Focus weeds for Harrison County Pest Resistance Project

The Harrison County Pest Resistance Project initially began in 2017 as a result of the introduction of Palmer amaranth in the county four years prior. As most farmers in the area are grappling with herbicide resistant weeds but do not have Palmer in their fields yet, the project was expanded to include waterhemp, marestail, and giant ragweed. Read below to learn more about these weeds and why they were included in this project.


Waterhemp, like Palmer amaranth, redroot pigweed, smooth pigweed, and Powell amaranth, is a member of the amaranth family (genus Amaranthus). While all pigweeds look fairly similar, waterhemp can be distinguished by its lack of hairs on leaves or stem, and petioles (leaf stems) that are shorter than the leaves. Stems of waterhemp can range from light green to red.

Waterhemp is a broadleaf species with seeds that can germinate from early spring into summer. Waterhemp is dioecious, meaning male and female flowers occur on separate plants. The cross-pollination required contributes to the high genetic diversity in waterhemp populations, which makes it more likely for resistance to develop and easy for it to spread. This extensive list of documented herbicide resistance makes waterhemp management challenging moving forward.

Palmer amaranth

Palmer amaranth is a member of the amaranth family and a close relative of waterhemp and other pigweeds. Several characteristics can help distinguish Palmer amaranth from its close relatives. Palmer amaranth, similar to waterhemp, lacks hair on its stems. Palmer has a symmetric growth pattern, often likened to a poinsettia.  In addition, if you grab the flower head of a female Palmer plant, it will be painful due to presence of sharp bracts.  Perhaps the best way to distinguish Palmer from other pigweeds is the presence of very long petioles, often longer than the length of the associated leaf.

A new introduction to Iowa, Palmer amaranth is a concern due to its fast growth rate, high seed production, and competitiveness with field crops. Palmer amaranth has a number of documented resistant biotypes. A free Palmer amaranth identification guide can be downloaded at https://store.extension.iastate.edu/product/14794.


Marestail, or horseweed, is a winter or sometimes summer annual species in the sunflower family. Marestail usually emerges in the fall or early spring.  Fall germinating plants over-winter as a small rosette and bolt the following spring. As a winter annual, marestail is primarily a weed in no- or reduced-tillage systems.

Marestail can produce 200,000 wind-dispersed seeds per plant, which are able to travel significant distances.

Marestail is most effectively controlled while still in the rosette stage. Fall herbicide applications can be very effective.

Giant ragweed

Giant ragweed, like its close relative common ragweed, is a member of the sunflower family. Giant ragweed is very competitive, growing up to 12 feet in height. Giant ragweed has large, opposite leaves with 3 to 5 lobes, often serrated at the edges. Giant ragweed has separate male and female flowers on the same plant. Male flowers are inconspicuous, yellow to green, and arranged in cylindrical spikes at the upper terminals; female flowers occur at leaf axils. Giant ragweed pollen is a significant contributor to late season hay fever. Giant ragweed germination occurs in a relatively short window, usually in April in Iowa.

Herbicide Resistance Screening

To confirm presence of resistant weed populations in Harrison County, the Harrison team collected seeds from suspected resistant populations. Seeds were grown in greenhouses at Iowa State University and treated with Roundup (glyphosate), Cobra (lactofen), and Callisto (mesotrione). Though we used Cobra and Callisto in these studies, these results can be assumed to apply to other PPO- and HPPD-inhibiting herbicides, respectively. Results from analyses of seeds collected in 2017 are found below.

In 2018, samples were once again collected from locations across Harrison County. Four waterhemp samples were submitted for molecular analysis to the University of Illinois Plant Clinic.  All four samples tested positive for resistance to glyphosate, while two of four samples tested positive for resistance to PPO-inhibitors. Additional samples were collected for testing at Iowa State University greenhouses. Results will be posted when they become available.


Populations of common weeds exhibited resistance to all three herbicides tested. All waterhemp populations sampled exhibited some degree of resistance to Roundup (HG 9) in this study, with survival between 36 and 75% 21 days after treatment (Table 1).  Waterhemp response to Cobra (HG 14) was variable, with plant survival between 10 and 100%.  Palmer amaranth showed moderate to high level of resistance to Roundup and Callisto (HG 27), but were completely controlled by Cobra.  Survival of giant ragweed to Roundup was between 21 and 78%, while one giant ragweed sample was highly resistant to Callisto.  All giant ragweed samples in this study were susceptible to Cobra.

These results confirm the presence of resistance to many of the most commonly used herbicides in corn and soybean production in weed populations in Harrison County. To reduce the spread and development of herbicide resistance, the use of diverse weed management programs are encouraged, including timely applications of herbicide mixtures containing multiple, effective mechanisms of action in addition to cultural and mechanical techniques where feasible. Due to high seed production in many weed species, all efforts to reduce and prevent seed production by resistant individuals will be highly beneficial.