Field studies were conducted in 2000, 2001, and 2002 at Urbana, IL, to examine the interference potential of common waterhemp that emerged at soybean growth stages VE, V2-V3, V4-V5, R1-R2, and R3-R4 in 19- and 76-cm row soybean. Soybean row width and common waterhemp emergence timing significantly influenced common waterhemp density, biomass, seed production, mortality, and soybean yield loss. Common waterhemp density declined as emergence timings were at later soybean growth stages. This decline happened at earlier growth stages in narrow-row soybean. Significant reductions in common waterhemp biomass and seed production occurred at the V2-V3 and V4-V5 emergence timings for the narrow- and wide-row soybean, respectively. Common waterhemp seed production was more than 23,000 seeds per plant at the VE emergence timing for both soybean row widths. Survival of common waterhemp that emerged after the V4-V5 soybean growth stage was less than 20% in both row widths. Common waterhemp interference reduced soybean seed yield at the VE, V2-V3, and the V4-V5 emergence timings. Row width affected the magnitude of yield reductions at these interference timings, with reductions being less in narrow-row soybean. This research suggests that control measures need to be implemented to common waterhemp plants that emerge before V4-V5 soybean to protect soybean yield and reduce common waterhemp seed production.

Corn and soybean growers across Indiana were surveyed during winter 2003/2004 to assess their perceptions about the importance of glyphosate-resistant weeds and management tactics to prevent development of resistant populations. The survey showed two intriguing observations. First, 65% of survey respondents expressed moderate or low levels of concern about weeds developing resistance to glyphosate, whereas 36% expressed a high level of concern. Second, when asked an open-ended question regarding the factors that contribute to development of glyphosate-resistant weeds, 58% of the responses included repeated use of the same mode of action. Other factors such as poor application techniques or timing (33%), unique weed characteristics (8%) and changes in tillage practices (1%) were also mentioned. The survey showed that even though a relatively low percentage of respondents were highly concerned about resistance, they still expressed a willingness to use field scouting, tank-mix partners with glyphosate for burn-down and postemergence weed control, and soil-applied residual herbicides as resistance management strategies. This survey also showed that growers who farm 800 ha or more were more concerned about glyphosate resistance and more likely to adopt resistance management strategies than smaller growers.

Herbicide-resistant crops, such as glyphosate-resistant (GR) soybean, allow for broad-spectrum, flexible weed control with minimal crop injury; however, the development of GR weeds, such as horseweed, has forced reliance on alternative herbicides for control of these weeds. While preplant (PP) herbicides provide excellent control of GR-horseweed, there are currently no POST herbicide control options within soybean. The objective of this study was to evaluate the efficacy of dicamba for the control of GR-horseweed when applied PP, POST, and sequentially in dicamba-resistant soybean. Dicamba applied PP at 600 g a.e. ha−1 provided 90 to 100% control of GR-horseweed 8 wk after application (WAA) across three field trials conducted in Ontario in 2011 and 2012. Similarly, sequential applications provided 91 to 100% control. This technology provides a much-needed POST option of dicamba to be applied as a rescue treatment to control weed escapes caused by late emergence or poor initial control following a PP herbicide application.

Soybean producers of South Carolina were surveyed in 2000 through mail and on-farm visits to determine which production practices limit seed yields the most. Production systems have direct implications on weed management practices, difficulties, and future problems. When asked to describe two extension and research focus areas that would improve current soybean production, the number one response was improved weed control strategies. Fifty-seven percent of the respondents indicated that weeds were the pest that most limited soybean yield. Sicklepod, Palmer amaranth, morningglory species, common cocklebur, yellow nutsedge, and Texas panicum were considered the most problematic weeds. More than half the producers (68%) used glyphosate-resistant soybean, with one-half of these farmers producing soybean in row widths > 51 cm. Of the farmers who grew conventional soybean, 10% used narrow rows (≤ 51 cm), whereas 22% used wide rows (> 51 cm). Only 27% of the farmers who planted glyphosate-resistant soybean used a soil-applied herbicide. Forty-three percent of farmers based the need for a postemergence herbicide on weed size, 21% used no repeatable method, 21% relied on days after soybean emergence, 11% depended on soybean size or groundcover, and only 4% used a combination of weed density and size. Thirty-five percent of the farmers did not rotate soybean with another crop. In fields where soybean parasitic nematodes reduced soybean growth or caused plant death, late-season weed infestations were often exacerbated, especially in the absence of nonresidual herbicides. Specific reasons for difficulty in managing weeds in soybean may include a diverse weed spectrum, which is difficult to control with a single herbicide, the use of wide-row soybean, the lack of crop rotation, the prevalence of nematodes that directly affect crop development and, in turn, weed management, the lack of an appropriate criterion for timing postemergence herbicide applications, and the greater attention paid to production of more valuable crops. Educational efforts addressing weed management problems and association of such problems with other production practices are needed if weed management is to be improved and weed-induced yield losses minimized.

The increased use of conservation tillage in cotton production requires that information be developed on the role of cover crops in weed control. Field experiments were conducted from fall 1994 through fall 1997 in Alabama to evaluate three winter cereal cover crops in a high-residue, conservation-tillage, nontransgenic cotton production system. Black oat, rye, and wheat were evaluated for their weed-suppressive characteristics compared to a winter fallow system. Three herbicide systems were used: no herbicide, preemergence (PRE) herbicides alone, and PRE plus postemergence (POST) herbicides. The PRE system consisted of pendimethalin at 1.12 kg ai/ha plus fluometuron at 1.7 kg ai/ha. The PRE plus POST system contained an additional application of fluometuron at 1.12 kg/ha plus DSMA at 1.7 kg ai/ha early POST directed (PDS) and lactofen at 0.2 kg ai/ha plus cyanazine at 0.84 kg ai/ha late PDS. No cover crop was effective in controlling weeds without a herbicide. However, when black oat or rye was used with PRE herbicides, weed control was similar to the PRE plus POST system. Rye and black oat provided more effective weed control than wheat in conservation-tillage cotton. The winter fallow, PRE plus POST input system yielded significantly less cotton in 2 of 3 yr compared to systems that included a winter cover crop. Use of black oat or rye cover crops has the potential to increase cotton productivity and reduce herbicide inputs for nontransgenic cotton grown in the Southeast.

Imazethapyr at 0.07 and 0.10 kg ai ha-1 applied preplant incorporated (PPI), preemergence (PRE), and postemergence (POST) was evaluated at two locations in 1988 and 1989 for safety to dry edible beans. Bean stunting, leaf crinkling, and interveinal chlorosis were evident from imazethapyr and varied from 1 to 57%. Imazethapyr significantly reduced bean height and delayed maturity. PPI and POST applications of imazethapyr at 0.07 and 0.10 kg ha-1 did not reduce bean seed yields compared to yields of the untreated control. Bean cultivar by herbicide interactions were significant for bean injury but varied with year and location.

The value of glyphosate has been compromised in some fields where weed populations have developed resistance or tolerant species increased. Three case studies related to reduced control from glyphosate are: (1) a weed population that has become resistant to glyphosate, with horseweed in Tennessee as an example; (2) a weed population increases due to lack of control in “glyphosate only” systems, with tropical spiderwort in Georgia cotton used as an example; and (3) the hypothetical resistance of common waterhemp to glyphosate in Illinois. For each of these case studies, an economic analysis was performed using a partial budget approach. This economic analysis provides the cost of control to the farmer when glyphosate fails to control these weeds and gives a critical time in years to compare different glyphosate resistance management philosophies (applicable only before resistance has evolved). The cost of glyphosate-resistant horseweed in cotton-soybean-corn rotation in Western Tennessee was calculated to be $30.46/ha per year. The cost of tropical spiderwort in cotton in southern Georgia was calculated to be $35.07/ha per year. The projected cost if common waterhemp were to develop glyphosate resistance in a corn-soybean rotation in southern Illinois was projected to be $44.25/ha per year, and the critical time was determined to be greater than 20 yr, indicating that a resistance management strategy would extend the value of glyphosate-resistant crops.

Eight field studies were conducted in soybeans at seven locations in Kansas over a 3-yr period to examine the efficacy of using reduced rates of the herbicides acifluorfen, bentazon, chlorimuron, and tank mixes of acifluorfen and bentazon. POST applications of these herbicides at 1/2X rates at 2 wk after planting (WAP) resulted in broadleaf weed control similar to that obtained from standard treatments of 1X rates applied at 4 WAP at six of seven studies with acifluorfen, bentazon, and acifluorfen plus bentazon and at five of seven studies with chlorimuron. One-quarter rates applied 2 WAP were equivalent to standard treatments for broadleaf weed control in four of seven studies with acifluorfen and chlorimuron, five of seven studies with bentazon, and six of seven studies with acifluorfen plus bentazon. One cultivation at 4 WAP, increased the broadleaf weed control with all herbicide treatments.

The significance of glyphosate and the appearance of glyphosate-resistant weeds have raised concerns about glyphosate sustainability. Resistance-prevention strategies, however, should first consider the mechanisms for resistance. For example, target-site resistance can provide virtual immunity, ensuring that every herbicide application successfully selects for resistance. However, metabolism and exclusion mechanisms provide lower magnitudes of resistance and are dependent on dosage. This discussion proposes that the relative risk of weed resistance is most highly correlated to mode of action (MOA), due to the respective principal mechanism for resistance. The development of data correlating agronomic practices with weed resistance vs. herbicide/MOA choices will be critical to the design of effective prevention strategies. Because resistance to glyphosate in weeds is typically of a low magnitude, using a high-dose strategy should minimize the potential for the selection of resistance and thus help to make the use of glyphosate sustainable. Maximizing weed control is the key to successful agronomic practice with limited weed resistance.

Barnyardgrass at 10 or more plants per 1 m of row caused over 80% root yield loss in spring-sown sugarbeets in California. The yield loss decreased to between 5 and 20% when barnyardgrass densities were one plant ever 2 to 3 m of crop row. The relationship between crop yield and barnyardgrass density was described by a rectangular hyperbolic function. The economic threshold density was in the range of one barnyardgrass plant every 5 to 20 m or row. Barnyardgrass plants growing at densities of less than one plant per 1 m in sugarbeets experienced only interspecific competition. Such individual barnyardgrass plants were estimated to produce from 30 000 to over 200 000 seeds, with an average of 100 000 seeds per plant. This seed production equated to a seed rain of about 4000 to nearly 20 000 seeds per m from a density of one weed every 10 m of crop row. These data suggest that using single-year economic thresholds for barnyardgrass is not a sound management practice because even low densities can produce large quantities of seed.

Studies were conducted in 1998 and 1999 in Ohio to determine the effect of postemergence (POST) application timing of glyphosate on weed control and grain yield in glyphosate-tolerant corn, and how this was influenced by corn planting date and the use of soil-applied herbicides. Glyphosate was applied based on giant foxtail height. Two applications of glyphosate provided better weed control than a single application, especially when applied to weeds 10 cm or less in early-planted corn. Yield was reduced occasionally with a single application on 5- or 10-cm weeds, because of weed re-infestation. Failure to control weeds before they reached a height of 15 to 30 cm also resulted in occasional yield loss. Application of atrazine or acetochlor plus atrazine prior to glyphosate did not consistently increase weed control or yield. Results suggested that glyphosate should be applied before weeds reach 15 cm in height to avoid corn grain yield loss.

Activator adjuvants for herbicides increase herbicide activity and encompass a wide variety of surfactant chemistry. Activator adjuvants may be included in the product formulation by the manufacturer or tank-mixed by the herbicide applicator. Activator adjuvant efficacy is a function of not only the adjuvant but also the herbicide, the particular weed species, and environmental conditions. Proposed modes of activator action include reduction of spray solution surface tension to enhance contact area, solibilization of the leaf cuticle, emulsifier action, increased spray retention, protection of the herbicide in the spray solution, promotion of rainfastness, acting as a cosolvent or copenetrant, modification of spray deposition on plant foliage, and enhanced movement on the foliage surface to areas of greater absorption.

Management of corn (Zea mays) row spacings and populations has been used for many years to increase corn productivity. In 1998 and 1999, nonirrigated corn was grown in 38-, 56-, and 76-cm row spacings at populations averaging 59,300, 72,900, and 83,900 plants/ha in Michigan. Glufosinate at 0.29 kg/ha was applied to common lambsquarters (Chenopodium album L.) averaging 5 cm in height in each plot. Corn population and row spacing did not influence weed emergence following application of glufosinate. Common lambsquarters biomass and seed production were reduced when grown under canopies of corn planted in populations exceeding 72,900 plants/ha. Common lambsquarters biomass was reduced as corn row spacings were reduced from 76 to 38 cm. Early-season interception of photosynthetic active radiation (PAR) by corn canopies increased as row spacings decreased, but differences were not apparent later in the season. Interception of PAR was similar throughout the season when corn populations exceeded 72,900 plants/ha. Corn yields were not affected by row spacing, but they were increased with corn populations of 72,900 plants/ha or higher.

The growth regulator herbicides 2,4-D and dicamba are used to control glyphosate-resistant horseweed before crops are planted. With the impending release of 2,4-D–resistant and dicamba-resistant crops, use of these growth regulator herbicides postemergence will likely increase. The objective of this study was to determine the effectiveness of various growth regulators on Indiana horseweed populations. A greenhouse dose–response study was conducted to evaluate the effectiveness of 2,4-D ester, diglycolamine salt of dicamba, and dimethylamine salt of dicamba on control of four populations of horseweed in the greenhouse. Population 66 expressed twofold levels of tolerance to 2,4-D ester and diglycolamine salt of dicamba. Population 43 expressed an enhanced level of tolerance to diglycolamine salt of dicamba but not to the other herbicides. Diglycolamine salt of dicamba provided the best overall control of populations 3 and 34. Additionally, a field study was conducted to evaluate standard use rates of 2,4-D amine, 2,4-D ester, diglycolamine salt of dicamba, and dimethylamine salt of dicamba on control of various sized glyphosate-resistant horseweed plants. Control of plants 30 cm or less in height was 90% or greater for all four herbicides. On plants greater than 30 cm tall, diglycolamine salt of dicamba provided 97% control while 2,4-D amine provided 81% control. Diglycolamine salt of dicamba provided the highest level of control of glyphosate-resistant horseweed, followed by dimethylamine salt of dicamba, 2,4-D ester and 2,4-D amine, respectively. This research demonstrates that horseweed populations respond differently to the various salts of 2,4-D and dicamba, and it will be important to determine the appropriate use rates of each salt to control glyphosate-resistant horseweed.

Vegetation development bordering the Middle Rio Grande, as with most major southwestern U.S. tributaries, has historically undergone rapid and dynamic change. The introduction of saltcedar (or Tamarisk, genus Tamarix) and other exotic species into this environment within the 20th century has contributed to this process. These plants are now an integral component of the riparian vegetation mix. Manpower, logistics, and financial resources constrain the degree to which a desired riparian habitat can be restored from saltcedar thickets on the Bosque del Apache National Wildlife Refuge near Socorro, NM. Saltcedar clearing is accomplished using a combination of herbicide, burning, and mechanical control techniques costing from $750 to $1,300/ha. Soil salinity and depth to water are the principal physical features limiting revegetation efforts. Cottonwood and black willow plantings and natural regeneration after timed irrigations have produced diverse habitats that support a wide array of faunal species in areas previously occupied by homogeneous saltcedar.

Twenty-one grass weeds have evolved resistance to herbicides in Latin America, particularly in rice, soybean, wheat, and orchards. Junglerice, the most widespread and economically important rice weed, evolved resistance to propanil, acetyl-coenzyme A carboxylase (ACCase)-inhibitor herbicides, quinclorac, and imazapyr in Central America, Colombia, and Venezuela. Some junglerice populations are resistant to at least three herbicide modes of action. Other herbicide-resistant (HR) rice weeds are barnyardgrass and gulf cockspur to quinclorac in Brazil, and saramollagrass to ACCase-inhibitor herbicides in Colombia and bispyribac in Venezuela. Populations of weedy rice resistant to imidazolinones are now emerging, most likely originated from gene flow from imidazolinone-resistant rice. Saramollagrass also became resistant to nicosulfuron in corn in Venezuela. Eight species associated with soybean are resistant to ACCase-inhibitor herbicides in Brazil (alexandergrass, goosegrass, and southern crabgrass) and Bolivia (Louisiana cupgrass, itchgrass, sudangrass, and two common wild sorghum species). Four more ACCase-inhibitor–resistant species (hedgehog dogtailgrass, wild oat, rigid ryegrass, and Italian ryegrass) are found in Chile infesting canola and wheat. ACCase-inhibitor–resistant hood canarygrass, littleseed canarygrass, and wild oat are important in wheat in Mexico. Resistance to acetolactate synthase (ALS)-inhibitor herbicides has been reported in itchgrass, goosegrass, and Mexican grass. Italian ryegrass populations resistant to glyphosate have been found in Chile and Brazil. Glyphosate resistance has also evolved in goosegrass in Bolivia and johnsongrass in Argentina. In general, little is done to prevent resistance evolution. An exception is the stewardship programs aiming to prevent gene flow from imidazolinone-resistant rice to weedy rice. Once resistance evolves, HR populations are mostly managed by shifting to herbicides with different modes of action and, in some cases, by slightly modifying agronomic practices. Propanil formulations containing a synergist are used to manage propanil-resistant junglerice. Increased no-till agriculture and planting of glyphosate-resistant crops are likely to select more glyphosate-resistant weeds.

Seeds of a putative 4-hydroxyphenylpyruvate dioxygenase (HPPD)-inhibiting herbicide–resistant tall waterhemp biotype from Henry County, IA, were collected from a seed corn field in fall 2009 after plants were not controlled following a POST application of mesotrione plus atrazine. The response of this biotype to various herbicide modes of action was evaluated in greenhouse and field tests. Under greenhouse conditions, the suspect biotype showed an eightfold decrease in sensitivity to mesotrione with a 50% control rate of 21 g ha−1 compared with 2.7 g ha−1 for the susceptible biotype. The biotype also had a 10-fold decrease in sensitivity to atrazine and a 28-fold decrease in sensitivity to thifensulfuron. Under field conditions, tall waterhemp was not controlled POST at the label rate of 1,100 g ha−1 of atrazine. Tall waterhemp control was less than 60% at the label rates of three commonly used POST HPPD-inhibiting herbicides in seed corn: 105 g ha−1 of mesotrione, 92 g ha−1 of tembotrione, or 18 g ha−1 of topramezone. Thus, this new tall waterhemp biotype is resistant to three herbicide modes of action: HPPD inhibitors, photosystem-II inhibitors, and acetolactate synthase (ALS) inhibitors.

Greenhouse and field experiments were initiated to evaluate rapeseed and mustard species as green manure crops for weed suppression. Under greenhouse conditions incorporating 20 g fresh wt leaf and stem tissue of rapeseed, two white mustards, and brown mustard into 450 g Sharpsburg, silty clay loam soil resulted in significant reductions in weed emergence, biomass, and height. Kochia, shepherd's-purse, and green foxtail emergences were reduced by all green manure crops. Redroot pigweed emergence was reduced by all green manure crops except brown mustard, and velvetleaf emergence was reduced by white mustards only. Kochia and shepherd's-purse fresh weights were reduced by all green manure crops, while redroot pigweed and velvetleaf fresh weights were reduced by brown mustard and white mustard var. Salvo. Green foxtail fresh weight was reduced by all green manure crops except rapeseed. With the exception of shepherd's-purse, no relationship between glucosinolate content of the incorporated green manure and suppression of weed growth was found. Under field conditions, early spring-planted green manure crops reduced early season weed biomass in soybean at one of the two locations. Mustard species as green manure crops reduced total weed biomass in soybean by 40% 4 weeks after emergence (WAE) and 49% 6 WAE. Soybean biomass and yield were sometimes reduced by the incorporation of green manure crops in treatments containing weeds; however, hand-weeded plots with green manure treatments yielded similar to hand-weeded plots without green manure.

A soil sampler, elutriator, and associated sample flushing device were designed and constructed for an intensive study of weed seedbanks. This equipment was used in 1993 to collect and process 4980 soil samples. The sampler was durable, core size was consistent, and sampling was efficient. Cores were approximately 200 cm3 and two people could take 120 cores/h. The elutriator separated weed seeds from 36 of these cores at a time. Washing required 60 to 75 min depending on soil texture. Seeds as small as 0.3 mm in diam were recovered and almost 100% of the seeds were recovered from samples spiked with barnyardgrass, redroot pigweed, velvetleaf, and witchgrass. The flushing device was used to transfer sample contents from strainers of the elutriator to propyltex bags for drying and storing. Equipment like this, plus improved technology for identifying and counting seeds, is needed to make weed seedbank studies more feasible.

Predicting crop yield reductions from weeds continues to challenge weed science. Useful methods based upon weed densities and upon area weeds influence have been developed. Results vary greatly from year to year and location to location for several reasons which are reviewed: 1) the shift in crop yield from a weed density-dependent to a weed density-independent relationship; 2) the effect of density on essential growth factor competed for; 3) differences among weed species in relative competitiveness for essential growth factors; 4) the differential effect of environmental conditions on the competitiveness of weed species; and 5) the effect of time of emergence on competition. To reduce the variability in prediction methods will necessitate developing a method based upon the factor(s) responsible for yield reduction.