Abstract: Plants of the genus Amaranthus are important agricultural weeds that compromise food production worldwide. Several biological characteristics make these plants thrive in the environment and cause significant yield losses in many crops. Among the seven most important Amaranthus species in the Americas, four have populations with resistance to more than one mode of action (A. hybridus, A. palmeri, A. retroflexus, and A. viridis). While multiple herbicide-resistance in Amaranthus species is widespread, chemical control remains as one of the most important tools against those weeds. In this review, we compiled data from multiple sources on the efficacy of different herbicides across the most common modes of action that are used in Amaranthus management. Both PRE and POST herbicides are discussed, as well as the key factors to be considered when using each one of them. Residual PRE herbicides bring several advantages when managing Amaranthus species. These herbicides can avoid weed interference in the initial stages of crop development and provide a more favorable situation for weed control in POST. In addition, including PRE herbicides allows for the addition of alternative modes of action that are not available as POST treatments. Most POST herbicides have limitations regarding weed size and herbicide resistance status. Applying POST herbicides at the early growth stage of weeds is crucial to obtain efficacy. Finally, weed management sustainability depends on herbicides. Therefore, herbicide use should be combined with other weed control methods to avoid herbicide resistance evolution.

Abstract Mixing herbicides is a common practice in agriculture, to optimize farm management practices, widen the weed control spectrum, enhance application efficiency, and manage herbicide resistance. Interactions between herbicides can occur within the spray tank, on the leaf surface, and/or inside the plant, resulting in physicochemical and physiological interactions. Additive, synergistic, or antagonistic effects can result from these interactions. Given the range in plant response and the potential for negative and positive outcomes for weed management, predicting herbicide interactions before using mixtures would be of great value. Therefore, the physiological responses in the literature can help to ensure efficient weed control and avoid unwanted effects in the field. This review compiles information on physiological and physicochemical interactions between herbicides, addressing the most known cases of synergism, antagonism, and additivity, as well as their physiological bases, and the methods for evaluating herbicide interactions. Reference models for herbicide interactions have been reported and they usually interfere interpretation of the mixture effect. Antagonistic interactions can increase the evolution of weed resistance by favoring the survival of individuals exposed to the herbicide. Physicochemical incompatibility in the spray tank usually causes herbicide antagonism, whereas both synergism and antagonism can result from increased or decreased uptake/translocation and from physiological changes in the plant. agriculture practices spectrum efficiency surface andor Additive synergistic value Therefore field additivity bases effect uptaketranslocation uptake translocation

Abstract Conyza species are important weeds in global agriculture, especially due to their capacity to evolve resistance to multiple herbicide mechanisms of action. We aimed to evaluate the frequency and distribution of resistance to glyphosate and chlorimuron-ethyl in Conyza spp. populations from Brazil. Seed samples were collected from grain production areas across nine Brazilian states over five consecutive years (2014 to 2018). Prior to resistance monitoring trials, dose-response assays were conducted to determine a single dose of glyphosate or chlorimuron-ethyl to discriminate resistant and susceptible populations. Resistance monitoring based on plant responses to the application of discriminatory doses of glyphosate (960 g ha-1) or chlorimuron-ethyl (20 g ha-1). Populations were classified as resistant, moderately resistant, or susceptible to either herbicide. While glyphosate resistance was highly frequent (71.2%) in all the five years, chlorimuron-ethyl resistant populations occurred at 39.8% of the total. The frequency of multiple resistance to both herbicides (35.3%) was proportional to the occurrence of chlorimuron-ethyl resistance (39.6%). Resistance to glyphosate and to chlorimuron-ethyl were found across all states evaluated.