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Fall Weed Control Research Advances

Friday, August 21, 2020
filed under: Weeds

Much of the research currently taking place at North Dakota State University’s North Central Research Extension Center at Minot revolves around weeds and how to keep those weeds from causing headaches for growers.  NDSU weed scientist Brian Jenks’ latest projects center around fall-applied herbicides. 
Controlling Winter Annuals
One of Jenks’ primary projects this year focuses on winter annual weeds. “Just like winter wheat, these weeds germinate in the fall, overwinter and then emerge in the spring,” he observes.  “In the spring, they grow, flower, set seed and die.”

Horseweed is among the most troublesome weeds for producers in western North Dakota. Jenks says horseweed has become a challenge because it is resistant to glyphosate, which is often applied in no-till fields in the spring to burn down the weeds prior to planting.

“If Roundup doesn’t control these weeds, including horseweed, a field could be a weedy mess,” Jenks points out. “Narrow leaf hawk’s beard is another new weed starting to show up in western North Dakota,” he adds.  

“We need something else — and farmers are looking for something else to control these winter annual weeds.  We want to find a way to control them in the fall, because they appear to be much easier to control in the fall than in the spring.”

That need led Jenks to exploration of a combination of sunflower and two very familiar herbicides: 2,4-D and dicamba.

“These herbicides are commonly applied in the fall; but they have certain rotational restrictions because they stick around in the soil for a short amount of time.  If they are applied in the fall and then the soil freezes, there isn’t time for that herbicide to break down.”

Labels require about four months from the application of dicamba to planting; for 2,4-D, it’s about a month.  But, Jenks says, frozen months don’t count.  And the months leading up to planting in North Dakota are almost always frozen.  That’s why he is challenging those labels.

“We think there is a chance the way the label is written that maybe not every crop was investigated — and that maybe you can plant before that four-month window ends,” Jenks explains.  “We have sprayed 2,4-D and dicamba in the fall and followed with spring-planted sunflower to see if we use different rates, or if we apply the herbicides at different times in the fall, if there will be enough carryover to injure the sunflower.”

So far, so good.  Jenks says with those two application timings for 2,4-D and dicamba, they have seen very little injury to the sunflower crop.  They have two years of data so far, but Jenks wants to continue this research for at least one more year, maybe two, before making a recommendation to farmers.

“We need at least three or four years of data to have confidence that a farmer can apply these herbicides and still be able to safely plant sunflower in the spring,” he says.  “Our goal is, after we get three or four years of data at a couple locations, to present that data to the chemical companies and see if they would be willing to adjust their label to allow this herbicide use.”

Jenks says changing the label would be a big benefit to producers trying to control weeds like horseweed and other winter annuals, including mustards, pennycress and shepherdspurse.  “If we can easily control these weeds in the fall, it will give producers a big advantage,” he states.  “If we have to wait until spring, it is extremely difficult — and, when it comes to sunflower, we have very few options to control those weeds in the spring.”
Controlling Kochia
Kochia can be a nightmare to control. Spartan is one of the most common herbicides growers use to control kochia; but when kochia emerges in the spring, Spartan only controls the weed as it germinates, Jenks notes.  If the kochia has already emerged when producers spray Spartan, it won’t provide complete control. 

“We need something in our burndown, such as Roundup, to control the kochia. But a lot of kochia has developed a resistance to Roundup.  We need a way to get rid of it, or at least thin it out so we’re dealing with a lower density of the weed.”

Valor might help, according to Jenks. When applied in the fall, he’s noticed that Valor has enough residual that in most cases it has reduced the amount of kochia that emerges in the spring.  For example: in a field where a grower doesn’t do anything to control kochia, the weed grows at about 100 plants per square foot.  Jenks says if that number can be reduced to 10 or even 20 plants per square foot, the benefits would be great.

“It would make it much easier to control the kochia in the spring.”
He points out that Spartan is a group 14 herbicide and a PPO inhibitor. Valor is also a PPO inhibitor.  Jenks thinks the two might deliver a power one-two punch to control kochia.  “In theory, we’d like to use Valor in the fall and rely on its residual activity to reduce the number of kochia that emerge in the spring,” he says.  “Then, in the spring, we could spray Spartan to control what’s left.  It could lead to some very clean fields.”
But, could PPO overload do more harm than good to sunflower fields?  It’s a question Jenks’ team is attempting to answer.  They applied Valor in the fall of 2019 and different Spartan products in the spring of 2020.  Jenks says the initial results are promising — plus, they did not notice any injury to the sunflower.
“This is good news,” says Jenks.  “It confirms what some people have told me they have seen in their fields.  Farmers have been doing this, even though there has never been any research on it.  But it’s not on the label, so producers are wondering if it’s something they should really be doing.  We only have one year of data so far, but it appears promising.”
Jenks hopes to continue this research for a year or two to account for different weather conditions. ?Spring 2020 was very dry, so he’s hoping for a more normal year to test his research in 2021. 
Controlling kochia is at the top of many producers’ wish-lists, Jenks states.
“Kochia can grow into a mini-Christmas tree.  It can grow very large, and it soaks up a lot of water,” he says.  “It grows really well in dry conditions.  When conditions are wetter, there isn’t a lot of kochia; but if it’s kochia versus sunflower, kochia often wins. 
“Spartan normally does a good job of controlling kochia; but it is a soil-applied herbicide.  It needs rainfall to activate it and to get into the top inch of soil where weed seeds germinate.  If we don’t get rain, it isn’t effective.”
Jenks hopes his research will prove using Valor in the fall and Spartan in the spring is an effective way to reduce the number of kochia plants farmers must deal with, come spring
       Though the weed was not widely reported in North Dakota last year, Brian Jenks reminds the state’s growers to also keep an eye out for Palmer amaranth.  “If you see a pigweed that looks taller than normal, contact your county agent to identify it.”  

—Jody Kerzman 
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