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Apply Glyphosate at Full Rates for Common Lambsquarters Control (05/16/19)

Weed seed from weed escapes was collected and evaluated in a greenhouse screening trial in the winter. Glyphosate (PowerMax) at 0, 32, and 64 fl oz/A was applied when seedlings were approximately 2 inches tall.

Weed seed from weed escapes was collected and evaluated in a greenhouse screening trial in the winter. Glyphosate (PowerMax) at 0, 32, and 64 fl oz/A was applied when seedlings were approximately 2 inches tall. Visible growth reduction was observed 7, 14, and 21 days after application.

Fifty-nine percent of the samples collected and evaluated in the greenhouse screening were common lambsquarters. Twenty-nine percent of samples were kochia, and 12% of samples were waterhemp. No common ragweed or redroot pigweed samples were collected in 2018.

Common lambsquarters, kochia and waterhemp control were classified based on their sensitivity to glyphosate. Weeds either resisted glyphosate at 32 and 64 fl oz/A (resistant), were sensitive to glyphosate (susceptible) or were controlled at 64 fl oz but not at 32 fl oz/A (rate dependent). Control was defined as greater than 90% visible growth reduction compared to the untreated check. Screening results are summarized in the following table.

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Kochia samples submitted were either resistant or rate dependent to glyphosate. Waterhemp samples were either glyphosate resistant or susceptible.

Results on lambsquarters were most surprising. First, twice as many lambsquarters samples were submitted compared to kochia and five times as many samples were submitted compared to waterhemp. Next, although lambsquarters was controlled with glyphosate, the time to achieve 90% control was much longer in some samples than others. Surprisingly, there were some samples not controlled with glyphosate at 32 fl oz/A.

The conclusions from the lambsquarters screening are critical for producers that consider lambsquarters their most important or second most important weed. First, use full rates of glyphosate combined with non-ionic surfactant and ammonium sulfate. Second, consider repeat applications, especially under hot and dry conditions when glyphosate absorption through the cuticle is difficult. Finally, use tank-mixtures with glyphosate delivering a second effective herbicide for common lambsquarters control. Discuss your observations with your county agent, ag-retailer, consultant or university specialist if you observe evidence of weed resistance.

Tom Peters

Extension Sugarbeet Agronomist

NDSU & U of MN

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