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22 Sobering Need-to-Know Facts About Herbicide-Resistant Weeds

1. Nothing new is coming

Ever notice the “new” herbicide products that come on the market? They really aren’t new.

Most of the time, they’re a mix of existing herbicide active ingredients. That’s a strategy the late Marshall McGlamery, University of Illinois Extension weed specialist, called “Can ’em and confuse ’em.” 

Other times, the compound actually contains a new active ingredient. Still, it belongs to an existing herbicide mode of action. The last new mode of action launched in the corn and soybean market was the HPPD inhibitor (Callisto, Balance Flexx) in the late 1990s. 

“The streak continues,” says Mike Owen, Iowa State University (ISU) Extension weed specialist. “There are no new herbicide modes of action available for 2014 or the foreseeable future.”

That’s not good news as far as resistant weeds are concerned. For now, you’ll have to make do with the existing chemistries to manage weeds.

2. You can’t diversify enough 

The good news is farmers are diversifying herbicide use. In Iowa, for example, use of nonglyphosate herbicides in Roundup Ready crops is 46% in soybeans and around 78% in corn. That’s according to 2011 Monsanto research. 

There’s still a ways to go, though. “In Iowa, there are around 8 million acres where just glyphosate is being used,” says ISU’s Owen. “That’s still too much. I do not think over last the last five to 10 years that we have pushed the bar very far in regard to adopting more diverse weed-management systems.” 

3. Pre's don’t always get applied 

Use of multiple and effective herbicide modes of action is a cornerstone of diversification. 

Part of this is applying residual preemergence herbicides that nix early-emerging weeds.

“I know that last winter and going into spring, a lot of people had a lot of good intentions to do the right thing,” says ISU’s Owen. “As the season developed, a lot of those good intentions went by the wayside.” 

Weather makes timely applications a struggle, adds Owen. Still, stick with it. 

“Year in and year out, preemergence herbicides will give you better control,” says Kevin Bradley, University of Missouri (MU) Extension weed specialist.

One option may be to consider early preplant residual applications, adds Owen. This could enable you to spread workload and to get more preemergence herbicides on fields for early applications.

4. Preemergence benefits are limited 

Numerous preemergence residual herbicides squelch weeds well. Still, they nix weeds for only 30 to 45 days. That’s a problem with season-long germinators like waterhemp.

“On average, waterhemp has five germination events, with the last one coming in late June or early July. No herbicide you apply in April or early May will last that long,” says ISU’s Owen. 

5. Fall applications don’t always work 

Fall herbicide applications are also an option, but most likely for those farmers south of I-80. 

“For controlling henbit and mustards, it is a good fit,” says ISU’s Owen. 

“On the other hand, if you are looking to eliminate the need for timely preemergence applications by going with postfall applications, your expectations are not correct. You cannot expect a fall-applied product to provide the needed level of residual control.”

6. You can’t use Liberty like RoundUp  

So far, the only weed in the U.S. that resists the glutamine-synthase inhibitor herbicide group (which includes glufosinate) is Italian ryegrass. Thus, the LibertyLink system that uses postemergence glufosinate (Liberty) can work well to forestall herbicide-resistant weeds. 

One caution, though. “If you use it like Roundup, it will not work. It isn’t Liberty’s fault when you spray 8-inch-tall waterhemp,” says MU’s Bradley. Instead, it’s best to apply it by the time weeds reach a 4-inch height. 

“It will break if you abuse it,” he says. 

7. Weed control via cover crops is mixed 

Cover crops can smother winter annuals, says MU’s Bradley. By the time waterhemp cranks up its late-season germination, though, cover crops are long gone. 

It’s also important to glean a complete kill from a cover crop. If they survive, they can compete with your main grain or oilseed crop in subsequent years. 

“Annual ryegrass is a species that I think you ought to think seriously about as a cover crop,” says Bradley. “In many parts of the U.S. and other countries, it is a serious problem. It is one of the world’s worst herbicide-resistant weeds. As a weed scientist, I have to ask, ‘Why go out and spread it around some more?’ ”

8. Weeds have weaknesses

You just have to exploit them. For example, some weed seeds have short lives. A 1994-1997 ISU study found just 15% of waterhemp seeds emerged over four years after being initially buried.  

You can turn this to your advantage. MU’s Bradley notes that a grower he worked with near Kansas City initially had waterhemp-infested fields so severe, soybeans could not be seen. 

“By the end of three years, he implemented everything we talked about,” says Bradley. These included steps like preemergence residual herbicides and multiple herbicide modes of action. 

“We started by not letting waterhemp produce seed,” he says. “I go by there once every summer, and I cannot find waterhemp on that farm.

“The ultimate objective must be to reduce the soil seed bank,” he adds. “If you have no other choice, that means rouging a field.”

9. Your combine is a giant planter

In weedy fields, a combine morphs into a giant seed spreader. 

In heavily infested areas, one option is to mow before harvest, says Bob Hartzler, ISU Extension weed specialist. In one case, an Iowa farmer hired his church’s youth group to pull female Palmer amaranth from a field mixed with it and waterhemp. 

“Such actions can greatly reduce the rate of Palmer amaranth spread from infested fields,” he says. “It also can reduce weed density the next year.” 

10. You can bury waterhemp seed

Waterhemp seeds do not emerge from 6- to 8-inch soil depths, says MU’s Bradley.

One way to place waterhemp seed at lower soil depths – and tread carefully on this one – is to moldboard plow the field.

Don’t do this on highly erodible soils. There are drawbacks to this, too, including the shattering of earthworm tunnels that aid water infiltration. 

In nightmare situations where waterhemp chokes your soybeans out and you’re up against a wall, though, it’s an option.

It takes a moldboard plow. Vertical tillage, disks, or field cultivators do not till the soil deep enough, says Bradley. 

Once you bury the seed, don’t moldboard plow the field again. This will simply spring the waterhemp seed back to the soil surface where it can emerge, says Bradley. 

11. Palmer amaranth should be feared

Be afraid. You’ve never dealt with anything like this before. 

It is a tough, competitive plant. Soybean yields can be cut up to 79% if 2.5 Palmer amaranth plants per foot of row are present. 

“The Southern and now Midwestern farmers will not get away with having this weed in their fields,” says MU’s Bradley. “It is the most competitive weed on a plant-for-plant basis.”

It also can grow quickly. In Southern states like Georgia, it can grow up to 3 inches per day. Growth rates likely aren’t that high in Midwestern states. MU studies show a 1.1-inch to 2.5-inch daily growth rate. Still, that’s a cut above waterhemp’s .75-inch-per-day to 1.25-inch-per-day growth rate that’s surfaced in MU trials.

Still, don’t get hung up on growth rates between the two. Both weeds need to be quickly controlled. Once your preemergence residual herbicide wears off and they start to grow, it’s best to spray them postemergence before both reach a 4-inch height.

“That doesn’t give you much time,” says Bradley.

12. Pigweeds are tough to distinguish

It’s tough to tell waterhemp and Palmer amaranth apart at the seedling stage. It’s easier when plants grow larger, such as during the flowering stage. 

“Waterhemp is a very leggy plant, with lots of space between leaves,” says ISU’s Hartzler. Meanwhile, most Palmer amaranth plants have a more dense canopy, with leaves at stem tips often tightly clustered. They resemble poinsettia plants.

Other differences include:

Head size.  Palmer amaranth produces a much larger seed head than waterhemp, often with terminal branches up to 2 feet long. The bracts of female Palmer amaranth seed heads are .25 inches long and sharp. 

Stalk thickness. Palmer amaranth has a thick stalk, similar to tree bark.

Height. Palmer amaranth is taller than waterhemp, reaching up to 10-foot heights. 

Leaves. Palmer amaranth often has leaves with petioles longer than the leaf blade. Palmer amaranth leaves may have a watermark on them, but that’s no identification guarantee.  “Sometimes, an oddball waterhemp plant can have a watermark, too,” says Hartzler.

13. Pigweeds can be co-ed

Pigweeds, like waterhemp and Palmer amaranth, are dioecious. 

This means they have both male and female plants. 

Unfortunately, these male pigweeds are prolific pollen producers. If they are herbicide-resistant, the pollen can transfer this resistance. 

“Pollen can remain viable up to 120 hours after pollen shed,” says MU’s Bradley. “Most waterhemp pollen will fertilize female plants within 165 feet of the pollen source, but it can travel up to a half mile.”

A recent paper authored by Colorado State University weed scientists showed that Palmer amaranth pollen can transfer the herbicide-resistance trait to other pigweed species like waterhemp and even pasture pigweeds like spiny pigweed.

“The danger in Missouri, with pastures between row-crop settings, is that resistance can be transferred to spiny pigweed in the middle of a pasture,” Bradley says. 

14. Pigweed hybrids are too common

Cattle producers have known for years that crossbreeding creates a better-performing calf due to hybrid vigor. If hybrid vigor occurred in pollen crosses between herbicide-resistant waterhemp and Palmer amaranth, an already bad situation could worsen. 

Hybridization doesn’t happen frequently between waterhemp and Palmer amaranth, but it has occurred.

“I have walked fields where I see some really weird plants that don’t add up,” says MU’s Bradley. “Leaves will look like waterhemp with a Palmer seed head.”

15. It’s harder to kill late-emerging waterhemp 

Are waterhemp harder to kill later in the year? MU’s Bradley thinks they are. 

“Farmers have said they are germinating later, and the waterhemp that germinates later is waxier and harder to kill,” says Bradley. “Waterhemp in my state is hard to kill anyway, no matter what time of year. A species can shift its biology, so there is no reason to believe it can’t occur.”

That extended germination period can cause headaches.

Preemergence herbicides do an excellent job nipping out early-emerging waterhemp during the first four to six weeks of the growing season. However, later-germinating waterhemp is where control difficulties lie. 

16. Waterhemp has one perk in your favor  

Waterhemp is not as competitive as Palmer amaranth. 

“Waterhemp is not very competitive on a plant-by-plant basis,” says MU’s Bradley. “Unfortunately, you can run a combine through it, and you know it doesn’t really even slow a combine down. Later in the season, it doesn’t cause as much yield loss compared to a weed like Palmer amaranth.”

17. Overlapping residuals give better control 

An overlapping residual program first consists of a preemergence residual-herbicide application followed by a postemergence mix of glyphosate (in Roundup Ready soybeans) and a residual herbicide.

“Cost increases, but you get better weed control from these programs and corresponding yield increase, as well,” says MU’s Bradley. “Overlapping herbicide programs won’t control all weed species, but they do match the biology of waterhemp and Palmer amaranth.”

18. Resistant weeds can wreak rental havoc 

Four years ago, the University of Arkansas held a “PIGposium” to herbicide-resistant Palmer amaranth management. 

“There were lots of landlords sitting down in front paying attention,” recalls Ford Baldwin, co-owner of Practical Weed Consultants, Austin, Arkansas. “No good farmer wants to take over a farm with resistance. Landlords can’t rent a good farm because of resistant weeds.” 

19. Waterhemp is the weed to beat in the Midwest 

Palmer Amaranth has engulfed many fields in mid-South states like Arkansas and Tennessee. It has surfaced – surprisingly – in states as far north as Michigan. 

Then again, waterhemp remains the weed to beat in Midwestern states like Missouri. “So far, in our geography, it has stayed mainly in river bottoms with sandy soils,” says MU’s Bradley. “In fields with mixed populations, waterhemp remains the predominant pigweed.”

20. Herbicide rates cannot be sliced

Saving herbicide rates initially appears to be a way to shave costs. Don’t do it.

“There is good evidence that reduced rates will predispose a weed population to resistance,” says ISU’s Owen. “When you start shaving rates, it is penny wise and pound foolish.”

21. You’ll see more crop injury 

In the pre-Roundup Ready days, chemical reps spent more time answering complaint calls than they did after herbicide-tolerant crops emerged. The advent of multiple-mode herbicide resistance has you diversifying chemistries, though. Some of these older ones have injury and carryover concerns under certain circumstances. “We’re dealing with things we haven’t dealt with the last 15 years,” says ISU’s Owen. 

There are ways to work around this. “If you have a field with low organic matter and high pH and used a product the previous year with potential for carryover, you may want to target that field for later planting,” says Owen.

A weed-infested field will choke yield potential much more than carryover or application injury. Carryover tends to be sporadic, affecting just parts of a field. Many times, crops snap back. 

22. You may need to cultivate

Eyeballs roll among farmers when ISU’s Owen brings up cultivation as a way to deal with herbicide-resistant weeds. Still, it’s an option for heavily weed-infested spots. 

“They’ll say, ‘I have 1,000 acres to cultivate,’” he says. “But they don’t have to do it all – just in certain areas.” 

What’s Coming Up

There is some new herbicide-tolerant trait technology on the way in the next few years. To find out more, check out 

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