Herbicide Resistant Marestail (horseweed) in Pennsylvania

Marestail (horseweed) resistant to glyphosate and Group 2 herbicides is present in PA and presents management challenges.
Herbicide Resistant Marestail (horseweed) in Pennsylvania - Videos


Marestail (also called horseweed) is one of the most challenging herbicide resistant weeds we face in Pennsylvania row crops. While some marestail in the state is not herbicide resistant, we hear a growing number of cases of glyphosate-resistant and ALS-resistant populations of this weed. Resistant marestail is especially challenging to manage after soybean planting, because effective herbicide options are extremely limited. Additionally, applications must be applied before the plants reach 4 inches tall for effective control. Therefore, optimal management of marestail in all row crops requires a proactive integrated approach. This video is Part 1 of a 2-Part video on marestail management. Part 1 covers important facts about marestail biology that growers should understand in order to best manage it. Part 2 covers the best management practices for marestail. We recommend watching both videos, starting with Part 1.


Annie Klodd

View Transcript

- [Narrator] Marestail, also called horseweed has been a problem weed in Pennsylvania for many years.

But the recent spread of herbicide resistant populations into our state has elevated producers concern about this weed.

Horseweed populations that are controlled tend to spread their seed for long distances, enabling it to infest vast areas rather quickly.

This video will discuss resistant horseweed and explain the recommended practices for managing it in our region.

Most of the herbicide resistant populations that we encounter in Pennsylvania are specifically resistant to Glyphosate or Round-Up, which is part of the herbicide site of action group nine.

There are also populations in Pennsylvania that are resistant to not only glyphosate, but also ALS inhibitor herbicides, site of action group two, which includes products like Classic, FirstRate, and others.

When small, resistant seedlings are not controlled with other effective methods, they can survive through the season and dominate the field.

Not all horseweed have the same herbicide resistance and many are not resistant at all.

A single population could be resistant to either just glyphosate, just ALS, both or neither.

However, most horseweed in Pennsylvania is resistant to glyphosate.

So it is good practice to treat all horseweed as if it is glyphosate resistant, so as not to rely on an herbicide that is not likely to be effective on them.

So, how do you know if horseweed on your land is resistant?

Just because a weed survives an herbicide application does not, in itself, mean that those plants are technically resistant.

Survival could be due to a number of issues including application or improper timing.

There are several clues that would indicate resistance.

First, if it survives multiple applications of an herbicide that is usually effective on horseweed when applied at the recommended rate and on seedlings that are under five inches tall.

Second, if the application was effective on surrounding horseweed plants and other weeds but not these.

Third, if the weed population gets larger every year despite proper applications.

Understanding the biology of a weed can aid greatly in knowing how to manage it.

Horseweed emerges both in the fall and the spring and seedlings that emerge in the fall will over winter and continue to grow in the spring.

They can grow to six feet tall and produce up to 200,000 seeds per plant.

These seeds are able to spread miles in the wind.

While they can be damaged by mowing, the base of the plant can still grow back and produce some seeds.

Lastly, horseweed is more common in no-till fields and reduced-till fields, compared to tilled fields.

A newly emerged horseweed plant has small lobed leaves, then forms a rosette.

It then begins to grow upward at a quick pace and mature plants have small white flowers at the tops of the stems.

The seeds that emerge are attached to small white bristles that help disperse them in the wind, similar to dandelion seeds.

Horseweed emerges in both the fall and the spring.

If fall emerging plants are not terminated before winter, most of them will survive into the spring and become much stronger and more difficult to terminate.

Another flesh emerges from April to early June.

This photo taken in August is of plants that emerged late and survived and choked out the crop by August.

Understanding weed biology also helps us understand the weaknesses of the plant that we can target in order to improve control.

First, it needs a lot of sun and doesn't do well in the shade.

We found that shading out horseweed with a cover crop or decreased rose basing challenges it's ability to survive.

Secondly, it's less of a problem in corn than soybean partly because of shading and partly because of increased herbicide options in corn.

Third, it doesn't tolerate tillage well.

So, in no-till fields, we need to think outside the box more for horseweed management.

Thank you for watching our video on herbicide resistant marestail.

Please watch part two, managing herbicide resistant marestail for best management practice recommendations.


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