Improving Neighbor Relations in the Face of Palmer Amaranth, a New Weed with New Challenges

Jason Hartschuh, Crawford County OSU Extension ANR Program Coordinator, The Ohio State University

The agronomic crops team has been working to educate farmers about the different weeds in the Amaranth (pig weed) family. Some people may ask, “Why the focus on pigweeds? We have had them for as long as I can remember.” Well not quite, we have had Redroot Pigweed but not the new, invasive Palmer amaranth. This species has been raising more cane across the south than a wild bull, devastating soybean and cotton farms. It has developed herbicide resistance to Glyphosate and protoporphyrinogen oxidase (PPO)-inhibiting herbicides, like Cobra. This means that once it shows up in soybean fields, you have lost the battle.

It has now been found in at least 5 areas of Ohio and has arrived by inadvertant movement of seed. Palmer amaranth seed moved north on equipment from the south, railroad cars, Conservation Reserve Enhancement Program (CREP) seed, and the biggest source, FEEDSTUFFS!!
This is where dairy farms come into the picture, especially those that feed any ingredient from the south. In multiple states, cotton seed and forage have been the source for the seed to come in and cause major economic problems. Not only has whole cottonseed been a problem but also cottonseed hulls and cottonseed meal, along with emergency forages from the south. We do not suggest that you eliminate cotton products from your ration, rather we do suggest that you be diligent about managing the risks. Cottonseed can be a good source of protein, fiber, and energy that complements other home grown feeds and helps keep milk fat at respectable levels.  

The first step is to learn to identify the plant. When Palmer amaranth is still small, it looks like other pig weeds from a distance. Look closer though. First is the petiole, or leaf stem, which is longer than the leaf itself. The leaves are often diamond shaped, with no hairs on the stem or leaves. The seed heads have at least one very large spike, often over 2 feet in length. One other notable characteristic, they can develop a base that measures over 2 inches across and is known for breaking harvesters and mowers.

Now that you can identify Palmer amaranth, how can it be controlled? Corn is the easiest. Start the year with a clean, weed-free field with tillage or a 2,4-D burn down. Then choose from the large selection of pre-emerge herbicides but be sure to use maximum label rates. Finally, scout for plants any that may emerge when corn is at the V4-V5 stages, and apply one of the numerous clean up herbicides with more residual if necessary to get you through the rest of the season.

Soybeans are where it gets tricky. THERE IS NO CLEAN UP HERBICIDE ONCE PALMER AMARANTH IS OVER 2 INCHES TALL. First consider applying manure nutrients for soybeans one year in advance, when you are planting corn, to clean up the first flush of seeds and decrease pressure. Second, believe in burn downs and residuals, even if you plan to plant conventionally. Use a spring burn down that includes a full rate of a growth regulating herbicide and one of the many residual products that works on amaranths. Seeds are easy to kill, but once Palmer amaranth can be seen, all bets are off. Finally, scout your fields. If you see even one seedling, apply some more residual herbicide with your glyphosate cleanup for other weeds.

Now that you have an idea how to identify Palmer amaranth and the control measures for it, do you spread manure on a neighbor’s ground? If so, it is time to go visit them and make sure they know you have fed a product from the south and explain to them how to identify Palmer amaranth. Printing off some of the resources from the following website ( can help facilitate the discussion and leave the landowner with some references. Palmer amaranth is controllable if you start in the spring and keep seed counts low by never having an outbreak. One plant can produce 500,000 seeds.

As you plan feed usage and manure applications, think about the ability to control the weeds in that manure. It may be time to change application strategies if you are applying manure before soybeans. If you believe you may have Palmer Amaranth, contact your local extension office to help you with a positive identification and management strategy. NOW IS THE TIME TO BE PROACTIVE, BEFORE PALMER AMARANTH IS PRESENT AND NEIGHBOR RELATIONS BECOME TENSE IN THE WAKE OF AN INVASION OF THIS WEED.