Used combines carry weed seed

Palmer amaranth is on the march. 

This isn’t a conspiracy project to sell new combines. It’s just a reminder—when you buy a used combine, you’re buying weed seed along with it, especially small weed seeds. And those might just be the ones you want to avoid most.

Lodged Palmer amaranth seeds may travel across state lines with the combine, said Kevin Bradley, weed scientist at University of Missouri Extension.

Bradley reminded growers that biosecurity and hygiene as it is associated with livestock farms is a mindset that works for crops, too.

“Know where your feed, seed and equipment come from,” he said. “It is not by accident that you see Palmer amaranth growing around hay bales, feedlots and near places where you have cleaned out your combine.”

Bradley considers Palmer amaranth the No. 1 weed to watch in Missouri, and he recommends exercising caution when bringing in materials from outside Missouri or from the Bootheel—where Palmer amaranth predominates. The weed can unknowingly be brought across state lines via used equipment, hay, straw and feed.

Bradley, speaking at MU Extension’s annual Crop Management Conference in Columbia recently, said Palmer amaranth is “not a weed for the faint of heart.” It is aggressively moving from the southern United States into the Midwest and north-central U.S., including locations in the northern half of Missouri.

The plant became resistant to glyphosate herbicide, the most-used weed control method in the state, about three years ago. Each plant can produce as many as 300,000 seeds per plant and can grow up to 7 feet in height.

It keeps soybean plants from growing by shading them. Its extensive root system robs plants of nutrients and water. The summer annual can reduce soybean yields by as much as 79 percent.

It is hard to identify Palmer amaranth in its early stages of growth. Many species within the pigweed family look similar. Palmer amaranth seedling cotyledons are narrow and green to reddish in color. Mature Palmer amaranth plant petioles are usually longer than the leaves. The leaves have a poinsettia-like arrangement when viewed from above. It is easier to identify by its towering seed head at the end of the growing season, but by then it’s too late.

Corn and soybean producers must control the weed by integrating cultural control practices like narrow row spacings, cover crops and tillage where appropriate in combination with multiple herbicide modes of action, especially in the form of pre-emergence herbicides.

Bradley recommends the MU Weed ID app to identify the weed (see Samples also may be sent to MU’s Plant Diagnostic Clinic (

“This cow’s out of the barn and I don’t know that we can ever close it,” Bradley said. “But we should try.”

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