Biorational management tactics to select against triazine‐resistant Amaranthus hybridus: a field trial

1. The individual and joint effectiveness of two biorational tactics (crop interference and exploitation of negative cross‐resistance to certain herbicides) in the management of triazine‐resistant Amaranthus hybridus L. (smooth pigweed) were estimated. Biorational tactics exploit biological idiosyncracies of resistant (R) genotypes to maximize fitness cost(s) of resistance. We quantified selection against triazine resistance by relative performance comparisons between lines having comparable nuclear genomes but either resistant or susceptible cytoplasm. Increasing soybean density by reducing row spacing (from 76 cm to 25 cm) did not significantly increase the fitness cost of resistance. 2. Low doses of bentazon (100 and 300 g active ingredient ha–1) did strongly increase the cost of resistance. Over 2 years, the mean relative performance of R genotypes in bentazon treatments was 0·40, compared to 0·60 in the absence of bentazon. Therefore, use of bentazon in soybean production has the potential to delay evolution of triazine resistance in maize–soybean rotations using triazines. 3. There was no consistent indication that increased soybean density and bentazon herbicide could act synergistically to increase costs of triazine resistance in Amaranthus hybridus. Nor were differences in response to biorational tactics evident between the two populations of origin (Maryland and Virginia, USA) from which experimental lines were derived. 4. Effects of the biorational tactics differed markedly between years, highlighting that resistance management depending primarily on these tactics would have widely variable results. Use of such tactics is likely to be most effective in the context of diversified weed management.