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Drought presents weed-control challenges for farmers

A sign near a parched field marks the spot of an experimental plot used in summer by the North Dakota State University Extension office and the Pierce County Ag Improvement Association. Sue Sitter/PCT

Persistent drought conditions in central and western North Dakota will present challenges for farmers putting crops in the ground.

Unfortunately, they may not cause any problems at all for weeds, according to a scientist with the North Dakota State University’s North Central Extension Research Center.

“There are some weeds that like drier conditions,” said Brian Jenks, a weed scientist with the center.

“One of our biggest challenging weeds is called kochia. Kochia actually thrives in drier conditions. So, we may see an abundant amount of kochia emerging in the next two to four weeks.”

“It’s usually one of the weeds that emerges first and unfortunately, it can be resistant to some of our common herbicides such as glyphosphate (known as Round Up),” Jenks said.

“We still expect emergence of some of the weeds that are right on the surface, such as winter annual weeds such as the mustard species or horseweed, prickly lettuce, any other number of weeds we call winter annuals,” Jenks added. “Those are the weeds that emerged in the fall. But there certainly can be weeds that are still emerging in the spring.”

“So, obviously with the dry conditions, we don’t expect as much weed emergence but there are still some weeds that will come and need to be controlled,” he said. “But kochia is by far the biggest concern we have because it just likes the dry conditions.”

A variety of herbicides together with no-till farming can help farmers use a flexible approach to combat weeds in drought conditions, Jenks said.

One type of weed killer, known as foliar herbicide, is applied to the leaves, or foliage, of weeds that have already emerged.

“Round Up (is one example),” Jenks said. “It’s taken up by the leaves.”

“Whereas other herbicides, which we refer to as soil-applied herbicides, are applied directly to the soil,” Jenks noted.

“Unfortunately, for soil-applied herbicides to be activated, we need a rainfall, either rain or a tillage pass to mix the herbicide into the soil, then the herbicide is taken up by the root as it germinates.”

“For example, a common (soil-based) herbicide that’s used in soybeans, peas or flax is called Spartan,” Jenks said.

“Farmers may think, ‘Should I apply this soil-applied herbicide?’ Because it needs rain to be activated.”

“If we don’t get a rain, that herbicide isn’t going to be in soil solution where the roots can take it up,” he added.

Jenks said farmers might hesitate to use soil-applied herbicides because they would consider putting the chemicals on dry ground a waste of money.

“If we don’t get a rain, it’s not going to do a farmer much good so that’s why a farmer may be hesitant to spray that soil applied herbicide in the absence of any rain that may or may not be coming, If we do get the rain, the herbicide will be activated and will control weeds,” Jenks said.

“So, that soil-applied herbicide is kind of like insurance. You want to have it in case you get the rain, but you hate to spend the money if you don’t get the rain,” Jenks said.

“Depending on the crop, a farmer will apply both a soil-applied and foliar herbicide,” Jenks noted. “For example, this time of year, a farmer may apply both. He may apply a foliar herbicide like Round Up and a soil-applied herbicide like Spartan in front of soybeans. What he’s doing is using the Round Up to control any weeds that are emerged but using a soil-applied herbicide like Spartan to control the weeds that may emerge later.”

“But we have seen that sometimes there’s more benefit to soil-applied herbicides than a farmer typically expects,” Jenks added. “If we have dry conditions, you don’t get 100 percent control but you might still get 30 or 40 percent control, which is better than nothing.”

Pierce County NDSU Extension Agent Brenden Klebe said many farmers in the area have chosen no-till methods to preserve soil moisture, which leaves them with few choices for weed control beyond herbicides.

“A lot of farmers I’ve talked with said there’s going to be a lot of no-till, which is great to conserve moisture in soil. But you’ve got to get on top of weeds that way with pre-emergent herbicides, even pre-seeding herbicides to get ahead of them,” Klebe said.

Manual weed control “goes to the tillage side of things,” Klebe said. “Tillage is a great tool to warm the soil up and get things opened up and get the sun shining on it, but our lack of moisture hurts that. Then, the sun beats on it too hard and you get a lot of dry areas. So, between the two, it’s kind of a catch 22,” he said of tillage and chemical control.

Klebe and Jenks said drought conditions could slow some weed growth, making herbicide application trickier.

“(Foliar) herbicide does not enter the leaves as readily and therefore sometimes it’s not as effective, “ Jenks said. “So, in that case, under those dry conditions we recommend that the grower may want to increase the water volume that they’re using. But that depends on the herbicide they’re using.”

“For example with the typical rate that a farmer may be applying, the amount of water they’re using per acre is about 10 gallons but he or she may want to consider increasing that water volume from 10 to 15 gallons per acre so there’s more water on the plant and it helps get that herbicide into the plant better.”

“There are some cases with some herbicides under dry conditions may want to use a different adjuvant,” Jenks said. “What an adjuvant does is helps the herbicide enter the plant more readily.”

“On the outer surface of the weed, there’s a waxy layer called the cuticle,” Jenks said. “What the adjuvant does is help the herbicide get through that cuticle more effectively. Some labels recommend a different adjuvant in drought conditions.”

Jenks said relying solely on one type of herbicide increased the likelihood weeds would develop resistance to it.

“We already have some weeds like kochia and horse weed that are resistant to Round Up,” Jenkins said. “So, we need other chemistries and other modes of action to control those weeds and that’s where the soil-applied herbicides come in. It helps control those weeds so we don’t have to rely solely on Round Up or other foliar herbicides.”

“Usually, the farmers are spraying (foliar-based chemicals) about a month after planting. Hopefully, we’ll get a rain by then and they’ll be able to come back in and spray. That’s usually when the weeds are no more than three to four inches tall. That’s usually in the first week of June.”

“Unfortunately, the other thing the drought does is sometimes in dry conditions you may not get every seed germinating at the same time so you may have uneven emergence,” Jenks noted. “Instead of having your crop emerge all at once, you may have 30 percent emerge one day, and then you may get another 20 percent another day, so you may have your crop emerge over a two-week period. So, your crop may not be at the same stage at the same time. That kind of makes things uneven throughout the season.”

“Unfortunately, there’s really nothing they can do about that,” Jenks said, noting some farmers might try to plant deeper in the soil to try to even things out.

“However,” Jenks added, “It’s entirely understandable that if a grower finds no moisture even at two inches, a grower faces a very big risk of not having good emergence of the crop unless you get a rain. That’s why some growers are hesitant to plant at all if they can’t plant into moisture.”

“There are a few parts of the state where growers have irrigation capabilities, but those are very few,” Jenks added. “We’re completely at the mercy of Mother Nature.”

“It’s going to be a very challenging spring if we don’t get any rain,” Jenks said.

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