Biorational management tactics to select against triazine-resistant Amaranthus hybridus: a field trial.
The individual and joint effectiveness of two biorational tactics (crop interference and the exploitation of negative cross-resistance to certain herbicides) in the management of triazine-resistant Amaranthus hybridus (smooth pigweed) were estimated. Seeds of both resistant and susceptible A. hybridus plants were collected at Blacksburg in Virginia, USA, and Beltsville, Maryland, and greenhouse tests and field trials were conducted at Kirksville, Missouri. Biorational tactics exploit biological idiosyncrasies of resistant (R) genotypes to maximize fitness cost(s) of resistance. Selection against triazine resistance was quantified by relative performance comparisons between lines having comparable nuclear genomes but either resistant or susceptible cytoplasm. Increasing soyabean density by reducing row spacing (from 76 cm to 25 cm) did not significantly increase the fitness cost of resistance. Low doses of bentazone (100 and 300 g/ha) strongly increased the cost of resistance. Over 2 years, the mean relative performance of R genotypes in bentazone treatments was 0.40, compared to 0.60 in the absence of bentazone. Therefore, the use of bentazone in soyabean production has the potential to delay evolution of triazine resistance in maize-soyabean rotations using triazines. There was no consistent indication that increased soyabean density and bentazone 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 from which experimental lines were derived. 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.