First class stubble management will be more important than ever this autumn to deal with high levels of seed return from problem grassweeds after the ‘double whammy’ of the winter deluge and spring drought; especially so, wherever weather worries get in the way of wheats being drilled as late as they need to be for the best control of black-grass, bromes and ryegrass.
Bayer weed control specialist, Tom Scanlon points to surprisingly high levels of black-grass infestation seen in many very late-drilled crops this summer where the weather got in the way of seedbed quality and/or peri and post-em spraying.
At the same time, he urges everyone to be particularly conscious of the serious grassweed legacy from poor and patchy spring crops established in a bone dry April on soils damaged by the wet winter; not to mention OSR that has struggled to shake-off the challenge of cabbage stem flea beetle larvae.
“It is imperative we bear down hard on grassweeds in the best annual control opportunity we have if we are to prevent them taking advantage of the major holes this past season has opened-up in our defences,” he stresses.
“We know to our cost how little it takes for the likes of black-grass, brome and ryegrass to bounce back from several years of apparently good control, and the problems this can cause us for many years to come when they do.
“That’s why it’s so worrying to see how many growers involved in the latest in our series of Roundup national grassweed management studies are looking to sow winter wheat before mid-October this year.
Alongside rotational ploughing, spring cropping and stale seedbeds, delayed drilling continues to stand out as one of the most widely-valued cultural grassweed control techniques in the 2020 study, run with almost 200 growers across the country by Briefing Media.
Despite this almost half the study growers are currently looking to sow winter wheat on high weed risk fields before mid-October this season, with more than one in 10 planning to do so in September. What’s more, this rises to 80% and 26% respectively on medium weed risk fields (Figure 1).
Figure 1: Winter Wheat Sowing Intentions – Autumn 2020
“After last autumn it isn’t surprising to find so many people anxious not to leave their drilling too late this time around,” Mr Scanlon says. “But, on high grassweed risk fields, in particular, all the evidence points to sowing before effectively eliminating the main annual grassweed flush as being a recipe for disaster.
“If the coming winter proves as unrelenting as the last one, you’re are certainly more likely to get your crop established. However, regardless of the weather, you are also far more likely to have a crop that’s full of black-grass, brome or ryegrass; a crop that will probably perform less well than most later-drilled ones or spring alternatives; and one which may need complete or partial spraying-off in the early summer to prevent a disastrous level of weed seed return.
“Although less immediately dangerous, drilling medium risk fields too early is equally inadvisable in most cases for the extra weed pressure it puts on your rotation,” Mr Scanlon points out. “As many people have found to their cost, it’s a seriously false economy to let problem grassweeds off-the-hook at any time. This is too easily done. And it will only reduce your future yields as well as increasing your costs and restricting your cropping options – not to mention encouraging resistance development.”
Mr Scanlon insists that not being tempted to drill wheat on medium or high weed-risk ground before mid-October is of fundamental importance in grassweed control. Not least for the extra time it gives to get rid of weeds ahead of drilling where the greatest non-selective herbicides pressure can be put on them.
Given the shorter interval between harvest and drilling where autumn sowing is earlier than ideal, he is adamant that the best stubble management is even more critical.
Wherever possible, he advises grower to employ the two glyphosate applications ahead of drilling widely acknowledged to be the optimum. The authoritative 2016 ADAS study for AHDB, for instance, shows overall black-grass control rising from just over 70% with one application to almost 95% with two (Figure 2).
Figure 2: Impact of Number of Glyphosate Applications on Black-grass Control
“Dry weather can seriously limit the decent flush of weed growth essential for the success of any pre-planting programme even in a low dormancy season,” he accepts. “So, where there’s insufficient time or moisture, you may have to rely on a single glyphosate spray to do the job. In which case you must make it count.
“It will be more important than ever to employ a modern Roundup formulation proven for its reliability under challenging dry conditions and short cultivation interval. All the more so, with the noticeably poorer performance being seen from many of the generic alkyl phosphate ester (APE) formulations replacing the now-withdrawn ethoxy tallow amines (ETAs).
“All our trial work shows we get the optimum control of black-grass with two pre-planting applications of 540g/ha of Roundup at the 2-3 leaf stage with a cultivation in between,” reports Mr Scanlon. “This is also the approach recommended to minimise the risk of resistance development.
“Where you’re only spraying once ahead of autumn drilling you need to increase the rate to 720g/ha to deal with better established, tillered weeds. This higher rate is also important where ryegrass or bromes are present.
“Attention to detail in spraying practice is every bit as important in making your pre-planting applications work as hard as they can. It really is vital not to rush the job, and get your water volume, nozzle type, spray pressure and boom height spot on.
“Adjuvants may help with hard water, in particular,” he adds. “But no amount can make up for insufficient glyphosate or an inferior formulation.”
In managing stubbles as effectively as possible this autumn, the correct integration of cultivation with pre-planting treatment is, of course, equally essential.
Black-grass, brome and rye-grass problems can essentially be addressed by the same overall approach – leaving shed seed on or very close to the soil surface to be predated by birds and small mammals then germinated and eliminated ahead of the next crop. Or full-inversion ploughing to a depth from which they struggle to emerge.
Meadow, soft and rye bromes differ from the others in needing a period on the soil surface to ripen before cultivation. But, like black-grass and ryegrass, providing there’s sufficient moisture sterile and great bromes will germinate readily in the autumn if left on the surface or under a straw mulch. As will volunteer oilseed rape.
So regardless of the weed challenge – or predicted dormancy status – Tom Scanlon finds the best approach is to avoid cultivating the ground immediately after combining, unless this is necessary – for soil remediation, OSR or cover/catch crop establishment or organic matter incorporation, for example.
In his experience, even the shallowest of cultivations at this stage will give little or no extra impetus to grassweed germination and, in most cases, only works against soil moisture preservation. Instead, a light roll to maximise seed-to-soil and seed-to-straw contact should be quite sufficient to ensure the best initial weed flush possible; especially where the combine has given a good chop and even spread of straw and chaff.
For the majority of growers min-tilling, ploughing or using a rotational plough ahead of autumn drilling without a catch crop, Tom Scanlon suggests the best approach is to cultivate after spraying-off the first flush of weed growth. And only then if soil conditions are right.
“It clearly depends on your particular soil and conditions, and no one size fits all,” he says. “But if you wait until after your first weed flush you’ll generally know you have sufficient moisture to do a decent job of cultivating. This will also enable you to get in with your glyphosate at the ideal 2-3 leaf stage of weed growth, then press or roll to give yourself another opportunity to spray-off further growth just ahead of drilling.
“Should conditions or time not allow a second stubble treatment, of course, you always have the option of including an approved glyphosate in the pre-em mix. This is something you should seriously consider wherever drilling is delayed for more than a few days after your pre-planting spray. A lot of black-grass can emerge in a very short time under the right conditions and it won’t be touched by most residuals. So, it can be a valuable extra string to your control bow.
“Gone are the days when stubbles were things we dealt with as rapidly and cheaply as we could between the far more important priorities of combining and drilling,” Tom Scanlon concludes. “They need to be valued to a far greater extent and treated far more carefully as a grassweed control essential. We ignore the most effective stubble management at our peril.”
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