Palmer amaranth recommendations for 2014 growing season
Hager said growers should not rely solely on glyphosate to control Palmer amaranth. “Molecular assays have indicated that resistance to glyphosate appears to be relatively common among recently identified Palmer amaranth populations in Illinois,” he explained.
Recommendations based on Palmer amaranth growth rate include:
- 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,” he explained.
- 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 (dicamba, 2,4-D), diphenylethers (acifluorfen, lactofen, fomesafen), glufosinate, glyphosate, and HPPD inhibitors (mesotrione, tembotrione, topramezone).
- 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.
In regard to Palmer amaranth seed production, Hager said research has demonstrated that female Palmer amaranth plants are capable of producing numbers of seed comparable to that of waterhemp (several hundred thousand to over one million). “Physically remove 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 said.
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