Relating soil herbicide residue levels to sugar beet stand and growth reduction

One common recommendation given to farmers who suspect herbicide carryover to sugar beet (Beta vulgaris L.) is to conduct laboratory tests to assess herbicide residue levels in the soil. While this is often a good practice, there are currently no guidelines on how these laboratory test results relate to sugar beet damage or yield loss. Greenhouse studies were conducted in 2022 and 2023 to assess the relationship between soil residue levels of six herbicides (flumioxazin, imazamox, mesotrione, metribuzin, rimsulfuron, and terbacil) and sugar beet stand and growth reduction. At levels below the limit of detection in the laboratory, rimsulfuron reduced sugar beet stand density by more than 50% and biomass by 25%. Sugar beet stand density and biomass were reduced by about 40% and 25%, respectively, at metribuzin residue levels below the limit of detection. Similarly, flumioxazin reduced stand density and biomass by 50% at residue levels below the limit of detection. Stand density and biomass reduction due to mesotrione exceeded 50% even at residue levels below the limit of detection. Imazamox and terbacil caused minimal sugar beet stand and biomass reduction at residue levels below the limit of detection. These results demonstrate that laboratory analysis of herbicide residue levels in the soil must be accompanied by a bioassay to determine the potential crop damage from herbicide carryover.