Herbicide Bioassays as Teaching Diagnostic Tools

A herbicide bioassay program was developed to help Ohio farmers and agrichemical dealers make better crop rotation and cultural management decisions following the 1988 drought. Surveys of custom applicators in three counties identified herbicide carryover as the major agronomic concern.

Most herbicides are degraded primarily by microbes such as bacteria, fungi, and algae.1 Soil conditions affecting microbial activity include pH, soil moisture levels, soil fertility, organic matter, texture, and soil temperature.2 Farmers and dealers alike were unclear about the potential of carryover following the drought. The bioassay test is one method to measure this threat.

For an herbicide bioassay, seeds of plants sensitive to certain herbicides are sown directly into the soil from herbicide treated fields. The plants grown in this suspect soil are then compared to plants grown in soil where herbicides haven't been applied. Farm agrichemical dealers collected soil samples and filled out a field data sheet from the suspect fields. A local technical school provided greenhouse space and student support so the bioassays could be conducted during the winter.

The objectives of this herbicide bioassay were to demonstrate:

1. The potential carryover concerns for five major herbicide families: triazine, dinitroanalines, isoxazolidinones, sulfonylureas, and imidazolinones.
2. The bioassay method for evaluating the threat of herbicide carryover.
3. The visible effects of herbicide carryover to rotated crops.

Nineteen agrichemical dealers, representing nearly all the farmers in a four-county area, received the herbicide bioassay demonstration results. The bioassay technique was taught to more than 600 farmers, custom applicators, and pesticide manufacturer representatives. Students at a local vocational school were also taught the bioassay method and participated in the project. Evaluation of the demonstrations by dealers and farmers was positive.

Herbicide carryover field prediction templates were also developed to help farmers manage potential herbicide carryover. These templates were designed to predict carryover risk to the rotational crop (corn or soybeans).

Following the Marion Agri-Day agronomy program, agrichemical dealers were surveyed about the change they'd seen in farmer's awareness, knowledge, and attitudes. Using a five-point Likert scale (0 = no change; 5 = a lot of change), dealers felt very strongly that farmers (4.6) and dealers (4.3) alike had become more aware of the herbicide carryover problem. They used the bioassay study results (4.0) to make recommendations about the following spring's cropping practices. Dealers also reported an increased number of questions from farmers about the effects of pH and organic matter on carryover of herbicides.

1. J. Ladlie, "Managing Herbicide Carryover and Selection," Midwest Growers Newsletter (Omaha, Nebraska: Midwest Ag Publications, 1988).

2. M. Loux, "Avoiding Herbicide Residue Problems," Agronomic Tips (Columbus: The Ohio State University, 1988).