One of the warmest winters on record in the Upper Southeast played havoc with virtually every crop grown in the region, and often not in ways clearly visible to growers or understandable to the experts.
Such is the case this year with a virtual explosion of horseweed and other weeds not usually considered to be too hard to manage.
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.
For sure glyphosate resistant horseweed, sometimes called marestail, can be a big problem, but there are a number of options for control, and some of these options just didn’t seem to work as well as expected this year, Everman says.
Some combination of glyphosate, 2,4-D and Valor have done a good job of controlling horseweed for the past couple of years. The biggest problems seem to occur when the weeds are sprayed late and get too big to manage, he notes.
“This year, even when we timed herbicide applications right, some of them didn’t seem quite right. Speaking at a recent field day, the North Carolina State weed scientist said, “I was there when we sprayed it, I did the mixing and the spraying and I put it all out there. I know I wasn’t wrong.”
“This year, with the warm winter, weeds had a chance to grow during time they are usually dormant. Even if growth was evident above the ground, these weeds were putting on roots and generally surviving the winter months much more efficiently than usual,” Everman adds.
Weeds were bigger
“When we sprayed fields at the normal burn-down time in the spring, some of those plants were a little bigger than typical for that time of year. Some growers probably did what they have done in past years, and what we recommended they do, but in many cases the horseweed was just a little too big to manage properly,” he adds.
If unusual weather patterns that affect the growth of both target plants and weeds continue to occur, growers are likely going to have to change their pest control tactics, including taking a different look at getting fields clean for planting time.
We may need to look at some options that are a little different, Everman says. One option is paraquat, though many growers don’t like to use it because of crop safety issues. “Used safely, paraquat can be a very effective product and works especially well with Valor in the tank-mix,” Everman says.
“In the spring, with daytime temperatures in the 50s and 60s, it’s safe to use paraquat-based burn-down products. It will take better coverage than when using glyphosate and other systemic herbicides, and it will need 15-20 gallons of water in the spray tank to get best results.
“Sharpen is a new product that showed good results last year when used as a burn-down herbicide. It is a member of BASF’s Kixor family of herbicides. When used as a burn-down material Sharpen should be tank — mixed with a preferred adjuvant.”
Everman says some North Carolina growers are going to a more full-season approach to manage weeds that impact crops in the spring and summer months. Some put down a pre-emergence application of Envive or Valor after they cut their wheat crop.
“If a grower gets good moisture after applying herbicides behind his wheat crop, he is likely to have fewer problems the following spring. With the warm winters we’ve had the past two years, that’s even more likely happen,” the North Carolina State scientist says.
This year North Carolina grain farmers harvested 870,000 acres of wheat. With prices good and confidence high, Dan Weathington, executive director of the North Carolina Grain Growers Association, says he expects 950,000 acres this coming season, and wouldn’t be surprised to see a million acres.
The rise in wheat acres, combined with more than a million acres of soybeans in the Tar Heel state, may encourage more growers to take a look at year-round weed control measures.
“One of the biggest issues with using herbicides in a double-crop situation is moisture,” Everman says.
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.