Marestail Nature and Distribution
Marestail, also known as horseweed, is a winter or summer annual that originated in North America and is a common problem in agricultural systems. This weed emerges during two periods, from March to June, and late summer to early fall. Marestail prefers soils that are undisturbed, such as reduced or no-till systems.
This weed overwinters in the rosette stage until spring, when the plant has rapid stem elongation (bolting) that can reach heights from 1.5 to 6 feet tall. Marestail is self-pollinating, but can also outcross within a population to increase the genetic diversity. Approximately 200,000 seeds are produced and spread mainly by wind from 20 to 400 feet from the parent weed. There is no dormancy period for these seeds, which can often germinate after a rainfall, but the seed is only viable for 2 to 3 years.
Little research has been done about the amount of yield loss due to marestail; however the weed competes with crops for light, nutrients, and water. Marestail can also be a host to the tarnished plant bug (an alfalfa pest) and a host for aster yellows (a viral disease transmitted by leafhoppers).
Marestail can be identified by its alternate, linear, and often notched leaves. Leaves are approximately 4 inches long and are attached directly to the stem. Stems are erect and non-branched, unless injured with herbicide or mowing. Both the stems and leaves are pubescent (having hairs). The flowers are small, white, and clustered while seeds are tan and cylindrical with a pappus, tiny hair-like bristles (often resembling dandelion seeds).
Marestail can often be confused with Shepherd’s purse or Virginia pepperweed when in the rosette stage.
Prevention and Control of Marestail
Due to the early germination of marestail, seedlings are already present before the crop is planted. They can be controlled by either tillage or early spring burndown applications. In field surveys, horseweed was found in 61 percent of no-till fields compared to 24 percent in reduced tillage fields and 8 percent in conventional fields.
Horseweed is most easily controlled when in the seedling or rosette stage. Fall or early spring application should be applied before the plant reaches 4 to 6 inches. Use of systemic herbicides along with a residual herbicide is recommended to control later emergence of marestail in the spring.
Failure to treat fields in the fall could result in a heavier population to control in the spring. If no fall treatment is applied, split applications of herbicides should be considered. Apply burndown herbicide and residual in the spring and then after the crop is planted, apply the rest of the residual herbicide. The addition of another herbicide may be necessary during the second application depending on weed density.
Marestail populations in the United States have been found to be resistant to gramoxone (PSI), chlorimuron (ALS), diuron (PSII), and glyphosate (EPSP) herbicides. This resistance is a concern, and therefore rotating modes of action or using tank mixes is important in managing resistant populations.