Herbicides that have residual soil activity are very useful in the low deserts, where weed seeds continue to germinate with each irrigation year-round. These herbicides can also be hazardous, however, when sensitive crops are planted into soil where they are still active. Determining the potential for crop injury from herbicides used on previous crops can be difficult. Injury potential is related to several interrelated factors such as soil type, irrigation practices, tillage, environmental conditions, organic matter and other conditions. Injury can vary from field to field, year to year and even be variable within the same field. Rotational crop restrictions on product labels must often cover many diverse conditions and geographic regions and are frequently much longer than needed. This link will give you a chart that contains the labeled and likely rotational crop interval for the major crops and herbicides used in the deserts as well as the usual soil persistence for each product.

Tests can be conducted prior to planting to evaluate the potential for crop injury. Many times soil samples are taken and sent to a lab for analysis, These tests are usually done using a High Performance Liquid Chromatograph (HPLC) or a Gas Chromatograph (GC) and can be costly, time-consuming and difficult to interpret. A simpler and more direct test can be done by using a bioassay or growing sensitive plants in pots containing soil from the questionable field. This technique is less expensive, requires little equipment and can be done by anyone. Bioassays are often conducted by growing species of plants that are known to be sensitive to a specific herbicide or class of herbicides. It is reasonable, however, to use the crop that is to be planted. If, for instance, lettuce will be grown, the seed used in the bioassay should be from the variety and lot number that will be used. Bioassays can be more accurate than more sophisticated lab tests in predicting potential crop injury. These tests are only good, however, if the soil sample collected is representative of what is in the field. Sample collection is very important and small amounts should be taken from several areas of the field. Herbicide concentration often varies within fields and separate samples should be collected and labeled to indicate in which part of the field they came from. Samples are normally taken from the top 2-4 inches from the surface. Soil must also be collected from similar soil types in the same vicinity that have not been treated for comparison.

We hope to start a pilot bioassay program this season to help growers evaluate potential herbicide injury using the new greenhouses at the Valley Agriculture Center. We would like to determine if such a program is needed and would be effective in identifying potential injury problems. If you have fields that you would like to have bioassayed for potential herbicide injury please contact either Barry Tickes (928-580-9902) or Marco Pena (928-782-5871) for guidelines on how to collect and drop off soil samples.

Bioassays: Yuma Ag Center

To contact Barry Tickes go to: btickes@ag.arizona.edu.

For questions or comments on any of the topics please contact Marco Pena at the Yuma Agricultural Center.
College of Agriculture, The University of Arizona, Tucson, AZ.