The current weather conditions have exposed both weeds and crop plants to extremely high temperatures and very little rainfall. In addition, these conditions have been accompanied by low humidity and high winds.
In this scenario, we expect most weed species to slow their growth and enter a kind of dormancy. We also observe that leaf cuticles thicken and density of leaf pubescence increases, resulting in difficulty for herbicides to penetrate.
Due to the weather stress, crop plants also will suffer. Production of mixed function oxidases (MFOs -- the "white blood cells" of the plant) will be substantially reduced.
All plants have their own defense system (including MFOs) to fight off "foreign substances," such as pathogens, viruses, and pesticides. MFOs function somewhat like the white blood cells in our own bodies.
Just like defense systems in our own bodies, MFO production is reduced when the plant is exposed to prolonged stresses. A lack of MFO production results in the plant being more susceptible to crop injury from postemergence herbicides. Due to the prolonged heat and drought stress currently being experienced throughout much of the Midwest, MFO production in the crop plant can be expected to be extremely low. For this reason, crops are highly susceptible to injury from postemergence herbicides.
Additionally, consider the current weather condition. Hot, dry conditions prevail over much of the Midwest. Not only will a crop plant be much more susceptible to postemergence herbicide injury, but most of the weed species will be essentially dormant during this timeframe.
Therefore, the likelihood of crop injury is dramatically increased, while the chances of consistent weed control are greatly decreased.
If herbicide applications must be made during hot, dry weather, we recommend that applications be made in the early morning. Once plants enter heat-induced dormancy, they require a period of cooler temperatures, accompanied with minimal sunlight, for recovery.
Most plants "recover" during the dark hours and cooler temperatures of night. Be sure to contact your local FS crop specialist for more information on herbicide performance in a hot, dry year.
Barry Nash is GROWMARK’s weed science technical manager. His e-mail address is email@example.com