Current Status of Herbicide Resistance in the Iberian Peninsula: Future Trends and Challenges

Abstract

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.

Herbicide Resistance Is Increasing in Spain: Concomitant Management and Prevention

“…Glyphosate (Group 9, HRAC legacy group G) is currently the only non-selective post-emergence herbicide registered in Europe [ 3 ]. In perennial woody crops, it is still the main (chemical) weed control tool used to manage weeds both under crop plants and along inter-rows [ 8 ].…”

“…Until the beginning of the 21st century, no cases of glyphosate were reported in this country. However, since then, with the first report for Conyza bonariensis , there has been a fast and steady increase in the number of cases at up to 18, particularly in perennial crops [ 3 ]. Unfortunately, the main response to glyphosate resistance has been to switch to another SoA to control glyphosate-resistant populations.…”

“…Multiple herbicide-resistant populations are common in different cropping systems [ 2 ]. Currently, L. rigidum is the most worrying and important herbicide-resistant weed species in Mediterranean Europe [ 3 ]. Oxyfluorfen, alone or mixed with glufosinate (Group 10, HRAC legacy group H) and glyphosate, were not only applied to control grass and broadleaf weeds in perennial crops but also to prevent and/or control glyphosate-resistant Lolium spp.…”

“…In a European context of herbicide dose reduction, particularly of residual herbicides from the re-registration process, a limited number of available SoAs, and few new ones that are incoming, resistance in winter annual crops is expected to increase in the future [ 1 , 3 ]. The biggest issues faced include simplified agronomic practices such as monoculture and the use of herbicides with the same SoA, as ACCase and ALS inhibitors are still predominant.…”

“…Spain ranks sixth in the world, and second in Europe (after France) in terms of unique herbicide-resistant weed cases [ 2 ]. According to the updated information provided in a recent review, it now ranks first in Europe, and fifth in the world, which is right behind the largest countries in the world in terms of both food production and surface area: Canada, USA, Brazil, and Australia [ 3 ].…”