Soybean is grown in rotation with corn or winter wheat crops, which is an essential management practice to maintain soil health and maximize crop yields. Though herbicides are critical for weed management, there is potential for damage to soybean from carryover herbicide concentrations in the soil in a rotation system. Soybean is an important agricultural trade commodity and most of the 100 counties in North Carolina are involved in its production. Simultaneously, global climate forecast models predict drier summers and intensified drought conditions, which are factors that favor herbicide persistence and carryover. It is, therefore, critical to recognize problems that soybean production could encounter in the near future and find ways to minimize them.

Varying soil types were collected from the Mountain, Piedmont and Coastal regions of North Carolina and herbicide persistence experiments were conducted. As hypothesized, persistence was influenced by a combination of soil and herbicide physicochemical properties and this resulted in different carryover concentration estimates for each soil type. Environmental conditions such as soil temperature were shown to influence herbicide degradation and, consequently, carryover concentrations.

Differences in herbicide bioavailability among soil types may determine the potential of herbicide residue to damage sensitive crops in a carryover scenario. Therefore, dose-response studies are being conducted where a range of herbicide concentrations will be investigated to determine bioavailability across soils and the threshold for soybean damage in each soil to identify those of concern. It is important to examine the interrelationships that exist between herbicide degradation, soil adsorption and bioavailability since, collectively, these may influence the potential for carryover damage to crops.

North Carolina adapted soybean germplasm will be screened for variation in carryover herbicide sensitivity to provide recommendation to the growers. Growth, development, seed yield, biomass, leaf area and other physiological responses will be measured. It would also be imperative to study the influence of factors like water deficit or elevated temperatures on soybean response to carryover residues. Findings from these studies will be disseminated to NC soybean producers to help them optimize herbicide use programs, save on pesticide input costs and improve soybean yield.