Biology, identification, & management of glyphosate-resistant horseweed (marestail, Conyza canadensis)

Glyphosate-resistant horseweed presents a new challenge for managing this weed in Montana's pastures, prairies and croplands.

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HORSEWEED (CONYZA CANADENSIS L. CONQUIST),

also known as marestail or Canadian fleabane, is a common broadleaf annual weed in prairies, pastures, croplands, and roadsides throughout North America. In December 2015, two biotypes resistant to EPSP inhibitors such as glyphosate (the active ingredient in Roundup® and other commercial products) were reported in Richland County, MT. These populations were found resistant to three to five times of the lethal dose for non-resistant populations of horseweed. While glyphosate-resistant populations of horseweed are new for Montana, resistant populations were first reported in the U.S. in 2001 from corn and soybean fields in Delaware. Since then, resistant populations have either evolved or dispersed throughout the Midwestern, Southeastern, and Mid-Atlantic U.S., as well as California. This recent discovery of glyphosate-resistant horseweed populations presents a new challenge for managing this weed in Montana.

Identification and biology

Horseweed can grow either as a winter or summer annual plant, reaching 1-5 feet high. Roots consist of a small taproot and numerous lateral roots. As a seedling, horseweed forms a basal rosette with sparsely hairy leaves. As the plant matures, it develops erect stems with typically unbranched lower portions, and may branch on higher portions. The dark green cauline leaves are simple, alternately arranged, crowded on the stem, and may be either sessile or have short, distinct petioles. Leaves are hairy, lance-shaped, and usually have entire margins, but may be somewhat toothed (Figure 1). Detailed photos of horseweed vegetative and reproductive structures can be found at the USDA Plant Database (http://plants.usda.gov/core/profile?symbol=coca5).

As a member of the sunflower family (Asteraceae), horseweed's inflorescence consists of numerous flowering heads each containing an outer ring of white female ray florets and yellowish-green perfect disk florets on small branches. The ray florets are often concealed by the involucres on the exterior of the flowering head. The small flowering heads range from 1?10-to 1?5-inch in height and produce an average of 60-70 fruits each, resulting in more than 200,000 seeds per plant. Horseweed fruits are small achenes, between 1?32-and 1?16-inch and topped with a 1?10- to 1?5-inch fluffy pappus that allows seed to travel easily with wind. Previous research has shown that horseweed seeds can reach altitudes of more than 450 feet and can travel hundreds of miles in a single flight at moderate wind speeds.

FIGURE 1 Diagram of horseweed. Image adapted from Ministry of Environment British Columbia. (1998). Illustrated flora of British Columbia. 1. Gymnosperms and Dicotyledons (Aceraceae through Asteraceae). G. W. Douglas (Ed.). Ministry of Environment, Lands and Parks.

While horseweed can grow as a winter or summer annual, most individuals in Montana germinate in the fall and overwinter as a basal rosette. The ability to overwinter as a basal rosette helps horseweed escape competition from summer annuals, species that germinate in spring and complete their life cycle in a single growing season. Bolting usually occurs in May, and flowers bloom around mid-July with fruits maturing approximately three weeks later. Approximately 80 percent of horseweed seeds are viable at maturity and germinate soon after falling off the mother plant. Seeds can remain viable for 2-3 years in soil. Horseweed seeds germinate best on the soil surface, and emerge at much lower rates even when buried only 1?3-inch below the soil surface.

Distribution and habitat

Horseweed is native to North America, but is distributed worldwide. Despite its widespread distribution, it grows primarily in temperate zones of the Northern Hemisphere. Horseweed occurs in its native range in all 48 contiguous U.S. as well as all provinces and territories of Canada, except Newfoundland and Yukon Territory. It has also been introduced to Hawaii and Alaska. Horseweed has been documented in most regions of Montana except the Bitterroot Valley, the Centennial Valley, the Helena Valley, and the Judith Basin. However, it is most frequently reported in Montana's Hi-Line, particularly in Phillips and Valley counties.

Horseweed can be found in pastures, prairies, croplands, urban areas, and floodplains as well as along roadsides and railroad right-of-ways. While this weed grows best in disturbed habitats, its seed does not germinate well if buried. Consequently, horseweed tends to be more problematic in conservation tillage and no-till farmlands, particularly on coarse well-drained soils, such as rocky, sandy, or loamy soils. It is drought tolerant, and occurs in dry regions and non- irrigated fields. While occasionally occurring in floodplains, horseweed is not flood tolerant.

Impacts

Horseweed is considered a weed in over 40 crops including potatoes, wheat, barley, pulse crops, and alfalfa. It is particularly problematic in reduced or no-till croplands, as well as pastures where vegetation is frequently disturbed, but soils are not. Relatively little data exists in Montana on yield loss due to horseweed infestation. However, one report from Michigan found that dense stands of horseweed reduced soybean yield in a no-till system more than 80 percent. In addition to yield losses from competition, horseweed is a wild host plant for the tarnished plant bug (Lygus lineolaris) and the alfalfa plant bug (Adelphocoris lineolatus), two major insect pests in alfalfa, as well as the asters leafhopper (Macrosteles quadrilineatus), a vector of the crop disease aster yellows. Horseweed leaves can irritate human skin, and the nostrils of grazing horses.

Despite these negative impacts, horseweed has several beneficial uses and functions. Tinctures of horseweed have been used in traditional medicine to relieve stomach pain by the Navajo and Ojibwe. In addition, horseweed produces a variety of chemicals with potential medical uses such as antioxidants, blood thinners, and astringents. Finally, horseweed may be an effective bio-accumulator of heavy metal in contaminated soils. One report in South Carolina found that horseweed could survive without injury on soil contaminated with cadmium, and accumulate 95 lbs of cadmium per ton of biomass.

What is Integrated Weed Management?

Integrated weed management (IWM) refers to the combined use of biological, cultural, mechanical, and chemical practices to manage weeds, so that reliance on any one management technique is reduced. The main goals of an IWM program are to 1) use preventive tools to maintain weed density at a level that does not harm the crop, 2) prevent shifts toward more difficult to control weeds, and 3) develop systems that maintain or improve crop productivity, farm revenues, and environmental quality.

The following five principles should be kept in mind when designing IWM programs that are effective at reducing opportunities for weeds to establish and become problematic.

1. Prevention is key. Not allowing weeds to establish is the most effective way to minimize weed problems in your crop fields

2. Increase natural weed mortality. It is unlikely that ALL weeds can be prevented from establishing. However, maximize the effectiveness of mortality factors such as seed predation, seed decay or desiccation. These factors already exist in fields and should be used in your favor.

3. Don’t feed the weeds. Crops and weeds compete for the same basic resources: sunlight, nutrients, water, and space. Make sure resources are captured by the crop and not the weeds.

4. Keep weeds off balance. Don’t let the weeds adapt to weed management practices. Using the same crop rotation generates predictable environmental conditions that are exploited by weeds.

5. Use as many “little hammers” as possible. Conventional cropping systems tend to rely on one or two “large hammers”, such as herbicides and tillage, for weed management. There is growing evidence supporting the merits of combining several different weed management practices or “little hammers” in an IWM program.

More information on IWM can be found in the MontGuides, Integrated Strategies for Managing Agricultural Weeds (MT200601AG) and Weed Seedbank Dynamics & Integrated Management of Agricultural Weeds (MT200808AG).

Integrated Management of Horseweed

Prevention

Given horseweed's prolific seed production and ease of dispersal, preventing populations from seeding is a key step in reducing its spread. Mowing horseweed when it is in the bud stage may be particularly effective in pastures, roadsides, and prairies. Because horseweed flowers in mid-summer, when crops are either actively growing or in the process of being harvested, mowing is not a viable option in croplands. However, given its shallow root system, horseweed can be hand weeded easily. This may be a valuable option for managing horseweed at early stages of invasion, on small farms, or to remove surviving individuals following other treatments hand weeding is impractical for large-scale management. As for all weeds, diverse crop rotations that avoid predictable environmental conditions are one of the most valuable measures to prevent the development of economically injurious populations. Horseweed is much less competing or problematic where competitive crops are grown. Winter cover crops in particular can help reduce horseweed density and the potential for seed production. Research conducted in Pennsylvania has shown that fall cover crops like rye, hairy vetch, or radish can successfully reduce horseweed density and fecundity.

Mechanical management

Horseweed seeds do not germinate well when buried, and tillage remains an effective treatment for reducing populations of this weed. However, in the water-limited environments of Montana, excessive reliance on cultivation could result in soil erosion and moisture loss. In no-till systems, where horseweed is more problematic, leaving larger amounts of crop residue on fields can reduce seedling emergence, but this approach is not as effective as tillage or herbicide application. Livestock do not graze horseweed because of the irritating defense chemicals it produces, and therefore grazing is not a viable management option.

Herbicide Resistance in Horseweed

In addition to glyphosate-resistant biotypes, populations of horseweed resistant to ALS inhibitors (e.g., cloransulam- methyl First Rate®), photosystem I electron diverters (e.g., paraquat Gramoxone®), and photosystem II inhibitors (e.g., atrazine), have been documented in the U.S. and Canada. There have also been five documented cases of multiple- herbicide-resistant horseweed biotypes in the United State and one in Canada, including cases of joint resistance. Multiple-herbicide-resistant biotypes are those resistant to two or more herbicide modes-of-action. Currently, the two glyphosate-resistant biotypes are the only documented cases of herbicide resistant horseweed populations in Montana. However, excessive use of herbicides for managing this weed may accelerate the evolution of herbicide-resistance including the selection of multiple-herbicide-resistant biotypes.

Preventing herbicide resistance

Herbicide resistance is the innate ability of a weed biotype to survive and reproduce after treatment with an herbicide dose that would normally be lethal. In Montana herbicide resistant biotypes of wild oat (Avena fatua), Persian darnel (Lolium persicum), green foxtail (Setaria viridis), kochia (Bassia scoparia), and Russian thistle (Salsola kali) have been found. Also, multiple-herbicide- resistance has been confirmed for kochia and wild oat biotypes.

To reduce the risk of selecting for herbicide-resistant biotypes, producers should rotate among herbicides with different modes-of-action applied either as tank mixes, premix formulations, or sequential applications. Also, producers should rotate management practices, such as the incorporation of timely cultivation, increasing seeding rate, and the use of competitive crop varieties. Finally, crop rotation is an excellent tool to prevent or retard the evolution of herbicide-resistant weed populations. More information on herbicide resistance can be found in the Montguide, Preventing and Managing Herbicide-resistant Weeds in Montana (MT200506AG).

Chemical management of glyphosate resistant horseweed

Herbicide-resistant horseweed can still be managed effectively with timely herbicide applications. Horseweed is most susceptible to herbicides as a seedling or a basal rosette, stages that typically occur in Montana in the fall. For resistant populations, post-emergent applications of growth regulators such as dicamba or 2,4-D ester are recommended. If herbicides are applied before May, a pre-emergent, soil residual herbicide such as atrazine can be applied to manage later emerging individuals. Control of horseweed is difficult once plants are taller than six inches. Farmers should carefully review and follow label instructions to minimize the risk of herbicide carry over injuries.

Research is underway to determine best management practices for controlling glyphosate-resistant horseweed in Montana; however, recommendations for the Midwestern U.S., where glyphosate-resistant horseweed has been a problem for longer, may be a good starting point for developing management strategies for this weed (Table 1).

TABLE 1. Recommended herbicide treatments for glyphosate-resistant and glyphosate-susceptible populations of horseweed. Refer to herbicide labels for application rates and additives. This table is adapted from “Controlling horseweed (marestail)” Michigan State University, accessed: 1/14/2016, http://fieldcrop.msu.edu/uploads/documents/Controlling%20Horseweed%20(Marestail).pdf.

Glossary

Achene: small, thin-walled, one-seeded fruit that does not split at maturity

Annual: plant that completes its life cycle in a single year

Basal rosette: early life stage of an annual or biennial in which a plant exists as a single whorl of leaves on the soil surface

Biotype: group of plants within a species that share a common trait absent in other populations of that species

Cauline leaf: leaf attached to aboveground portions of a stem

Disk florets: tubular florets in the center of a flowering head of plants in the Asteraceae family

Entire margin: leaf margins that are untoothed

EPSP: an enzyme (5-enolpyruvylshikimate-3-phosphate synthase) produced by plants and microorganisms that catalyzes a chemical reaction essential for their survivorship.

Inflorescence: flowering stalk of a plant

Involucres: bracts (diminutive leaf-like structures) along the sides of the flowering head of plants in the Asteraceae family

Ray florets: outer ring of floral structure on the flowering heads of plants in the Asteraceae family

Pappus: modified calyx (outermost ring of flower structures; the ring of sepals) on fruits in the Asteraceae family, consisting of bristles, awns, scales or short crown on the tip of an achene

Perfect flowers: containing both male and female reproductive structures

Petiole: stalk of a leaf attaching it to the stem of a plant

Sessile: leaves attach directly to the stem; lacking petioles

Disclaimer

Information in this document is provided for educational purposes only. Reference to commercial products or trade names does not imply an endorsement of them by MSU Extension. Common chemical and trade names are used in this publication for clarity for the reader. Inclusion of a common chemical or trade name does not imply endorsement of that particular product or brand of herbicide and exclusion does not imply non-approval. This publication is not intended to replace the product label.