Weed shifts in wheat country

Published on: 13:14PM Jun 26, 2010
Chuck Foresman
Weed scientists have been talking about changing weed patterns for years. A variety of cultural changes are causing weed shifts in all crop systems. In wheat country, for example, prickly lettuce, Persian darnel and cheatgrass are becoming bigger problems. 
There are several potential explanations for these shifts:
  • These weeds have not typically been targeted by herbicide programs. Wild oat, Italian ryegrass and foxtails have gotten the most attention in cereal crop weed control programs, and so these weeds spread beyond pastures and ditches to cropland.
  • Conservation tillage practices have allowed some weeds to compete more. For example, cheatgrass roots aren’t very deep, and are often controlled by tillage. Reducing tillage allows cheatgrass to spread.
  • Over-reliance on one herbicide mode of action supports weed shifts, especially for weeds that adapt easily. Relying on only one mode of action year after year selects for more tolerant or resistant biotypes that spread.
  • Unvaried crop rotations also support weed shifts, especially in wheat country where fallow in included in the rotation. Seeds can remain viable in the soil for two to three years, germinating when weather and competitive conditions are best for them.
Plus, populations of some problem weeds, such as downy brome or cheatgrass have gotten so high that even if a small percentage survive, they can still cause damage and substantially increase the soil weed seed bank.
These patterns have been adopted across agriculture, supporting weed shifts in all crops. What changes in weed pressure are you seeing in your fields?