Infestation of weed species in pre-planting of soybean in succession to winter crops

This study aimed to evaluate different crops and plant species planted after soybeans for one year, in terms of their potential to inhibit the occurrence of weed species. The following crops that were planted as second crop after soybeans were evaluated: (1) corn (Zea mays) planted at spacing of 90 cm between rows, intercropped with Brachiaria ruziziensis in the inter-rows; (2) sunflower (Helianthus annuus); (3) crambe (Crambe abyssinica); (4) radish (Raphanus sativus); (5) rapeseed (Brassica napus); and (6) winter fallow - no plantation after soybeans. Phytosociological characterization of weed species was carried out at the pre-planting of soybeans in the following cropping season. Estimations of relative abundance, relative frequence, relative dominance and Importance Value Index were made for each species present. Areas were also intra-characterized by the diversity coefficients of Simpson and modified Shannon-Weiner, and areas were compared using the Jaccard similarity coefficient for presence-only, by multivariate cluster analysis. In the short‑term (a single cropping season), cultivation of winter crops do contribute for lower occurrence of weed species at the pre-planting of soybeans on the subsequent cropping season. The suppressive effects depend both on the species grown in the winter and in the amount of straw left on the soil by these winter crops. Radish was more efficient in inhibiting the occurrence of weed species and rapeseed showed composition of infestation similar to that observed at the area under fallow.

Interactions between soybean and weeds in a replacement series system, considering the effects of water stress

Due to the increase of water deficiency in many farm regions and its meaning on weed interference, competitive interactions between soybean and three weeds were evaluated under water stress (20 to 40 days after transplanting) and no stress conditions. Three independent experiments were carried out in a growth chamber, being each one composed by the weeds Alternanthera tenella, Tridax procumbens or Digitaria ciliaris, along with the crop, in which soil water condition and plant composition effects were evaluated while in competition. A replacement series system was used, including both monoculture of each species and a mixture with a ratio of 50% between weed and soybean. A completely randomized design was used in factorial arrangement, with treatments distributed in three levels for plant composition factor (soybean and weeds monocultures, in addition to the soybean + weed mixture) and two levels for the water factor (with or without stress), amounting six treatments in each experiment. Soybean dry mass was higher than weed dry mass, when growing without water stress. However, under water stress conditions, the dry mass of soy was reduced in all experiments, mainly in the D. ciliaris comparative experiment. Water restriction was also significant in the plants' photosynthesis reduction in most of the experiments, reducing leaf area duration and efficiency of water use. Analysing all variables shows greater weed tolerance than soybean when submitted to water deficit and with distinct changes of their interactions and mechanism of competition, in each experiment.

Factors affecting Cyperus difformis seed germination and seedling emergence

Specific knowledge about the dormancy, germination, and emergence patterns of weed species aids the development of integrated management strategies. Laboratory studies were conducted to determine the effect of several environmental factors on seed germination and seedling emergence of Cyperus difformis. Germination of freshly harvested seeds was inhibited by darkness; however, when seeds were subsequently transferred to complete light they germinated readily. Our results showed that 2 wk of cold stratification overcome the light requirement for germination. Seeds of C. difformis were able to germinate over a broad range of temperatures (25/15, 30/20, 35/25, and 40/30 ºC day/night). The response of germination rate to temperature was described as a non-linear function. Based on model outputs, the base, the optimum and the ceiling temperatures were estimated as 14.81, 37.72 and 45 ºC, respectively. A temperature of 120 ºC for a 5 min was required to inhibit 50% of maximum germination. The osmotic potential and salinity required for 50% inhibition of maximum germination were -0.47 MPa and 135.57 mM, respectively. High percentage of seed germination (89%) was observed at pH=6 and decreased to 12% at alkaline medium (pH 9) pH. Seeds sown on the soil surface gave the greatest percentage of seedling emergence, and no seedlings emerged from seeds buried in soil at depths of 1 cm.

Reduction of seed production of red rice escapes in clearfield rice

Roguing is a practice used to reduce the seed source of red rice escapes to control in Clearfield-rice areas. However, there is great difficulty in performing it in large and heavily infested rice fields. This objective of this work was to evaluate the effects of the use of imazamox herbicide, applied in different rates and times, on plants of Clearfield-rice and red rice. Four experiments were conducted during the 2007/08 and 2008/09 growing seasons, in completely randomized block design and treatments arranged in factorial design, using three replications per treatment. The treatments had increasing rates of imazamox, application times and rice cultivars. The rice cultivars tested were IRGA 417, IRGA 422 CL, Avaxi CL and Puitá INTA CL. The variables evaluated were the number of panicles m-2, number of grains panicle-1, spikelet sterility in rice and red rice; and, rice grain yield and its components. The imazamox reduced the seed production of red rice escapes in a simulated situation of commercial Clearfield-rice area. The greater percentage reductions were obtained when this herbicide was applied at final formation of the panicle or panicle exertion of the red rice plant escapes to control. The Puitá INTA CL cultivar has high level of resistance to imazamox, independent of rate and application times tested, becoming the only alternative to the use of this practice.

Corn and soybean yields as affected by cover crops and herbicide timing under no-tillage system

To achieve better results in the no-tillage system (NTS), it is important to properly manage the cover crop prior to planting by using herbicides, usually glyphosate. The effect of glyphosate on plant coverage is slow, and plants take a few days to die completely. Thus, when applying the herbicide on the same day of planting soybean or corn, cover crops are still alive and standing, causing initial shading on seedlings of the crop and delaying its establishment. Therefore, this study aimed to evaluate the effect of distinct cover crops and their timing of desiccation prior to planting soybean or corn, on crop yield and yield components. Two experiments were installed, one for soybean and another for corn. Each experiment consisted in combining three cover crops (Brachiaria brizantha, common bean or millet) chemically desiccated at two timings before planting the crop (15 or 0 days before planting) under no-tillage system (NTS). Experiments were installed in a completely randomized block design with five replications. Brachiaria brizantha produced the highest amount of biomass; common bean and millet as cover crops allowed higher soybean grain yields; herbicide application under common bean, millet and Brachiaria brizantha 15 days before planting soybean allowed higher crop grain yields; desiccation timing of common bean did not affect corn grain yield; Brachiaria brizantha should be desiccated 15 days before planting corn to allow maximum grain yield; when millet was used as a cover crop, glyphosate application at planting of corn allowed the highest grain yield.

Residual herbicides in weed management for glyphosate-resistant soybean in Brazil

In agricultural production systems where the glyphosate-resistant soybean crop (Glycine max) is grown and the practice of crop rotation with alternative herbicides is not adopted, the exclusive and continuous use of glyphosate has led to the occurrence of resistant weed populations that may limit or compromise the benefits of this technology. Thus, the efficacy of weed management programs, including the use of residual herbicides (sulfentrazone, flumioxazin, imazethapyr, diclosulan, chlorimuron and s-metolachlor) applied in preemergence and followed by in-crop postemergence applications of glyphosate (PRE-POST) were compared to glyphosate postemergence only programs - POST. The study was conducted across nine locations during the 2009/2010 and 2010/2011 growing seasons. PRE-POST programs were efficient in the control of Amaranthus viridis, Brachiaria plantaginea, Bidens pilosa, Commelina benghalensis, Eleusine indica, Euphorbia heterophylla and Raphanus raphanistrum, with the level of control being similar when comparing the program with two applications of glyphosate POST. Some PRE-POST programs were not efficient in controlling Cenchrus echinatus, Ipomoea hederifolia and Ipomoea triloba. Sulfentrazone and diclosulam PRE-POST programs improved the control of Ipomoea triloba compared to sequential applications of glyphosate alone. No significant differences in soybean yield were observed between any of the herbicide treatments or study locations. The use of residual herbicides in preemergence followed by glyphosate in-crop postemergence provides consistent weed control and reducing early season weed competition. Furthermore, these programs utilize at least two herbicide modes of action for herbicide use diversity, which will be needed to stay ahead of resistance build-up, regardless of when weeds may appear.

Interference of Sorghum sudanense and Eleucine indica in the soybean and corn cultivation

The natural infestations are composed of numerous species that compete for environmental resources such as water, light, nutrients and space. The objective of this study was to evaluate the interference of mixed infestations Sorghum sudanense (sudangrass) and Eleusine indica (goosegrass) in the presence of soybean and corn. The experimental design was completely randomized with four replications and the experimental units consisted of plastic pots with a volume capacity of 8 L. The treatments were associations of plants S. sudanense and E. indica in the proportions 8:0, 6:2, 4:4,2:6 and 0:8, respectively, corresponding to 100, 75, 50, 25 and 0% S. sudanense and the reverse for E. indica. In all treatments remained constant four soybean or corn plants per experimental unit. The variables analyzed in the weeds were shoot dry weight, root, total and height of plants. The competitive analysis was accomplished through diagrams applied to replacement series experiment and indexes of competiveness. The results indicated that E. indica was more competitive than S. sudanense in mixed infestations with corn. Rather, S. sudanense was more competitive than E. indica, in mixed infestations with soybean, demonstrating differences in competitiveness among the weeds.

Competition periods of crabgrass with rice and soybean crops

Determining the periods of weed competition with crops helps the producer to choose the most appropriate time to use weed control practices. This strategy allows for the reduction of the number of herbicide applications, reducing costs and the environmental impact of pesticides. The objectives were to determine the period before the interference (PBI) of crabgrass (Digitaria ciliaris) competing with flooded rice, the critical period of interference prevention (CPIP) of crabgrass with soybean and the effects of competition on the grains yield and their components. Experiments were conducted with the coexistence of BRS Querência rice cultivar with crabgrass, for periods of 0, 7, 14, 21, 28, 35, and 110 days after emergency (DAE) and Fundacep 53RR soybean cultivar, whose periods of coexistence and control of crabgrass were 0, 7, 14, 21, 28, 35, and 156 DAE. Rice can grow with crabgrass infestation until 18 DAE, while soybean should remain free from the presence of crabgrass in the period between 23 and 50 DAE. The grain yield and its components, in the crops studied, are affected when grown with crabgrass.

Phytosociological variability of weeds in soybean field

Understanding spatial distribution of weeds in the crop enables to perform localized herbicide applications, increasing the technical and economic efficiency of operations and reducing environmental impacts. This work aimed to characterize the spatial and phytosociological variability of weeds occurring in soybean commercial field. It was conducted in an agricultural area located at the municipality of Boa Vista das Missões - RS, during the 2010/2011 harvest season. The area, that had been managed under no-tillage with soybean monoculture (summer) for five years, was divided in regular squares of 50 x 50 m (0.25 ha), totalizing 356 points. For species identification, 0.5 x 0.5 m sample squares were used. During the survey, 1,739 individuals were identified, distributed in 19 species of 13 families. The weed species Cardiospermum halicacabum, Digitaria horizontalis, Urochloa plantaginea and Raphanus raphanistrum showed the highest population variation in the area; however, only C. halicacabum, U. plantaginea and R. raphanistrum stood out based on the Importance Index Value (IVI). Localized management strategies considering the spatial variability of weed species placed in the Magnoliopsidas and Liliopsidas group show a high potential for use in soybean crop. The results show that the sampling method through regular grid was capable of characterizing the occurrence, population density and spatial variability of weed species in soybean crop.

Effect of plant density and seed position on mother plant on physiological characteristic of cocklebur (Xanthium strumarium) seeds

Experiments were conducted in 2010 to determine the influence of plant density and seed position on the mother plant on seed physiological characteristics of cocklebur (Xanthium strumarium). Cocklebur burs were collected in fall of 2010 from Research Farm of University of Agricultural Sciences and Natural Resources of Gorgan, Iran. The experiment was established as factorial arrangement using a completely randomized design with three replications. The factors included different densities of cocklebur (0, 2, 4, 6 and 8 plant m-2) and the top and bottom parts of the canopy. Non dormant seeds were used for determining cardinal temperatures and tolerance to salinity and drought stresses. Base, optimum and ceiling germination temperatures were estimated between 7.09 to 12.33, 32 to 35 and 44 to 45 respectively in different treatments. Salinity stress up to 300 Mm and osmotic potential 8 bar inhibited the germination completely. Comparison of base temperatures and sigmoid equation coefficients showed that seeds produced in the top had higher germination than those that produced at the bottom of the mother plant. It seems plant densities through seed position on the mother plant affect seed quality. Likewise changes of light quality and quantity in shade environment increased seed dormancy in matured seeds. Shade environment affect seed germination on mother plant that increased dormancy of seeds maturing under shade be an adaptive response that reduces the probability of germination of offspring under unfavorable (shade, competitive) conditions.

Does the resistance to glyphosate herbicide affect the competitive ability of ryegrass with soybean?

The objective this work was to investigate the competitive ability between resistant and susceptible ryegrass biotypes and of these with soybean crop. Four experiments were carried under greenhouse, in a completely randomized design with four replications, in 2011 and 2012. Treatments were arranged in additive series and replacement series assay. In each series, the proportions among ryegrass resistant and susceptible plants related to soybean were: 100:0, 75:25, 50:50, 25:75 e 0:100. Leaf area and shoot dry mass were evaluated. Competitiveness statistical analysis consisted in applying diagrams to the replacement series and alternative interpretations of the competitiveness indexes. The soybean crop had equivalent competitiveness to the susceptible ryegrass biotype and inferior to the resistant biotype, while the biotypes, both susceptible and resistant to glyphosate, present equivalent competitive ability. In general, the intraspecific competition is more harmful to ryegrass when in competition with soybean, while interspecific competition is predominant for culture.

How glyphosate may affect transgenic soybean in different soil and phosphorus levels

The technology that employs genetic modifications brought a significant increase in the utilization of glyphosate. Transgenic soybean has been suffering injury, even though it possesses a resistance mechanism to glyphosate. Currently, there are only a few studies on the dynamics of glyphosate in transgenic soybean planted in soils with different textures interacting with phosphorus concentrations. This study focused on assessing the effects of glyphosate in transgenic soybean plants on different types of soil and at different phosphorus levels. The experimental design was completely randomized, in factorial design: 2 x 6 x 3, that being 2 soil types, 6 doses of glyphosate and 3 levels of phosphorus, and four replications. Plants were cultivated for thirty days in pots with two types of soil, one being clayey (Red-Yellow Latosol) and the other sandy (Quartzarenic Neosol). They received one, two, and three times the maintenance dose of fertilization of phosphorus, corresponding to: 170, 250 and 330 kg of P2O5 ha-1 to QN, and 380, 460 and 540 kg P2O5 ha-1 to RYL, respectively. Glyphosate was applied at six different doses: 0, 1,200, 2,400, 12,000, 60,000 and 120,000 g ha-1 of active ingredient. Plant height, a and b chlorophyll, and shoot were lower for the plants that received lower doses of glyphosate, regardless of the type of soil. Greater availability of phosphorus and lower amount of glyphosate used in Quartzarenic Neosol soil provided for less phytointoxication symptoms in transgenic soybean.

Volunteer RR® corn management in roundup ready® soybean-corn succession system

The present study evaluated the effects of cover crops (Pennisetum glaucum, Crotalaria spectabilis and Urochloa ruziziensis) associated with the application of herbicides {glyphosate; (glyphosate + haloxyfop-R); (glyphosate + fluazifop-p-butyl); (glyphosate + imazethapyr) and (glyphosate + imazaquin)} in soybean desiccation management for volunteer RR® corn control. The experiment was conducted under field conditions at Sinop-MT, during the 2013/2014 crop season, in a randomized complete blocks design with factorial scheme and four replications. The following parameter were evaluated: dry matter of cover crops and ground coverage rate, control of volunteer RR® corn present at the time of desiccation, dry matter, height and intoxication level on soybean plants caused by herbicides at 7, 14 and 28 days after emergence (DAE), control of volunteer RR® corn derived from emergence fluxes subsequent to desiccation management and soybean yield. The joint application of (glyphosate + haloxyfop-R) provided the best level of volunteer RR® corn control present at the time of desiccation. Satisfactory control (80%) of volunteer corn was obtained with the application of (glyphosate + imazethapyr). This treatment displayed an additional residual effect of imazethapyr, which efficiently controled volunteer RR® corn derived from fluxes subsequent to desiccation management, especially in treatments performed under U. ruziziensis straw. None of the herbicides used in desiccation management caused any significant effect on dry matter, height and phytotoxicity of soybean plants at 7, 14 and 28 DAE nor on grain yield.

The Allelopathic Effect of Eucalyptus Leaf Extract on Grass Forage Seed

This study aimed to evaluate the allelopathic effect of various concentrations of an aqueous extract of eucalyptus leaves on Urochloa decumbens and Panicum maximum seeds. The extract was prepared from Eucalyptus urograndis leaves that were milled and mixed with distilled water in a 1:9 milled leaves: water ratio to obtain an extract with a defined concentration of 100%. In addition, dilutions of 50%, 25% and 12.5% were prepared, and a 0% dilution was used as a control. The experiment followed a completely randomized design, with four replicates, each of 50 seeds of U. decumbens and 50 seeds of P. maximum, arranged on filter paper moistened with each concentration of extract in a Gerbox plastic box. The results demonstrated the allelopathic potential of E. urograndis aqueous extracts applied to the seeds of U. decumbens and P. maximum. The 50% and 100% concentrations of leaf extract most strongly inhibited the germination, vigor and seedling growth of U. decumbens and P. maximum. The germination speed index and the root length were the characteristics that were most affected by the potentially allelopathic substances contained in the eucalyptus extracts at all concentrations.

CHEMICAL CONTROL OF WHITE CLOVER IN SOYBEAN CROPS

ABSTRACTWhite clover is tolerant to many herbicides, making difficult a chemical control of this species during soybean crop establishments. The objective of this research was to select herbicides applied postemergence to control white clover in soybean and know the effects of this control on soybean yield. Seven herbicides were assessed, applied postemergence, with or without sequential application of glyphosate, and two control treatments (no control and total control of white clover). Glyphosate (with two sequential applications), fomesafen (with a sequential application of glyphosate), chlorimuron-ethyl and lactofen have shown a satisfactory control of white clover (above 80%). The lower control efficiency has resulted in lower production of soybeans.