Weed survey findings & pesticide re-certification upcoming

By Tony Nye - OSU Extension

Each fall just before harvest, the OSU weed science program conducts a statewide driving survey evaluating the frequency and distribution of problematic weed species in Ohio. According to Alyssa Essman, visiting professor in weed science at The Ohio State University, diagonal transects are driven through the top 45-50 soybean producing counties. 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.

In 2022 over 4200 Ohio fields were surveyed. Roughly 57% of fields were clean, or at least free of the ten weeds evaluated. The most common weed in 2022 was giant ragweed, present in 12% of fields when combined across rating levels. Waterhemp was the second most frequent weed, in 11% of fields, followed by marestail in 10% of fields. Grass/foxtail spp. were found in 9% of fields and volunteer corn in 8% of fields.

A yearly weed survey is also conducted for Clinton County and the local results are similar to the statewide results. In 2022, 110 fields were observed with 43% rated clean. Giant Ragweed was most common and present in 22% of the fields surveyed. Volunteer corn was the second most prevalent in 15.5% of fields followed by marestail in 11% of fields. Grasses such as foxtail were present in 9% of fields. Common ragweed and common lambsquarters were present in slightly less than 5.5% of fields. Pigweed species such as redroot pigweed and waterhemp were evident in less than 4.5 % of fields, but I am aware there were some producers that found and hand pulled waterhemp from several fields prior to the survey so beware that the number could be higher for the presence of waterhemp.

Giant ragweed continues to be one of the most common and troublesome weeds in Ohio. It has a fast growth rate and is an extremely competitive plant. One of the first weeds to emerge each spring, giant ragweed can germinate through early summer. Continuous no-till practices and comprehensive herbicide programs can reduce populations over time. Ohio giant ragweed populations have been identified with resistance to group 2 (ALS inhibitors) and group 9 (glyphosate) herbicides, and multiple resistance to both group 2 and 9 herbicides. Resistance to these herbicides decreases control options for giant ragweed, especially in non-GMO soybeans. Effective giant ragweed control programs include a combination of herbicide modes of action and both pre- and postemergence applications. As producers start making plans for weed control for next season, consider these general recommendations weed scientists from OSU, Purdue, and across the corn belt have established for management of giant ragweed:

Effective burndowns reduce giant ragweed pressure at the time of planting. Examples of effective burndowns include a group 4 (2,4-D or dicamba) herbicide plus either a group 9 (glyphosate) or 22 (paraquat) herbicide. Check labels for restrictions on plant-back intervals for 2,4-D and dicamba.

An effective residual product with the burndown application or at plant can reduce population pressure through the time of the post application. Full rates of chlorimuron or cloransulam (group 2) containing products tend to be most effective. Where giant ragweed is resistant to group 2 herbicides, fomesafen (group 14) can be used, but can be more variable and will restrict fomesafen use postemergence.

Giant ragweed will likely require multiple postemergence applications. Two pass programs should include an initial application based on weed size followed by a second application 3-4 weeks later.

Soybeans tolerant to glufosinate, dicamba, or 2,4-D can receive applications of these herbicides postemergence. Glufosinate followed by glufosinate is an option in the LibertyLink system. In the Xtend or Enlist systems, the second application may need be a group 14 herbicide (or glufosinate for Enlist) based on label restrictions for application timings.

In non-GMO soybean production, group 14 herbicides (fomesafen, lactofen) can be used postemergence. Control can be variable and overuse increases selection pressure for resistance. OSU research has shown that fomesafen followed by lactofen 3-4 weeks later is the most effective approach.

Producers should have received notice; however this is a reminder to anyone needing pesticide and fertilizer recertification for 2023 that the meeting in Clinton County will be held this Thursday, Dec. 15 in the Clinton County Extension Community Room, 111 S Nelson Ave. in Wilmington from 12:30– 4:30 p.m.

Pesticide recertification will be first from 12:30 p.m. to 3:30 p.m. followed by fertilizer certification at 3:30 p.m. There will be a fee of $25 for Pesticide Recertification and $10 for Fertilizer Recertification. For those of you needing both certifications your fee will be $35. Registration Deadline is Monday Dec. 12. To RSVP, you may call or stop by the Extension office at 111 S. Nelson Ave. in Wilmington. Office hours are 8 a.m. to 4:30 p.m.

Tony Nye is the state coordinator for the Ohio State University Extension Small Farm Program and has been an OSU Extension Educator for agriculture and natural resources for over 30 years, currently serving Clinton County and the Miami Valley EERA.


By Tony Nye

OSU Extension