Thoughtful management – not haphazard control efforts – will be the key to keeping Palmer amaranth from taking over fields and ruining yield potential this summer, University of Illinois weed sciences researcher Aaron Hager suggests.
Simply attempting to control Palmer amaranth, he said, can lead to ineffective herbicide applications, substantial crop yield loss, and increased weed infestations. If ignored or otherwise not effectively managed, Palmer amaranth can reduce corn and soybean yield to nearly zero.
Developed in accordance with the unique growth characteristics of Palmer amaranth, the University of Illinois Weed Science Department created a full list of suggestions for weed management. The goals of the recommendations are twofold: to reduce the potential for Palmer amaranth to negatively impact crop yield, and to reduce Palmer amaranth seed production.
Three principles of Palmer amaranth management:
Prevention is preferable to eradication. Palmer amaranth seeds can be transported in several ways, making prevention of seed introduction extremely challenging, Hager advises, but using tactics to prevent Palmer amaranth infestation is the first priority.
Higher herbicide costs. It is not uncommon for annual herbicide costs to at least double once Palmer amaranth becomes established. There are simply no soil- or foliar-applied herbicides that will provide sufficient control of Palmer amaranth throughout the entire growing season.
Control of Palmer amaranth should not be less than 100%. "In other words, the threshold for this invasive and extremely competitive species is zero," Hager said. "Female Palmer amaranth plants produce tremendous amounts of seed, and in less than five years a few surviving plants can produce enough seed to completely shift the weed spectrum in any particular field."
Recommendations based on germination and emergence characteristics:
Control all emerged Palmer amaranth plants before planting corn or soybean. Burn-down herbicides or thorough tillage are effective tactics to control emerged Palmer amaranth plants before planting.
Keep in mind, however, that glyphosate will not control glyphosate-resistant Palmer amaranth and that growth regulator herbicides such as 2,4-D or dicamba are most effective on Palmer amaranth plants fewer than 4 inches tall, Hager says. If pre-plant scouting reveals Palmer amaranth plants taller than 4 inches, consider using tillage instead of herbicides to control the plants.
Apply a full rate of an effective soil residual herbicide no sooner than seven days prior to planting and no more than three days after planting. Many soil residual herbicides that are effective for controlling waterhemp are also effective for controlling Palmer amaranth.
Related: How To Identify Palmer Amaranth
In soybeans, products containing sulfentrazone or flumioxazin have provided effective control of Palmer amaranth. Application rates of products containing these active ingredients should provide a minimum of 0.25 lb ai/acre sulfentrazone or 0.063–0.095 lb ai/acre flumioxazin.
Recommendations based on Palmer amaranth growth rate:
Begin scouting fields within 14 to 21 days after crop emergence. "We recommend this interval even for fields previously treated with a soil residual herbicide applied close to planting," Hager explains.
Apply herbicides before 4-inch height. Foliar-applied herbicides must be applied before Palmer amaranth plants exceed 4 inches in height. The effectiveness of most foliar-applied herbicides dramatically decreases when Palmer amaranth plants are taller than 4 inches. Postemergence herbicides that demonstrate control or suppression of Palmer amaranth include synthetic auxin herbicides, diphenylethers, glufosinate, glyphosate, and HPPD inhibitors.
Consider including a soil residual herbicide during the application of the foliar-applied herbicide. A soil residual herbicide applied with the foliar-applied herbicide can help control additional Palmer amaranth emergence and allow the crop to gain a competitive advantage over later-emerging weeds.
Fields should be scouted 7 to 14 days after application of the foliar-applied herbicide to determine herbicide effectiveness; if the soil residual herbicide included with the post application is providing effective control; and if additional Palmer amaranth plants have emerged.
"If scouting reveals additional Palmer amaranth plants have emerged, make a second application of a foliar-applied herbicide before Palmer amaranth plants are 4 inches tall," Hager said.
Hager also suggests physically removing any remaining Palmer amaranth plants before the plants reach the reproductive growth stage. Plants should be severed at or below the soil surface and carried out of the field.
"Severed plants can root at the stem if left on the soil surface, and plants can regenerate from stems severed above the soil surface," he explains.