Herbicide resistance is a solvable problem
By Doug Rich
All weeds, including herbicide-resistant weeds, have weaknesses. The best way to control herbicide-resistant weeds is to target those weaknesses.
Kevin Bradley, Extension weed specialist at the University of Missouri, said he is probably biased but he thinks herbicide resistance is the No. 1 pest problem right now and it is a severe problem in soybean production. Bradley said there are resistant-weed problems in corn but it is nowhere as significant as the problems that occur in soybeans.
Lowell Sandell, weed science Extension education specialist at the University of Nebraska-Lincoln, said he tends to see more problems in soybeans than corn because the chemical toolbox is not as large for soybeans, particularly with post-emergence products. Sandell said in corn, even with glyphosate-resistant weeds, there is still a pretty good selection of tools to control waterhemp and marestail (horseweed).
In Missouri the top three herbicide-resistant weeds are waterhemp, marestail, and giant ragweed. Waterhemp and marestail top the list of weed problems in Nebraska with herbicide-resistant kochia being the main problem in the western half of the state.
“But it is a solvable problem,” Bradley said. “There are some management practices we have been doing over the past several years that are having an impact. The No. 1 management practice is the use of multiple, alternative effective herbicide modes of action on the weed in question.”
Sandell said rotating modes of action and/or using at least two effective modes of action in an herbicide application are the top practices for managing herbicide-resistant weed populations. Using two effective modes of action with post-emergence products is very useful.
Should growers drop one mode of action completely? Bradley and Sandell said “no.”
“I think in our geography glyphosate has almost no efficacy on waterhemp anymore,” Bradley said. “But in most cases it should still be in the mix because it still has efficacy on a variety of other weeds that are out there.”
Sandell said in any given field there are probably two to three primary weed species and a handful of other weed species, and glyphosate works well on those other weed species.
“Glyphosate still has utility in the spectrum of weeds that it does control but if growers have a resistant-weed population, they need to use at least two effective modes of action,” Sandell said.
Growers need to use the biology of the weed to their advantage. Timeliness of a chemical application at the labeled rate is essential.
For example, giant ragweed emerges very early and needs an effective burndown for best control. Bradley said this is not the case with waterhemp, which will germinate throughout the growing season. The best way to control waterhemp is to apply an effective pre-emergence residual herbicide as close to planting as possible.
“Something new in Midwest weed management systems in soybeans is not only do we put a pre-emergence residual herbicide out at planting but we come in later and put an overlapping residual over the top of the soybeans to eliminate any waterhemp that might emerge throughout the growing season,” Bradley said.
Sandell said fall applications work best for marestail control and the key to success with waterhemp is to use pre-emergence herbicides at the full-labeled rate.
The use of cover crops is a popular topic right now and it can play a limited role in controlling herbicide-resistant weeds. Bradley said the effectiveness of cover crops depends on the weeds being targeted and the cover crop a grower chooses to use. Not all cover crops are going to produce enough biomass over the ground to inhibit the germination of weed seeds in the summer.
“But anything you can do to break up the continuous cycle of production, in our case soybean on soybean production, is in effect another cultural practice that will help control weeds,” Bradley said.
Sandell some of the problems with weed resistance occur because growers keep doing the same thing over and over.
“Planting cover crops can certainly provide a number of benefits beyond weed control, but if growers incorporate a cover crop planted in the fall and terminate it prior to planting corn or beans, does that really fundamentally fix what they are doing in terms of crop rotation?” Sandell said. “I am not sure that it does.”
If growers decide to use a cover crop, Bradley and Sandell said they should pay particular attention to the species they choose for a cover crop. Bradley said cover crops do well on winter annual weeds but not always so well on summer annual weeds.
Cover crop research he has conducted in Missouri shows that cereal rye and wheat used as cover crops have some ability to reduce waterhemp emergence the following summer. Bradley said growers could plant rye or wheat in the fall, let it grow throughout the winter, terminate it in the spring, and then plant soybeans.
Researchers have looked at radishes and Austrian winter peas but they did not see any reduction in the emergence of summer annual weeds from these cover crops.
“Ones that I have concerns about are vetch and annual or Italian rye grass (Lolium multiflorum) species that have a potential for herbicide resistance to develop,” Sandell said. “On the vetch side about the only products that work for termination are 2,4-D and Dicamba, which can be a challenge ahead of soybeans.”
Tillage is an option for weed control in certain situations. For example, tillage works very well for pigweed control because it will not germinate from deep in the soil. Just turning the soil over one time will bury that seed.
“I don’t usually recommend moldboard plowing, but it works for pigweed,” Bradley said. “If you have a really bad field and it will cost a lot to achieve adequate weed control, growers can consider moldboard plowing once and starting over with effective management from that point forward to get that situation under control.”
Sandell said some of the rolling ground in eastern Nebraska is very erosion prone and growers need to think very hard whether or not they should do any tillage. In a central area with flat soils the utility of tillage goes up quite a bit.
“I would hate to see people go back to full-width tillage unless it is an extraordinary situation,” Sandell said.
A survey conducted recently by BASF shows that 76 percent of the growers in the survey have already changed their weed management program to address resistance. Two-thirds of the growers participating in the survey said they would be applying a pre-emergence herbicide this season, more than half of the growers are planning to add an additional herbicide to their existing program and 47 percent said they plan to use overlapping residual herbicides to control resistant weeds.
The message coming from universities and the commercial side is starting to change how growers approach weed management. Growers are developing proactive weed management systems that attack weeds at their weakest point.
Doug Rich can be reached by phone at 785-749-5304 or by email at firstname.lastname@example.org.