Fall scouting for soybean weed management
As soybeans approach maturity now is the time to evaluate this year’s herbicide program. Weeds that escaped 2011’s herbicides are currently setting seed, which will be the source of problems in subsequent years. Most weeds are still green with leaves and seed heads attached so they are very easy to identify. Proper identification of weeds that survived this year’s program will allow selection of herbicides for next year’s crop and for future planning in that particular field. Also be sure to look below the canopy to see what weeds have emerged this fall.
There are several reasons that weeds are not managed by an herbicide program. The first is timing of application as it relates to weed size and weed lifecycle. Herbicide labels list the weeds controlled and the average size of weeds for various herbicide rates. For example the Roundup Powermax label lists the maximum height for control of giant ragweed at 12 inches for the 22 oz rate. Allowing ragweed to exceed this height reduces the efficacy of the herbicide, so escapes are more likely to happen. Increasing the rate of glyphosate can help increase control, but a better option is to apply the herbicide when weeds are smaller.
Application timing can also occur to early and miss certain weeds that are late emergers. Weeds like waterhemp and morningglory can emerge well into July and August, past the time of most POST herbicide applications. There isn’t much to do about late emergers unless a residual herbicide is included with the POST. There are very few herbicides labeled for this purpose and they are very limited in the number of weeds they control. One example would be to include Warrant with a POST application to control pigweeds.
The second reason for weed escapes is an herbicide program that is not comprehensive enough. With the advent of Roundup Ready production systems, farmers shifted to a single application of glyphosate. However, certain weeds like marestail can survive a postemergence application of glyphosate. Marestail management requires a spring herbicide application with a strong burndown combination and a residual herbicide with activity against marestail.
The third reason weeds can escape an herbicide program is resistance. In Ohio many weeds are resistant to both glyphosate and ALS-inhibiting herbicides. This list includes marestail, giant ragweed, and common ragweed. Other weeds like waterhemp and common lambsquarters are showing less susceptibility to glyphosate. A shift from a single application of glyphosate to a PRE + POST program can reduce the impact of herbicide resistant weeds on crop yields. Adding a PRE with several modes of action and another herbicide mode of action to the POST can greatly improve control of resistant weeds. For example adding a PPO inhibitor like Flexstar, Cobra, or Phoenix to a POST program can control glyphosate-resistant giant ragweed and waterhemp. However, utilizing a contact herbicide like a PPO or Ignite often comes with shorter weed height limits and different application methods. Be sure to read the label.