Marestail (Horseweed) Management

 

Herbicide resistant marestail (horseweed) is widespread in Pennsylvania. Glyphosate (Roundup)-resistant marestail is prevalent throughout the state, with ALS-resistance (Group 2; i.e. FirstRate, Classic) in some areas. It is particularly challenging in no-till and reduced-tillage systems, where integrating multiple tactics often becomes necessary. To prevent resistant populations from establishing, management programs need to avoid reliance on glyphosate and ALS herbicides, diversify herbicide modes of action (MOA), and integrate appropriate chemical and non-chemical control tactics.

 

 

This winter annual weed emerges in both fall and spring (September-November and April-early June in Pennsylvania). Most fall-emerged plants survive the winter and keep growing in the spring, at which point they become harder to control. Seedlings have small lobed leaves that grow into rosettes. The rosettes “bolt” into upright stems that produce up to 200,000 seeds and reach up to 6 feet tall. The seeds are wind-blown long distances, spreading to neighboring farms, counties, and states. Once plants grow above 5 inches tall, they are very difficult to control with herbicides alone.

 

 

Marestail is more prevalent in no-till, reduced-till, and perennial production systems compared to annually tilled fields, because it thrives in landscapes where soil is not disturbed. While it can infest all row crops and forages, it is most challenging to control in soybeans due to limited herbicide options and soybean’s summer annual life cycle. Winter wheat and winter annual cover crops develop thick canopies in the early spring, shading out spring-emerging horseweed. But in Pennsylvania, soybean and corn planting happens after marestail has already established. Marestail can often recover and produce viable seeds following damage by ineffective herbicide application, grazing or mowing.

 

 

1) Control fall-emerged marestail before the winter with a fall burndown. If fall seedlings are not controlled, they overwinter and are more challenging to control in the spring. If planting a cereal rye cover crop, 2,4-D and dicamba may be used.

 

2) Control all horseweed prior to cash crop planting. Effective POST options are limited, especially in soybean.

 

3) Use multiple effective herbicide modes of action (MOA). Vary MOAs in tank mixes and between applications. Using herbicides with different MOAs prevents selection of weeds resistant to certain herbicides like glyphosate or ALS.

 

4) Target marestail under 5 inches tall when they are most susceptible to herbicides. Scout early and regularly.

 

5) Include residuals at planting to prevent late-spring emerging horseweed plants.

 

6) Shade out spring marestail with winter annual crops or cover crops, narrow row spacing, or high seeding rates.

 

7) Hand-remove mature marestail before flowering, before it has a chance to produce and disperse its seeds.

 

8) Include corn or small grains in rotation, to increase herbicide options and compete with marestail for sunlight. Small grains and alfalfa also allow for mowing and harvesting as control options.

 

 

A high-biomass winter annual cover crop can suppress a high percentage of marestail in the spring. If it is left on the field after termination, it can also shade out marestail that emerges after cash crop planting. A vigorous species like cereal rye should be planted after fall harvest and allowed to grow for several weeks into the spring. This allows it to accumulate biomass so that it can function as a weed suppressive mulch that the crop is later planted into. In general, 4000 lb or more dry matter is necessary prior to termination, in order to actively suppress emerging weeds in the spring. Higher biomass can be achieved by earlier planting dates and later termination dates. Before planting cover crops for weed suppression, please contact Penn State Extension for more information, as results depend on various management decisions and individual farm factors.