Sorghum halepense (L.) Pers is a common and noxious worldwide weed of increasing distribution in many European countries. In the present review, information on the biology, ecology, agricultural, economic and environmental impact of johnsongrass is given, and the current status of this weed in Europe is discussed. Furthermore, special attention is given to the important role of field trials using glyphosate to control weeds in arable and perennial crops in many European countries. Some of the factors which affect control efficacy and should be taken into account are also discussed. Finally, several non-chemical alternative methods (cultural, mechanical, thermal, biological, etc.) for johnsongrass management are also presented. The adoption of integrated weed management (IWM) techniques such as glyphosate use, crop rotation, and deep tillage is strongly recommended to control plant species that originate from both seed and rhizomes.

The evolution of herbicide resistance in weeds has emerged as one of the most serious threats to sustainable food production systems, which necessitates the evaluation of herbicides to determine their efficacy. The first herbicide resistance case in the Iberian Peninsula was reported about 50 years ago, wherein Panicum dichotomiflorum was found to be resistant (R) to atrazine in Spanish maize fields. Since then, herbicide resistance has evolved in 33 weed species, representing a total of 77 single-herbicide-resistance cases in this geographic area: 66 in Spain and 11 in Portugal. Changes in agricultural practices, namely the adoption of non-tillage systems and the increased use of herbicides, led to the selection of weed biotypes resistant to a wide range of herbicides. Nowadays the most important crops in Spain and Portugal (maize, winter cereals, rice, citrus, fruits, and olive orchards) are affected, with biotypes resistant to several mechanisms of action (MoAs), namely: ALS inhibitors (20 species), ACCase inhibitors (8 species), PS II inhibitors (18 species), and synthetic auxin herbicides (3 species). More recently, the fast increase in cases of resistance to the EPSPS-inhibiting herbicide glyphosate has been remarkable, with 11 species already having evolved resistance in the last 10 years in the Iberian Peninsula. The diversity of resistance mechanisms, both target-site and non-target-site, are responsible for the resistance to different MoAs, involving point mutations in the target site and enhanced rates of herbicide detoxification, respectively. More serious are the 13 cases reported with multiple-herbicide resistance, with three cases of resistance to three–four MoAs, and one case of resistance to five MoAs. Future research perspectives should further study the relationship between management strategies and the occurrence of TSR and NTSR resistance, to improve their design, develop monitoring and diagnostic tools for herbicide resistance, and deepen the study of NTSR resistance.

The invasive weed Amaranthus palmeri is spreading throughout Spain, with Catalonia being one of the most affected regions. For this species, acetolactate synthase‐inhibiting herbicide‐resistant populations have been reported, and now glyphosate resistance is also suspected. Glyphosate targets and inhibits the enzyme 5‐enolpyruvylshikimate‐3‐phosphate synthase (EPSPS), but A. palmeri has evolved different resistant mechanisms leading to plant survival. One of the most effective is the EPSPS overexpression due to copy number variation (CNV). Gene copies accumulate within the EPSPS cassette that is an extrachromosomal circular DNA displaying unique structural polymorphisms. This study aims to determine the response to glyphosate of a suspected resistant population collected from a roadside and investigate the resistance mechanism involved. The herbicide bioassay confirmed that 40% of the plants survived glyphosate applied at 540 g a.i. ha⁻¹. No known mutations endowing glyphosate resistance were found at EPSPS amongst confirmed resistant plants, while in most of them (70%) specific molecular markers revealed the presence of the EPSPS cassette. All these results indicate that this population is glyphosate resistant and it is very likely that the EPSPS gene CNV is the main resistance mechanism. This is the first case of glyphosate resistance in A. palmeri in Europe whose introduction is likely due to importation of contaminated seed with glyphosate‐resistant Palmer amaranth from the Americas. This introduction poses a significant danger to summer crops in our continent.

Fall panicum ( Panicum dichotomiflorum Michx.) is an annual allochthonous grass weed currently spreading in maize ( Zea mays L.) fields in Spain. In the summer of 2022, poor control of P. dichotomiflorum with the acetolactate synthase (ALS)-inhibiting herbicide nicosulfuron was reported in two maize fields: Gerb, Catalonia (GB-R), and Sodeto, Aragon (SO-R), with a history of repeated nicosulfuron use. While target-site resistance (TSR) has previously been reported in this species, non–target site resistance (NTSR) to ALS inhibitors has not yet been documented. The objectives of this study were to (1) confirm and quantify nicosulfuron resistance in the putative resistant populations GB-R and SO-R relative to the susceptible population TS-S; (2) characterize the presence of TSR mutations in the ALS gene; and (3) assess the potential involvement of NTSR via enhanced metabolism mediated by cytochrome P450 monooxygenases (P450s) and glutathione S -transferase (GSTs). Dose–response assays confirmed that GB-R and SO-R populations were 29- and 37-fold more resistant than the TS-S population. However, co-application of nicosulfuron with the P450 inhibitor piperonyl butoxide (PBO) and the GST inhibitor 4-chloro-7-nitrobenzofurazan (NBD-Cl) significantly reduced resistance in both populations. In GB-R, GR ₅₀ values declined from 66.3 g ai ha ^–1 (nicosulfuron alone) to 15.8 g ai ha ^–1 with PBO and 6.5 g ai ha ^–1 with NBD-Cl, both well below the recommended field rate (40 g ai ha ^–1 ), corresponding to 76% and 90% resistance reductions, respectively. Similarly, in SO-R, GR ₅₀ values declined from 83.8 g ai ha ^–1 to 16.8 g ai ha ^–1 with PBO and 15.7 g ai ha ^–1 with NBD-Cl, representing 80% and 81% resistance reductions. Sequencing of the ALS gene revealed no known target-site mutations, and ALS enzyme activity did not significantly differ between R and S populations. These results suggest that nicosulfuron resistance in two Spanish P. dichotomiflorum populations is potentially associated with enhanced metabolism mediated by P450 and GST enzymes, providing the first worldwide evidence of NTSR in this species.