The IPM team has received questions about how to evaluate possible crop injury from herbicides that were applied in previous seasons. As we know residual soil activity is very useful when we needed but we must be very careful when the following crop is sensitive. Potential injury is affected by soil type, watering methods, weather, tillage, organic matter, or the combination of these factors.
The following is a chart that “Minimum Recroping Interval” contains the rotational crop interval and soil persistence of herbicides for the major crops and herbicides used in the desert. You can find this chart in the PCA Study Guide Section VI-26.
During 2014-2018 we ran an HPLC (High Pressure Liquid Chromatograph) in our Lab at the Ag. Center to quantify soil and plant herbicide residues free of charge. Currently we don’t have this equipment available so, we helped growers and PCAs evaluate potential herbicide injury using our greenhouses at the Yuma Ag Center (YAC). A basic bioassay can be done just by growing sensitive plants in pots containing soil from the field in question. Even though bioassays are often done with plants that are sensitive to the herbicide used, it is reasonable to use the crop and variety that will be planted.
Also, it is very important to collect small amounts (top 2-4 inches) from several areas of the field. Herbicide concentrations often changes within fields, so separate samples should be collected and labeled to indicate which block they come from.
We are also doing some field evaluations right now with bensulide and metolachlor in spinach at the UA YAC to answer some of your questions and results will be shared soon in this newsletter. If you have a suggestion or herbicide injury test or bioassay you want to run, please contact us at AZVegIPM-Team@email.arizona.edu.