Abstract: Background: Weeds pose a significant challenge to Argentine crops, leading to herbicide resistance and environmental concerns. Objective: To identify key weed problems, assess management practices, and herbicide use. Methods: A web-based survey system, with closed questions, was distributed from March to August 2020 among 147 agricultural stakeholders related to soybean, corn, wheat, and sunflower crops in Argentina. Results: Notable troublesome weeds included Conyza bonariensis (L.) Cronq., Digitaria sanguinalis (L.) Scop., Eleusine indica (L.) Gaertn., Amaranthus hybridus L., and Sorghum halepense (L.) Pers. Herbicide usage was as follows: 86% used burndown fallow chemical control, 51% adhered to labeled application rates, 40% applied herbicides at the labeled growth stage, and 37% rotated modes of action. Non-chemical controls were also employed, with 67% favoring crop rotation and 36% focused on preventing weed seed production. Glyphosate was the dominant herbicide, employed by over 90% of respondents. For summer crops, frequently used herbicides included paraquat, 2,4-D (80% of respondents), atrazine (60% of respondents), sulfentrazone, flumioxazin, S-metolachlor (60% of respondents), and clethodim (65% of respondents). In winter cereals, 2,4-D, flurochloridone, flumioxazin, and pyroxasulfone were the top choices. Conclusions: This survey underscores the high reliance on chemical control in Argentina's major crops. The findings provide crucial insights for regional policy planning, emphasizing the importance of integrating diverse weed management tactics. It also highlights the need for proactive integrated management strategies at the field level to mitigate and prevent weed issues in Argentina, offering a valuable approach for analyzing weed problems in extensive crops worldwide. Abstract Background concerns Objective practices use Methods webbased web based system questions 202 14 soybean corn wheat Argentina Results L. L (L. Cronq Cronq. Scop Scop. Gaertn Gaertn. Pers follows 86 51 rates 40 stage 37 action Nonchemical Non 67 36 production 90 respondents paraquat 2,4D 24D D 2,4 2 4 80% 80 (80 respondents, , respondents) 60% 60 (60 sulfentrazone flumioxazin Smetolachlor S metolachlor 65% 65 (65 . cereals 2,4D, D, flurochloridone choices Conclusions Argentinas s planning tactics worldwide 20 1 (L 8 5 3 6 9 4D 24 2, (8 (6 (

Abstract Background Agriculture in South America (SA) had a great expansion in the last decades and weed control changed accordingly with region and crop practices. Objective The objective of this review is to present the history of herbicide use and discuss the main changes in weed management in SA. Methods Herbicide use quantities were obtained from official institutions and commercial organizations in Argentina, Brazil, Chile, Paraguay and Uruguay. Data were summarized per active ingredient, herbicide mode of action or crop. The evolution of the cultivated area of the main crops in each country, and the crop and weed management associated to it were considered to discuss the importance and the consequences of the main herbicides used. Results In 2019 the most used herbicides in Brazil were glyphosate, 2,4-D, atrazine, paraquat and diuron representing 62, 15, 7, 5 and 2% of the total amount used. In Argentina, the increasing selection of herbicide resistant populations (4 cases/year), resulted in utilization of older chemistries. Weed control in Uruguay is traditionally benefited from crop/pasture rotations but recently is also facing problems of continuous cropping systems. Agriculture in Chile is more diverse, but similar patterns and problems of herbicide use are present. Conclusions Intensification of agriculture, no-tillage, glyphosate resistant crops, and herbicide resistant weeds were the most important drivers of herbicide use changes in SA. Integrated weed management is unpostponable to provide sustainable increasing food production in SA.

Abstract Herbicide resistance in weeds is an evolutionary process. Although there is a great global diversity of weeds, independent origins of herbicide resistance evolution have been shown to converge into similar molecular and physiological resistance mechanisms in geographically distant weed populations. Amaranthus species have shown an extraordinary ability to evolve herbicide resistance and invade new environments at a global scale, which represents an opportunity for identifying adaptive evolutionary patterns. The most frequent cases of herbicide-resistant Amaranthus species have been identified in North America, where A. hybridus, A. palmeri, A. tuberculatus and A. retroflexus comprise more than 90% of them. Meanwhile, A. retroflexus, A. hybridus and A. palmeri have been the most reported species in South America. Around 70% of the cases of herbicide-resistant Amaranthus species have been identified in global soybean and corn crops. The higher fecundity and adaptability of plants to a broad range of environments would make populations more likely to persist and be selected for herbicide resistance. Co-evolution of multiple herbicide resistance mechanisms at the plant and/or population level is evident in weed species. For Amaranthus spp., resistance cases highlight evolutionary responses to herbicide use with clear patterns of selection for multiple herbicide resistance in particular regions and spread to new areas within and between global cropping systems. Seed-mediated gene flow is an important component to the spread of herbicide resistant Amaranthus spp. populations. Reduction of the intensity of herbicide selection by combining diverse and integrated weed control practices should be a common goal in weed management programs.