By: Alyssa Essman

Each fall the OSU weed science program conducts a preharvest survey evaluating the frequency and distribution of problematic weed species across the state. Transects are driven through the top 45-50 soybean producing counties and visual ratings are given for ten weed species in each soybean field encountered. The weeds evaluated during this survey were: marestail, giant ragweed, common ragweed, waterhemp, Palmer amaranth, redroot pigweed, volunteer corn, common lambsquarters, grasses/foxtail spp., and velvetleaf. Over 4200 fields were surveyed this past fall, approximately 57% of which were “clean”, or free of the ten weeds evaluated. The most encountered weed in 2022 was giant ragweed, present in 12% of fields when combined across rating levels. Information about this was covered in a previous C.O.R.N. article. Waterhemp was the second most frequently encountered weed, in 11% of fields. We have seen this number steadily rise in recent years. Waterhemp was identified in 6.5% of the fields evaluated in 2021. In 2020, pigweed species (waterhemp, Palmer amaranth and redroot pigweed combined) were found in 8% of fields.

This increase in the presence of waterhemp is concerning, as it is a very prolific weed and can take over fields in just a couple of growing seasons. It is less competitive on a plant for plant basis compared to some other summer annual species, but what it lacks in competitive ability it makes up for in seed production. Waterhemp can produce anywhere from 100,000 to 1 million seeds per plant depending on environmental conditions and nearby competition. It can also grow very quickly, upwards of an inch per day, and emerge longer throughout the season than many other summer annual species. Waterhemp male and female flowers are on separate plants. This means that waterhemp populations are very genetically diverse, which increases the risk of developing herbicide resistance.

Across the US, waterhemp has developed resistance to seven site of action groups (SOA #2, 4, 5, 9, 14, 15, and 27). Refer to the herbicide classification chart for site of action group descriptions. We have been conducting preliminary screenings of Ohio waterhemp and Palmer amaranth populations for potential resistance to relevant herbicides for several years. Results of the screen of populations collected in 2022 are summarized below.

Overall, populations varied in their sensitivity to the herbicides included in this screen. Several populations were well controlled by all of the herbicides. A few populations were not at all controlled by the 1x rates of atrazine or glufosinate. It is likely that some magnitude of resistance to these herbicides exists within the populations screened. Based on these results, we will be doing further screenings of these populations in order to better define the magnitude and potentially confirm resistance.

Information on the results of prior herbicide screenings can be found in the articles from 2021 and 2022, as well as a video and PDF. In recent years we have also submitted populations to a regional project that screens waterhemp populations for resistance to glufosinate, 2,4-D and dicamba. Samples submitted for 2022 did not show any of the populations as having a reduced response to these herbicides.

The good news is that although waterhemp plants produce many seeds, the seeds are relatively short lived. Three to five years of diligent control measures can help eradicate this weed. This includes preventing further introductions from contaminated equipment or seed sources, comprehensive herbicide programs, and late-season monitoring and removal of escapes. Cultural and mechanical practices such as narrow-row spacing, fall-seeded cover crops, and deep tillage as needed can also help reduce population densities. Herbicide programs that utilize more than two effective sites of action, and a management plan that is diverse and includes multiple control tactics, are generally most effective for waterhemp management.