Managing Herbicide Resistant Marestail (horseweed)
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 2 of a 2-Part video on marestail management. Part 2 covers the best management practices for marestail in our state. Part 1 covers important facts about marestail biology that growers should understand in order to best manage it. We recommend watching both videos, starting with Part 1.
- [Instructor] These are the best management practices that we recommend in general for horseweed in Pennsylvania.
I'll go into each individually.
First, start clean in the spring by controlling horseweed prior to crop planting.
Horseweed is particularly difficult to control after the crop has been planted.
This is especially true in soybean, where effective post herbicide options are extremely limited.
So controlling it prior to crop planting means applying a burndown to emerged plants in the fall prior and in the spring.
Burndown herbicide options include one pint of 2,4-D ester, which if applied seven days before planting, was found to control 75 to 85% of horseweed.
This can be combined with other modes of action like Gramoxone, Liberty, or Glyphosate, or a group two herbicide like FirstRate.
However, keep in mind that for Glyphosate or ALS-resistant horseweed, those group two and nine herbicides will not be effective options.
When using herbicides to control horseweed, use multiple effective modes of action.
Over reliance on one or two modes of action selects for a resistant populations.
Varying modes of action also enables us to target the weed from multiple angles and prevents introduction of new resistant weeds.
Modes of action can be varied in the tank mix or rotated between applications and years.
For example, if a field has glyphosate-resistant horseweed and we apply glyphosate plus 2,4-D in a burndown, those are two different modes of action, but they are not two effective modes of action.
Glyphosate will be ineffective, leaving only one effective mode of action, the 2,4-D.
A better option would 2,4-D ester Gramoxone and Liberty, because that gives us three effective modes of action.
Number three, control seedlings before they reach five inches tall when they're most susceptible to herbicides.
We can catch weeds early by scouting early and often with a five-inch tall soda can for comparison.
Four, include residuals at planting to prevent late-spring emerging horseweed plants.
Residual options should be sprayed at planting and should again include multiple modes of action, like groups two, five, 14, and 15.
For ALS-resistant horseweed, only groups five, 14, and 15 will be effective.
Some example options of mixes are listed on the left.
Mixes that include groups two and 14 are Sonic, Envive, Afforia, and Valor XLT.
Other tank mixes with multiple modes of action include Trivence, Broadaxe, Authority, and Fierce XLT.
Metribuzin, which is group five, can also be incorporated.
Number five, rotations of corn or small grains with soybeans increases options for effective herbicides and varies planting times from year to year, which throws off the growth cycle of horseweed.
Small grains enables harvesting as a control option to slow horseweed growth my mowing it off.
Adding alfalfa to the rotation also presents an opportunity to cut off horseweed during its peak growth, weakening the plants.
Number six, planting the crop in a way that creates a shady environment between the rows makes it difficult for horseweed to survive.
Horseweed does not do well in shaded environments because it needs high amounts of sunlight to germinate and grow.
Narrow row spacing and higher planting densities and selecting varieties with faster canopy closure all help decrease the amount of light that hits the soil, making it harder for horseweed to grow.
Cover crops may almost be considered to out-compete early spring horseweed as it grows and to create a thick suppressive mulch.
Number seven, do not rely on post applications for horseweed control, as the options are too limited to be effective.
This is especially true in soybean.
Even if marestail is not herbicide-resistant, trials at Penn State and in other states continue to show us that the only effective post options for soybean are Liberty, glyphosate, and ALS herbicides like Classic, Synchrony, and First Rate.
However, if the horseweed population is resistant to glyphosate, then glyphosate is no longer a viable option.
If the population is both glyphosate and ALS-resistant, then Liberty is our only effective post option.
The take-home message of this is that especially for soybean, if it's not controlled in the burndown, it will not be controlled in the crop.
Number eight, because horseweed emerges in the fall and then overwinters into hardier plants in the spring, controlling it in the fall after harvest is very important.
This can be achieved with a fall burndown using the burndown recommendation given earlier in this video.
If planting a cover crop, a fall burndown of 2,4-D and Dicamba may be applied to the cereal rye cover crop to terminate any horseweed that appears in the cover crop.
Penn State weed scientists do extensive research into the best ways to suppress weeds using cover crops.
For horseweed we've found that a thick, fast-growing cover crop can suppress a high percentage of horseweed by the time of spring burndown.
Here's how the system works.
In the fall, a vigorous cover crop species is planted, like cereal rye.
It overwinters, and in the spring it is allowed to grow for up to two weeks prior to cash crop planting, at which time it's terminated with either burndown or a roller-crimper.
Then it dries out on the field, creating a thick mulch that the crop is then planted into.
A general pattern we've found is that thicker cover crops can suppress high percentages of horseweed, up to 70% suppression by a cover crop that supplies 80% ground cover.
If you're considering using cover crops to suppress weeds, please contact Penn State Extension first for more information.
Even in no-till fields, there are mechanical control options for managing horseweed.
Unfortunately, horseweed is more common in no-till fields because it survives well in environments where this is low soil disturbance.
If horseweed plants escape control and grow too large to manage with herbicides, they should be hand-pulled to prevent seed dispersion.
It's very important to not let these plants produce seed, because each plant can produce up to 200,000 seeds and spread quickly.
Cultivation is also a mechanical option for plants that are too large for effective herbicide control but are under about a foot tall.
Mowing can also be used to weaken the plants and slow their growth, but it's not likely that mowing will prevent the plants from producing seeds completely unless it's performed very frequently.
Thank you for watching our video on horseweed management.
Further information can be found in our 2017 Mid-Atlantic Weed Management Guide.
You can also visit the Integrated Weed Management Resource Center for more info.