Here are some reminders about timing and management practices that can help improve control of herbicide resistant weeds this year.
Weeds emerge year-round with different species having different emergence patterns. Knowing the species present in your fields is extremely important for proper management. For instance, marestail emerges primarily in the fall with some seedlings also emerging in early spring. Kochia and giant ragweed ― the first summer annual weeds to emerge in the spring ― have a short emergence window. (Most seedlings emerge within three to four weeks.) Conversely, common waterhemp and Palmer amaranth start emerging at soybean planting time and continue to emerge until August.
The greatest crop rotation benefit comes from using crops with distinct agronomic practices such as planting/harvest dates. Rotating corn and soybeans planted and harvested at similar times and using glyphosate as the primary option for weed management for both crops does little good in terms of herbicide resistance management. Adding a crop with a different phenology (e.g., winter wheat) or one that would “force” you to select herbicides other than glyphosate (e.g., grain sorghum) would be an effective strategy. If you are using a soybean-corn rotation, rotating traits (e.g., Liberty Link) also can be effective.
To reduce the selection of herbicide-resistant weeds, rotate herbicide sites of action and, when possible, use mixtures with multiple sites of action in each application. When rotating from corn to soybean, try to use herbicides with different sites of action.
Scout your fields multiple times throughout the season, particularly fields or areas with a past history of heavy weed pressure. Shortly after an herbicide application, evaluate the quality of the application. By constantly scouting, unpleasant surprises can be avoided in July/August after soybeans close canopy.
Be sure to tackle problems early as you are trying to avoid having fields that are overrun with resistant weeds before you take action. If you have just one or two weeds above the canopy. Stop and hand pull. Treat field borders and edges to keep weeds from spreading over the field. Spot treat trouble areas with appropriate herbicides. You can also use target tillage or cultivation to stay ahead of trouble spots.
When you do spray, be sure to mix herbicides appropriately with multiple sites or modes of action. In the past we have saved money by using a cut rate of herbicide, but that may have helped develop the resistance. Current recommendation is to not cut down on herbicide rates.
Chris Proctor, UNL Weed Science Educator, provided the bulk of the information used in this week’s column.
Announcement: The Platte County Extension Office will be closed on April 28th for the Arbor Day Holiday.