Palmer amaranth has been increasing around the Midwest and Conservation Reserve Enhancement Program (CREP) seedings can apparently be one source of new infestations. Much of the seed for CREP seeding comes from Kansas where Palmer amaranth and other pigweeds are abundant, and it has become apparent over the past decade that it is possible to have Palmer amaranth seed present in the seed mixtures used to establish CREP areas. We so far have one major Palmer amaranth infestation in far southern Ohio, and it was our assessment that a CREP seeding about five years ago was probably the source of Palmer amaranth there. We also identified Palmer amaranth in several other CREP seedings about 7 years ago, but none of these appeared to gain a foothold in nearby crop fields. We were made aware of the presence of Palmer amaranth in another new CREP area last week, and we are assuming that the source of the Palmer there was also contamination of the seed mix. The Palmer amaranth infestation in far southern Ohio is definitely resistant to glyphosate, but we don’t yet know whether this new one is resistant.
One of our assumptions about Palmer amaranth based on experiences in the southern United States is that it will likely develop glyphosate resistance in crop fields when exposed to repeated applications of glyphosate. It’s important that we err on the side of preventing new infestations of Palmer amaranth in Ohio so that we don’t have similar experiences here. We are working with several agencies to better determine whether seed used to establish CREP does contain Palmer amaranth seed, and if so, what steps need to be taken to make sure this stops. Our recommendation at this point for landowners and growers is to scout recently established CREP areas for the presence of Palmer amaranth. Plants that are not yet producing viable seed (small black seed when you shake or smash seedheads in your hand) should be removed from the field immediately to prevent seed production. Herbicides applied now are not likely to adequately prevent the development of viable seed. Where plants are already producing viable seed, it would probably be best to just leave them where they are to prevent spread of seed. Intensive monitoring of any infested areas and adjacent fields should obviously also occur for the next several years. There is information on identification and management of Palmer amaranth on the OSU and Purdue weed science websites, including short videos on identification that can be reviewed prior to crop harvest. Click here to access the OSU Extension page on Palmer amaranth.