In its latest Crop Tour Field Report, AGCO shows best practices to maximise effectiveness of a weed-control program. They examined the effects of ground speed, sprayer nozzles, droplet size, carrier volume and sprayer boom height.
“When it comes to controlling weeds, there’s this battle going on between productivity, efficacy and drift control,” says Darren Goebel, AGCO director of Global Agronomy and Farm Solutions. “Growers want to get over as many acres as they can without having to stop and refill the sprayer. But with today’s contact herbicides, they need to get good coverage while also ensuring that the product they’re spraying reaches its intended target.”
Application timing crucial for weed-control efficacy
Using demonstration plots of LibertyLink crops – cotton in Georgia and soybeans in Illinois – the AGCO team examined the effects of ground speed, sprayer nozzles, droplet size, carrier volume and sprayer boom height.
Jason Lee, AGCO agronomist and farm solutions specialist, says that regardless of treatment, the biggest takeaway from the plots in Illinois was the importance of weed-control timing. Because the plots did not initially experience much weed pressure, the agronomists waited for more weeds to emerge, which allowed some troublesome waterhemp plants to get quite large.
We should control weeds when they’re small: four inches or less in height
“The problem with waiting for a full flush of weeds before spraying is that some weeds will get way too big and really hard to control,” Lee explains, adding that applicators should read and follow all product label instructions. “We should control weeds when they’re small: four inches or less in height.”
The AGCO team applied Liberty herbicide at speeds ranging from 5 to 15 mph and with carrier volumes ranging from 10 to 20 gallons per acre. Sprayer nozzles creating droplets of various sizes from medium to extremely coarse also were evaluated.
“From what we saw, as long as we were using the appropriate nozzle to give us the correct droplet size for the speed we were running, we’d get the right coverage,” Goebel says. “We thought maybe there would be a bigger effect due to speed, but we really didn’t see any issues as long as we were doing everything else right.”
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Contact herbicides should be applied when weeds are still small: four inches or less in height, says AGCO. - Photo: AGCO
Coverage a key consideration when spraying contact herbicides
With weed-control products such as Liberty, which is a contact herbicide, getting adequate coverage is the most important factor, Goebel says. Achieving proper coverage will vary based on sprayer nozzle type and carrier volume.
At lower carrier volumes, such as 10 gallons per acre, using nozzles that create smaller droplets will help improve control. “However, smaller droplet sizes increase the risk of drift. Higher carrier volumes provide better coverage but require more tank fill-ups. Applicators need to strike that balance between productivity efficacy and drift,” he adds.
“If you’re going to use nozzles that produce larger droplets, you really should bump the water up some more to ensure you’re getting good coverage
Based on what he observed in the Georgia cotton plots, Goebel recommends applying at least 15 gallons of carrier volume per acre, using nozzles that produce a coarse droplet. “If you’re going to use nozzles that produce larger droplets, you really should bump the water up some more to ensure you’re getting good coverage,” he says.
Sprayers equipped with pulse width modulation systems help ensure applicators are maintaining constant pressure, rate and droplet size no matter the speed at which the application occurs.
Use correct boom height to increase coverage
Another application factor the AGCO team evaluated was boom height. Goebel says that the coverage penalty for improper height can be severe. The team compared the coverage and weed control achieved with booms at 20 inches and 60 inches above the target.
“Booms often get set too high because applicators don’t want them to accidentally hit the ground. But if you’re spraying four to five feet from the target, you’re not going to get proper coverage, and you’ll also likely get more drift,” he says, noting that boom-height control systems take the guesswork out of setting heights.
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Boom height is crucial for achieving effective weed control. The height above targeted weeds should equal the spacing of nozzles to ensure proper coverage, says AGCO. Booms set too high also can lead to significant evaporation of product and issues with drift control. - Photo: AGCO
“Ideally, for 110-degree spray angles, your boom height above the target should equal your nozzle spacing. So, if your nozzles are 20 inches apart, your boom should be approximately 20 inches above the target. At 60 inches, we only achieved about 30% control. The spray simply didn’t make it to the target due to a combination of drift and evaporation.”
“If you’re trying to get over a lot of acres quickly on a hot, sunny afternoon, boom height is really going to have a big impact,” he concludes. “These products need to be applied well, and having a good understanding of application equipment setup is the first step in ensuring an effective weed-control program.”