A proven method for understanding opponents is to put yourself in their shoes and see the world as they see it – in this case, to think like a weed. What do weeds need to thrive? What plant characteristics and cultural practices are beneficial to herbicide-resistant weeds? What management tactics have proven effective against them?

Weeds are plants out of place that compete with crops for light, moisture and nutrients in their efforts to flower, create seed and multiply. The nature of plants, whether desirable or not, is to produce seed to expand and prolong the species.

Repetition breeds resistance

“Resistant weeds have thrived in fields where the same management practices are used year after year,” says Kevin Bradley, University of Missouri Extension weed scientist. “Weed resistance has developed where growers have applied the same herbicide(s) continually, and/or planted the same crop each year, often using the same row spacing and tillage practices, as well.”

Resistance starts as a few weeds survive the repetitive herbicide treatment. The survivors multiply and produce more and more resistant seeds. Given various locations and growth conditions, resistant species can germinate throughout the season, which results in multiple weed flushes. Plants that emerge later can escape preemergence applications and dodge postemergence treatment from contact herbicides.

The most aggressive and prolific herbicide-resistant weeds, such as Palmer amaranth and waterhemp, are dioecious (male and female flowers on separate plants).“Male flowers produce pollen, which is wind-disseminated to fertilize females that produce the seed,” Bradley says. “The pollen can transfer the resistance trait to a previously herbicide-susceptible female in another area. A single male Palmer amaranth or waterhemp plant can transfer genetics to a whole field. The female will then produce more than 200,000 seeds, some of which will be resistant.”

Certain weeds have the ability to spread seed aerially. Research has shown that marestail seed can reach altitudes of 450 ft. and travel 100 miles in a single flight with moderate winds.

Resistant weeds also spread mechanically on equipment from field to field and region to region. For example, in fall 2010, Christy Sprague, Michigan State University Extension weed specialist, was called to a farm infested with glyphosate- and ALS-inhibitor-resistant Palmer amaranth, a weed not native to Michigan. Sprague went into detective mode to figure out how a weed generally considered a problem in the South and Mid-South had made it to Michigan.

“In 2011 and 2012, we received more reports of glyphosate-resistant Palmer amaranth on farms four to five counties away from the first one,” Sprague says. “A common factor was that manure from dairy farms had been applied to many of the infested fields. We speculated that the seed may have been brought in with hay or feed from another state. A lot of Michigan dairies feed cotton seed.”

In one exceptional case, Palmer amaranth appeared only on the edge of a field. “We speculated that it may have blown off a truck from a corn seed delivery,” Sprague says. “It turned out that cotton seed had been hauled in the truck before it was loaded with corn seed.”

By 2012, glyphosate-resistant Palmer amaranth had been found in nine Michigan counties, with a lot of it from manure applications. “The Palmer amaranth in Michigan looks like it can be as aggressive here as it is in the South,” Sprague says.

Last summer, Mike Owen, Iowa State University Extension weed specialist, found that 60% of waterhemp populations sampled in Iowa were resistant to three or more herbicide modes of action. The samples were gathered as part of a study sponsored by the Iowa Soybean Association.

“In Iowa, it’s likely that you may have two or three different types of resistance in the same waterhemp population,” Owen says. “Farmers have to understand that we already have widespread herbicide resistance in Iowa. We clearly need to do something by going back to integrated pest management practices and diversifying as much as possible.”

 

Defensive tactics

“The number-one thing in a resistance management program is to be aware of the weeds you have in your fields and how they got there,” says Sprague says. “It’s very important to identify weed problems correctly.

“If you see a patch of weeds from the combine seat that should have been controlled, you really need to investigate what species is there, maybe collect some seeds, gather some samples to test and find out if they’re resistant.”

There is no simple solution to the weed resistance challenges growers face today, says Owen. “In the past, every time we had a problem, a different site of herbicide action came along to take care of it. The solution to triazine resistance was ALS-inhibitors, which were then used on 85-90% of soybean acres. When weeds evolved with ALS-inhibitor resistance, we turned to glyphosate. Unfortunately, there are no new sites of herbicide activity in the development pipeline. And even if new active sites of action were discovered, they would not represent stand-alone solutions for glyphosate resistance.”

That’s why growers should incorporate as much diversity as possible in weed management programs, Sprague advises. “Use herbicides with different residuals and different effective modes of action, tillage, cover crops, crop rotation – there are many practices that can fit in a program.”

The tactics don’t have to be fancy, she adds. “If you are in the field and see suspect weeds early in the season, pull them before they go to seed. They are a lot easier to manage when they are small and immature.”

There is one tool that every herbicide-resistance management plan needs: “Soil-applied preemergence herbicides are a must,” Owen says. Relying solely on postemergence applications is no longer an option.

“Many growers don’t understand or believe that early season weed control is absolutely essential to protect crop yields. They say, ‘Man, that’s going to cost a lot of money.’ But a preemergence program doesn’t cost them anything in the long run,” he says. “When you consider the extra yield brought by better weed control, and with current bean prices, you can afford to buy a really good residual weed management program for $20-30/acre.”

Sprague cautions famers who employ diverse programs to remain vigilant about scouting for escapes. “Now that we have new infestations in the upper Midwest and understand how easily the weeds spread, we really need to keep our eyes open to the possibility of having resistant weeds,” she says. “Our focus now needs to be on preventing them from becoming widespread problems that are very difficult to control.”