It's a No-win Situation: Large Weeds and Hot, Dry Weather

By Bob Hartzler, Department of Agronomy

For a variety of reasons, including unfavorable weather for herbicide activity and the presence of herbicide resistant weeds, many fields have weeds that have survived early-season control tactics. Waterhemp is the number one issue across the state, but lambsquarters, giant ragweed, marestail and others also are present.  Because most agronomists are unwilling to throw in the towel at this time, there is a search for the magic recipe that will provide effective control of these weeds. Unfortunately, in most situations these rescue treatments will be unsuccessful and may cause more harm than good. In addition to failing to control the target weed, they can cost the farmer more than the just the expense of the herbicide application. Reasons to carefully consider the value of late-season treatments include:

• Poor performance — Much of Iowa has been dry, and high temperatures are forecast for the upcoming week. These conditions significantly reduce the consistency of all postemergence herbicides. Where glyphosate resistance is suspected, the PPO inhibitors (Group 14 herbicides) are the primary option for controlling waterhemp in soybean. Maximum size for most of these products is typically 4 inches or smaller, thus the PPO inhibitor herbicides are unlikely to provide control of the waterhemp found in the majority of fields.
• Crop injury — The risk of injury increases with applications made during periods when the crop is stressed and with increasing maturity of the crop. Herbicide combinations that are often used for rescue treatments also are more difficult for crops to tolerate. Finally, corn and soybeans across the state are entering reproductive stages, thus there is greater potential to significantly reduce yield potential than with applications made earlier in the season.
• Herbicide carryover — The PPO inhibitors commonly used for late-season rescue treatments can be persistent compounds. The reduced time period between application and planting next year's crop increases the likelihood of phytotoxic residues being present. Dry conditions the remainder of the season would greatly increase the probability of carryover injury to rotational crops.

The desire to take action when weeds survive the primary control program is understandable. However, many of the rescue treatments that are being considered have little chance of providing effective control. Carefully evaluate the risks associated with applications made at this time, and use the experiences from this year to develop more consistent weed control programs for next year.

Bob Hartzler is a weed science extension specialist and professor in the Department of Agronomy. He can be reached at 515-294-1164 or e-mail hartzler@iastate.edu.