It won’t take long to notice when resistant weeds set up camp in your fields. When they tower over your corn or soybeans, rob yields and double or triple your herbicide costs, it’ll be a rude awakening. Some weeds can grow 2" to 3" per day and produce millions of seeds. Even if you have a handle on weeds for the time being, take note because resistance is headed to your fields.
Since the early 1980s, herbicide resistant weeds have been on the rise. Today, more than 450 species are known globally—competing with crops for sunlight and nutrients, decreasing yields and farmers’ profits.
Before herbicides such as glyphosate (Roundup, Touchdown, etc.) and glufosinate (Liberty) became popular, even a mix of herbicides, tillage and cultivation couldn’t prevent weed escapes. After these herbicide groups hit the main stage, farmers could control outbreaks of problem weeds with preplant applications to start the season strong.
With the introduction of herbicide-tolerant crops in 1995, the use of glyphosate skyrocketed. Many farmers used it for preplant and post-emergent control, which meant weeds had the same herbicide group sprayed on them at least twice per year.
Repeat use of a single mode of action aided the select few weeds that were naturally resistant to herbicides to reproduce and build their population, creating “Superweeds.”
The Weed Science Society of America defines Superweeds as “slang used to describe a weed that has evolved characteristics that make it more difficult to manage due to repeated use of the same management tactic. Over dependence on a single tactic as opposed to using diverse approaches can lead to such adaptations.”
Glyphosate-tolerant corn and soybeans don’t cross pollinate with weeds and create a mutant weed. The strongest survive, and in this case, it’s the weeds with natural resistance that amass in numbers. Farmers selected for the resistant population by overspraying a single mode of action.
“There are those rare individuals that are naturally resistant,” says Larry Steckel, Extension weed specialist at the University of Tennessee. “If you keep spraying the same herbicide group, you select for resistance.”
For the past 20 years, that’s what many farmers have done. Due to repeat use of glyphosate, the U.S. alone has 14 glyphosate-resistant weeds.
Some weeds might have a single attribute that causes them to be resistant, but others are resistant in multiple ways, says Bill Johnson, professor of weed science at Purdue University. The most detrimental are those resistant to multiple herbicide groups because it lowers the number of effective herbicide combinations.
New technology, such as dicamba, 2,4-D and HPPD-tolerant soybeans, leaves some farmers expecting a cure-all, but that’s not the case. Farmers need to use adequate burndown, a pre-emergent with residual and post-emergent application. In some cases, tillage might be worth considering.
Unfortunately, since resistance is a natural occurrence, it is likely there will be isolated incidences. “You are still going to eventually get resistance, but you can slow it,” Steckel explains.
Resistant weed infestations start small but can spread fast thanks to cultural practices and reaction time.
Resistant weeds can spread the following ways:
- Weed seeds hitchhike on harvest equipment or get blown out the back of a combine and across the field. Cleaning a combine takes considerable time and is costly. Many farmers don’t clean their equipment when going from one field to the next because they don’t see the immediate benefit. When one field has resistant weeds, you run the risk of spreading seeds through a contaminated combine.
- Resistant genes are also carried by pollen. Wind, insects and birds carry pollen over long distances, which allows resistant seeds to be produced more frequently since only males need to have the resistant gene. If you miss a weed, even one sensitive to herbicides, it can still produce resistant seeds since pollen from a resistant plant can be carried a long way.
- Migratory birds can spread weed seed via their feathers or through digestion. Since these birds travel through several different states during their migration, they can introduce new weeds or resistant weeds that previously weren’t an issue.
It’s hard to get ahead of weed resistance when certain weeds are so prolific. In perfect sunlight and water, palmer amaranth and waterhemp can produce up to 1 million seeds. In competitive situations, such as with crops, both palmer amaranth and waterhemp still produce hundreds of thousands per plant. Giant ragweed, another noxious weed, only produces 5,000 seeds per plant. When you compare, it’s easy to see why those two weeds have caused so much damage.
Harvest weedy fields last to mitigate risk of spreading resistant weeds. “Don’t run the combine through patches of resistant weeds either,” Johnson says.
Be smart about herbicide rotations, he adds. If you use 2,4-D in corn, don’t use it in soybeans the following year and vice versa. Switch herbicide groups frequently so weeds don’t have time to build resistant populations in your fields.
Watch new technology too. Dicamba and 2,4-D options are exciting because they offer control, but they’re in the same herbicide category, Group 4, so they act on the same part of the weed to kill it. Repeat use of only these two herbicides could quickly kill this technology’s efficacy.
Southern farmers have more severe weed infestations, so they’re able to provide warning as weeds move north. Steckel has firsthand experience with resistant weeds and knows the key weeds to keep an eye on.
“Palmer amaranth is the perfect weed. It has a high photosynthetic rate and can grow 2" to 3" per day,” Steckel says. “You can’t kill it over 3" tall. It can produce 1 million seeds with light and space and resistant male plants pollinate [non-resistant] females.”
With palmer amaranth, the risk of spreading is increasing as combines are sold, seed is moved and farmers fail to rotate herbicides.
After palmer, Steckel says waterhemp, johnsongrass, goosegrass and Italian ryegrass are troublesome. Waterhemp is native to the Corn Belt, so it’s a bigger problem since it didn’t have to migrate.
“Italian ryegrass is very troublesome in wheat and has a lot of resistance,” Steckel says. “In Australia, it is resistant to everything, and we are starting to see it here, hurting corn when it is not burned down.
“Goosegrass has gained in the past three to four years, especially in cotton, where both Roundup and Liberty are not working,” he adds.
Some farmers call glyphosate a “good grass-killer,” but if you don’t manage it properly, it might stop working on grasses too.
It is essential to use the available weed management tools appropriately and be mindful of spreading the problem with harvesting equipment so you don’t end up harvesting more weeds than crops.
Steps to a Clean Combine
Take the time to clean your combine to help reduce the spread of resistant weeds from field to field.
When it comes to combine cleanout, have an objective, such as removing weed seeds. Mark Hanna, Iowa State Extension agricultural engineer, shares steps to clean out combines.
Thoroughly cleaning a combine can take a full day. Hanna provides a faster cleaning method, but it can still take four to six hours. Certain parts of the combine will be dirtier than others. In some situations, you might need to do a more thorough cleaning where you dismantle parts of the combine.
- When harvesting, run the unloading auger for at least one minute to empty it.
- Open sieves in the shoot and turn the cleaning fan on full blast to get rid of as much as possible.
- Unhook the grain platform or corn head.
- Open the elevator doors; rock trap door and any other access doors.
- After you make sure everyone is out of harms way (there could be flying particulates), start the combine and run it a few minutes to clean seed and cobs out. (This is the “self-cleaning” step.)
- Use compressed air, both vacuum and blowing, to get dust and particulates off the combine. Work from front to back and top to bottom until you get all the debris you can off the combine without disassembly.
Since this is time consuming, plan ahead. Prioritize specific fields at harvest to make scheduling combine cleaning easier. Harvest weedy fields at the end of your schedule so you only have to clean out the combine once. Don’t let weed seeds hitchhike and cause major problems next year.