Herbicide resistance in Bromus spp.: a global review

This review summarizes what is currently known about herbicide resistance in Bromus spp. worldwide. Additional information on the biology and genetics of Bromus spp. is provided to further the understanding of resistance evolution and dispersal of the different species. Cases of herbicide resistance have been confirmed in Bromus catharticus Vahl., Bromus commutatus Schrad.; syn. Bromus racemosus L., Bromus diandrus Roth, Bromus japonicus Thunb.; syn. Bromus arvensis L., Bromus madritensis L., Bromus rigidus Roth; syn. Bromus diandrus Roth ssp. diandrus, Bromus rubens L., Bromus secalinus L., Bromus sterilis L., and Bromus tectorum L. in 11 countries. Bromus spp. populations have evolved cross and multiple-resistance to six herbicide sites of action: acetyl-coenzyme A carboxylase, acetolactate synthase, photosystem II, very long-chain fatty acid, 5-enolpyruvylshikimate-3-phosphate synthase, and 4-hydroxyphenylpyruvate dioxygenase inhibitors. Resistance mechanisms varied from target-site to non-target-site or a combination of both. Bromus spp. are generally highly self-pollinated, but outcrossing can occur at low levels in some species. Bromus spp. have different ploidy levels, ranging from diploid (2n = 2x = 14) to duodecaploid (2n = 12x = 84). Herbicide resistance in Bromus spp. is a global issue, and the spread of herbicide resistance alleles primarily occurs via seed-mediated gene flow. However, the transfer of herbicide resistance alleles via pollen-mediated gene flow is possible.