Wild oat and green foxtail are the two most abundant weeds and the most economically important herbicide-resistant grass weeds in the northern Great Plains of western Canada. Farmers in this region rarely proactively manage these weed species to prevent or delay the selection for herbicide resistance; they usually increase the adoption of integrated weed management practices only after intergroup herbicide resistance has evolved. The effectiveness of herbicide and nonherbicide practices to proactively or reactively manage herbicide-resistant wild oat and green foxtail are described, based on a decade of field trials, field-scale experiments, and field surveys in western Canada. Nonherbicide weed management practices or nonselective herbicides applied preplant or in crop, integrated with less frequent selective herbicide use in diversified cropping systems, have mitigated the evolution, spread, and economic impact of herbicide-resistant wild oat and green foxtail.

Blackgrass is the most important herbicide-resistant weed in Europe, occurring in 10 countries. Enhanced metabolism is the most common mechanism, conferring partial resistance to a wide range of herbicides, but acetyl-CoA carboxylase (ACCase) target-site resistance also occurs widely. Recently, acetolactate synthase (ALS) target-site resistance conferred by a Pro197 mutation was identified in blackgrass in England and is of concern because of increasing use of sulfonylurea herbicides such as mesosulfuron + iodosulfuron. Resistance management strategies encourage (1) greater use of cultural control measures such as plowing, crop rotation, and delayed drilling; (2) reduced reliance on high-risk herbicides (ACCase, ALS); and (3) use of mixtures and sequences of herbicides with different modes of action. A key message is that, as weeds are relatively immobile, preventing and managing herbicide resistance is largely within the individual farmer's own control. In practice, financial and environmental pressures limit the scope for more cultural control, and the European Community Pesticide Review will result in fewer alternative herbicides being available. Consultants often feel unable to recommend lower risk but weaker herbicide strategies to farmers because the amount of blackgrass remaining might be unacceptable. This dilemma is exemplified by the recent introduction of a formulated mixture of mesosulfuron + iodosulfuron, which has given outstanding control of blackgrass. Farmers expect new herbicides to become available, but this optimistic view appears misplaced. A primary aim is to continue to encourage farmers to integrate cultural and chemical control in a long-term strategy.

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.

Arkansas has been the leading state in rice production in the United States for many years. Barnyardgrass is the dominant weed in Arkansas rice. Propanil was the first highly effective herbicide for weed control in rice and has been used in Arkansas since 1959 as the primary herbicide for rice weed control. By 1989, its continual use led to the development of propanil-resistant barnyardgrass, which had spread to 16 of the 38 rice-producing counties in Arkansas by 1992. Arkansas rice growers are dependent on herbicides for the control of weeds in this drill-seeded crop. The residual herbicides thiobencarb, molinate, and pendimethalin mixed with propanil applied early postemergence improved control of propanil-resistant barnyardgrass. But it was quinclorac, introduced in 1992, that became the real replacement treatment for propanil-resistant barnyardgrass. Then in 1999, a barnyardgrass biotype with resistance to both quinclorac and propanil was confirmed in Craighead County, Arkansas. Additionally, problems with quinclorac drift to other crops, especially tomato, led to restrictions on application of quinclorac in Arkansas by 1994. Fortunately, alternative herbicides for barnyardgrass control were developed, and clomazone was introduced in 2000. Clomazone is currently the standard herbicide for annual grasses in rice, including barnyardgrass. Herbicides recently developed for rice allow a broad range of options for a resistance management program, based on rotational and sequential herbicide applications. These include fenoxaprop and cyhalofop (both acetyl-CoA carboxylase [ACCase] inhibitors), bispyribac and penoxsulam (acetolactate synthase [ALS] inhibitors), and imazethapyr and imazamox (also ALS inhibitors for imidazolinone-resistant rice). From a global standpoint, there is considerable evidence that barnyardgrass has the propensity to develop resistance to most of these herbicide groups. Therefore, efforts to manage and monitor for herbicide resistance in this species need to be diligently continued. Research on nonchemical options is in progress utilizing weed-suppressive rice breeding lines to control barnyardgrass.

In most world crop-production areas, the evolution of herbicide-resistant weeds is becoming a major issue. This problem has become most severe across the Australian dryland crop-production region, where herbicide-resistant weed populations are threatening crop-production profitability and sustainability across 20 million ha. Widespread herbicide resistance has forced changes in agronomic and herbicide practices. This problem is particularly evident in Western Australia, where the frequency and distribution of herbicide-resistant weed populations appear to be greater than anywhere else in the world. Judicious use of herbicide mixtures and rotations can reduce the selection pressure for evolved resistance to any one specific herbicide. Additionally, agronomic practices, such as the double knockdown (preseeding sequential application of nonselective herbicides), increased seeding rates, and targeting of weed seed production to prevent seedbank inputs, are needed to reduce the selection pressure on all herbicides by reducing in-crop weed populations. However, these techniques are not without problems or limitations, and their weed control efficacy is inferior to that of most in-crop selective herbicides. The adoption by Australian farmers of the current limited technology is clear evidence of the value placed on the use of these alternate crop weed-control practices. The continued evolution of herbicide resistance more than justifies continuing research and development efforts to produce integrated strategies and smarter herbicide use so as to achieve sustainable weed management.

Littleseed canarygrass is a major weed of winter-season crops, although it is most dominant in wheat-growing regions in the Indo-Gangetic Plains of India, Pakistan, Nepal, and Bangladesh. Resistance in this species to photosystem II–inhibiting herbicide isoproturon was first recorded in 1992, and has since spread to several Indian states covering more than a million ha. Genetic studies and resistance characterization from multiple locations indicate independent evolution of resistance due to continuous use of isoproturon and monoculture rice–wheat-cropping system. Isoproturon-resistant biotypes were found cross-resistant to diclofop, but not to chlortoluron, which has the same mode of action as isoproturon. The isoproturon-resistance mechanism is metabolic degradation, mediated by P-450 monooxygenase enzymes. This type of resistance could become serious and lead to the evolution of multiple resistances to herbicides of different modes of action. Adoption of fenoxaprop-P, clodinafop, and sulfosulfuron in isoproturon-resistant areas since 1997 initially led to high yields, but resulted in a weed flora shift which eventually reduced yields and increased the cost of weed management. Although isoproturon recommendation has been withdrawn from rice–wheat cropping zones, resistance in littleseed canarygrass is spreading in other areas where isoproturon has been used for several years because it is inexpensive and has broad-spectum weed control. Management factors, such zero or minimum tillage, early planting after rice harvest, and alternative herbicides provide effective control of resistant biotypes. However, lower efficacy of these herbicides has been observed in the field, although multiple resistances have yet to be confirmed. Herbicide rotations, mixtures, and sequences are beneficial, but only in the short term. Improved cultivation practices are also helpful; however, no current single system is sustainable. An integration of tillage method, planting time, varietal selection, crop rotation, timing and method of herbicide application, optimum dose, and sanitation practices is crucial in managing herbicide-resistant littleseed canarygrass.

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.

Weed management is evolving to include cultural tactics that reduce weed populations. This study near Brookings, SD, evaluated the effect of crop sequence and tillage on seedling emergence of common sunflower across years. In the third and fourth years of the study, seedling density was sevenfold greater after 2 yr of soybean with tillage compared with a 2-yr sequence of canola and winter wheat with no-till. Apparently, canola and winter wheat enhanced the natural decline of common sunflower seed density in soil, leading to fewer seedlings in following years. In the first year of the study, tillage increased seedling emergence of common sunflower compared with no-till; seedlings rarely emerged in canola or winter wheat. Most seedlings of common sunflower emerged in May, with more than 90% of seedlings emerging between May 7 and June 4. Cool-season crops grown with no-till may affect weed seed survival in soil in the western Corn Belt.

Planting date of soybean may be one factor that affects the crop's ability to compete with weeds. Field experiments were conducted over 3 yr at two locations in Illinois to determine whether planting date affects optimal weed management strategies and the critical time of weed removal (CTWR) in glyphosate-resistant soybean. Across planting dates, a PRE application of metolachlor plus metribuzin followed by a single glyphosate application at a 10- or 20-cm weed height at Monmouth or at a 20-cm weed height at Urbana was effective at protecting yield. At Monmouth, higher yields occurred with sequential glyphosate applications than with a single application in the early planting in 2 yr when weeds were removed at a height of 10 cm. Across planting dates, highest yields at Monmouth occurred with a single glyphosate application when weeds were 20 to 30 cm tall. At Urbana, the CTWR was not affected by planting date and occurred between 176 and 290 growing-degree days (GDD) after planting, corresponding to the V1 to V2 stage of soybean development and a weed height of 11 to 19 cm. Applying glyphosate near the CTWR at Urbana (10-cm weed height) required a sequential application to prevent yield loss with early and middle planting dates in 1 of 3 yr. Overall, planting date did not affect optimal weed management strategies at either location.

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.

Studies were conducted at three locations in North Carolina in 2004 to evaluate density-dependent effects of glyphosate-resistant (GR) corn on GR cotton growth and lint yield. GR corn was taller than GR cotton as early as 25 d after planting, depending on location. A GR corn density of 5.25 plant/m of crop row reduced late season cotton height by 49, 24, and 28% at Clayton, Lewiston–Woodville, and Rocky Mount, respectively, compared to weed-free cotton height. At Clayton, GR corn dry biomass per m crop row and GR corn seed biomass per m of crop row decreased linearly with increasing corn density. The relationship between GR corn and GR cotton yield loss was described by the rectangular hyperbola model with the asymptote (a) constrained to 100% maximum yield loss. The estimated coefficient i (yield loss per unit density as density approaches zero) was 9, 5, and 5 at Clayton, Lewiston–Woodville, and Rocky Mount, respectively. The examined GR corn densities had a significant effect on cotton yield, but not as significant as many other problematic grass and broadleaf weeds.

Field experiments were conducted to evaluate weed control and cotton response to glyphosate or glufosinate applied alone or with residual herbicides applied in the last POST-directed application (LAYBY) in glyphosate- and glufosinate-resistant cotton. Glyphosate (0.86 kg ae/ha) or glufosinate (0.47 kg ai/ha) were applied alone over the top of glyphosate- or glufosinate-resistant cotton early POST (EPOST) followed by (fb) late POST (LPOST) fb one of the herbicides applied either alone or with a residual herbicide at LAYBY. Glyphosate- and glufosinate-based treatments were applied only to glyphosate- and glufosinate-resistant cotton, respectively. Residual herbicides evaluated included prometryn (1.12 kg ai/ha), fluometuron (1.12 kg ai/ha), diuron (1.12 kg ai/ha), oxyfluorfen (1.12 kg ai/ha), pendimethalin (1.0 kg ai/ha), prometryn + trifloxysulfuron (1.33 kg ai/ha + 12 g ai/ha), or linuron + diuron (0.56 + 0.56 kg ai/ha). Glyphosate-and glufosinate-based weed management systems with and without residual LAYBY herbicides resulted in little to no injury to cotton. Three applications of glyphosate or glufosinate alone provided better full-season control of most species when compared to two applications of either herbicide. The addition of a residual herbicide to glyphosate or glufosinate at LAYBY did not improve cotton yields, but did improve overall control of barnyardgrass and yellow nutsedge and reduced weed dry biomass present at time of cotton harvest when compared to three applications of glyphosate or glufosinate alone.

Three studies were conducted to develop pollen tests for the screening of acetyl coenzyme-A carboxylase (ACCase) target-site resistance in a biotype of johnsongrass. The assays were based on germination of johnsongrass pollen in media supplemented with clethodim. Two different methods were used to evaluate pollen germination—a visual assessment and a spectrophotometric assay. The response of pollen to the germination media was linear for 16 h. At 6 h after treatment, absorbance at 500 nm was nearly 0.5; consequently, 6 h was chosen to conduct the pollen assays using the spectrophotometer. Both assessment methods differentiated the susceptible (S) and resistant (R) biotypes. Pollen from the susceptible biotype of johnsongrass was strongly inhibited by increasing concentrations of clethodim, with a GR50 of 25.8 ± 0.6 (SE) µM and GR50 of 16.4 ± 1.7 (SE) µM clethodim by visual assessment and spectrophotometric assessment, respectively. Minimum R/S values were > 3.9 by visual assessment and > 6.1 by spectrophotometric assessment. ACCase target-site resistance is expressed in johnsongrass pollen.

A reason given by cash-grain farmers for not using manure from neighboring livestock operations is that manure may cause greater field weediness. To address this concern, trials were established in corn on 11 cash-grain farms, in which manure from six nearby dairy farms was spread for the first time in at least 10 yr. A split-plot design was used in which manured and nonmanured treatments were established as whole-plots, and split-plot treatments were either with or without the farmer's regular weed control. In the multisite analysis, weed seedling density at the time of corn emergence was not greater in the manured vs. nonmanured treatments. At 7 to 8 wk following planting, weed density was not greater in the manured plots. Just before corn canopy closure, weed biomass also did not differ between manured and nonmanured treatments. Although neither weed species richness nor species diversity differed significantly between manured and nonmanured treatments, these measures did have significant environment-by-manure interactions, indicating that weed species distributions responded differently to manure across the different trial environments. However, farmers' weed control practices were highly successful in both the manured and nonmanured plots. Large portions (280 m2) of all whole plots were visually inspected for introduced weed species after all weed control practices had been completed. The manured treatments did not differ significantly in the set of species observed, suggesting that manure did not introduce new weed species. Thus, this exploratory study showed that, contrary to some farmers' concerns, an application of dairy manure neither increased field weediness nor required alterations in the farmers' weed control programs.

Forty-three Spanish populations of hairy fleabane, sampled from perennial crop locations, were studied under controlled and field conditions to confirm and characterize glyphosate resistance. In the initial screening, under controlled conditions, significant differences in glyphosate response between locations and among plant progenies within location were observed. From the initial screening, six populations (five potentially resistant [R] and one susceptible [S]) were selected, and a dose–response experiment was conducted to determine the resistance factor. The resistance factor was close to 10× for the most resistant population. In addition, the glyphosate response of R and S populations was dependant on phenological stages: the glyphosate rate required for control increased as a function of plant age. Finally, the resistance was confirmed with field trials conducted in five locations (one S and four R previously studied under controlled conditions). The field trials were also used to find alternative solutions for Spanish farmers to control resistant hairy fleabane populations.

Creeping bentgrass creates a dense, high-quality playing surface on golf courses, but it often encroaches adjacent areas of Kentucky bluegrass. Mesotrione can control creeping bentgrass in Kentucky bluegrass, but more information is needed regarding the effect of herbicide rate and number of applications on creeping bentgrass control and the impact to Kentucky bluegrass. Field experiments were conducted to determine the effect of application rate and number of applications on creeping bentgrass control. One application of mesotrione controlled 7 to 43% of creeping bentgrass in Kentucky bluegrass, and two applications of mesotrione controlled 39 to 88% as rates increased from 70 to 1,120 g ai/ha. Gaps present in the canopy after the creeping bentgrass died reduced overall turfgrass quality 2 to 6 wk after treatment (WAT) before recovering. These data indicate the capability of mesotrione to selectively control creeping bentgrass while providing excellent safety to Kentucky bluegrass.

Spring wheat tolerance to DE-750 applications was studied in Balcarce, Argentina, during 2002 and 2003. Rates of DE-750 evaluated were 6 and 12 g ae/ha tank-mixed either with metsulfuron at 4 g ai/ha or 2,4-D at 240 g ae/ha and applied at three different Zadoks growth stages: Zd 15/22, Zd 16/23, and Zd 32. The first two Zadoks stages corresponded to Nerson's elongated apex with two or more visible primordia and the glumes differentiation stage, respectively. Dicamba plus metsulfuron and picloram plus 2,4-D were included as comparative treatments and applied at the same growth stage. In both 2002 and 2003, Baguette 10 tolerated the DE-750 plus metsulfuron and DE-750 plus 2,4-D mixtures sprayed during the first two applications (Zd 15/22 and Zd 16/23). However, DE-750 plus metsulfuron (12 g/ha + 4 g/ha) and DE-750 plus 2,4-D (6 g/ha + 240 g/ha and 12 g/ha + 240 g/ha) sprayed during the last application (Zd 32) caused significant yield reductions in the 2002 spring season. On the other hand, picloram plus 2,4-D at the last growth stage reduced yield 39% in 2002 but did not affect yield in 2003. We suggest that an excess of precipitation after application and above-normal air temperatures during grain fill may have increased crop injury with herbicides applied at the last growth stage. In short, the effect of different DE-750 mixtures with metsulfuron or 2,4-D on wheat yield could be the result of the interactions among environmental conditions, and application timing.

Recent changes in herbicide registrations and governmental restrictions on field burning raised many management questions for Kentucky bluegrass seed producers, particularly the extent to which useful lives of their stands might be shortened by decreasing crop yields or increasing weed pressure. Tests conducted over the lives of two grass seed stands (1993–1997) evaluated three contrasting methods of postharvest residue management (vacuum sweep, bale/flail chop/rake, and field burn) and 13 herbicide treatments. Downy brome was the primary weed at both the Madras and LaGrande, OR, sites. In nontreated checks and the four least effective herbicide treatments, downy brome populations increased exponentially over time, with year-to-year increases in density averaging 13.1-fold. Competition had easily detected effects on Kentucky bluegrass seed yield at densities of 30 downy brome plants/m2, and crop stands were destroyed beyond 100 to 200 weeds/m2. Both PRE terbacil at 840 g/ha and early POST (EPOST)/late POST (LPOST) split-applied primisulfuron at 20 g/ha per application contained downy brome during the first 2 yr but not the third, when crop injury from terbacil forced reduction in terbacil rate and changes in weed populations overcame primisulfuron. PRE terbacil followed by LPOST primisulfuron, EPOST terbacil plus primisulfuron followed by LPOST primisulfuron, and EPOST/LPOST split-applied terbacil plus primisulfuron achieved excellent control of downy brome until the final years of the study, when control became increasingly erratic as primisulfuron-resistant downy brome proliferated in specific individual plots. Injury from combination terbacil plus primisulfuron treatments reduced yield relative to safest treatments in early years when downy brome population densities were low.

Research was conducted to evaluate absorption, translocation, and metabolism of foliar-applied trifloxysulfuron in flue-cured tobacco. The majority of 14C-trifloxysulfuron was absorbed by 4 h, with an accumulation in the plant of 43% of the radioactivity after 72 h. Translocation of radioactivity did not significantly differ between harvest timings of 4 to 72 h after treatment. Not more than 4% of applied 14C-trifloxysulfuron moved out of the treated leaves of tobacco, whereas less than 1.9% accumulated in any one part. Tobacco metabolized 14C-trifloxysulfuron rapidly, with 60.9% of the absorbed herbicide remaining in the parent herbicide form 4 h after treatment, whereas only 12.1% remained after 72 h. These data suggest that limited absorption and translocation, as well as rapid metabolism, are the basis for tobacco tolerance to foliar-applied trifloxysulfuron and illustrate the potential safe and effective use of trifloxysulfuron in tobacco for POST weed control.