Genomic-based epidemiology can provide insight into the origins and spread of herbicide resistance mechanisms in weeds. We used kochia (Bassia scoparia) populations resistant to the herbicide glyphosate from across western North America to test the alternative hypotheses that 1) a single EPSPS gene duplication event occurred initially in the Central Great Plains and then subsequently spread to all other geographical areas now exhibiting glyphosate-resistant kochia populations or that 2) gene duplication occurred multiple times in independent events in a case of parallel evolution. We used qPCR markers previously developed for measuring the structure of the EPSPS tandem duplication to investigate whether all glyphosate-resistant individuals had the same EPSPS repeat structure. We also investigated population structure using simple sequence repeat (SSR) markers to determine the relatedness of kochia populations from across the Central Great Plains, Northern Plains, and the Pacific Northwest. We found that the original EPSPS duplication genotype was predominant in the Central Great Plains where glyphosate resistance was first reported. We identified two additional EPSPS-duplication genotypes, one having geographic associations with the Northern Plains and the other to the Pacific Northwest. The EPSPS duplication genotype from the Pacific Northwest seems likely to represent a second, independent evolutionary origin of a resistance allele. We found evidence of gene flow across populations and a general lack of population structure. The results support at least two independent evolutionary origins of glyphosate resistance in kochia, followed by substantial and mostly geographically localized gene flow to spread the resistance alleles into diverse genetic backgrounds.

ABSTRACT The evolution of glyphosate-resistant and -tolerant weeds has caused changes in weed management around the world. Residual herbicides are crucial tools for weed management, but the rate of adoption by soybean growers remains very low in Brazil. In this research, we used glyphosate tolerant Ipomoea triloba as a model weed species to evaluate the advantages of using residual herbicides on soybeans in multiple years and locations of transition and Cerrado regions of Brazil, rather than relying only on post-emergence control. Most residual herbicides provided enough residual activity to allow a longer application window in post-emergence. Treatments with residual herbicides increased overall weed control, preventing weed interference and increasing soybean yield. When two residual herbicides were used as opposed to only one, a better I. triloba control was achieved, reflecting in higher crop yield, especially in conditions of high weed infestation. The use of pre-emergence herbicides allows growers to have a longer application window for the post-emergence treatment, which is particularly important in Brazilian Cerrado large fields when logistic could be an issue.

ABSTRACT The evolution of glyphosate-resistant and -tolerant weeds has caused changes in weed management around the world. Residual herbicides are crucial tools for weed management, but the rate of adoption by soybean growers remains very low in Brazil. In this research, we used glyphosate tolerant Ipomoea triloba as a model weed species to evaluate the advantages of using residual herbicides on soybeans in multiple years and locations of transition and Cerrado regions of Brazil, rather than relying only on post-emergence control. Most residual herbicides provided enough residual activity to allow a longer application window in post-emergence. Treatments with residual herbicides increased overall weed control, preventing weed interference and increasing soybean yield. When two residual herbicides were used as opposed to only one, a better I. triloba control was achieved, reflecting in higher crop yield, especially in conditions of high weed infestation. The use of pre-emergence herbicides allows growers to have a longer application window for the post-emergence treatment, which is particularly important in Brazilian Cerrado large fields when logistic could be an issue.

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