Phytosociology in agricultural areas submitted to distinct wintercropping management

Studies related to weed dynamics are essential for agricultural sustainability in tropical soils, as their interference can cause significant yield losses, especially in crops with lower competitive ability. This study aimed to assess the composition of weed communities in the third cropping season in areas submitted to distinct wintercroppings for two consecutive years. Evaluations were made in terms of species composition, level of infestation, and severity of occurrence of each weed species in relation to the others within the same area. The wintercropping management systems were: (1) Brachiaria ruziziensis; (2) corn intercropped with B. ruziziensis; (3) corn and (4) cowpea. Plant communities under these wintercrop systems were evaluated in the third year in terms of relative frequency, relative dominance, relative abundance and importance index for each species. Areas were also compared in terms of species composition by the Sorensen's similarity coefficient. Two years of distinct wintercrop managements were not enough to cause a significant shift in terms of weed composition and severity in a given area. There is also evidence that some weed species are more favored by specific crops, although the causes of this selective behavior are not fully explored in this article. The presence of B. ruziziensis, either alone or intercropped with corn, resulted in lower similarity coefficients when compared to areas where this species was not present. It is suggested that several mechanisms briefly discussed may be involved in the impact of the presence of B. ruziziensis on the weed community.

Further molecular characterization of weed-associated begomoviruses in Brazil with an emphasis on Sida spp

Begomoviruses are whitefly-transmitted, single-stranded DNA viruses that are often associated with weed plants. The aim of this study was to further characterize the diversity of begomoviruses infecting weeds (mostly Sida spp.) in Brazil. Total DNA was extracted from weed samples collected in Viçosa (Minas Gerais state) and in some municipalities of Alagoas state in 2009 and 2010. Viral genomes were amplified by RCA, cloned and sequenced. A total of 26 DNA-A clones were obtained. Sequence analysis indicated the presence of 10 begomoviruses. All viral isolates from Blainvillea rhomboidea belonged to the same species, Blainvillea yellow spot virus (BlYSV ), thereby suggesting that BlYSV may be the only begomovirus present in this weed species. Four isolates represent new species, for which the following names are proposed: Sida yellow blotch virus (SiYBV), Sida yellow net virus (SiYNV), Sida mottle Alagoas virus (SiMoAV) and Sida yellow mosaic Alagoas virus (SiYMAV). Recombination events were detected among the SiYBV isolates and in the SiYNV isolate. These results constitute further evidence of the high species diversity of begomoviruses in Sida spp. However, the role of this weed species as a source of begomoviruses infecting crop plants remains to be determined.

Reflectance indices as a diagnostic tool for weed control performed by multipurpose equipment in precision agriculture

Several tools of precision agriculture have been developed for specific uses. However, this specificity may hinder the implementation of precision agriculture due to an increasing in costs and operational complexity. The use of vegetation index sensors which are traditionally developed for crop fertilization, for site-specific weed management can provide multiple utilizations of these sensors and result in the optimization of precision agriculture. The aim of this study was to evaluate the relationship between reflectance indices of weeds obtained by the GreenSeekerTM sensor and conventional parameters used for weed interference quantification. Two experiments were conducted with soybean and corn by establishing a gradient of weed interference through the use of pre- and post-emergence herbicides. The weed quantification was evaluated by the normalized difference vegetation index (NDVI) and the ratio of red to near infrared (Red/NIR) obtained using the GreenSeekerTM sensor, the visual weed control, the weed dry matter, and digital photographs, which supplied information about the leaf area coverage proportions of weed and straw. The weed leaf coverage obtained using digital photography was highly associated with the NDVI (r = 0.78) and the Red/NIR (r = -0.74). The weed dry matter also positively correlated with the NDVI obtained in 1 m linear (r = 0.66). The results indicated that the GreenSeekerTM sensor originally used for crop fertilization could also be used to obtain reflectance indices in the area between rows of crops to support decision-making programs for weed control.

Resistance to ACCase inhibitors in Eleusine indica from Brazil involves a target site mutation

Eleusine indica (goosegrass) is a diploid grass weed which has developed resistance to ACCase inhibitors during the last ten years due to the intensive and frequent use of sethoxydim to control grass weeds in soybean crops in Brazil. Plant dose-response assays confirmed the resistant behaviour of one biotype obtaining high resistance factor values: 143 (fenoxaprop), 126 (haloxyfop), 84 (sethoxydim) to 58 (fluazifop). ACCase in vitro assays indicated a target site resistance as the main cause of reduced susceptibility to ACCase inhibitors. PCR-generated fragments of the ACCase CT domain of the resistant and sensitive reference biotype were sequenced and compared. A point mutation was detected within the triplet of aspartate at the amino acid position 2078 (referred to EMBL accession no. AJ310767) and resulted in the triplet of glycine. These results constitute the first report on a target site mutation for a Brazilian herbicide resistant grass weed.

Efficiency of a reduced herbicide rate for Brachiaria brizantha control in sugarcane

At different growth stages, weeds present different sensitivities to herbicides. Thus, the registered herbicide rate may be reduced under specific conditions, while maintaining satisfactory weed control. This study evaluated the efficiency of reduced rates of the formulated herbicide mixture Velpar K WG® (hexazinone + diuron) + Volcane® (MSMA) for Brachiaria brizantha control at different growth stages. Optimum weed control efficiency was obtained when applying 50% of the recommended rate in younger plants (plants with one to four leaves). In late applications, it is necessary to increase the herbicide rates and, under these conditions, 90% of the recommended rate for (diuron + hexazinone) + MSMA was estimated to be the most economical one.

Influence of seed size and ecological factors on the germination and emergence of field bindweed (Convolvulus arvensis)

An understanding of seed germination ecology of weeds can assist in predicting their potential distribution and developing effective management strategies. Influence of environmental factors and seed size on germination and seedling emergence of Convolvulus arvensis (field bindweed) was studied in laboratory and greenhouse conditions. Germination occurred over a wide range of constant temperatures, between 15 and 40 ºC, with optimum germination between 20 and 25 ºC. Time to start germination, time to 50% germination and mean germination time increased while germination percentage and germination index decreased with an increase in temperature from 20 ºC, salinity and osmotic stress. However, germination was tolerant to low salt (25 mM) or osmotic stress (0.2 MPa), but as salinity and osmotic stress increased, germination percentage and germination index decreased. Seeds of C. arvensis placed at soil surface showed maximum emergence and decreased as seeding depth increased. Seeds of C. arvensis germinated over a wide range of pH (4 to 9) but optimum germination occurred at pH 6 to 8. Under highly alkaline and acidic pH, time to start germination, time to 50% germination and mean germination time increased while germination percentage and germination index decreased. Increase in field capacity caused decreased time to start germination, time to 50% germination and mean germination time but increased germination percentage and germination index. Bigger seeds had low time to start germination, time to 50% germination and mean germination time but high germination percentage and germination index. Smaller seeds were more sensitive to environmental factors as compared to larger or medium seeds. It can be concluded that except for pH, all environmental factors and seed sizes adversely affect C. arvensis as regards seed germination or emergence and germination or emergence traits, and larger seeds result in improved stand establishment and faster germination than small seeds, regardless of moisture stress or deeper seeding depth.

Seed germination and seedling emergence of velvetleaf (Abutilon theophrasti) and Barnyardgrass (Echinochloa crus-galli)

Abutilon theophrasti and Barnyardgrass (Echinochloa crus-galli) are major weeds that affect cropping systems worldwide. Laboratory and greenhouse studies were conducted to determine the effects of temperature, pH, water and salinity stress, and planting depth on seed germination and seedling emergence of Velvetleaf and Barnyardgrass. For Velvetleaf, the base, optimum and ceiling germination temperatures were estimated as 5, 35 and 48 ºC, respectively. Seed germination was sensitive to drought stress and completely inhibited by a potential of -0.6 MPa, but it was tolerant to salinity. Salinity stress up to 45 mM had no effect on the germination of Velvetleaf, but germination decreased with increasing salt concentration. Drought and salinity levels for 50% inhibition of maximum germination were -0.3 MPa and 110 mM, respectively. Seed germination of Velvetleaf was tolerant to a wide range of pH levels. For Barnyardgrass, the base, optimum and ceiling germination temperatures were estimated as 5, 38 and 45 ºC, respectively. Seed germination was tolerant to drought stress and completely inhibited by a potential of -1.0 MPa. Salinity stress up to 250 mM had no effect on seed germination. Drought and salinity levels for 50% inhibition of maximum germination were -0.5 MPa and 307 mM, respectively. A high percentage of seed germination was observed at pH=5 and decreased to 61.5% at acidic medium (pH 4) and to 11% at alkaline medium (pH 9). Maximum seedling emergence of Velvetleaf and Barnyardgrass occurred when the seeds were placed on the surface of the soil or at a depth of 1 cm.

Management of Bidens pilosa and Commelina benghalensis in organic corn cultivation under no-tillage

Mowing is one of the most important methods used to control weeds in organic farming, under the no-tillage system. This study aimed to evaluate the effects of three weed management techniques on weed development, using the weeds Bidens pilosa and Commelina benghalensis, in competition with organic corn {mowing at the three-leaf stage (14 days after corn emergence - DACE), mowing at the three- and six-leaf stage (14 and 25 DACE), and no mowing. Single cultivation with no mowing was also evaluated for these weeds. Mowings performed at 14 and 25 DACE prevented the production of B. pilosa seeds, ensuring efficient control of this species. However, the use of this technique was shown to be inefficient in the control of C. benghalensis.

Use of herbicides on turfgrass

In crop production, weeds must be controlled so as not to adversely affect crop yield and crop quality. Thus, a low level of weeds infesting a field, in most instances, is not a problem. Except in sod or seed production, turfgrass does not have a yield component. The value of turfgrass is its inherent aesthetic quality and usability. Aesthetic quality is the beauty and value that turfgrass adds to a managed landscape. Usability can be the durability of a sport field, trueness of golf putting green roll, or reduction in soil loss from water runoff or wind. Any weed presence in turfgrass can decrease the aesthetic quality and usability of turfgrass. Utilizing herbicides is the only way to completely control weeds in a turfgrass stand. While it is possible to reduce weed populations using cultural or mechanical management practices, it is impossible to completely eliminate weeds as can be accomplished with herbicides. This manuscript will review the major herbicides used in turfgrass in the United States with respect to their modes of action, herbicide family, and primary use in turfgrass.

Effect of weed interference on soybean cultivars at two sowing times

This study aimed to evaluate the effects of weed interference on soybean cultivars at two sowing times in Urutaí, GO. The treatments were arranged in split-plots, and the sowing times (November 16 and December 16, 2009) were allocated in the plots; the soybean cultivars [BRSGO Amaralina, P98C81 (semi-late cycle) BRSGO Raissa, BRSGO Indiara, P98Y11 (median cycle) and BRSGO 7560, BRSGO Caiapônia, Emgopa 302RR (early cycle)] were allocated in the split-plots; and the coexistence or non coexistence of soybean cultivars with weeds, throughout their cycle, was allocated in the split-plots. Non coexistence was established by manual weeding. The experiment was arranged in randomized blocks with four replications. It was verified that the optimal time for sowing soybeans was the month of November, and that under these conditions, the cultivars had higher competitive ability against weeds. Late sowing affected the cycle, development, and yield of the soybean cultivars; this effect was greater under the influence of the weed community.

Weed interference periods in the 'Fécula Branca' cassava

This study aimed to determine the periods of weed interference in the first cycle of 'Fécula Branca' cassava. The experiment was arranged in a randomized block design, with four repetitions. The treatments consisted of eight periods of weed control (25, 50, 75, 100, 125, 150, and 175 days after planting - DAP) and eight periods of coexistence between the weed community and the crop (25, 50, 75, 100, 125, 150, and 175), besides control without weeds and control with weeds until harvest (322 DAP). The predominant weed species with higher relative importance were: Avena sativa, Sorghum halepense, Conyza Canadensis, Euphorbia heterophylla, Raphanus raphanistrum, and Commelina benghalensis. It was concluded that, accepting losses of 1% for root and starch production, the period before interference (PBI) was 66 and 70 DAP; the total period of interference prevention (TPIP) was 88 and 91 DAP and the critical period of interference (CPI) was between 66-88 and 70-91 DAP, respectively. For losses of 5% for root and starch production, the PBI was 87 and 88 DAP, and the TPIP 80 and 81 DAP, respectively; in this case, there was no CPI. Considering the average prices of R$ 218.90 t-1 and R$ 1,191.84 t-1, paid in 2012 for root and starch production, respectively, and control cost of R$ 300.00 ha-1 , the root and starch production for the period prior to economic loss (WEEPPEL) could be estimated to be 20 and 24 DAP, respectively.

Limited occurrence of resistant radish (Raphanus sativus) to AHAS-inhibiting herbicides in Argentina

Radish has developed feral and weedy biotypes, which is a concern for agriculture around the world. In Argentina, it is one of the most widespread and troublesome crop weeds. In Brazil, this species has developed herbicide-resistance to acetohydroxyacid synthase (AHAS) inhibiting herbicides. The objective of this study was to record the presence of herbicide-resistant weedy radish plants in Argentina. In spring 2008, we found a small population of radish at the end of the flowering stage in an imidazolinone-tolerant canola field treated with imazethapyr. Screening and dose-response tests were conducted to two successive generations. They proved the biotype resistant status, and showed extensive survival (between 50 and 80% of control) to the application of a double dose of four AHAS‑inhibiting herbicides from two different chemical families (imidazolinones and sulfonylureas). Dose-response assays exhibited very high resistance for imazethapyr (LD50 = 2452.5 g a.i. ha-1, GR50 = 2926.9 g a.i. ha-1) and intermediate for metsulfuron (LD50 = 3.0 g a.i. ha-1, GR50 = 43.2 g a.i. ha-1). The acquisition of cross-resistance to different herbicide families would confer an adaptive and invasive advantage in agricultural environments to this biotype. Due to the herbicide rotation conducted in the field, the dispersion of this biotype was restricted. This is the first report of resistance in weedy radish in Argentina.

Correlation of soil properties with weed ocurrence in sugarcane areas

Soil properties can influence weed community composition and weed density agricultural area. Knowing this relationship would allow to choose the best strategy for the control of such plants. This study aimed to investigate the correlation between weed density and chemical and physical attributes of soil in three areas (UCO, USC, and UPA) for commercial sugarcane cultivation in Campos dos Goytacazes, RJ. Grids of 40 m x 40 m were established in the areas, and soil samples were collected at the intersection points for physical and chemical analysis and evaluation of the soil seed bank (SSB), followed by a phyto-sociological survey of the weeds present. Samples were collected during two periods: February/March and June/July, 2010. SSB presented the greatest number of species per vegetation evaluated in the two sampling periods. Clay content had a positive effect leading to greater weed density in all areas (UCO, USC and UPA) in at least one of the densities (0-10 and 10-20 cm). On the other hand, sand content, when significant, presented a negative correlation with plant density in all the SSB areas analyzed. The pH negatively influenced the density of the species found through the phyto-sociological survey at USC and UPA. Cyperus rotundus, dominant in all areas, correlated positively with phosphorus, potassium, and clay content and negatively with pH and high sand content.

Initial growth and competitive ability of yellow nutsedge and irrigated rice

Weeds cause significant reduction in the irrigated rice crop yield. Cyperus esculentus (yellow nutsedge) is adapted to irrigate environment. Information on the competitive ability of the weed to the culture, and their environmental adaptation, are scarce. In this study, we sought to determine the initial growth and competitive ability of yellow nutsedge and irrigated rice, as a function of cultivar growth cycle. Initial growth and competition studies were conducted in a randomized complete design in a greenhouse in the agricultural year 2010/11. For the initial growth study, the treatments consisted of a factorial combination of a biotype of yellow nutsedge and two rice cultivars in the function of the vegetative cycle (BRS Querência: early cycle - IRGA 424: intermediate cycle) and six evaluation times (10, 20, 30, 40, 50, and 60 days after emergence). Were evaluated: plant height, leaf area, aboveground dry biomass and root dry biomass. In the competitive ability study in the replacement series, the cultivar BRS Querência (early cycle) and yellow nutsedge were utilized and tested in different proportions of competition (100:0, 75:25, 50:50, 25:75, and 0:100). Were evaluated leaf area and aboveground dry biomass. In general, rice cultivars have an adaptive value equivalent to yellow nutsedge. IRGA 424 cultivar has less height than weed, becoming the weed control more important in this cultivar. For rice crop, intraspecific competition is more important, whereas for the weed, interspecific competition is the most pronounced.

Eucalyptus growth in silvopastoral system under different crown dia meters

The aim of this study was to evaluate the effect of different crown diameters on the early growth of eucalyptus intercropped with Brachiaria decumbens in a silvopastoral system. The experiment was conducted in a B. decumbens established pasture, where hybrid eucalyptus urograndis (clone GG100) was planted, spaced 8 x 3 m. A randomized block design was used, with six replicates. Treatments consisted of five crown diameters (0.0, 1.0, 1.5, 2.0, and 3.0 m) surrounding the eucalyptus plants. Five weeding hoes were performed throughout the experiment, according to the different crown diameters, aiming to maintain the eucalyptus plants free from B. decumbens interference. At 90, 180, 270, and 360 DAP, the height and the diameter of the eucalyptus plants were evaluated, and at 360 DAP, surface biomass and leaf area were evaluated. At 90 DAP, it was verified that the non-weeded plants had lower growth, compared to those submitted to crowns. Crown diameters of 2.51 and 2.64 m allowed greater growth in height and diameter at ground level of eucalyptus plants, respectively, in all periods evaluated. Biomass production and leaf area per plant at 360 DAP were also influenced by the different crown diameters. It was concluded that crown diameter around 2 meters provided favorable conditions for early growth of eucalyptus and less involvement in the area occupied by forage.