Site Preparation

glyphosate (numerous product names)

Rate Read label carefully for crops listed and geographic location.

Time Apply lower rates for actively growing annual weeds.

Remarks Consult label for higher rates and application time for perennial weeds. Additional surfactant or ammonium sulfate fertilizer sometimes improves control when weeds are growing slowly. Inhibits production of three amino acids and protein synthesis.

Caution If repeat treatments are necessary, do not exceed 10.6 lb ai/a (10.6 quarts) in 1 year. Grazing is prohibited. Repeated use of glyphosate has caused resistant biotypes to develop in some cropping systems.

Herbicide resistance management Repeated use of glyphosate in at least one orchard in western Oregon has selected for a resistant biotype of annual ryegrass. Overreliance on herbicides with a single site of action for orchard floor maintenance increases the risk of selecting for resistance in other weed species, and threatens the long-term usefulness of glyphosate for weed control in orchards and other crops. Several alternative, nonselective herbicides, listed below, have different sites of action and can be applied in rotation with glyphosate to reduce the risk of selecting for weeds that are resistant to glyphosate. Refer to "Managing Herbicide to resistant Weeds" in "Section C. Agrichemicals and their Properties" in this handbook for more information.

Steps to avoid or manage glyphosate resistance

  1. Use other means to manage weeds such as cultivation and mowing in orchards.
  2. Use preemergence herbicides where possible. Consider use of other nonselective herbicides such as glufosinate or paraquat with PPO inhibitors for burndown control.
  3. To delay development of resistance, use higher glyphosate rates and do not cut the rate.
  4. If continuing to use glyphosate in orchards or vineyard with resistant weeds, then tank mix glyphosate with other herbicides and make the application when the weeds are small.
  5. Do not let weeds go to seed.

Site of action Group 9: inhibits EPSP synthase

Chemical family None generally accepted