Three years of field trials have been carried out in Zaragoza, Spain, using different biodegradable mulch materials in processing tomatoes. The aim was to evaluate weed control with several biodegradable mulches as alternatives to black polyethylene (PE) mulch. The treatments were rice straw, barley straw, maize harvest residue, absinth wormwood plants, black biodegradable plastic, brown kraft paper, PE, herbicide, manual weeding, and unweeded control. Assessments focused on weeds and on crop yield. A laboratory study showed that 1 kg/m2 of organic mulch was sufficient to cover the soil for rice, barley straw, and maize harvest residue. The most abundant weed species in the field were purple nutsedge, common purslane, common lambsquarters, and large crabgrass and a change in weed composition was observed between treatments and years. Most weed species were controlled by the mulching materials except that purple nutsedge was controlled only by paper mulch. The other species were well controlled by PE and biodegradable plastic and also by some of the organic mulch treatments. Best weed control and lowest weed biomass were achieved by paper followed by PE and biodegradable plastic. The best organic mulch was rice straw and the worst weed control was from absinth wormwood. Tomato yield was highest for PE followed by paper, manual weeding, biodegradable plastic, and rice straw and was clearly related to weed control. Paper, biodegradable plastic, and rice straw are potential substitutes for PE and herbicides.

The influence of spotted knapweed on surface runoff and sediment yield was determined under simulated rainfall conditions near Garrison, MT. Comparisons were made in 12 paired plots: one of each pair dominated by bunchgrass and the other dominated by spotted knapweed. Runoff and sediment yield were 56% and 192% higher, respectively, for the spotted knapweed, rather than the bunchgrass vegetation types. Spotted knapweed invasion onto bunchgrass range of western Montana is thus detrimental to the protection of soil and water resources.

Estimates of seed viability using the imbibed seed crush test, a method performed by applying pressure to imbibed seeds, were compared with estimates obtained from using the imbibed seed crush test supplemented with tetrazolium staining. The seeds of three weed species, giant foxtail, green foxtail, and yellow foxtail, were collected from three different crops and tested by each method. The results from the two approaches were strongly and significantly correlated. The imbibed seed crush test requires considerably less skill and time to perform and is a reasonable alternative to tetrazolium staining to test the seed viability of newly produced foxtail seeds.

A classification of herbicides according to their sites of action was developed to help maintain the usefulness of herbicides as a tool in crop production to delay the selection of herbicide-resistant weeds. The classification includes a numbering system for the site of action, the chemical family, and the common name of the herbicide. The United States of America trade name and package mixes also are included.

The objectives of this study were to determine the efficacy and risk of controlling weeds at reduced herbicide rates under various environmental and biotic conditions, through analysis of published data on the use of below-labeled rates of herbicides. A database was established by extracting information from previously published papers on weed control at below-labeled rates of herbicides in crop production systems over large geographical and temporal scales. The database was then analyzed to evaluate the efficacy and risk of using herbicides at various reduced rates under different management systems. Using below-labeled herbicide rates in conjunction with interrow cultivation is an effective way of reducing herbicide input in agricultural systems while maintaining satisfactory weed control. There are greater opportunities for herbicide reduction using preemergence (PRE) than preplant incorporated (PPI) or postemergence (POST) herbicides, in coarse-textured than in fine-textured soils, and in corn than in soybean or wheat. The success of reducing herbicide rates does not depend on whether the herbicides are applied in conventional or conservation tillage systems or whether they are used with or without adjuvants. The above conclusions are based on studies conducted in experimental fields where weed pressures may be subjectively chosen to be high. Greater potential for herbicide reduction may exist at locations or in cropping systems were weed pressure is low.

A telephone survey was conducted with growers in Iowa, Illinois, Indiana, Nebraska, Mississippi, and North Carolina to discern the utilization of the glyphosate-resistant (GR) trait in crop rotations, weed pressure, tillage practices, herbicide use, and perception of GR weeds. This paper focuses on survey results regarding herbicide decisions made during the 2005 cropping season. Less than 20% of the respondents made fall herbicide applications. The most frequently used herbicides for fall applications were 2,4-D and glyphosate, and these herbicides were also the most frequently used for preplant burndown weed control in the spring. Atrazine and acetochlor were frequently used in rotations containing GR corn. As expected, crop rotations using a GR crop had a high percentage of respondents that made one to three POST applications of glyphosate per year. GR corn, GR cotton, and non-GR crops had the highest percentage of growers applying non-glyphosate herbicides during the 2005 growing season. A crop rotation containing GR soybean had the greatest negative impact on non-glyphosate use. Overall, glyphosate use has continued to increase, with concomitant decreases in utilization of other herbicides.

Weed management is a perennial challenge for growers, and continual innovation is essential to maintain the effectiveness of management technologies. The first generation of herbicide-resistant crops revolutionized weed control. However, weeds are adapting to crop systems that rely on a single mode of herbicide action. Crops with resistance to multiple modes of herbicide action could help maintain weed management. GAT/HRA is a new multiple herbicide–resistance technology for corn, soybean, and other crops. GAT/HRA combines metabolic glyphosate inactivation with an acetolactate synthase (ALS) enzyme that is insensitive to ALS-inhibiting herbicides. The mechanism to inactivate glyphosate is the glyphosate N-acetyltransferase enzyme, which transforms glyphosate into a nonphytotoxic metabolite. The gat gene is derived from a naturally occurring soil bacterium and optimized by repetitive gene shuffling and screening. The resistance mechanism to ALS-inhibiting herbicides is a double-mutant, highly resistant ALS (HRA) that is insensitive to all five classes of ALS herbicides. GAT/HRA crops will maintain natural tolerance to selective herbicides and thus provide more weed management options for growers to help deter weed spectrum shifts and delay the evolution of herbicide-resistant weeds.

Crop consultants in Arkansas and Mississippi were sent a direct-mail survey in fall of 2011 with questions concerning weed management in rice. The goal of the survey was to document the extent of imidazoline-resistant rice hectares, the herbicides most commonly recommended in rice, the weeds perceived to be most troublesome in rice including those resistant to herbicides, and suggested areas of research and educational focus that would improve weed management in rice. When appropriate, results from this survey were compared to a similar survey conducted in 2006. Completed rice surveys were returned by 43 consultants, accounting for 179,500 ha of scouted rice or 38% of the rice hectarage in Arkansas and Mississippi. Imidazolinone-resistant rice was grown on 64% of the hectares, and this technology was used continually for the past 5 yr on 11% of the rice hectares. Of the area planted to imidazolinone-resistant rice, 42% of this hectarage was treated solely with an acetolactate synthase (ALS)-inhibiting herbicide. Consultants listed improved control options for barnyardgrass and Palmer amaranth as the most important research and educational need in rice. The top five weeds in order of importance were (1) barnyardgrass, (2) sprangletops, (3) red rice, (4) northern jointvetch, and (5) Palmer amaranth. From a predetermined list of research and educational topics, consultants gave the highest ratings of importance to (1) control of herbicide-resistant weeds, (2) strategies to reduce the occurrence and spread of herbicide-resistant weeds, and (3) development of new economical herbicide-resistant rice varieties which was comparable to economical weed control options. Findings from this survey point to the overuse of imidazolinone-resistant rice and a lack of preemptive resistance management strategies such as crop rotation and use of multiple effective herbicide modes of action by some growers, which has likely contributed to selection for the ALS-resistant barnyardgrass and rice flatsedge recently confirmed in Arkansas and Mississippi rice.

A simulation model is used to explore management options to mitigate risks of glyphosate resistance evolution in Palmer amaranth in glyphosate-resistant cotton in the southern United States. Our first analysis compares risks of glyphosate resistance evolution for seven weed-management strategies in continuous glyphosate-resistant cotton monoculture. In the “worst-case scenario” with five applications of glyphosate each year and no other herbicides applied, evolution of glyphosate resistance was predicted in 74% of simulated populations. In other strategies, glyphosate was applied with various combinations of preplant, PRE, and POST residual herbicides. The most effective strategy included four glyphosate applications with a preplant fomesafen application, and POST tank mixtures of glyphosate plus S-metolachlor followed by glyphosate plus flumioxazin. This strategy reduced the resistance risk to 12% of populations. A second series of simulations compared strategies where glyphosate-resistant cotton was grown in one-to-one rotations with corn or cotton with other herbicide resistance traits. In general, crop rotation reduced risks of resistance by approximately 50% and delayed the evolution of resistance by 2 to 3 yr. These analyses demonstrate that risks of glyphosate resistance evolution in Palmer amaranth can be reduced by reducing glyphosate use within and among years, controlling populations with diverse herbicide modes of action, and ensuring that population size is kept low. However, no strategy completely eliminated the risk of glyphosate resistance.

A survey of county extension agents was conducted in 1998 to determine the most troublesome weeds in corn, cotton, forages and pastures, peanut, small grains, soybean, tobacco, and vegetables in Georgia. The most troublesome weed statewide averaged over all crops was sicklepod. It was the most troublesome weed in cotton and soybean and among the four most troublesome weeds in corn, peanut, tobacco, and vegetables. Sicklepod was found in each of the nine climatological districts and in all the crops surveyed. Perennial nutsedge species were the second most troublesome weeds in Georgia. They ranked as the most troublesome weeds in tobacco and vegetables and were among the top five most troublesome weeds in corn, cotton, peanut, and soybean. Pigweed species were ranked third averaged over all the crops surveyed and were the second most troublesome weeds in cotton and vegetables and among the top five most troublesome species in corn, soybean, and tobacco. Morningglory species were listed as troublesome in six of the eight crops surveyed and ranked fourth overall. Similarly, Texas panicum was found in all districts and was the fifth most troublesome weed species. Texas panicum was the most troublesome weed in corn and among the top five most troublesome weeds in peanut, soybean, and tobacco. Florida beggarweed was the most troublesome weed in peanut, the second most troublesome weed in tobacco, and a top-10 weed species in corn, cotton, soybean, and vegetables, resulting in a ranking of sixth overall. Wild radish, large crabgrass, and tropic croton were the seventh through the ninth most troublesome weeds. Wild radish was the most troublesome weed of small grains and the sixth most troublesome weed of vegetables. Large crabgrass was the second most troublesome weed of forages and pastures and was reported in six other crops. Tropic croton was a troublesome weed in seven of the eight crops surveyed and was among the top five most troublesome weeds of cotton and peanut. The 10th most troublesome weed overall was bahiagrass, the most troublesome weed of forages and pastures.

Studies were conducted to evaluate density-dependent effects of Palmer amaranth on weed and peanut growth and peanut yield. Palmer amaranth remained taller than peanut throughout the growing season and decreased peanut canopy diameter, although Palmer amaranth density did not affect peanut height. The rapid increase in Palmer amaranth height at Goldsboro correspondingly reduced the maximum peanut canopy diameter at that location, although the growth trends for peanut canopy diameter were similar for both locations. Palmer amaranth biomass was affected by weed density when grown with peanut. Peanut pod weight decreased linearly 2.89 kg/ha with each gram of increase in Palmer amaranth biomass per meter of crop row. Predicted peanut yield loss from season-long interference of one Palmer amaranth plant per meter of crop row was 28%. Palmer amaranth seed production was also described by the rectangular hyperbola model. At the highest density of 5.2 Palmer amaranth plants/m crop row, 1.2 billion Palmer amaranth seed/ha were produced.

The productivity and native species diversity of Great Plains grasslands have been substantially reduced by past management that facilitated the establishment of invasive exotic weeds and displacement of native species. Management strategies are needed to rapidly restore the productive capacity and biological diversity of these degraded grasslands. Critically important phases of the grassland restoration process are the reintroduction and establishment of native species. Weed interference is the primary constraint to successful establishment of native plants. The goal of our research is to develop strategies that use multiple technologies, including herbicides, to expedite grassland revegetation with native grasses and forbs. Imidazolinone herbicides (AC 263,333, imazapyr, and imazethapyr) were used successfully to improve establishment of native perennial grasses (big bluestem, switchgrass, little bluestem) and selected forbs (blackeyed-susan, purple prairieclover, Illinois bundleflower, trailing crownvetch, and upright prairie coneflower) on cropland and as components of a strategy to revegetate leafy spurge-infested rangeland with native tallgrasses. Imazethapyr at 70 or 110 g ai/ha applied at planting resulted in stands of big bluestem and little bluestem that were similar or superior to stands established where atrazine was applied. Seedling grasses were susceptible to imazapyr at two of three study sites. Imazapyr at 560 g ai/ha plus sulfometuron at 100 g ai/ha applied in fall was the optimum treatment for suppression of leafy spurge and exotic cool-season grasses and establishment of big bluestem and switchgrass on degraded rangeland sites. Establishment of selected forbs was improved by PRE treatment with AC 263,222 or imazethapyr at 70 g ai/ha. This research provides evidence that the imidazolinone herbicides can be important components of integrated weed management strategies designed to reverse deterioration of grasslands by reestablishing native species, improving grassland productivity, and decreasing the prevalence of exotic weeds.

Plant characteristics of red rice ecotypes obtained from Arkansas, Louisiana, Mississippi, and Texas, including 11 strawhulled, five blackhulled, two goldhulled, and one brownhulled type, were evaluated under field conditions. Most ecotypes were uniform and stable but manifested considerable genetic variability. Red rice plants had pubescent leaves, were taller with lighter green color, and produced more tillers and panicles per plant than rice cultivars ‘Lemont,’ ‘Mars,’ and ‘Maybelle.’ Most ecotypes were highly susceptible to seed shattering starting about 14 d after anthesis when seed moisture was more than 25%. Seeds of most ecotypes were highly dormant at harvest. Rice cultivars had a larger flag leaf and more total leaf area per plant at anthesis and produced more seeds per panicle than red rice. Some red rice ecotypes had plant characteristics closely related to cultivated rice suggesting natural hybridization with rice.

Field experiments were conducted in 1997 and 1998 near Columbia and Novelty, MO, and Urbana, IL, to evaluate crop injury, weed control, corn yield, and net economic returns provided by weed control programs in glyphosate-resistant corn. The herbicide programs evaluated included acetochlor preemergence (PRE) followed by (fb) glyphosate with or without atrazine postemergence (POST) and total POST programs consisting of single and sequential applications of glyphosate alone and tank-mixed with actochlor, atrazine, or both. Metolachlor PRE fb dicamba plus atrazine POST and metolachlor plus atrazine PRE were included for comparison. In the total POST treatments, mid-post (MPOST) applications provided better control than early-post (EPOST) applications on weeds that germinated throughout the growing season such as shattercane and common cocklebur, but also resulted in yield reductions of up to 23% caused by early-season weed competition. The addition of atrazine to glyphosate POST generally increased control of common cocklebur, morningglory species, and common waterhemp. EPOST or PRE fb EPOST applications generally provided higher yields than MPOST treatments, although MPOST treatments often provided equal or greater weed control at midseason. Treatments including two herbicide applications tended to provide greater weed control, yield, and profit than those with a single application. Input costs for glyphosate-resistant corn are slightly higher than nontransgenic hybrids. However, net economic returns are similar and the use of glyphosate POST allows greater flexibility in POST weed management decisions.

Field experiments were conducted at Vegreville and Lacombe, AB, to determine the influence of barley (Hordeum vulgare) variety and seeding rate on interference of wild oat (Avena fatua) with barley. Barley variety and seeding rate affected barley density, height at maturity, and seed yield, as well as wild oat shoot dry weight and seed yield in most experiments, but there was no variety by seeding rate interaction. As expected, the semidwarf varieties Falcon and CDC Earl were the shortest. Barley seedling emergence and subsequent plant densities varied among varieties, locations, and years. The hull-less varieties Falcon and CDC Dawn had the poorest emergence in most cases, whereas AC Lacombe and Seebe had the highest emergence. Wild oat shoot dry matter and seed production was highest in the Falcon, CDC Dawn, and CDC Earl plots, suggesting that these were the least competitive with wild oat. Barley yield loss from wild oat interference also tended to be highest in these varieties. Poor emergence of Falcon and CDC Dawn and the shorter stature of Falcon and CDC Earl likely contributed to their relatively poor competitiveness with wild oat. Increasing the seeding rate improved the competitiveness of all varieties, as evidenced by reduced wild oat shoot dry matter and seed production and, in some cases, improved barley yields.

Ecological studies improve the understanding of crop losses caused by weeds and interactions among weeds and crops. Knowledge of weed-crop interactions can aid in developing more efficient weed management systems. Also, knowledge of the duration of weed interference and the length of the weed-free period required to prevent crop losses is essential for planning cost-effective control practices.

Development and utilization of dicamba-, glufosinate-, and 2,4-D-resistant crop cultivars will potentially have a significant influence on weed management in the southern United States. However, off-site movement to adjacent nontolerant crops and other plants is a concern in many areas of eastern North Carolina and other portions of the southeastern United States, especially where sensitive crops are grown. Cotton, peanut, and soybean are not resistant to these herbicides, will most likely be grown in proximity, and applicators will need to consider potential adverse effects on nonresistant crops when these herbicides are used. Research was conducted with rates of glufosinate, dicamba, and 2,4-D designed to simulate drift on cotton, peanut, and soybean to determine effects on yield and quality and to test correlations of visual estimates of percent injury with crop yield and a range of growth and quality parameters. Experiments were conducted in North Carolina near Lewiston-Woodville and Rocky Mount during 2009 and 2010. Cotton and peanut (Lewiston-Woodville and Rocky Mount) and soybean (two separate fields [Rocky Mount] during each year were treated with dicamba and the amine formulation of 2,4-D at 1/2, 1/8, 1/32, 1/128, and 1/512 the manufacturer's suggested use rate of 280 g ai ha−1 and 540 g ai ha−1, respectively. Glufosinate was applied at rates equivalent to 1/2, 1/4, 1/8, 1/16, and 1/32 the manufacturer's suggested use rate of 604 g ai ha−1. A wide range of visible injury was noted at both 1 and 2 wk after treatment (WAT) for all crops. Crop yield was reduced for most crops when herbicides were applied at the highest rate. Although correlations of injury 1 and 2 WAT with yield were significant (P ≤ 0.05), coefficients ranged from −0.25 to −0.50, −0.36 to −0.62, and −0.40 to −0.67 for injury 1 WAT vs. yield for cotton, peanut, and soybean, respectively. These respective crops had ranges of correlations of −0.17 to −0.43, −0.34 to −0.64, and −0.41 to −0.60 for injury 2 WAT. Results from these experiments will be used to emphasize the need for diligence in application of these herbicides in proximity to crops that are susceptible as well as the need to clean sprayers completely before spraying sensitive crops.

Cover crop management with a roller/crimper might reduce the need for herbicide. Weed suppression from a rolled cereal rye cover crop was compared to no cover crop with and without postemergence herbicide application in no-till soybean. The experiment was designed as a two-way factorial with rye termination and soybean planting date as the first factor and weed control treatment as the second. Cereal rye was drill-seeded in late September and managed using glyphosate followed by a roller/crimper in the spring. Soybean was no-till seeded after rolling and glyphosate was applied postemergence about 6 wk after planting to half the plots. Rye biomass doubled when delaying rye kill by 10 to 20 d. Weed density and biomass were reduced by the rye cover crop in all site–location combinations except one, but delaying rye kill and soybean planting date only reduced both weed density and biomass at a single location. The cover crop mulch provided weed control similar to the postemergence herbicide in two of four locations. Treatments did not affect soybean grain yield in 2007. In 2008, yield at Landisville with rye alone was equal to those yields receiving the postemergence herbicide, whereas at Rock Springs, it was equivalent or less. The net added cost of a rye cover crop was $123 ha−1 with or $68.50 ha−1 without a postemergence herbicide application. A rolled-rye cover crop sometimes provided acceptable weed control, but weed control alone did not justify the use of the cover crop. The potential for reduced herbicide use and other ecosystem services provided by a cover crop justify further refinement and research in this area.

Conservation tillage systems for cotton production were monitored for their influence on horseweed populations. Without fall tillage, horseweed became established and was present the following spring. Disking in the fall or spring controlled horseweed for the subsequent cotton crop.

Field studies were conducted in 2005 and 2006 to determine the most effective chemical options within three individual herbicide-based burndown programs, glyphosate, paraquat and glufosinate, for controlling glyphosate-resistant horseweed in Mississippi. Burndown treatments were applied April 5, 2005 and March 15, 2006 to horseweed plants 15 to 30 cm in height. Glyphosate at 0.86 kg ae/ha alone provided 60 to 65% horseweed control 4 wk after treatment (WAT). Control 4 WAT ranged from 73 to 74% when the glyphosate rate was increased to 1.25 kg/ha. Glyphosate at 0.86 kg/ha applied in combination with 2,4-D at 0.84 kg ae/ha or dicamba at 0.28 ae/ha maximized control of horseweed (≥ 90%) 4 WAT and soybean yield. Horseweed control 4 WAT with paraquat alone at 0.84 kg ai/ha ranged from 55 to 63% and control did not improve by increasing the rate to 0.98 kg/ha. Addition of 2,4-D or dicamba to paraquat maximized horseweed control both years (78 to 89%), whereas soybean yield was maximized with addition of dicamba or metribuzin at 0.42 kg ai/ha. Glufosinate applied alone at 0.47 kg ai/ha resulted in at least 88% control of horseweed and maximized soybean yield. Results indicate that effective management of glyphosate-resistant horseweed can be obtained in glyphosate-resistant soybean in glyphosate-, paraquat-, and glufosinate-based preplant weed control programs.