Most Wanted: Weeds Edition

Know what you’re up against and then target its weaknesses

Waterhemp, marestail, palmer amaranth, giant ragweed and Italian ryegrass have been found guilty. Their crime: stealing nutrients, sunlight and water from your crops, leading to lower yields and profits. These culprits have been sentenced to a life in exile from crop fields across the U.S.

The only way to catch these dangerous fugitives is to regularly drop in on your fields for a walk-through. Track weed growth patterns, suspected locations and seedling and mature appearances and then eradicate them from your fields.

“Different weeds have different emergence patterns and levels of competition,” says Jeffrey Gunsolus, University of Minnesota professor, Extension agronomist and weed scientist. “You have to decrease early season weed competition.”

While all weeds are problematic, waterhemp, marestail, palmer amaranth, giant ragweed and Italian ryegrass are especially dangerous because they have documented cases of resistance to multiple herbicide groups, which makes them more difficult to control. In addition to resistance, they have the natural ability to evade herbicide control methods.

Use the attached information to figure out what weeds you’re up against, and then plan weed control timing and herbicide application.

“Weed resistance is something you ought to respect, it’s going to keep happening if we do the same thing over and over,” says Kevin Bradley, University of Missouri Extension weed scientist. “Figure out what’s your most problematic weed and target some of its weaknesses.”

Topping the most wanted list is waterhemp—a weed that has quickly spread across much of the Midwest. Waterhemp’s growth cycle is opposite of the spraying window, so it can usually dodge the herbicide bullet. Be on the lookout for this summer annual when it’s small because it won’t stay that way for long. It can grow up to 2.5" per day and will reach 9' tall if left unchecked. Seeds are viable for four to five years, but the seed won’t grow from low soil depths. Waterhemp has been proven resistant to six herbicide groups in one or more U.S. states. Both resistant and herbicide-sensitive waterhemp are tough to control but not invincible.

Second in the lineup is marestail, which is common in row crops and pastures and tricky to manage. As its namesake suggests, this weed runs like wild horses throughout the countryside and is covered in hair from stem to leaf tip. Left to its own devices, marestail will grow to 6.5' tall during summer and winter months. Control measures are limited because marestail is resistant to five herbicide groups. Scout frequently to catch marestail before it reaches 4"—it’s easiest to kill with herbicides before canopy.

The next fugitive, giant ragweed, is more than just a tricky allergen; it’s an extremely competitive weed found across the U.S. So competitive in fact, as few as two plants per square meter can reduce corn yield by 37%, and one per square meter can reduce soybean yield by 52%, according to the University of Illinois. This monster of a weed grows quickly and can reach a towering height of 16' in fertile soils. Due to its highly competitive nature, you want to catch giant ragweed early. It should be manageable because it’s only resistant to two herbicide groups.

Cousin to waterhemp, palmer amaranth is making waves across the South and stealing yields as it inches its way north. At only 6.5’ tall, what it lacks in height, it makes up with seed production and resistance. In a single season, it can produce nearly 1 million seeds and is resistant to six herbicide groups. Seeds have a relatively short shelf life of six years and can’t grow in deep soils, much like waterhemp. Use an effective pre-emergent herbicide on this weed early because it doesn’t fare well in shade canopy, which can reduce germination by 74%, according to the University of Missouri.

Rounding off the most wanted list is Italian ryegrass, the only grass weed to make the list. Many northern farmers might not view this weed as a threat, but farmers in the South warn of its ability to steal yield, especially in corn fields where effective herbicides are limited. When identifying Italian ryegrass, look for a single weed or clumps; it’s not picky. If you see membranous ligules (thin outgrowth at the junction of leaf and leafstalk) and small auricles (or even no auricles), it could be Italian ryegrass. This problematic weed is resistant to six herbicide groups in one or more states. You can control Italian ryegrass through carefully planned herbicide combinations and tillage.

These weed fugitives are at large and dangerous to corn, soybeans, cotton and wheat. Keep an eye out for any weed that fits these descriptions, and if evidence indicates you could have resistance, plan your attack.

“Think beyond herbicides,” Bradley advises. “In the current situation, we can’t think a herbicide is going to bail us out every time. It needs to be an integrated approach.”

While it’s important to address weed issues early, don’t give up later in the season. “When you see weed patches, map those in the combine,” Gunsolus says. “Shift your focus to managing the weed seed bank.”

Take note of the weed types, when you sprayed and what you sprayed to determine if they escaped herbicide or are resistant. If you spot the same weed in various stages of death it’s a good indication of resistance. If a variety of weeds appear healthy, it’s likely they dodged the herbicide bullet.

Your reward for capturing these dangerous fugitives is reduced seed banks, preserved yield and lower herbicide costs. Join the fight against resistance for a better future for your fields.

The next story in the Billion Dollar Bind series will provide best practices to control tricky weeds on your farm.
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