Horseweed (Conyza canadensis) occurs throughout the continental United States and most of Canada. It has long been one of the most problematic weeds across all crops in agriculture, but its economic impact became greater when it developed resistance to glyphosate-containing herbicides (e.g., Roundup).
Horseweed is a winter annual. It often germinates in late summer or early fall and forms a rosette to overwinter. The overwintering rosette allows the plant to bolt quickly in the spring and
When discussing herbicide-resistant weeds, it is important to remember that only some populations of the species are herbicide resistant, not the entire species. Glyphosate-resistant horseweed can tolerate normal rates of glyphosate. Treated weeds may be chlorotic, stunted, and have excessive rosetting of the foliage. However, these plants can still produce viable seed and perpetuate the population.
How do weeds become herbicide-resistant?Weeds can become resistant to herbicides by one or a combination of mechanisms. Some weeds have altered leaf surfaces that limit the amount of herbicide absorbed into foliage or stems. Some weeds have adapted to reduce the amount of herbicide translocated to target sites within the plant. For example, glyphosate is most effective when it kills plant roots, thus reduced translocation
Individual plants do not respond to herbicides by becoming stronger or resistant. Remember that herbicide resistance occurs to a population, not individual plants. Consider, for example, a 1-acre field with a million weed seedlings. An herbicide is applied, killing 99.99 percent of the weeds. Fewer than 100 weeds survive because they randomly expressed a gene for a slightly thicker cuticle, which prevented herbicide absorption. Those few weeds grow poorly, but
Where did glyphosate-resistant horseweed come from?
Glyphosate-resistant populations of horseweed most likely developed in no-till crops that utilized “Roundup-Ready” technology. These crops rely primarily on Roundup (glyphosate) for controlling weeds while the crop establishes. After annual repeated applications of the same herbicide, a few horseweed plants randomly expressed some genetic property that allowed them to resist normal rates.
Nursery growers in the Midwest have recently reported difficulty in controlling horseweed in field-nursery crops. Field-grown nursery crops share some similarities to no-till agronomic crops. Most importantly, soils in field nurseries are not tilled after planting, at least within the planting row. While many nurseries use preemergent herbicides in the spring and fall, glyphosate is routinely used for spot-spraying and maintaining weed control throughout the growing season. Some nurseries use glyphosate exclusively to periodically remove weed infestations and forego using any preemergent herbicides. Either way, this reduction in tillage and reliance on glyphosate for weed control has led to expanding populations of glyphosate-resistant horseweed in nursery fields.
Controlling glyphosate-resistant horseweed
Farmers deal with glyphosate-resistant horseweed in agronomic crops (corn, soybeans, etc.) by using a combination of 2,4-D and other phenoxy-herbicides, as well as a variety of ALS-inhibiting herbicides. However, these solutions are not viable for nursery crops due to the likelihood of plant injury, as well as the limited labeled applications for using these products in nursery production.
Considering the prolific seed production and fertility of this weed, its ability to germinate throughout the growing season, and its tolerance to glyphosate, the only practical method for controlling horseweed is through an effective
Experiments were conducted in a horseweed-infested field in Wooster, Ohio. The field was first plowed and disked. Herbicides were spray-applied during the first week of October while the field was still weed-free. All herbicides were applied with a spray volume of 40
In addition to horseweed, the most prevalent weed species observed in the plots included annual bluegrass (Poa
Results were different for the spring application of preemergent herbicides. Marengo and SureGuard still provided effective control, although the spring application of Marengo was not quite as effective as the fall application (see Fig. 2). Princep and Tower provided excellent control after spring application, in contrast to poor control after fall application. Finally, Gallery and Goal provided relatively poor control following spring application, despite providing excellent control in fall application.
Herbicides and rates applied to a field soil to determine efficacy on preemergence horseweed control.
Year-round horseweed control
The easiest way to control horseweed is through cultivation. Recall that horseweed becomes problematic in no-till agronomic crops and in nursery fields that lack cultivation (within tree rows). Disking or light cultivation should be used wherever it is practical and efficient. When cultivation is not possible, apply preemergent herbicides to