In weed science literature, models such as concentration addition, independent action, effect summation, and the parallel line assay technique have been used to predict and analyze whole-plant response to herbicide mixtures. Although a joint action reference model is necessary for determining whether the herbicide mixture provides less than (antagonistic), equal to (zero-interaction or additive), or greater than (synergistic) expected control, model selection often occurs with little regard to the model's underlying biological assumptions. The joint action models of concentration addition (CA) and independent action (IA) are the appropriate models to consider for analysis of herbicide mixtures of two active components. CA assumes additivity of dose, with herbicides differing only in potency, whereas IA assumes multiplicativity of effects, in which herbicides behave independently and sequentially within the plant. CA and IA predicted mixture responses were computed for a sample mixture data set of mesotrione plus atrazine. IA predicted lower mixture responses than CA; for example, mesotrione at 17.5 g ha−1 + atrazine at 140 g ha−1 was predicted to provide 45% (IA) compared with 53% (CA) control of Palmer amaranth. Joint action claims of synergism and antagonism were shown to be dependent on the reference model selected. Although mesotrione plus atrazine combinations were synergistic under IA assumptions, analysis under CA assumptions indicated mesotrione plus atrazine to be synergistic, additive, and antagonistic according to the selected effective concentration (ECx ) level and fixed-ratio mixture. Because it is not possible to determine the appropriate joint action model on the basis of fit of predicted to observed mixture data, the appropriateness of underlying biological assumptions was considered for the sample mixture data set. Additionally, we provide decision criteria to aid researchers in their selection of an appropriate joint action reference model.

The sharp decline in the area of lupin grown in Australia is partly attributed to the failure to control herbicide-resistant weeds in narrow-leaf lupin crops grown with the conventional 25-cm-wide row spacing. Growing lupin with wider row spacing allows for interrow weed control by nonselective herbicides using a sprayshield or physical methods. During 2003 to 2006, two experiments conducted at five sites evaluated the efficacy of interrow weed control techniques in narrow-leaf lupin crops grown in 55- to 65-cm-wide rows within the Western Australia wheatbelt. Interrow herbicides were applied POST using sprayshields, intrarow herbicides were banded on lupin rows at seeding, and interrow weeds were mowed using a garden mower. The main weed species at each site was rigid ryegrass, blue lupin, or wild radish. Paraquat plus diquat applied on the interrow of the lupin crop with sprayshields controlled up to 100% of weeds between rows, leading to increases in lupin grain yield in most of the sites. Glyphosate alone, a mixture of glyphosate plus metribuzin, and glyphosate followed by paraquat plus diquat also controlled interrow weeds, but did not increase lupin grain yield at any site. Thus, paraquat plus diquat is a better choice for interrow weed control in wide row lupin than glyphosate. Mowing did not improve weed control, but mowing followed by paraquat plus diquat increased lupin grain yield at one site. Regression models predicted that there was a strong relationship between weed biomass and lupin grain yield.

Chinese sprangletop, a C4 species, is one of the most important grass weeds of seeded rice in Asia. Chinese sprangletop biology was studied by growing it alone and in competition with 4 and 12 rice plants. Rice competition did not affect the height of Chinese sprangletop, and the weed grew taller than rice, regardless of the competition. Compared with Chinese sprangletop grown alone, competition from rice reduced Chinese sprangletop leaf number, leaf production rate, tiller number, tiller production rate, leaf area, shoot biomass, relative growth rate, and net assimilation rate. Leaf area and shoot biomass of Chinese sprangletop when grown in competition with 12 rice plants was only 16% and 13%, respectively, of the leaf area and biomass of the weed grown alone.

A study was conducted in summer fallow fields near Davenport, WA, and Pendleton, OR, in 2007 and 2008 to evaluate the POST weed control efficacy of herbicide treatments applied with a light-activated, sensor-controlled (LASC) sprayer compared to the broadcast application of glyphosate. The LASC application of glyphosate alone (at all rates) and in mixture with pyrasulfotole plus bromoxynil or 2,4-D had weed control (≥ 88%) and dry weight (≤ 6% of control) similar to the broadcast application of glyphosate across locations and years. Tumble pigweed and prickly lettuce control with bromoxynil, 2,4-D, or carfentrazone plus dicamba, was 12 to 85% less than glyphosate applied alone with LASC or broadcast sprayer. Overall, none of the tested alternate herbicides was promising enough to replace glyphosate under present conditions.

Cryogens are defined as substances that produce low temperatures. In this study, cryogens refer to salts added to snow or ice to cool underlying soil, resulting in reduced weed establishment. In laboratory experiments, bags of ice mixed with cryogens were able to reach temperatures as low as −17 C. In soil-filled pots stored at 4 C, bags of cryogenic salts filled with ice chips reduced the soil temperatures to below 0 C and reduced the establishment of weeds significantly without salinity effects. The cryogen magnesium chloride-6-hydrate (MC) that was effective in pot experiments was tested in an oat field in 2008 and 2009. Plastic bags containing concentrated solutions of MC, perforated at the top, were placed on bare soil just before snowfall. Contact of snow with MC was expected to decrease the surface soil temperature enough to cause freezing injury to seeds in the soil. Although overall effects on weed establishment were small, the cryogenic effect did significantly reduce corn spurry establishment in 2008, and significantly reduced overall weed establishment in both years. These results show that weed management with cryogenic salts is possible in principle, but requires further technical improvements to be practical in the field.

We studied the emergence phenology of large and smooth crabgrass in lawn and bare soil environments and identified ornamental plants as phenological indicators that predict the progress of emergence. From 2002 to 2004, we monitored emergence of large and smooth crabgrass in field plots to estimate the dates of first emergence, and 25, 50 and 80% emergence. Each year, we monitored 74 taxa of ornamental plants to determine dates of first and full bloom. We compiled dates of weed emergence and ornamental blooming to create a biological calendar of phenological events for each year, ordered by average cumulative degree days (DD) (January 1 start date, 10 C base temperature). Ornamental plant flowering events that occurred in a regular sequence before crabgrass emergence events were identified as the phenological indicators. We also evaluated DD and rule-based models for predicting crabgrass emergence and optimum time of PRE herbicide application. In general, smooth crabgrass reached each emergence stage earlier than large crabgrass. Differences in emergence between environments were not consistent over years for the two species. There was no consistent pattern in parameters for DD models predicting emergence events for either crabgrass species or environment. For published DD models, the deviation between observed and predicted emergence events ranged from 0 to > 60 d. Published rule-based predictions, though accurate in some cases, were sometimes difficult to implement. The order of ornamental plant blooming and crabgrass emergence events was generally consistent over years (R2 = 0.977). The biological calendar provided useful crabgrass emergence predictions using real-time field-based indicators of sequential biological events that can help managers plan and optimize management strategies.

Diclofop-resistant Italian ryegrass is widespread in southwestern North Carolina, and growers have resorted to using acetolactate synthase (ALS) inhibitors such as mesosulfuron and pyroxsulam to control this weed in wheat. In the spring of 2007, mesosulfuron failed to control Italian ryegrass in several wheat fields. Seed were collected from six fields in two counties and greenhouse studies were conducted to determine response to mesosulfuron and the acetyl-CoA carboxylase (ACCase) inhibitors diclofop and pinoxaden. All populations were resistant to diclofop and cross-resistant to pinoxaden. Five of the six populations were resistant to diclofop, pinoxaden, and mesosulfuron. An additional study with two biotypes confirmed cross-resistance to the ALS inhibitors imazamox, mesosulfuron, and pyroxsulam. Resistance to mesosulfuron was heritable.

Vinegar can supplement the existing intrarow weed control options of organic farmers. However, there are two primary limitations to its use in vegetable crops. First, it is costly. Second, vinegar applications that contact the crop can cause injury and yield loss. The aim of this research was to use vinegar to control intrarow weeds in bell pepper and broccoli in a way that product costs would be reduced and crop injury would be minimized. Banded applications were shielded and directed below the crop canopy to reduce weed control costs and minimize contact with crop foliage. Organic paints applied to crop stems were evaluated as potential physical barriers to crop stem injury. Four field trials were conducted in 2009, two in transplanted bell pepper and two in transplanted broccoli. A single application of 200-grain vinegar (20% acetic acid) at 700 L ha−1 was applied when weeds were in the cotyledon to six-leaf stage. Applications were made to crops with the lower stems coated in one of two stem protectants, or left uncoated. Hand-weeded and weedy treatments were included for comparison. One day after vinegar application, in-row weed control was 100% in both pepper trials and greater than 96% in the broccoli trials. Two weeks after application, 75% fewer weeds germinated in the vinegar-treated areas compared with the areas that were hand weeded. Neither stem protectant prevented crop injury. Despite pepper foliar injury of less than 5%, stem injury 2 wk after application contributed to a measurable yield reduction. Broccoli injury was limited to instances where overspray contacted the crop canopy. With vinegar, high levels of weed control and the extended duration of that control relative to hand weeding could facilitate improved organic intrarow weed control. However, crop injury must be reliably reduced. Alternative stem protectants may merit evaluation.

Common cocklebur is a new weed in irrigated maize grown for forage in the hot, dry region of northwest Pakistan. We conducted experiments in the Khyber Pakhtunkhwa Province, Peshawar, Pakistan, during 2006 and 2007 to evaluate the interaction of common cocklebur density and maize density on biomass, leaf area index (LAI), and plant height of forage maize. Seven common cocklebur densities (0, 2, 4, 6, 8, 10, and 12 plants m−2) in maize planted at four densities (5, 7.5, 10, and 12.5 plants m−2) were evaluated. An ANOVA for both years revealed significant main effects and interactions for all variables. Regression of measured variables against common cocklebur density showed that maize biomass declined linearly as common cocklebur density increased from 0 to 12 plants m−2, with an increasing rate of decline for high maize densities and low maize densities. Combined data for all maize densities revealed that the relationship between maize biomass and common cocklebur biomass fit a linear function, with 1.28 to 1.35 kg ha−1 loss in maize biomass for each kilogram per hectare increase in common cocklebur biomass from about 1,500 to 3,200 kg ha−1. Above 8 to 10 common cocklebur plants m−2, weed biomass declined, presumably due to intraspecific competition. An increase in common cocklebur density decreased maize LAI about 0.15 to 0.3 units for each additional common cocklebur plant per square meter in 2006, and 0.11 to 0.24 units in 2007. Common cocklebur LAI increased in a linear fashion as density of the weed increased. Results suggest that the effect of common cocklebur interference on maize biomass was associated with a change in allocation of resources, resulting in increased crop height growth at the expense of a reduction in LAI and presumably potential light interception by the crop as common cocklebur density increased.

Texasweed is an annual broadleaf plant belonging to the Euphorbiaceae family and is an emerging problem in southern U.S. rice fields. Field studies were conducted in 2008 and 2009 to study the effect of flood depth on Texasweed survival and growth. The trearments were five flood depths: 0, 10, 15, 20, and 30 cm and two Texasweed growth stages: two- to three-leaf stage and four- to five-leaf stage. The experiment was conducted in a completely randomized split-plot design with three replications. Flooding conditions were created by placing potted plants in 1.3 m by 0.7 m by 0.7 m polyvinyl chloride troughs. The effect of flood depth on Texasweed growth and fruit production was evaluated using ANOVA and regression analysis. Texasweed plants were able to survive in floods up to 30 cm; however, growth and fruit production were reduced. Increasing flood depths resulted in increased plant height and greater biomass allocation to stem. Texasweed plants produced adventitious roots and a thick spongy tissue, secondary aerenchyma, in the submerged roots and stem, which may play a role in its survival under flooded conditions. The recommended flood depth for rice in Louisiana is 5 to 10 cm. A 10-cm flood in the present study caused about 30 and 15% biomass reduction in two- to three-leaf and four- to five-leaf stage Texasweed, respectively. The results, thus, suggest that flooding alone may not be a viable option for Texasweed management in drill-seeded rice. However, appropriate manipulation of flooding could enhance the effectiveness of POST herbicides. This aspect needs further investigation.

In 2007, populations of Italian ryegrass were observed surviving applications of glyphosate under field conditions in southeast Arkansas. At least 10 reports of Italian ryegrass escaping glyphosate applications followed in subsequent years in Arkansas. These were unconfirmed reports of resistance from county agents, consultants, and farmers. The objectives of this research were to confirm resistance to glyphosate in a suspected resistant population collected in 2007 (Desha 2007) and to determine the level of resistance of a putative glyphosate-resistant population collected in 2009, both from Desha County, AR. Other objectives were to determine the resistance frequency in these populations, to determine whether the 2009 population was also acetolactate synthase (ALS) or acetyl-CoA carboxylase (ACCase-resistant), and to determine the effect on plant size as it relates to dose–response to glyphosate. The Desha, AR, 2007 population exhibited a low level of resistance to glyphosate. The estimated glyphosate dose that would control this population 50% was 1,260 g ae ha−1, compared with 190 g ae ha−1 for the susceptible check. In 2009, a population of Italian ryegrass (Des03) was identified that survived a glyphosate application of 1,740 g ae ha−1 made in the field, which is twice the commercial use rate for glyphosate. Dose–response experiments determined that an estimated 3,890 g ae ha−1 glyphosate was required to obtain 50% biomass reduction of Des03; this was 23 times that of the susceptible standard. Neither growth stage nor glyphosate rate evaluated affected the level of resistance observed in the Des03 population. This population was determined to be more than 70% resistant at the levels reported. In addition to glyphosate, Des03 was also resistant to diclofop, a commonly used herbicide in wheat in Arkansas and other areas. As a result, alternative management strategies for Italian ryegrass are currently being explored.

Methyl bromide has been widely used as a broad-spectrum fumigant for weed control in polyethylene-mulched bell pepper. However, because of environmental hazards, the phase-out of methyl bromide requires development of alternative weed management strategies. Brassicaceae plants produce glucosinolates which are hydrolyzed to toxic isothiocyanates following tissue decomposition, and therefore can be used as a cultural strategy. Field experiments were conducted in 2007 and 2009 to study the influence of soil amendment (‘Seventop’ turnip cover crop vs. fallow) and the effect of initially planted yellow nutsedge tuber density (0, 50, and 100 tubers m−2) on the interference of yellow nutsedge in raised-bed polyethylene-mulched bell pepper. Total glucosinolate production by the turnip cover crop was 12,635 and 22,845 µmol m−2 in 2007 and 2009, respectively, and was mainly contributed by shoots. In general, soil amendment with the turnip cover crop was neither effective in reducing yellow nutsedge growth and tuber production nor in improving bell pepper growth and yield compared to fallow plots at any initial tuber density. Averaged over cover crops, increasing initial tuber density from 50 to 100 tubers m−2 increased yellow nutsedge shoot density, shoot dry weight, and tuber production ≥ 1.4 times. However, increased tuber density had minimal impact on yellow nutsedge height and canopy width. Compared to weed-free plots, interference of yellow nutsedge reduced bell pepper dry weight and marketable yield ≥ 42 and ≥ 47%, respectively. However, bell pepper dry weight and yield reduction from 50 and 100 tubers m−2 were not different. Light was the major resource for which yellow nutsedge competed with bell pepper. Yellow nutsedge shoots grown from initially planted 50 and 100 tubers m−2 caused up to 48 and 67% light interception in bell pepper, respectively. It is concluded that yellow nutsedge interference from initial densities of 50 and 100 tubers m−2 are equally effective in reducing bell pepper yield and that soil biofumigation with turnip is not a viable management option for yellow nutsedge at these densities.

With increasing incidence of glyphosate-resistant weeds worldwide, greater farmer awareness of the importance of glyphosate stewardship and proactive glyphosate-resistance management is needed. A Web-based decision-support tool (http://www.weedtool.com) comprising 10 questions has been developed primarily for farmers in western Canada to assess the relative risk of selection for glyphosate-resistant weeds on a field-by-field basis. We describe the rationale for the questions and how a response to a particular question influences the risk rating. Practices with the greatest risk weighting in western Canadian cropping systems are lack of crop-rotation diversity (growing mainly oilseeds) and a high frequency of glyphosate-resistant crops in the rotation. Three case scenarios are outlined—low, moderate, and high risk of glyphosate-resistance evolution. Based on the overall risk rating, three best-management practices are recommended to reduce the risk of glyphosate resistance in weeds.

Agroecosystems are inherently complex, and practices aimed at managing one component of the system can have unintended consequences for other components of the system. Management decisions, therefore, can be improved by assessing and understanding the multivariate nature of agricultural systems and the multifunctional character of particular agricultural management practices. The act of simultaneously assessing and evaluating multiple characteristics or functions in agriculture also can be a valuable education and extension activity, because it draws on active and experiential methods of learning and because the process effectively reveals important functions and tradeoffs associated with agroecosystems and their management. Here we introduce a tool (the spider plot) and present a case-study exercise in which we used this tool to evaluate the multiple characteristics and functions of different cover crops within a field day workshop format. We also provide examples of how this approach could be used to assess other management practices or properties of agroecosystems and communicate multivariate concepts within a weed science classroom or extension environment.

The replication of experiments over multiple environments such as locations and years is a common practice in field research. A major reason for the practice is to estimate the effects of treatments over a variety of environments. Environments are frequently classed as random effects in the model for statistical analysis, while treatments are almost always classed as fixed effects. Where environments are random and treatments are fixed, it is not always necessary to include all possible interactions between treatments and environments as random effects in the model. The rationale for decisions about the inclusion or exclusion of fixed by random effects in a mixed model is presented. Where the effects of treatments over broad populations of environments are to be estimated, it is often most appropriate to include only those fixed by random effects that reference experimental units.

Summer leguminous cover crops can improve soil health and reduce the economic and environmental costs associated with N fertilizers. However, adoption is often constrained by poor weed suppression compared to nonlegume cover crops. In field experiments conducted in organic vegetable cropping systems in north-central New York, two primary hypotheses were tested: (1) mixtures of legume cover crops (cowpea and soybean) with grasses (sorghum–sudangrass and Japanese millet) reduce weed seed production and increase cover crop productivity relative to legume monocultures and (2) higher soil fertility shifts the competitive outcome in favor of weeds and nonlegume cover crops. Cover crops were grown either alone or in grass–legume combinations with or without composted chicken manure. Under hot, dry conditions in 2005, cowpea and soybean cover crops were severely suppressed by weeds in monoculture and by sorghum–sudangrass in mixtures, resulting in low legume biomass, poor nodulation, and high levels of Powell amaranth seed production (> 25,000 seeds m−2). Under more typical temperature and rainfall conditions in 2006, cowpea mixtures with Japanese millet stimulated cowpea biomass production and nodulation compared to monoculture, but soybeans were suppressed in mixtures with both grasses. Composted chicken manure shifted competition in favor of weeds at the expense of cowpea (2005), stimulated weed and grass biomass production (2006), and suppressed nodulation of soybean (2006). In a complementary on-farm trial, cowpea mixtures with sorghum–sudangrass suppressed weed biomass by 99%; however, both common purslane and hairy galinsoga produced sufficient seeds (600 seeds m−2) to replenish the existing weed seedbank. Results suggest that (1) mixtures of cowpeas with grasses can improve nodulation, lower seed costs, and reduce the risk of weed seed production; (2) soybean is not compatible with grasses in mixture; and (3) future costs of weed seed production must be considered when determining optimal cover crop choices.

A survey of 109 fields was conducted across western Canada in spring 2007 to determine the extent of ALS-inhibitor and dicamba (synthetic auxin) resistance in kochia. Weed seedlings were collected from fields in three provinces of western Canada and transplanted into the greenhouse. Seeds were harvested from selfed plants, and the F1 progeny were screened for resistance to the ALS-inhibitor mixture thifensulfuron–tribenuron or dicamba. All kochia populations were susceptible to dicamba. ALS inhibitor–resistant kochia was found in 85% of the fields surveyed in western Canada: 80 of 95 fields in Alberta, six of seven fields in Saskatchewan, and all seven fields in Manitoba. For the 93 ALS inhibitor–resistant populations, the mean frequency (±SE) of parental plants classified as resistant was 61 ± 3%. Most of the resistant populations (87%) were heterogeneous and contained both resistant and susceptible individuals. ALS sequence data (Pro197 and Asp376 mutations) and genotyping data (Trp574 mutation) obtained for 87 kochia parental (i.e., field-collected) plants confirmed the presence of all three target-site mutations as well as two mutational combinations (Pro197 + Trp574, Asp376 + Trp574) in resistant individuals.

Glyphosate-resistant Amaranthus species are a recognized risk to U.S. agriculture. With affected cropland exceeding 1.2 million ha, this epidemic is particularly pertinent to agricultural regions that utilize an intensive glyphosate-based management program to control weedy pests. Before 2006, Texas had no identified glyphosate-resistant populations. Two independent common waterhemp populations exhibiting poor control by glyphosate were identified in Wharton County and Fort Bend County, TX in 2006 and 2008, respectively. The objective of the present research was to characterize the level of glyphosate resistance (50% lethal dose [LD50] and 50% reduction in growth rate [GR50]) in each population. Resistance levels in four putatively glyphosate-resistant common waterhemp biotypes selected from these two populations were compared with confirmed glyphosate-resistant and -susceptible common waterhemp populations under greenhouse conditions. The LD50 value for the susceptible population (736 g ae ha−1) was equivalent to the 0.9× labeled rate of glyphosate, whereas the putatively resistant lines exhibited a broad range of resistance with LD50 values ranging from 3.5 to 59.7× the labeled rate of glyphosate. The GR50 value for the most resistant line was 2.5-fold greater than the susceptible biotype (317 g ae ha−1 of glyphosate). These results confirm the first documented case of a glyphosate-resistant weed species in Texas.

Seashore paspalum has high salinity tolerance, suggesting sodium chloride might have potential as a selective grassy weed herbicide. The objective of this research was to investigate sodium chloride rate and application timing for smooth crabgrass control and seashore paspalum and common bermudagrass injury. Five rates of sodium chloride (244, 488, 976, 1,952, or 3,904 kg ha−1) were compared with quinclorac at 0.84 kg ai ha−1 for controlling multileaf or multitiller smooth crabgrass. Sodium chloride at ≥ 976 kg ha−1 provided excellent control (90 to 100%) of multitiller smooth crabgrass from 7 to 28 d after treatment, but ≥ 1,952 kg ha−1 was required to achieve excellent control of multileaf populations. Furthermore, 976 kg ha−1 of sodium chloride applied to multitiller smooth crabgrass caused minimal seashore paspalum injury (0 to 6%), comparable to quinclorac, but was more injurious when applied earlier in the spring for multileaf smooth crabgrass control. Common bermudagrass injury increased with sodium chloride rate and was > 20% from sodium chloride at 488 and 976 kg ha−1 at both application timings. Overall, sodium chloride was most effective and safe on seashore paspalum when applied for smooth crabgrass control at the multitiller growth stage, whereas bermudagrass injury might be excessive at minimum rates required for control.

Crop diversity may improve tolerance to weed interference and reduce the need for herbicides. This experiment measured weed interference in corn as affected by the preceding crop in two studies. The first study, based on interference of the resident weed community, compared dry pea, soybean, canola, and spring wheat for effect on corn tolerance to weeds. Prominent weeds were green and yellow foxtail. The second study examined corn tolerance to a uniform infestation of foxtail millet as affected by dry pea, soybean, spring wheat, and corn as preceding crops. Each plot was split into weed-free and weed-infested subplots in both studies. Corn was most tolerant to weed interference following dry pea; compared with soybean, dry pea improved corn tolerance more than twofold. Corn also yielded the highest in weed-free conditions following dry pea; compared across 4 yr, corn yielded 7 to 23% more following dry pea than following either soybean or spring wheat. Crop diversity has helped producers reduce herbicide inputs in the Great Plains and may provide an additional benefit of reducing weed impact on crop yield.