The overall objective of this study was to identify common patterns in thespatial distribution of the major weed species present in the corn-growingregion of central Spain, exploring the scale dependence of these patternsand the possible associations or dissociations between individual species.Weed density was assessed in 16 commercial fields using digital imagesacquired in a 9-m by 9-m sampling grid. A set of six species was found inall the fields: black nightshade, common cocklebur, fierce thornapple,johnsongrass, purple nutsedge, and velvetleaf. Spatial analysis by distanceindices and inverse distance weighting interpolation methods were performedto create weed distribution maps. The results showed aggregated spatialdistribution patterns for all individual species regardless their lifecycle, annual or perennial. Some associations and dissociations amongspecies were found in the analysis of interactions. Nevertheless, thespatial patterns of co-occurrence of weed species were field-specific andtherefore cannot be considered general patterns of weed co-occurrence. Inorder to explore the scale dependence of these results, an additional studywas conducted in an experimental field located in the same area using a1.0-m by 0.75-m sampling grid. Although this resolution allowed for a betterdefinition of the positions of the weed patches and weed-free gaps, theresults obtained revealed similar patterns to those observed with a coarsersampling resolution.

Clethodim resistance was identified in 12 rigid ryegrass populations fromwinter cropping regions in four different states of Australia. Clethodim hadfailed to provide effective control of these populations in the field andresistance was suspected. Dose–response experiments confirmed resistance toclethodim and butroxydim in all populations. During 2012, the LD50 of resistant populations ranged from 10.2 to 89.3 g ha−1, making them 3 to 34–fold more resistant to clethodimthan the susceptible population. Similarly, GR50 of resistantpopulation varied from 8 to 37.1 g ha−1, which is 3 to 13.9–foldhigher than the susceptible population. In 2013, clethodim-resistantpopulations were 7.8 to 35.3–fold more resistant to clethodim than thesusceptible population. The higher resistance factor in 2013, especially inmoderately resistant populations, could have been associated with lowerambient temperatures during the winter of 2013. These resistant populationshad also evolved cross-resistance to butroxydim. The resistant populationsrequired 1.3 to 6.6–fold higher butroxydim dose to achieve 50% mortality and3 to 27–fold more butroxydim for 50% biomass reduction compared to thestandard susceptible population. Sequencing of the target-site ACCase geneidentified five known ACCase substitutions (isoleucine-1781-leucine,isoleucine-2041-asparagine, aspartate-2078-glycine, andcysteine-2088-arginine, and glycine-2096-alanine) in these populations. Innine populations, multiple ACCase mutations were present in differentindividuals. Furthermore, two alleles with different mutations were presentin a single plant of rigid ryegrass in two populations.

Edamame, a specialty food-grade soybean popular among health-consciousconsumers, is growing in popularity worldwide. Despite a well-developedsoybean industry, most edamame consumed in the United States is importedfrom Asia. Considerable interest exists in growing edamame domestically;however, weed interference is a major problem, and until recently, only asingle herbicide was registered for use on the crop. The objectives of thiswork were (1) to compare effectiveness of weed management treatments thatutilize herbicides currently registered for use on edamame or that may beregistered in the near future, (2) to determine the significance of edamamecultivar on performance of these treatments, and (3) to identify potentialrelationships between the crop and weed. Ten different weed managementtreatments were tested in three edamame cultivars over a 3-yr period. Allweed management treatments increased marketable pod yield relative to thenontreated control, but only treatments with saflufenacil or S-metolachlor combinations were comparable to thehand-weeded weed-free treatment. Of the treatments studied, S-metolachlor followed by imazamox was among thegreatest yielding, had the least weed density and biomass, and did notreduce crop population density. Also, cultivars differed in theirweed-suppressive ability. Path analysis indicated certain relationships wereconsistent across cultivars, such as weed population density having a directnegative association with crop biomass; however, other edamame–weedinteractions were not identical across cultivars. Although more improvementsare needed, the vegetable industry is beginning to have nascent weedmanagement options in edamame, which will likely reduce reliance on handweeding and result in crop-production costs that are more competitive in theglobal market.

Nitrous oxide (N2O) is a potent greenhouse gas with implicationfor climate change. Agriculture accounts for 10% of all greenhouse gasemissions in the United States, but 75% of the country's N2Oemissions. In the absence of PRE herbicides, weeds compete with soybean foravailable soil moisture and inorganic N, and may reduce N2Oemissions relative to a weed-free environment. However, after weeds arekilled with a POST herbicide, the dead weed residues may stimulate N2O emissions by increasing soil moisture and supplying carbonand nitrogen to microbial denitrifiers. Wider soybean rows often have moreweed biomass, and as a result, row width may further impact how weedsinfluence N2O emissions. To determine this relationship, fieldstudies were conducted in 2013 and 2014 in Arlington, WI. A two-by-twofactorial treatment structure of weed management (PRE + POST vs. POST-only)and row width (38 or 76 cm) was arranged in a randomized complete blockdesign with four replications. N2O fluxes were measured fromstatic gas sampling chambers at least weekly starting 2 wk after plantinguntil mid-September, and were compared for the periods before and after weedtermination using a repeated measures analysis. N2O fluxes werenot influenced by the weed by widthinteraction or width before termination, after termination,or for the full duration of the study at P ≤ 0.05. Interestingly, weobserved that POST-only treatments had lower fluxes on the sampling dayimmediately prior to POST application (P = 0.0002), but this was the onlyincidence where weed influenced N2O fluxes, andoverall, average fluxes from PRE + POST and POST-only treatments were notdifferent for any period of the study. Soybean yield was not influenced by width (P = 0.6018) or weed by width (P = 0.5825), but yield was 650 kg ha−1higher in the PRE + POST than POST-only treatments (P = 0.0007). Theseresults indicate that herbicide management strategy does not influence N2O emissions from soybean, and the use of a PRE herbicideprevents soybean yield loss.