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Marestail/horseweed
EARLY CONTROL: Marestail is shown at the pre-bolt stage. Pulling or cultivating small marestail plants can be an effective control method, as long as the plants are young.

Act soon to manage invasive marestail in pasture

As the weed becomes a bigger problem in western Nebraska, consider steps for early control.

By Gary Stone

Early Detection and Rapid Response is a concept to identify potential invasive species before or just as the establishment of the invasive species is taking place. An integrated pest management plan can be developed to manage, contain and eradicate the invasive species before it can spread further, avoiding costly, long-term control efforts.

A native species that has become a major problem in the Nebraska Panhandle and Sandhills rangeland is marestail or horseweed.

With the advent of no-till farming and repeated use of glyphosate products, marestail has become glyphosate-resistant and a major weed problem in corn and soybean-producing areas. Marestail also is resistant to some ALS inhibitor (amino acid synthase) and PPO (cell membrane disruption) herbicides.

Some have speculated that marestail's increase in pastures and rangeland is because of recent cool, wet springs, causing an increase in seed germination. While these plants may not be glyphosate-resistant, ranchers are concerned they are losing valuable grazing potential because of this plant.

Several questions need to be answered about marestail in grazing situations: Does the high density of marestail in pasture and rangeland affect desirable forage and discourage grazing? Will it be profitable to apply herbicide to manage this weed in rangeland situations?

Problem with marestail

Marestail is a winter and summer annual, reproducing or spreading by seed. Seed viability is generally two to three years, although some studies have found seed viable after 20 years in pastures.

The seed doesn't need dormancy to germinate. Seeds germinate on or near the surface of the soil and form a rosette. Seed is generally spread by wind. Mature plants bolt and form a series of seed heads and have a taproot system.

Marestail can grow in height from 1 to 6 feet or taller. Total seed production is proportional to plant height, with taller plants producing up to 230,000 seeds. Marestail can be found in grasslands and disturbed sites, including riparian areas.

The plant is adaptable to a broad range of soil conditions and types. Marestail contains allelopathic chemicals, which can inhibit germination and growth of several other plant species. Herbicide-resistant biotypes have developed over the years.

Marestail management

Having well-established grasses and forbs on a well-maintained pasture or rangeland with proper grazing and rotational grazing techniques help minimize marestail's effects. Scouting, monitoring and proper identification are key factors for management. Infestations can occur rapidly.

Pulling or cultivating small marestail plants can be an effective control method, if the plants are young — before they bolt and form seed heads — and if the area affected is small. Mowing after the bolt stage but before or at flowering can help reduce seed production and reduce seed spread.

Since marestail can germinate in fall or spring, selection of the proper herbicide for the situation is important. Herbicide is a costly option, unless infestations are localized and small.

The cost of a herbicide application may not yield the economic benefits desired, as herbicide applications will only have an effect for one growing season. Spring applications of herbicides for marestail may be better, usually from April through early May. After this, herbicide efficacy drops off.

Marestail treated in fall with products that don't have any soil residual activity will not control plants that germinate the next spring. Marestail plants should be in the seedling to rosette stage and no larger than the five-leaf stage for effective control.

Careful consideration must be made when using herbicide. Herbicides may injure other beneficial forbs and plants in the pasture or rangeland ecosystem, and not have an effective cost benefit.

Some herbicides that have shown to have some control or suppression of marestail include 2,4-D ester, aminopyralid (Milestone), dicamba, triclopyr + fluroxypr (PastureGard HL), clopyralid (Stinger), chlorsulfuron (Telar XP), picloram + 2,4-D (Graslan L — restricted use pesticide). Tank-mix combinations of products may enhance control, and combining products with different modes of action will help reduce herbicide resistance.

Herbicide disclaimers and recommendations

Consult the specific herbicide label for instructions on how to treat marestail for timing of application and rates. It is important to follow the product label unique to each individual herbicide. When the herbicide label allows, the use of a non-ionic surfactant may improve control.

Be sure to select a product labeled for the site. Read, understand and follow all label instructions when using any pesticide. Create an integrated pest management plan combining different control strategies to manage marestail and promote the desired plant community you want.

Stone is a Nebraska Extension educator at the Panhandle Research and Extension Center.

Source: Panhandle Research and Extension Center, which is solely responsible for the information provided and is wholly owned by the source. Informa Business Media and all its subsidiaries are not responsible for any of the content contained in this information asset.
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