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• North Carolina State University Weed Scientist Wes Everman says he has gotten more calls about difficult to manage horseweeds this year than he has gotten concerning Palmer amaranth.
WES EVERMAN, North Carolina State University weed scientist, says some growers are going to a more full-season approach to manage weeds that impact crops in the spring and summer months.
Hit pigweed when small
“This past winter I tried to talk about spraying pre-emergence herbicides when you are fairly sure you will get some rain. Otherwise, I tell growers to save it and hit pigweed when they are small with a herbicide like Prefix. This gives a grower control, plus residual,” he points out.
If a grower is thinking about using a herbicide treatment after soybean harvest, they should be very careful in what material they use. Late applications of Flexstar, for example, can have a carryover effect in corn or grain sorghum in the spring.
“So, knowing crop rotation and understanding the residual and carryover affects of herbicides used in the fall are critical,” he adds.
Often herbicides are used post harvest in soybeans to manage Palmer amaranth and other weeds that escaped during the end of the growing season. Typically, weeds at this growth stage are difficult to control and expectedly control is often not very good.
One of the most common scenarios is the application of diphenylether herbicides such as Flexstar, Cobra and Ultra Blazer for pigweed plants not controlled by a previous herbicide application.
In non-GMO and glyphosate-resistant soybean varieties, these herbicide active ingredients can provide good to excellent control of small pigweed, but control is often poor when weeds exceed 5 -6 inches.
“Once pigweed get more than 6-8 inches tall about the only thing you can do is use a ‘wrap around’ — wrap your hands around the big pigweeds and pull them out of the ground,” Everman says.
Horseweed or marestail is another common problem in a wheat-soybean double-crop.
In Virginia, Soybean Specialist David Holshouser says a number of growers had to cope with residual marestail in soybean fields.
“Once these weeds get 30 inches tall or taller, they are difficult to control with any herbicide,” he says. Growers facing this problem this summer had a couple of options, he adds.
One option is mechanical cultivation prior to planting beans — not a good option for many because of potential moisture loss and loss of conservation-tillage program payments.
The second option is to spray with glufosinate, commonly sold as Ignite or now Liberty. It will usually suppress fairly large marestail, but if weeds get too tall, two applications may be needed, and that will be a problem, unless the grower planted LibertyLink beans, Holshouser says.
The Virginia specialist says warm winter weather may be a contributor to the increasing number of marestail escapes, but he says the main culprits are lack of use of a burndown and/or tillage before planting wheat and inadequate weed control during the growing season.
With wheat prices hovering around $8 a bushel and soybean prices consistently in the $12-15 per bushel range, the incentive is high to keep weeds under control in both crops. Managing weeds year around may be time consuming and sometimes costly, but with prices high, keeping fields clean year around may be more cost effective than ever before.