Disappointing weed control in many crops over the past season makes it vital for growers across the country to step-up their stubble management efforts this autumn.
The cold wet spring really let problem weeds like sterile brome, annual meadow grass, ryegrass, scutch and thistles off the hook. It was well into April before most people could get on the land. By which time the majority of weeds were far too well-grown for most herbicides to touch them.
At the same time, short and thinner-than-ideal crops in many cases have given weeds just the space they need to develop strongly. Equally uncompetitive have been very late-sown spring crops struggling to find sufficient moisture for most of the summer. As a result many farms face especially worrying threats from annual and perennial weeds in the coming season.
“This makes eliminating as many of them as possible ahead of winter cereal emergence more important than ever,” stressed Roundup technical specialist, Barrie Hunt. “From a nice-to-have extra, stubble treatment has become one of the single most valuable weapons in our weed management armoury. Which means we have to get it right.”
Getting it right, in Mr Hunt’s experience means managing stubbles with a clear understanding of weed biology, taking sufficient care in glyphosate spraying and having the patience to hold-off on autumn drilling until a decent flush of weed growth has been dealt with.
“First and foremost, the right cultivation approach is essential,” he insisted. “You have two options with annuals like sterile brome, ryegrass and annual meadow grass. Either bury the seed to a depth from which little, if any, will emerge or keep it near the surface, encourage it to germinate with a stale seedbed and spray it off before drilling.
“Ploughing down can be a good solution where these weeds are especially troublesome. But you need to plough sufficiently well to ensure complete seed and seedling burial. And you shouldn’t plough again for 4-5 years if you don’t want to bring viable seed back up to the surface to cause you future problems.
“Ploughing won’t work either if repeated deep cultivation over the years means your annual weed seeds are spread throughout the soil profile. That’s why the stale seedbed approach is so valuable – either as standard practice or in the years between rotational ploughing.”
For the best results here, Barrie Hunt generally advises a light surface cultivation as soon after combining the previous crop as possible, with good consolidation to maximise moisture retention and seed-to-soil contact.
He recommends leaving oilseed rape stubbles undisturbed for several weeks, however, to encourage the most complete germination of shed seeds, pointing out that any significant burial will increase dormancy and future volunteer problems. Cultivation should similarly be delayed in fields where scutch and other perennial weeds are problematic, allowing them regrow sufficiently for the greatest glyphosate uptake and activity.
“Whenever or however you cultivate, you should aim for at least one good spray of glyphosate ahead of drilling,” said Hunt. “More care and less speed are essential here to make the most of it.
“With time at such a premium and effective weed control so crucial, less-than-ideal treatment is something you can’t afford these days. Quite apart from wasting time and money and putting extra pressure on in-crop chemistry, an incomplete kill is the fastest route to resistance development.”
The short autumn window between harvesting and drilling on many units makes it vital to guard against conditions that can compromise glyphosate uptake and activity. In particular, hot and dry weather, rainfall within a few hours of spraying, and hard water.
This season, in particular, it’s also vital to be aware that, following the withdrawal of European approval for ethoxylated tallow amine (ETA) formulations, the performance of many glyphosates may fall below expectations. This is because products reformulated with the most widely-used alternative to tallow amines perform noticeably less well than their predecessors.
“Trials comparing a range of reformulated glyphosates against our Roundup benchmark confirm anecdotal evidence of markedly lower performance than original ETA formulations from growers and agronomists,” Barrie Hunt reported. “The greatest risk here is clearly on perennial weeds, at lower glyphosate rates, with higher water volumes and under more challenging environmental conditions.
“In contrast, modern Roundup formulations are proven to give better control of weeds under challenging conditions. Their reliability is enhanced by rainfastness in as little as an hour. And annual grass weeds can be cultivated just six hours after treatment for the greatest planting flexibility.
“As well as the most effective formulation, it’s important to use the right rate of glyphosate for the job,” he added. “You typically need 540 g/ha to control grassweed seedlings up to early tillering and as much as 1800 g/ha for the most difficult perennials, remembering that adjuvants will not make up for any lack of active ingredient.”
Correct glyphosate targeting is another stubble management essential. For which operators need to spray at the right speed, height and pressure with the most appropriate nozzles and water volumes.
When workloads are stretched and time is short it’s tempting to crack-on with stubble spraying at 14-16 km/hour with the boom set high enough to ensure good ground clearance and coarse or air induction nozzles to minimise drift risk. But this can seriously interfere with activity.
In Barrie Hunt’s experience, water volumes as low as 100 litres/ha are fine providing care is taken, enabling faster treatment of large areas and getting higher concentrations of glyphosate onto the leaf.
Given its translocated action, he normally recommends a relatively coarse spray in the interests of drift reduction. But, with thin seedling annual grassweed leaves he warns against nozzles giving the coarsest sprays as they will limit droplet numbers, reducing coverage and the chances of hitting all the targets.
“For the best seedling weed targeting and least drift risk a boom height of around 50 cm and forward speed of 12 km/hour to minimise turbulence effects is about your best approach,” he suggested. “Especially if you marry this with 03 flat fan (FF) nozzles operated at 2.5 bar.
“Of course, the most effective pre-planting control fundamentally depends on having enough moisture in the stubble to stimulate a strong flush of annual weed seed germination and perennial weed regrowth. So, in addition to the right cultivation and spraying approach the third and final ingredient for stubble management success is patience.
“Thankfully, this year’s early harvest will give you valuable extra time ahead of drilling. Even so you may well have to sacrifice your earliest winter cereal drilling aspirations to make sure you get the pre-planting kill you need,” pointed out Barrie Hunt. “Believe me, it will more than make-up for any loss in growing time.
“Bear in mind too that where drilling has to be delayed for more than a few days after stubble spraying, the approval of modern Roundups for post-planting, pre-emergence application and their broad tank-mixing compatibility gives a valuable extra string to your stubble management bow.”