Abstract Background: Lettuce-production areas in Brazil are frequently infested by pigweed plants, which can interfere with production by competing for resources or releasing allelopathic compounds in the environment. Objective: The present study evaluated the effect of redroot pigweed (Amaranthus retroflexus) density and distribution on lettuce crop (Lactuca sativa L.). Methods: The cultivars Lídia, Verônica, Lucy Brown and Salad Bowl were used. After transplanting, these plants coexisted until harvest with four pigweed plants (equivalent to 16 plants m-2) located within or outside planting rows, or with one pigweed plant located at the center of the plot (4 plants m-2), as well as a control without weed. The experiment was conducted in an open and semi-controlled area, with treatments arranged in a 4x4 factorial scheme, in a completely randomized experimental design with three replicates. Evaluations included: the number of leaves, fresh matter (lettuce only), height, leaf area and dry matter of lettuce and redroot pigweed plants. Results: The coexistence with redroot pigweed caused yield losses of up to 45% for cultivar Lídia; 41% for Salad Bowl; 33% for Verônica; and 28% for Lucy Brown. Conclusions: The density of 16 plants m-2 of redroot pigweed was the one that most negatively affected the growth of the cultivars, in which the positioning in the planting lines resulted in greater interference to the crop growth. Among the cultivars tested, Lucy Brown showed greater tolerance to coexistence with redroot pigweed plants, while the cultivar Lídia was the most sensitive to the imposed interference.

Abstract Background: Plants synthesize compounds of specialized metabolism to defend themselves against biotic and abiotic stresses. These compounds could be used as models for pesticide development. Among the species that have not yet been studied for the potential to produce active compounds is the kidney leaf mud plantain (Heteranthera reniformis). Objective: The goal was to evaluate the effect of hexanic, ethyl acetate and methanolic extracts of H. reniformis leaves on cress (Lepidium sativum) seed germination to determine which one have the highest biological activity; and to evaluate different concentrations of extract with the highest biological activity on lettuce (Lactuca sativa), barnyard grass (Echinochloasp.) and giant arrowhead (Sagittaria montevidensis) germination. Methods: The seeds were placed on petri dishes containing two sheets of germitest paper, and after the application of the extract, they were sealed and placed in growth chambers with controlled temperature and photoperiod. The variables analyzed were 1st and 2nd germination count, germination speed index and length of the aerial part and roots. Results: The ethyl acetate extract provided greater phytotoxicity on cress than the other extracts. In general, the highest concentration of ethyl acetate extract was the most efficient in reducing variables for all species. Conclusions: The ethyl acetate extract of H. reniformis presents inhibitory activity on the seeds of cress, lettuce, giant arrowhead and barnyard grass, but this activity was dependent on the concentration of the extract and the species studied. H. reniformis synthesizes compounds with phytotoxic activity and purification of extracts is required to isolate, identify and characterize the action mechanism of the compounds with herbicide activity, so that in the future these can be used as models for the development of herbicides.

Abstract Background: The application of Acetyl CoA carboxylase (ACCase) inhibitors prior to sowing of grass crops may result in crop damage due to residual effect of these herbicides in soil. The hypotheses was that different corn hybrids and ACCase herbicides can result in different plant-back intervals after burndown treatments. Objective: This work was planned to determine safe plant-back periods for three corn hybrids where ACCase herbicides clethodim and haloxyfop-methyl are applied. Methods: Herbicides were sprayed at 21, 14, 7, 2 and 0 days before sowing (DBS) at 192 g a.i. ha-1 (clethodim) and 120 g a.i. ha-1 (haloxyfop-methyl). Individual experiments were carried out with three hybrids: KWS9110 and 2B587 with glyphosate and glufosinate tolerance and Enlist with aryloxyphenoxypropionate tolerance. Results: Haloxyfop-methyl resulted in higher damage potential compared to clethodim for the hybrids KWS9110 and 2B587. For these hybrids, significant crop stand and grain yield reductions were observed mainly when haloxyfop was applied closer to sowing date (7, 2 and 0 DBS). Conclusions: Based on the effects on corn grain yield, a safe plant-back period for haloxyfop was estimated to be 8 and 11 days for the hybrids KWS9110 and 2B587, respectively. For the Enlist corn hybrid, there were no yield decreases, even when haloxyfop was applied at 0 DBS. Clethodim did not affect corn yield of any hybrid in any application time.

Abstract Background: Substantial losses to crops can occur due to the presence of sourgrass in agroecosystems, which is promoted by its seed dispersal ability. Environmental factors can affect sourgrass germination and emergence. Objective: The objective of this study was to determine the effects of water restriction, salinity, and depth of sowing on the sourgrass germination and emergence processes. Methods: Experiments using sourgrass seeds were conducted under controlled conditions simulating osmotic and saline stress and seeding depths. The data collected was submitted to analysis of variance and adjusted to models by regression analysis. Results: Smaller osmotic potentials reduced germination, normal seedlings, and increased quiescence, with no germination occurring from -0.80 MPa. Salinity caused a decrease in germination from 20 mM NaCl, influencing the percentage of normal and abnormal seedlings and quiescent seeds. Seed depth influenced the emergence of seedlings, with the maximum percentage of emergence (83%) in the seeds placed on the substrate surface. The maximum depth from which sourgrass seedlings emerged was 7.6 cm. Conclusions: The results lead to the conclusion that the sourgrass germination process tolerates low osmotic potentials and salinity levels. Despite being small sized seeds, they can emerge from high depths. These characteristics justify its occurrence in several regions and agroecosystems, especially in perennial and no-till crops lacking soil cover.

Abstract Background: Competition with weeds and the lack of selective herbicides registered for birdsfoot trefoil crops affect their development and limit their productivity. Objective: This study aimed to evaluate selectivity of post-emergent herbicides for weed control in birdsfoot trefoil crops. Methods: Experiments were carried out in a greenhouse and on a field, in both vegetative and reproductive development stages of birdsfoot trefoil crops. Treatments were composed of post-emergent herbicides recommended for soybean, corn and garlic cultivation. Phytotoxicity (PHYTO), plant height (HEI), shoot dry matter (SDM) and root dry matter (RDM) were evaluated. Data were submitted to the analysis of variance and when means were significant, they were compared by the Duncan’s Test (p≤0.05). In order to determine post-emergent herbicides which were selective to the crop, those that provided PHYTO below or equal to 15% were recommended, since they did not significantly interfere with morphological variables HEI, SDM and RDM, by comparison with the control. Results: Unlike results found in the vegetative stage, birdsfoot trefoil plants were more tolerant to phytotoxic effects of herbicides imazethapyr, chlorimuron-ethyl, cloransulam-methyl and fomesafen in the reproductive stage. The variables were negatively affected by the herbicides bentazon, glyphosate and carfentrazone-ethyl, with PHYTO higher than 40% at 20 days after treatment in the vegetative stage. Conclusions: Herbicides imazethapyr, chlorimuron-ethyl and flumetsulam, when applied as post-emergent ones, regardless of the development stage of the species, are selective to birdsfoot trefoil crops, without affecting crop development, whereas herbicides cloransulam-methyl and fomesafen are not selective to them.

Abstract Background: Talquezal (Paspalum virgatum L.) has become one of the main pasture weeds due to its association with the Urochloa brizantha death syndrome, a disease that causes the progressive death of clumps of susceptible grasses in patches, which leads to weed colonization and pasture degradation. Understanding the weed emergence pattern is essential in decision making for management strategies. Objective: Thus, this study aimed to evaluate the emergence pattern of talquezal seedlings according to its origin and the amount of straw covering the soil. Methods: The experiment was carried out in a greenhouse in a completely randomized design, with six replications and treatments arranged in an 8 × 3 factorial scheme with eight amounts of Urochloa brizantha straw covering the seeds (0, 0.5, 1.0, 2.0, 4.0, 6.0, 8.0, and 10.0 ton ha-1) and three collection sites (Rondonópolis, Redenção, and Cacoal). The effect of the origin and amount of straw was evaluated by the percentage of emergence, relative emergence frequency, mean emergence time, emergence synchrony, and emergence speed index of seedlings obtained over 35 days after sowing. Results: Talquezal seeds showed high seedling emergence vigor regardless of their origin under amounts of straw lower than 4.0 ton ha-1. Conclusions: The result suggests that the amounts of U. brizantha straw higher than 4.0 ton ha-1 on the soil surface, the lower is the percentage of Talquezal seedling emergence.

Abstract Background: Palyno-morphological and foliar epidermal studies plays a very important role in the correct identification and differentiation of two weeds of genus convolvulus. Objective: The aim of the study is to determine the medicinal uses of the two important weeds for correct systematic identification. Methods: Both the qualitative and quantitative features were measured with the help of Light microscopy (LM) and scanning electron microscope (SEM). Results: Variations were observed in both the weeds, i.e Convolvulus arvensis (bindweeds) and Convolvulus prostatus (soft bindweed) in epidermal cells, stomatal size and number, guard cell shape and size, subsidiary cell and diversity of trichomes. Paracytic type of stomata was found in both the weed species. Stomata shape in Convolvulus arvensis is elliptical and oval to oval oblong in Convolvulus prostratus. Weed species have a difference in size, shape, polarity and exine ornamentation. Tricolporate pollen type was observed. The shape of pollen grains is Prolate, Perforate. By using these Palyno-morphological characteristics a taxonomic key is prepared for the identification of these weed plants. Conclusions: High fertility rate i.e Convolvulus arvensis 96% and in Convolvulus prostatus is 90 % shows the weed species are well adapted in the area. Systematics studies of the weed play a very important role not only incorrect identification but also differentiation with other weed plants and subsequently for the conservation purposes.

Abstract Background: Information on the carryover of herbicides applied to the destruction of cotton stalks, as well as the withdrawal period necessary to prevent the development and productivity of the crop in succession from being affected are limited in the literature. Objective: The objective was to identify the carryover effect promoted by herbicides used in the management of the destruction of cotton stalks and to estimate the host free period for sowing soybean in succession. Methods: Two individual experiments were conducted simultaneously, one for single application and the other for sequential application of herbicide treatments. The experiments were installed in a factorial scheme (15x5), in a randomized block design with four replications. The first factor evaluated was herbicide treatments and the second factor was five soybean sowing times after application (0, 30, 60, 90 and 120 days). The herbicides used in the treatments were 2,4-D, glyphosate, saflufenacil, [imazapic + imazapyr], dicamba, fluroxypyr and sulfentrazone. Results: As this work was conducted, the results provide a carryover indicator. It is concluded that the single application and the sequential application of the glyphosate + dicamba + saflufenacil treatment has great carryover potential for soybean crops, with the host free period for sowing the crop exceeding 120 days after application. Conclusions: The treatments 2,4-D, 2,4-D + glyphosate, glyphosate + saflufenacil + fluroxypyr had the lowest host free period intervals, even when in sequential application. The treatment with application of 2,4-D alone showed the lowest carryover potential for soybean.

Abstract Background: Inconsistent control of Barnyardgrass has been reported repeatedly by farmers in major rice growing areas of Iran and Turkey. Objective: Thus, a greenhouse study was conducted to investigate the resistance of Barnyardgrass to acetyl CoA carboxylase (cyhalofop-butyl) and acetolactate synthase (penoxsulam, bispyribac-sodium) inhibiting herbicides. Methods: The seeds were sown in pots in a greenhouse and after screening, were sprayed with various rates of cyhalofop-butyl, penoxsulam and bispyribac-sodium herbicides at 3-4 leaf stage. Four weeks later, the above ground biomass was cut, dried in an oven and weighted. The results were then analyzed in the R software (drc package) using a four or three parameter log-logistic function. All experiments were repeated twice. Results: While 30% of biotypes collected from Turkey were not controlled by ACCase inhibitors at twice the recommended rates, no resistance was observed in Iran’s samples. Only one biotype exhibited cross-resistant to penoxsulam and bispyribac-sodium in Iran, which was due to consecutive application of herbicides with modes of action similar to these herbicides. In contrast, several Turkish biotypes showed cross-resistance as well as multiple resistance to ACCase and ALS inhibitors. Conclusions: The rapid expansion of herbicide resistance in both countries indicate the necessity of adopting integrated weed management practices to hinder the further evolution of resistance in future.

Abstract Background: Megathyrsus maximus (Jacq.) is a perennial weed that affects many crops. In Argentina, sugarcane is the most affected. Objective: Study effective techniques to break dormancy and the effect of environmental factors on the germination of this weed. Methods: Experiments were carried out in cabinet incubators twice, with five replicates per treatment. The experimental unit was made up for 50 seeds. Results: Seed dormancy was strongly associated with the presence of glumes. Manual extractions of glumes and immersion in sulphuric acid were the most effective techniques for breaking dormancy. Megathyrsus maximus did not depend on light to germinate, and it did so both under a 12 h-light-dark photoperiod and in complete darkness, with maximum mean germination percentages of 73 and 76%, respectively. Mean germination percentage (G) and coefficient of germination (CG: number of germinated seeds per day) showed that this weed responded to a wide range of temperatures, the optimal varied between 25 and 35 °C. Both mean germination percentages and CG decreased as osmotic potential became increasingly negative (0 MPa to -0.6 MPa), and as sodium chloride solution concentrations increased (10 to 130 mmol L-1). No germination was observed at -0.8 MPa and with a 150 mmol L-1 sodium chloride solution. Conclusions: The optimal germination conditions for M. maximus can be found in central sugarcane areas in Argentina, since soils are in ideal conditions and are kept under irrigation. In marginal areas, M. maximus germination would depend on rainfall and certain sodium chloride concentrations in the soil.

Abstract Background: Parthenium weed (Parthenium hysterophorus L.) is an invasive weed that has invaded vast regions of Pakistan in a relatively very short period of a decade or two, threatening the crop fields of the agrarian fed country. Parthenium hysterophorus L. is native of central South America and Gulf of Mexico, has now turned out to be a weed of global significance due to its alarming invasions and profuse spread in approximately all parts of the world. Its invasion is probably due to the contamination of its seeds in the imported grains from other countries of the world. Objective: During comprehensive sampling from Pakistan and Australia, it was observed that parthenium weed accessions exhibited several distinct morphological features present at different geographical regions. Therefore this study focuses on the use of plastid DNA barcodes (psbA-trnH) to evaluate the extent of variations in nucleotide sequences between the parthenium weed sampled accessions. Methods: The variability or genetic diversity was evaluated through sequencing of the amplified products and data was subjected to phylogenetic analysis in Molecular Evolutionary Genetic Analysis (MEGA; version 6.06) software. Results: In Maximum Likelihood tree, mainly two clades with three subdivisions are evident which showed increased heterogenity. The results of sequence based markers showed 12 haplotypes of P.hysterophorus populations (having two parsimony informative sites) with 10 indels and a few SNPs (single nucleotide polymorphisms). Conclusions: The results advocate that there have been multiple introductions of parthenium weed into Pakistan.

Abstract Background: Soybean cultivation has been an option used to diversify the production system and perform herbicide rotation in irrigated rice crops in the lowland soils of the Rio Grande do Sul State, southern Brazil. However, elevated salinity levels have been detected in these soils that can inhibit plant growth due to the reduction of the osmotic potential of the soil solution and can causes toxicity. The combination of stress factors in the plantation areas can intensify deleterious effects, such as changes in salinity associated with herbicides that trigger oxidative stress in crops. Objective: This study aimed to evaluate osmotic potential, cell damage, and enzyme activities of the antioxidant metabolism on soybean after treatment with herbicides and salinity stress conditions. Methods: For this purpose, completely randomized design was used in a factorial scheme with three replicates. The A factor included four herbicide treatments, as follows: control (no herbicide), sulfentrazone, S-metolachlor, and sulfentrazone + S-metolachlor. The B factor was represented by the following three salinity levels: 0 (control), 60, and 120 mM NaCl, which were applied 24 hours after soybean sowing. Results: The results showed a significant alteration in the osmotic potential of soybean plants, mainly at higher salt concentrations. Although an increase in the lipid peroxidation has been detected in some treatments, antioxidant enzyme action combined with osmotic adjustment to reduce oxidative damage were mechanisms found to be employed by plants to reduce hydrogen peroxide levels. Conclusions: We concluded that herbicide treatment, in combination with saline stress, can alter physiological and biochemical processes of soybean plants.

Abstract Background: The pre-emergence herbicides diclosulam and flumioxazin are used to weed control in soybean. We hypothesized management of the new emergence of weeds in the early development stages of the soybean could alter the periods of interference. Thus, the period prior to interference allows and increases the canopy closure spaces before the critical period and disfavors the development of weeds inside the vegetative canopy. Objective: This study aimed to evaluate whether the period prior to interference of soybean cultivars was altered due to the application of residual pre-emergence herbicides. Methods: Two experiments were conducted with the soybean cultivars ‘NA 5909 RG’ and ‘P95R51’. One day before sowing in the main plots, the herbicide treatments paraquat (control without residual herbicide), paraquat+diclosulam, and paraquat+flumioxazin were applied. In the sub-plots, the increasing periods of coexistence of soybean with weeds of 0, 7, 14, 21, 28, 35, 42, and 49 days after the emergence of soybean were allocated. Results: The control without residual herbicide had the lowest period prior to interference and the most considerable reductions in grain yield when compared with residual herbicides in both experiments. The presence of residual herbicide increased the period prior to interference because they controlled the initial emergence of the major weeds in the experimental area. Conclusions: The use of the herbicides with residual activity diclosulam and flumioxazin increased the period prior to interference of ‘NA 5909 RG’ and ‘P95R51’ in areas infested mainly with horseweed and radish.

Abstract Background: The study of both intra- and interspecific competitions in Wheat (Triticum aestivum L.) agroecosystem is quite complex and it is essential to understand the influence of nitrogen in these competitions. Objective: The aim of this study was to evaluate the effect of different competitive models on nitrogen balance in wheat ecosystem agriculture (Chamran cultivar). Methods: A field experiment was performed using split-plot based on randomized complete block design (RCBD) with three replications. The main and sub-plots consisted of different nitrogen rates (control, 50, 100 and 150 kg nitrogen ha-1) and different competitive patterns (no, intraspecific, interspecific competition and intra- and interspecific in combination) were applied to the sub-plots, respectively. Weed density in the interspecies competition was applied through the equal planting of two narrow leaves (Avena sativa L.) and broadleaf (Sinapis arvensis L.) weed species. Results: The dry matter (DM) accumulation and nitrogen content (NC) at anthesis was higher than that at maturity stage for all competition treatments as well as nitrogen rates. The trend of nitrogen translocation efficiency (NTE) and nitrogen harvest index (NHI) was different from dry matter translocation efficiency (DMTE), and both were highest in interspecific competition treatments. Moreover, the highest grain yield in no competition treatment resulted in a lower dry matter and NTE. Conclusions: It was concluded that the capacity for nitrogen accumulation in the stem, was associated with a high nitrogen uptake, nitrogen assimilation and high post-anthesis nitrogen remobilization efficiency. In additions, high NHI could be used in the development of cultivars with the desired N balance.

Abstract Background: Fleabane plants (Conyza spp.) resistant to glyphosate herbicide are found infesting areas in the Central-West and Southeast regions of Brazil, but its management is not a concerning problem for summer crops in these regions as in the South region of Brazil. Objective: To evaluate the management of adult fleabane plants at the summer soybean crop and off-season (autumn-winter), monitoring new emergences to assess the dynamics of plants from November 2018 to March 2020, and the residual effect of herbicides applied at the end of the rainy season in the Cerrado biome, in Brasília, DF, Brazil. Methods: Four field experiments were developed from November 06, 2018 to April 02, 2020; all under randomized block design, with four replications, in factorial or split-plot arrangement, with visual weed control evaluations (0% to 100%) and seedling counting. Results: The highest fleabane infestations in soybean crops were found before sowing, and were partially or fully controlled by pre-sowing chemical/management treatments, which did not interfere with the soybean seed yield. The fleabane biological cycle ended in November-December (1st year), and December-January (2nd year). Conclusions: The most significant fleabane emergence in the areas was found in February and March. The treatments with saflufenacil and glufosinate-ammonium, followed by 2,4-D, were the most efficient and fastest control of adult fleabane plants.