed on the ‘out-of-crop’ part of the year between the ripening of one crop and the emergence of the next,” added Monsanto Crop Protection technical manager, Barrie Hunt. “With the revolutionary chemistry of little more than 10 years ago now giving barely 20-40% black-grass control in many cases, I’d go so far as to say this will be essential to the viability of many combinable crop rotations.”

In-crop herbicide choice and efficacy remain important considerations, the specialists agree; especially so in winter oilseed rape after wheat or wheat after late-harvested roots where the out-of-crop windows are so short.  However, in most combinable cropping transitions they see very valuable windows for effective pre-planting control.

“You also have the opportunity to extend your windows,” James Clarke  pointed out. “After all you don’t have to drill your wheat on the first available day or cultivate your rape stubble as soon as the combine has left the field. 

“Delaying autumn wheat drilling to give you time to spray off two good flushes of germinating black-grass ahead of planting can pay huge dividends.  And where your weed problems are particularly acute or your soil conditions don’t lend themselves to decent later autumn seedbeds you always have the option of spring sowing which allows you to tackle even more weed growth in the meanwhile.

“Cultivating too early can be as harmful as drilling too early,” he warned. “Black-grass will germinate perfectly well under a mulch of chopped straw. So if all you’re doing by cultivating right behind the combine is losing precious moisture, you’d be better off preserving it to get both a good weed seed chit and for your next crop. In many cases, it may be preferrable to hold off on your cultivation for up to four weeks.

“It’s all a matter of thinking about the out-of-crop windows available and making the very most of them. As well as flexibility in timing at both ends of the window, flexibility in cultivation type to address the particular weed problems you face is important. Equally, you have the flexibility to adjust your rotation to prevent as well as cure problems.

“More radically too, if things get really bad you can create yourself a different window with different opportunities.  For instance, silaging a badly-infested cereal eliminates seed return as well as giving extra time for optimising pre-planting control.

There are many advantages of aiming weed control at the out-of-crop period. Not least, the particularly cost-effective opportunities that cultivations and non-selective herbicides together offer for  reducing weed populations without increasing selection pressure for resistance.

Unlike so many actives, there are currently no known cases of weed resistance to glyphosate in the UK. And Barrie Hunt is adamant that using the chemistry correctly is vital to keep it this way as well as ensuring the greatest value for money.

In this respect, he underlined the importance of making every pre-harvest, pre-planting or post-plant/pre-emergence application count; using the right rate of the most reliable glyphosate; not reducing active ingredient rates when adding adjuvants; and, applying it to the greatest effect.

“There are currently more than 150 professional glyphosate products on the UK market,” he explained. “They vary in their active ingredient loading from 36%  right up to 72%. They’re based on no less than four different salts, all with their own properties. And they come in an unbelievable range of formulations, from simple tallow amine adjuvants to power surfactant complexes.

“So it’s not surprising that modern Roundups are proven to be 10% more effective than less well-researched glyphosates on tough annuals and perennials, 20% better under hot/dry and cool/dry conditions, 25% better with rain in less than two hours and 25% better in hard water.

“Even they have to be used at the right active ingredient rates for the weeds in question, though,” he continued. “For instance, 540g/ha of glyphosate is needed for black-grass seedlings, other annual grass weeds and OSR volunteers of up to six leaves while heavy infestations of couch and other perennials require 1440 g/ha. Adjuvants simply will not make up for an insufficient dose of glyphosate.

“It’s also important to get Roundup water volumes, nozzles and spray timing right. For stubble treatment bear in mind that lower water volumes give the best results with the correct nozzles; flat fan nozzles are most suitable for targetting seedlings; and, populations should be treated before their density restricts spray coverage.”

Both James Clarke and Barrie Hunt see effective out-of-crop weed control as essential where grass weeds are problematic.

Failure to create the right windows to do this and take full advantage of them is, they have no doubt, simply not an option with every 100 ears/m2 of black-grass known to knock wheat yields by 1 t/ha, growing herbicide resistance across the country straining an increasingly limited chemical arsenal, and feed wheat values at little more than £100/t.