Distribution and frequency of resistance to four herbicide modes of action in Lolium rigidum Gaud. accessions randomly collected in winter cereal fields in Spain

Lolium rigidum is the most prevalent and damaging grass weed of winter cereals in Spain. L. rigidum infestations are frequently treated with herbicides and, consequently, populations have evolved resistance. The objective of this study was to determine the extent and frequency of herbicide resistance in L. rigidum populations in Spain to the selective herbicides chlortoluron, diclofop-methyl and chlorsulfuron, commonly used for its control in-crop, and to glyphosate. The response to these herbicides was evaluated on 123 accessions surveyed randomly across cereal cropping areas of the regions of Castile and León, Catalonia and Andalusia. The fresh weight and the frequency of undamaged plants were calculated for each accession and herbicide. At the regional level, higher frequencies of accessions displaying resistance occurred in Catalonia, an intensively cropped region with a greater herbicide selection pressure. Of concern is that in this region the 60% of the accessions displayed some level of resistance to the ALS-inhibiting herbicide chlorsulfuron. The 6.9% of the accessions found in Castile and León with some resistance to glyphosate could also indicate an incipient problem of resistance to this herbicide. For the other herbicides and regions the majority of the accessions remained susceptible. The possible mechanisms of herbicide resistance development in L. rigidum accessions (target-site versus non-target-site resistance) and their variation among regions was discussed. This study can be used to generate herbicide resistance-management schemes for farmers, based upon the herbicide the site and the potential for resistance development.

Research highlights
► This is the first random and entirely blind survey carried out in Spain to determine the extent of the spread of herbicide-resistant Lolium rigidum.
►  The survey detected regional differences in the levels of resistance to herbicides. The highest levels of resistance were found in Catalonia, this area could be considered of high risk for Group B (ALS inhibitors) resistance.
► The presence of resistance hotspots is supported by autocorrelation data.
►  In the case of glyphosate, most of the accessions were susceptible but 6.9% of them displayed some resistance (R? and RR), probably reducing herbicide performance at field dose, which could indicate an incipient problem of resistance.
► There are two primary mechanisms of herbicide resistance in weeds: target-site or monogenic resistance, not dose-dependent and putatively represented by undamaged plants in our study; and non-target-site or polygenic resistance, dose-dependent and represented by plants that are damaged but nevertheless alive. Data were discussed according to the multiple inheritance mechanisms of the herbicide resistance character.
► The Spanish scenario in the case of the herbicides studied is not as serious as is in Australia. Nevertheless, Spain must be very cautious: if these extremely useful molecules are inadequately handled, there is a risk of losing them.