Abstract
Sugar beet (Beta vulgaris L.) is a major agronomic crop in Idaho and southeastern Oregon, but it is known to be sensitive to herbicides and weed pressure. Glyphosate-resistant weeds present an increasing challenge in glyphosate-resistant sugar beet systems. In addition, herbicide drift from neighboring fields is assumed to injure this crop. The main objectives of this project were to evaluate integrated weed management strategies in sugar beet as well as assess the impacts of herbicide drift on sugar beet yield and quality. For Chapter 1, a 2-year field study was conducted to evaluate weed suppression from a cover crop of volunteer wheat. Six cover crop treatments were combined with three herbicide treatments to determine if volunteer wheat reduces the number of glyphosate applications needed per season to control weeds. By sugar beet planting time, the various treatments with volunteer wheat had significantly more biomass than the fall-planted wheat and 77.7–96% lower weed biomass. However, the treatments with volunteer wheat reduced sugar beet stand density by up to 36.6%. In addition, combining glyphosate with ethofumesate and s-metolachlor resulted in similar weed control as two applications of glyphosate. This shows that integrating volunteer wheat and residual herbicides into a sugar beet rotation system could provide weed suppression and reduce the need for glyphosate, although alternative methods should be tested to preserve sugar beet stand densities and consequent yield. In Chapter 2, the impacts of simulated herbicide drift on sugar beet yield and quality were evaluated. Herbicide drift injury is often observed in sugar beet, but it is not known how final yield will be affected. To investigate this, four herbicides commonly encountered in drift scenarios were applied to sugar beet seedlings at 25%, 12.5%, 6.25%, and 3.125% of the field use rate to mimic the low concentrations that would occur in drift situations. An untreated control was included as well. Each of these herbicides caused visible injury that resulted in yield loss even when the plants recovered. Tissue samples were taken two weeks after the herbicide treatments to identify specific concentrations of herbicides in the leaves to relate with potential yield loss. Furthermore, biomass samples two weeks after the treatments provided a moderate prediction of final yield. After four months of storage, rimsulfuron and fluroxypyr increased the electrical conductivity, while thifensulfuron-methyl reduced sucrose and root biomass. This shows that recovery from herbicide drift does not guarantee normal yield and quality in sugar beet, and early assessment of drift impacts can predict the degree of yield loss and thus inform economic decisions.