Glufosinate is a key herbicide to manage glyphosate-resistant weeds mainly because it is a broad-spectrum herbicide, and transgenic glufosinate-resistant crops are available. Although glufosinate use has increased exponentially over the past decade, the treated area with this herbicide is far less than that with glyphosate. This is because glufosinate often provides inconsistent performance in the field, which is attributed to several factors including environmental conditions, application technology, and weed species. Glufosinate is also highly hydrophilic and does not translocate well in plants, generally providing poor control of grasses and perennial species. In the soil, glufosinate is rapidly degraded by microorganisms, leaving no residual activity. While there have been concerns regarding glufosinate toxicology, its proper use can be considered safe. Glufosinate is a fast-acting herbicide that was first discovered as a natural product, and is the only herbicide presently targeting glutamine synthetase. The mode of action of glufosinate has been controversial, and the causes for the rapid phytotoxicity have often been attributed to ammonia accumulation. Recent studies indicate that the contact activity of glufosinate results from the accumulation of reactive oxygen species and subsequent lipid peroxidation. Glufosinate disrupts both photorespiration and the light reactions of photosynthesis, leading to photoreduction of molecular oxygen, which generates reactive oxygen species. The new understanding of the mode of action provided new ideas to improve the herbicidal activity of glufosinate. Finally, a very few weed species have evolved glufosinate resistance in the field, and the resistance mechanisms are generally not well understood requiring further investigation.

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The rapid spread of glyphosate-resistant sourgrass populations generates concern in the agricultural production sector in Brazil. Nonetheless, there is not much information related to the frequency and dispersion of sourgrass throughout recent years. We investigated the frequency and dispersion of glyphosate-resistant sourgrass populations in Brazilian agricultural regions as part of a larger-scale weed resistance monitoring study. A discriminatory rate of 960 g ae ha−1of glyphosate was used on plants at the 2- to 3-tiller stage, originating from 2,593 populations of sourgrass sampled in 329 counties in 14 Brazilian states between 2012 and 2015. The dispersion of sourgrass populations originated in western Paraná State, next to the Paraguay border, where the first resistance case was reported. Its dispersion to the central region of Brazil, mainly in soybean-producing areas, is most likely a consequence of agricultural equipment movement and wind-mediated dispersal. Glyphosate-resistant sourgrass populations were found in every geographical region across all Brazilian states tested. These data highlight the importance of an appropriate weed resistance monitoring program to track the evolution and dispersion of resistance to mitigate these issues by focusing efforts regionally and raising awareness among stakeholders in each region.

Herbicides play an important role in preventing crop yield losses due to both their weed interference ability and their capacity for increasing soil conservation in no-till systems. Group A herbicides or acetyl-CoA carboxylase (ACCase) are essential tools the selective management of glyphosate resistance in grass weed species. In this review, we describe important aspects of ACCase biology and herbicides targeting this enzyme, along with a discussion on stewardship programs to delay the evolution of herbicide resistance which can evolve either through target site and/or non-target site mechanisms. Sixteen-point mutations have been reported to confer resistance to ACCase inhibitors. Each mutation confers cross resistance to a different group of herbicides. Metabolic resistance can result in resistance to multiple herbicides with different mechanisms of action (MoA), and herbicide detoxification is often conferred by cytochrome P450 monooxigenases and glutathione-Stransferases. Regardless of whether resistance mechanisms are target or non-target site, using herbicides with the same MoA will result in resistance evolution. Therefore, while field surveys and resistance mechanism studies are crucial for designing reactive management strategies, integrated weed management plays a central role in both reactive and proactive mitigation of herbicide resistance evolution.

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Glufosinate-resistant Lolium perenne L. spp. multiflorum biotypes from Oregon exhibited resistance levels up to 2.8-fold the field rate. One resistant biotype (MG) had an amino acid substitution in glutamine synthetase 2 (GS2), whereas the other (OR) exhibited the wild-type genotype. We hypothesized that the amino acid substitution in GS2 is involved in the resistance mechanism in MG and that non-target site resistance mechanisms are present in OR. OR metabolized glufosinate faster than the other two biotypes, with >75% of the herbicide metabolized in comparison to 50% in MG and the susceptible biotype. A mutation in GS2 co-segregating with resistance in MG did not reduce the enzyme activity, with results further supported by our enzyme homology models. This research supports the conclusion that a metabolism mechanism of glufosinate resistance is present in OR and that glufosinate resistance in MG is not due to an altered target site.

-Several cases of herbicide resistance in goosegrass have been confirmed worldwide. Reports of control failures after glyphosate application have been observed, especially in the Midwest region of Paraná State. The objective of this study was to evaluate the existence of goosegrass populations resistant to glyphosate. For this, 25 populations collected in two consecutive seasons (2013/ 2014 and 2014/2015) were sown and grown in greenhouse. Glyphosate dose-response curve experiments were performed using doses of 0, 60, 120, 240, 480, 960, 1,920, 3,840, 7,680 and 15,360 g a.e. ha -1 . The application stages were from two to three tillers (E1) for the populations of 2013/2014 and E1 and five to six tillers (E2) for the populations of 2014/2015. Furthermore, three of the populations supposedly considered resistant in these experiments (populations 7, 19 and 25) have had their F1 submitted to the herbicide dose-response test in order to verify whether the resistance was inheritable. With the results obtained in this study, it was concluded that the populations 19 and 25, from Campo Mourão and Luziânia (Midwest of Paraná) are the first confirmed cases of goosegrass resistant to glyphosate in Brazil (RF = 3.99 to 6.81), following all the criteria for confirmation of new weed resistance cases. (2013/2014 e 2014/2015) foram semeadas e cultivadas em casa de vegetação. Experimentos de curva de dose-resposta de glyphosate foram realizados utilizando-se as doses de 0, 60, 120, 240, 480, 960, 1.920, 3.840, 7.680 e 15.360 g e.a. ha -1 Keywords: Eleusine indica, dose-response, application timming, resistance factor. RESUMO -Diversos casos de resistência de capim-pé-de-galinha têm

Glufosinate inhibits glutamine synthetase (GS), a key enzyme for amino acid metabolism and photorespiration. Protoporphyrinogen oxidase (PPO) inhibitors block chlorophyll biosynthesis and cause protoporphyrin accumulation, a highly photodynamic intermediate. Both herbicides ultimately lead to plant death by a massive accumulation of reactive oxygen species (ROS) through different mechanisms. We investigated a potential synergistic effect by the mixture of the two herbicide mechanisms of action (MoAs). The tank mix between a low rate of glufosinate (280 g ai ha−1) with an ultra-low dose of saflufenacil (1 g ha−1) provided enhanced herbicidal activity compared with the products applied individually on Palmer amaranth (Amaranthus palmeri S. Watson). The synergism between the two herbicides was also confirmed by isobole analysis and field trials. The herbicide combination provided high levels of efficacy when applied at low temperature and low humidity. Mechanistically, glufosinate caused a transient accumulation of glutamate, the building block for chlorophyll biosynthesis. Consequently, inhibition of both GS and PPO resulted in greater accumulation of protoporphyrin and ROS, forming the physiological basis for the synergism between glufosinate and PPO inhibitors. While the synergy between the two herbicide MoAs provided excellent efficacy on weeds, it caused low injury to PPO-resistant waterhemp [Amaranthus tuberculatus (Moq.) Sauer] and high injury to both glufosinate-resistant and glufosinate-susceptible soybean [Glycine max (L.) Merr.]. Glufosinate enhances the activity of PPO inhibitors through glutamate and protoporphyrin accumulation, leading to increased levels of ROS and lipid peroxidation. The synergism between the two herbicide MoAs can help to overcome environmental effects limiting the efficacy of glufosinate. Future research is needed to optimize the uses for this herbicidal composition across different cropping systems.