Hunt Is On for Palmer Amaranth

By Pam Smith
DTN Progressive Farmer Crops Technology Editor

DECATUR, Ill. (DTN) -- Aaron Hager is on a mission. The University of Illinois weed scientist is determined to get farmer boots in the field this spring and summer in the search of Palmer amaranth.

"While weed scouting is always important, knowing if this particular weed is present is critical to control. It's hard to stress how aggressive it is. Trust me, you do not want to underestimate it," Hager told DTN.

Palmer amaranth, aka pigweed, was once considered a Southern problem. The weed with desert origins has reached epidemic status in states such as Alabama, Arkansas, Georgia, North Carolina, Mississippi and Tennessee over the past decade. More recently, the weed has migrated north into Missouri, Illinois, Indiana and as far north as Michigan. The original Michigan outbreak is thought to have come from seed that hitchhiked on cottonseed destined as dairy feed.

Hager calls the summer annual broadleaf the "thug of weeds." It has shown an ability to muscle its way into areas and survive where no one thought possible. "Research has demonstrated that Palmer amaranth has a higher growth rate and is more competitive than other pigweed species," Hager said. "Growth rates approaching 3 inches per day and yield losses of 78% (soybean) and 91% (corn) attributed to Palmer amaranth interference have been reported in the scientific literature."

Weed scientists are hustling to get a fix on this spread. Part of the problem, Hager said, is at early growth stages the plant resembles waterhemp and other pigweed species. A cousin to pigweed, waterhemp is also romping across the Midwest landscape and has also shown an ability to adapt to herbicides.

"The need to distinguish between Palmer and the other pigweed species is important because of the aggressiveness of the weed," Hager said. "Proper management of Palmer amaranth populations can help reduce the potential for seed production that will augment the soil seedbank and perpetuate the population in future growing seasons," he said.

Hager described the cotyledon leaves of Palmer amaranth as being relatively long compared with other Amaranthus species. "Like all weedy Amaranthus species in Illinois, the true leaves -- those produced after the cotyledon leaves -- of Palmer amaranth have a small notch in the tip. Occasionally, a single hair can be found in the leaf notch. This hair may not be present in each leaf notch of a Palmer amaranth plant, but tends to be more noticeable on Palmer leaves than on waterhemp leaves. The stems and leaves have no or few hairs and the stems feel smooth to the touch. Leaves are alternate on the stem and are generally lance-shaped or egg-shaped with prominent white veins on the underside," he said. "As plants become older, they often assume a poinsettia-like appearance and sometimes have a white or purple chevron on the leaves. Leaves are attached to the stem by petioles that are usually longer than the leaf blade."

To assist weed management practitioners in accurately identifying Palmer amaranth, Hager said researchers in the University of Illinois crop sciences department have developed molecular biology tools that can differentiate among most of the common pigweed species and identify hybrids. Growers in that state are being asked to submit samples if they think they have discovered Palmer amaranth.

"Yes, these weeds can hybridize," Hager said. "In reality, it doesn't happen with much regularity, and when it does, the offspring are often sterile. Palmer is most likely to hybridize with spiny amaranth, and waterhemp is more likely to hybridize with smooth pigweed."

Early and accurate identification of Palmer amaranth plants, coupled with implementation of an integrated management program, are essential to reduce the potential for crop yield loss due to interference, he added. "The late spring has allowed a lot of weeds to get a foothold and we all need to be monitoring fields."

Pamela Smith can be reached at Pamela.Smith@telventdtn.com

(GH/AG)

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