Evaluating potential herbicide injury to rotational crops

By Barry Tickes, UA Area Agriculture Agent

Herbicides with residual soil activity are very useful in the low deserts where weed seeds continue to germinate with year-round irrigation. These herbicides can also be hazardous 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 including soil type, irrigation practices, tillage, environmental conditions, organic matter, and other conditions. Injury can vary from field to field and year to year, and 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, "Recroping Interval", contains a chart that has the labeled and likely rotational crop interval for major crops and herbicides used in the deserts plus 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. The tests are usually done using a High Performance Liquid Chromatograph or a Gas Chromatoaph 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 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 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 to 4 inches from the surface. Soil must also be collected from similar soil types in the same vicinity not treated for comparison.

We are conducting a bioassay program this season to help growers evaluate potential herbicide injury in greenhouses at the Yuma Agricultural Center. Contact me, or Marco Pena at (928) 782-5871, for guidelines on how to collect and drop off soil samples.

Contact Tickes: (928) 580-9902 or btickes@ag.arizona.edu