Glyphosate-Resistant Palmer Amaranth Problems Developing in Ohio – What Dairy Producers Need to Know

Dr. Mark Loux, Extension Specialist, Department of Horticulture and Crop Science, The Ohio State University

We have plenty of glyphosate-resistant weed populations in Ohio.  Resistance currently is known to occur in four weed species here – marestail (horseweed), giant ragweed, common ragweed, and waterhemp - and many of these populations are also resistant to acetolactate synthase (ALS) inhibitors (Classic, FirsRate, etc.).  The good news is that our resistance problems are overall less severe than in the southern United States, where the now widespread occurrence of Palmer amaranth has had a substantial impact on crop yields and profitability of cotton and soybean growers and forced a semi-permanent change in the amount of herbicide that has to be used.  As we expected though, glyphosate-resistant Palmer amaranth populations are starting to show up here in Ohio.  We are already working hard to educate grain producers and their advisors and suppliers about this weed in hopes of curtailing the spread.  The purpose of this article is to inform dairy producers about the problem, that they may be inadvertently facilitating the spread of this weed, and the actions that can be taken to avoid doing so.

Palmer amaranth is in the Amaranthus or pigweed family.  The most abundant pigweed species in Ohio are redroot and smooth pigweed, which are essentially identical to each other with regard to identification.  Most preemergence herbicides have substantial activity on these pigweed species, and they are usually well controlled by a combination of preemergence and postemergence herbicides.  We have screened a few redroot and smooth pigweed populations for resistance to herbicides, and so far, we have not observed  glyphosate resistance.  Palmer amaranth (aka Palmer pigweed) has been fairly accurately characterized as “pigweed on steroids”.  In addition to the glyphosate resistance, this weed’s rapid growth, large size, extended duration of emergence, prolific seed production, and general tolerance of many POST herbicides makes it a much more formidable weed to deal with than the other pigweeds.  The POST herbicides that have activity on Palmer amaranth must be applied when the weed is less than 3 inches tall.  Palmer amaranth has overall more potential to reduce yield if not controlled well, compared with the other pigweeds.  It’s probably not possible for us to overestimate how severe a problem this weed can be based on these characteristics and the problems that have occurred in the south, where some growers finally resorted to hiring crews of laborers to remove plants from fields at great expense.

So why are we trying to get information to dairy producers about this weed?  At this point, we know of several major infestations of Palmer amaranth in Ohio, and it is starting to be found in additional fields.  As far as we can tell, the source of the first major infestation may have been a Conservation Reserve Enhancement Program (CREP)/wildlife type seeding, where the seed of the desirable species was apparently contaminated with Palmer amaranth seed.  However, for the newer infestations that are occurring, the source appears to be contaminated cottonseed brought in from the southern US for use as animal feed here.  Manure from these animal operations is being spread on crop fields, where the Palmer amaranth can then became established.  For example, there appears to be somewhat of an epicenter of new Palmer amaranth infestations in an area southwest of Columbus, bordered roughly by Midway on the north and Washington CH on the south.  There is a dairy in the area that has been using cottonseed products for feed and a local grower has been transporting these products to the dairy from somewhere in the south.  There are Palmer amaranth plants in a number of fields in the area and also on the grounds of the dairy.  Cottonseed feed products also have been the source for major infestations of Palmer amaranth in Michigan and Indiana. 

A primary goal over the next decade for Ohio agribusiness and growers, and OSU weed scientists, has to be the prevention of additional infestations of Palmer amaranth.  This weed has more potential to impact the profitability of our corn and soybean production than our other resistance problems.  Some things for dairy producers to consider relative to this developing problem include:

  1. Take a few minutes to familiarize yourself with the problem.  The OSU weed science website ( has information on Palmer amaranth, including a short video on identification and an 11-minute video that explains the risk from this weed. 
  2. If you are feeding cottonseed feed products, be aware that there is at least some likelihood that they are contaminated with Palmer amaranth seed.  Make your feed supplier aware of this and ask what steps are being taken to ensure that these products are not contaminated.  Scout areas where manure is stored or spread for the presence of Palmer amaranth plants. 
  3. Early identification and management are keys to preventing major infestations.  The OSU weed scientists can help with identification, either via digital photos emailed to us or with visits to fields.  We would appreciate being informed ( of any new infestations of Palmer amaranth as soon as possible. 
  4. Where Palmer amaranth plants are found, it is essential that growers take all steps possible to prevent weed seed production.  This can include tillage, mowing, and also removal of surviving plants by hand.  The result of not doing so could be substantial increases in weed management costs and loss of income in future years.