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.

Nomenclature: Green foxtail, Setaria viridis (L.) Beauv. SETVI; wild oat, Avena fatua L. AVEFA.

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.

Nomenclature: Iodosulfuron-methyl-sodium (proposed), [methyl 4-iodo-2-[3-(4-methoxy-6-methyl-1,3,5-triazin-2-yl)-ureidosulfonyl]benzoate, sodium salt]; mesosulfuron-methyl (proposed), [methyl 2-[3-(4,6-dimethoxypyrimidin-2-yl)ureidosulfonyl]-4-methanesulfonamidomethylbenzoate]; blackgrass, Alopecurus myosuroides Huds. ALOMY.

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.

Nomenclature: Bispyribac; glyphosate; imazapyr; nicosulfuron; propanil; quinclorac; alexandergrass, Brachiaria plantaginea (Link) A. S. Hitchc. BRAPL; barnyardgrass, Echinochloa crus-galli (L.) Beauv. ECHCG; common wild sorghum, Sorghum verticilliflorum (Steud.) Stapf. and saccharatumS. (L.) Moench; goosegrass, Eleusine indica (L.) Gaertn. ELEIN; gulf cockspur, Echinochloa crus-pavonis (Kunth) J. A. Schultes. ECHCV; hedgehog dogtailgrass, Cynosurus echinatus L. CYXEC; hood canarygrass, Phalaris paradoxa L. PHAPA; Italian ryegrass, Lolium multiflorum Lam. LOLMU; itchgrass, Rottboellia cochinchinensis (Lour.) W. D. Clayton. ROOEX; johnsongrass, Sorghum halepense (L.) Pers. SORHA; junglerice, Echinochloa colona (L.) Link ECHCO; littleseed canarygrass, Phalaris minor Retz. PHAMI; Louisiana cupgrass, Eriochloa punctata (L.) Desv. ex Hamilt. ERBPO; Mexican grass, Ixophorus unisetus (Presl) Schult. SETUN; rigid ryegrass, Lolium rigidum Gaudin. LOLRI; saramollagrass, Ischaemum rugosum Salisb. ISCRU; southern crabgrass, Digitaria ciliaris (Retz.) Koel. DIGSP; sudangrass, Sorghum sudanense (Piper) Stapf. SORSU; weedy rice, Oryza sativa L. ORYSA; wild oat, Avena fatua L. AVEFA; canola, Brassica napus L; corn, Zea mays L; rice, Oryza sativa L; soybean, Glycine max (L.) Merr; wheat, Triticum aestivum L.

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.

Nomenclature: 2,6-bis[(4,6-dimethoxy-2-pyrimidinyl)oxy]benzoic acid, proposed common name bispyribac-sodium; clomazone; cyhalofop; fenoxaprop; imazamox; imazethapyr; molinate; pendimethalin; 2-(2,2-difluoroethoxy)-6-(trifluoromethyl-N-(5,8-dimethoxy[1,2,4]triazolo[1,5-c]pyrimidin-2-yl))benzenesulfonamide, proposed common name penoxsulam; propanil; quinclorac; thiobencarb; barnyardgrass, Echinochloa crus-galli (L.) Beauv. ECHCG; rice, Oryza sativa L; tomato, Solanum lycopersicum L.

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.

Nomenclature: Chlortoluron; clodinafop; diclofop; fenoxaprop-P; isoproturon; sulfosulfuron; littleseed canarygrass, Phalaris minor Retz. PHAMI; rice, Oryza sativa L; wheat, Triticum aestivum L.

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.

Nomenclature: Glyphosate, paraquat, glufosinate, triazine, atrazine, chloracetamide, bipyridiliums.

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.

Nomenclature: Common sunflower, Helianthus annuus L. HELAN; canola, Brassica napus L; soybean, Glycine max (L.) Merr; winter wheat, Triticum aestivum L.

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.

Nomenclature: Glyphosate; metolachlor; metribuzin; paraquat; soybean, Glycine max (L.) Merr.

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.

Nomenclature: Palmer amaranth, Amaranthus palmeri S. Wats. AMAPA; peanut, Arachis hypogaea L. ‘Perry’.

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.

Nomenclature: Glyphosate; corn, Zea mays L., ZEAMX, ‘DKC 69-71RR’; cotton, Gossypium hirsutum L. ‘FM 989RR’, ‘ST 4892RR’.

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.

Nomenclature: Diuron; fluometuron; glufosinate; glyphosate; linuron; oxyfluorfen; pendimethalin; prometryn; trifloxysulfuron; barnyardgrass, Echinochloa crus-galli (L.) Beauv. ECHCG; yellow nutsedge, Cyperus esculentus L. CYPES; cotton, Gossypium hirsutum L. ‘DPL 555BG/RR’, ‘Fibermax 958LL, 966LL, and 989RR’.

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.

Nomenclature: johnsongrass, Sorghum halepense (L.) Pers. SORHA.

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.

Nomenclature: Corn, Zea mays L.

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.

Nomenclature: Flazasulfuron; fluroxypyr; glyphosate; glufosinate; hairy fleabane, Conyza bonariensis (L.) Cronq. ERIBO.

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.

Nomenclature: Mesotrione; creeping bentgrass, Agrostis stolonifera L. AGSST; Kentucky bluegrass, Poa pratensis L. POAPR.

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.

Nomenclature: DE-750¹ (4-amino-3,6-dichloropyridine-2-carboxilic acid); dicamba; metsulfuron; picloram; 2,4-D butyl ester; spring wheat, Triticum aestivum L., ‘Baguette 10’.

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.

Nomenclature: Dicamba; metribuzin; oxyfluorfen; primisulfuron; terbacil; downy brome, Bromus tectorum L. BROTE; Kentucky bluegrass, Poa pratensis L. POAPR.

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.

Nomenclature: Trifloxysulfuron; flue-cured tobacco, Nicotiana tabacum L. ‘NC 72’.

Annual bluegrass is one of the most difficult-to-control weeds in creeping bentgrass putting greens. Field trials were conducted in 2003 and 2005 to evaluate bispyribac-sodium for annual bluegrass management in creeping bentgrass greens maintained at a 3 mm mowing height. Bispyribac-sodium applied weekly at 12 or 24 g ai/ha controlled annual bluegrass 86% 12 wk after initial treatment (WAIT). In 2003, bispyribac-sodium applied at 12 and 24 g/ha/wk injured creeping bentgrass approximately 15 and 50% by 4 WAIT, respectively. However, injury was transient and was not evident by 12 WAIT. In 2005, the 12 and 24 g/ha/wk injured creeping bentgrass 15 and 85% by 8 WAIT, respectively, and was still evident throughout the trial. Putting green quality was reduced when compared to nontreated creeping bentgrass by the same treatments. The removal of annual bluegrass caused soil exposure until creeping bentgrass grew over the bare areas, contributing to decreased quality evaluations. Management of annual bluegrass in creeping bentgrass putting greens is possible with bispyribac-sodium. However, these results indicate bispyribac-sodium can cause excessive injury when applied to creeping bentgrass mowed at 3 mm.

Nomenclature: Bispyribac-sodium; annual bluegrass, Poa annua L. POANN; creeping bentgrass, Agrostis stolonifera L. AGSST, ‘Penncross’.

Field studies were conducted during the 2002 thru 2004 growing seasons at two locations in the south-central Texas cotton-production region to evaluate trifloxysulfuron and trifloxysulfuron plus prometryn in combination with either S-metolachlor or glyphosate or both for weed control and cotton response. Cotton leaf burn (13 to 19%) was noted in 2002 at one location with trifloxysulfuron plus prometryn applied late POST-directed (LPDIR). Herbicide combinations that included trifloxysulfuron controlled barnyardgrass, hemp sesbania, yellow nutsedge, Palmer amaranth, smooth pigweed, ivyleaf morningglory, pitted morningglory, and smellmelon at least 80% in most instances. Glyphosate applied early POST over-the-top (EPOTT) and mid-POST over-the-top (MPOTT) at 0.84 kg ai/ha followed by trifloxysulfuron plus prometryn at 1.1 kg ai/ha applied LPDIR controlled the above-mentioned weeds plus Texas panicum at least 94%. No other herbicide systems provided effective control (greater than 79%) of Texas panicum. Higher cotton yields in 2002 were obtained with herbicide systems that included glyphosate alone or glyphosate plus S-metolachlor applied EPOTT or LPOTT followed by trifloxysulfuron plus prometryn applied LPDIR, whereas in 2003, none of the herbicide systems increased yield over the nontreated check.

Nomenclature: Glyphosate; S-metolachlor; trifloxysulfuron; prometryn; trifloxysulfuron; barnyardgrass, Echinochloa crus-galli L. ECHCG; hemp sesbania, Sesbania exaltata (Raf.) Rydb. Ex A.W. Hill SEBEX; ivyleaf morningglory, Ipomoea hederacea (L.) Jacq. IPOHE; Palmer amaranth S. Wats, Amaranthus palmeri S. Wats AMAPA; pitted morningglory, Ipomoea lacunose L. IPOLA; smellmelon, Cucumis melo L. var. Dudaim Naud CUMME; smooth pigweed, Amaranthus hybridus L., AMACH; Texas panicum, Panicum texanum Buckl. PANTE; yellow nutsedge, Cyperus esculentus L. CYPES; cotton, Gossypium hirsutum L. ‘Deltapine 422BRR’, ‘Deltapine 434RR’, ‘Deltapine 436RR’, ‘Deltapine 555RR’, ‘Stoneville 4892BR’.

Bioassay experiments were conducted to determine the phytotoxicity of methanol and ethyl acetate extracts of hairy vetch and cowpea residues on the germination and radicle elongation of three vegetable crops and three weed species. The species tested included common chickweed, redroot pigweed, wild carrot, tomato, corn, and cucumber. The extracts of both species were dissolved in methanol to yield seven concentrations ranging from 0 to 8 g/L. Germination was significantly reduced by methanol and ethyl acetate extracts of hairy vetch extracts except for corn and tomato. Common chickweed and wild carrot were the only species that showed consistent reduction in germination with the methanol and ethyl acetate cowpea extracts. The radicle growth of most species, with the exception of corn and cucumber, was reduced by the extracts of both cover crops. Corn and cucumber radicle elongation was stimulated at low concentrations of the extracts; however, these observations were not significantly different among treatments. This study demonstrated that methanol and ethyl acetate extracts of hairy vetch and cowpea contained allelopathic compounds and that their phytotoxicity is likely species specific. Future studies should focus on the identification and isolation of the allelochemical(s) found in the methanol and ethyl acetate extracts of the hairy vetch and cowpea residues.

Nomenclature: Hairy vetch, Vicia villosa Roth; cowpea, Vigna unguiculata (L.) Walp; common chickweed, Stellaria media (L.) Vill. STEME; redroot pigweed, Amaranthus retroflexus L. AMARE; wild carrot, Daucus carota L. DAUCA; corn, Zea mays L; cucumber, Cucumis sativus L; tomato, Lycopersicon esculentum Mill.

A callus induction and plantlet regeneration system for croftonweed was developed by studying the influence of explant type (leaf, stem, and nodal segment) and different concentrations of plant growth regulators. The leaf was a better explant for callogenesis compared to the stem. The highest callus induction frequency (87.2%) was obtained from leaf segments on Murashige and Skoog's medium (MS medium) supplemented with 0.5 mg/L (2,4-dichlorophenoxy)acetic acid and 2.0 mg/L 6-benzylaminopurine (BA), and 71.6% differentiation along with a multiplication rate of 4.1 adventitious shoots per callus was achieved with a combination of 0.5 mg/L 1-naphthaleneacetic acid (NAA) and 1.0 mg/L BA. In addition, MS medium supplemented with 0.5 mg/L NAA and 1.0 mg/L BA was the best medium for axillary shoot regeneration from nodal segments. Rhizogenesis of cultured shoots was satisfactorily obtained in half-strength MS without any growth regulators. The regenerated rooted plantlets were successfully acclimatized in soil where they grew normally without showing any morphological variation. These studies provide the prerequisite system for the development of genetic engineering in the future and propagating croftonweed rapidly for further study.

Nomenclature: Croftonweed; Ageratina adenophora (Spreng.) King & H. E. Robins. EUPAD.

Foramsulfuron has recently been registered for weed control in corn in Ontario, but there is very little information on the rate of foramsulfuron required to obtain at least 90% weed control. Our objective was to determine the foramsulfuron rates giving at least 90% weed control while maintaining crop yield loss due to weed interference and injury at less than 5%. Ten field trials were conducted at five Ontario locations (Exeter, Harrow, Ridgetown, Woodslee, and Woodstock) in 2001 and 2002 to evaluate the effectiveness of foramsulfuron at rates ranging from 8.75 to 140 g ai/ha. To obtain a reduction in biomass of 90% (I₉₀) at 78 d after treatment (DAT), foramsulfuron must be applied to common lambsquarters at 68 g/ha and to common ragweed at 86 g/ha, respectively. For green foxtail a foramsulfuron rate of 25 g/ha was required to achieve 90% control. The application of foramsulfuron caused injury to corn at 7 DAT at Ridgetown and Woodstock only, but did not exceed a rating of 10%; by 14 and 28 DAT no corn injury was recorded at any location. Corn yield of at least 95% of a weed-free check was obtained at Woodstock when foramsulfuron was applied at 70 g/ha. At Exeter and Woodslee yield was 90% of the weed-free check at a foramsulfuron rate of 35 g/ha. Finally, at Harrow and Ridgetown, corn yield was lowered at all foramsulfuron rates because of broadleaved weed interference. Tank-mixing foramsulfuron with dicamba plus prosulfuron improved common lambsquarters and common ragweed control and final corn yield was improved by more than 20% when compared with an application of foramsulfuron alone. Thus, these results show that weed control with foramsulfuron is species specific and that tank mixtures with a broadleaf herbicide may be required for broad-spectrum weed control and to protect the full yield potential of corn.

Nomenclature: Foramsulfuron; common ragweed, Ambrosia artemisiifolia L. AMBEL; common lambsquarters, Chenopodium album L. CHEAL; green foxtail, Setaria viridis (L.) Beauv. SETVI; corn, Zea mays L.

Studies were conducted to evaluate absorption and translocation of ¹⁴C-glyphosate in glyphosate-resistant (GR) cotton. Both commercial GR cotton events [glyphosate-resistant event 1, marketed as Roundup Ready®, released 1997 (GRE1), and glyphosate-resistant event 2, marketed as Roundup Ready Flex®, released 2006 (GRE2)] were evaluated at the four-leaf and eight-leaf growth stages. Plants were harvested at 1, 3, 5, and 7 d after treatment (DAT). Glyphosate absorption, as a percentage of applied, increased over time with 29 and 36% absorption at 7 DAT in four-leaf GRE1 and GRE2 cotton, respectively. In eight-leaf cotton, glyphosate absorption (33% at 7 DAT) was not different between events. Glyphosate translocation patterns were not different between events or harvest timings and exhibited a source–sink relation. Observed translocation differences between cotton growth stages were probably due to reduced glyphosate export from the treated leaf of eight-leaf cotton. An additional study compared absorption and translocation of ¹⁴C-glyphosate and ¹⁴C-sucrose in 5- and 10-leaf GRE2 cotton. Averaged over trials, ¹⁴C compounds, and growth stages, cotton absorbed 28% of the applied dose at 14 DAT. On the basis of the percentage of ¹⁴C exported out of the treated leaf, glyphosate and sucrose translocation patterns were similar, indicating that glyphosate may be used as a photoassimilate model in GRE2 cotton.

Nomenclature: Glyphosate; cotton, Gossypium hirsutum L.

Weed control and potato response to halosulfuron applied alone POST and with rimsulfuron or S-ethyl dipropyl carbamothioate (EPTC) were evaluated in 2004 and 2005 near Paterson, WA. Potatoes were injured and exhibited chlorosis and stunted growth after halosulfuron applications of 18, 26, and 35 g/ha. Potato height was reduced 33 and 20% in late May by halosulfuron at 18 or 26 g/ha in 2004 and 2005, respectively. Halosulfuron applied alone failed to control hairy nightshade and large crabgrass. Total tuber yield and U.S. no. 1 yield were reduced 10% in halosulfuron-treated plots because of poor weed control and possibly herbicide injury. Tank-mixing rimsulfuron with halosulfuron improved control of hairy nightshade and large crabgrass and increased potato yield. Tank-mixing EPTC at 2 kg/ha with halosulfuron improved early-season hairy nightshade control, but weed control was poor at row closure. Rimsulfuron applied alone at 18 or 26 g/ha controlled hairy nightshade and large crabgrass without potato injury and resulted in the greatest potato yields.

Nomenclature: Halosulfuron; rimsulfuron; hairy nightshade, Solanum sarrachoides Sendt. SOLSA; large crabgrass, Digitaria sanguinalis L. Scop. DIGSA; potato, Solanum tuberosum L. ‘Umatilla’.

Meadow foxtail is a rhizomatous grass widely grown for hay and pasture in wet meadows of the western United States and Canada. Two sulfonylurea herbicides, chlorsulfuron and metsulfuron-methyl, were evaluated for their effects on meadow foxtail biomass. Both herbicides were applied at four doses, 0.035, 0.070, 0.105, and 0.140 kg ai/ha, together with a control at each of two sites in October 2003. Treatments were replicated four times at each site and arranged in a randomized complete block design and sampled in July 2004 and 2005. Meadow foxtail biomass depended on site (P  =  0.001) or year (P  =  0.001), but not herbicide treatment (P  =  0.182). No biomass production losses resulted from applying up to 0.14 kg/ha of either chlorsulfuron or metsulfuron-methyl on meadow foxtail, even in relatively high-pH soils.

Nomenclature: Chlorsulfuron; metsulfuron-methyl; meadow foxtail, Alopecurus pratensis L.

Studies were conducted to determine injury potential to rotational crops from carryover of herbicides used in watermelon production. Treatments included halosulfuron, ethalfluralin, and sulfentrazone alone; halosulfuron in tank mixtures with bensulide, clomazone, ethalfluralin, and naptalam; and a tank mixture of naptalam and bensulide. Sulfentrazone applied at 224 g ai/ha to watermelon severely reduced spinach emergence, but did not reduce emergence of broccoli, cabbage, or wheat. Residues of sulfentrazone applied to watermelon at 450 g/ha stunted growth of broccoli and cabbage and was the only treatment that reduced wheat stand. Injury to broccoli, cabbage, and spinach increased as the halosulfuron rate increased. Ethalfluralin did not reduce stand or cause injury to any of the four rotational crops. Naptalam plus bensulide did not reduce stand of the four crops and caused either slight or no injury. Residues of sulfentrazone and halosulfuron can injure vegetables following crops in which these herbicides are used, and caution should be taken particularly with spinach, broccoli, and cabbage in this respect.

Nomenclature: Bensulide; clomazone; ethalfluralin; halosulfuron; naptalam; sulfentrazone; broccoli, Brassica oleracea var. botrytis (L.) ‘Everest’, ‘Green Sprouting Calabrese’; cabbage, Brassica oleracea var. capitata (L.) ‘Early Jersey Wakefield’; spinach, Spinacia oleracea (L.) ‘Cypress’, ‘F-380’; watermelon, Citrullus lanatus (Thunb.) ‘Jubilee’, ‘XIT 101’; hard red winter wheat, Triticum aestivum (L.) ‘Jagger’.

Two Italian ryegrass populations from Mississippi, Tribbett and Fratesi, were suspected to be tolerant to glyphosate. A third population from Mississippi, Elizabeth, known to be susceptible to glyphosate, was included for comparison. Plants were treated with the isopropylamine salt of glyphosate at 0, 0.11, 0.21, 0.42, 0.84, 1.68, 3.36, and 6.72 kg ae/ha. GR₅₀ (herbicide dose required to cause a 50% reduction in plant growth) values for the Tribbett, Fratesi, and Elizabeth populations were 0.66, 0.66, and 0.22 kg/ha, respectively, indicating that the Tribbett and Fratesi populations were threefold more tolerant to glyphosate compared with the Elizabeth population. These three populations were also treated with diclofop at 0, 0.13, 0.25, 0.5, 0.75, 1, and 2 kg ai/ha. Diclofop GR₅₀ values for the Tribbett, Fratesi, and Elizabeth populations were 0.25, 0.28, 0.21 kg/ha, respectively, indicating similar tolerance to diclofop in the three populations. Response of all three populations to clethodim rate (0, 0.02, 0.03, 0.05, 0.06, 0.08, 0.09, and 0.13 kg ai/ha) was measured. Clethodim GR₅₀ values for the Tribbett, Fratesi, and Elizabeth populations at the small growth stages were 0.016, 0.023, 0.014 kg/ha, respectively, and at the large growth stage were 0.04, 0.034, 0.02 kg/ha, respectively.

Nomenclature: Clethodim; diclofop; glyphosate; Italian ryegrass, Lolium multiflorum Lam. LOLMU.

Quinoclamine has been evaluated for POST control of liverwort in nursery crop production. Nearly all previous research has assumed that high spray volumes (e.g., > 935 L/ha) were required for quinoclamine to be effective. To test this assumption, quinoclamine was applied to liverwort in a factorial treatment arrangement of three rates (1.4, 1.9, and 3.8 kg ai/ha), three spray volumes (374, 1,112, and 1,871 L/ha), and two spray pressures (276 and 414 kPa). Control was influenced primarily by quinoclamine rate, although there was a trend for greater control with higher spray pressure. Absorption and translocation studies using ¹⁴C-quinoclamine established that ¹⁴C absorption into liverwort thalli approached 70% of the amount applied within 9 h after application. Although liverwort lacks vascular tissue, ¹⁴C is readily translocated away from the site of entry and tended to accumulate at thallus margins. Neither absorption nor translocation was influenced by spray volume. The perceived requirement for high spray volume may be a misdirected assumption on the basis of the high proportion of inert ingredients within the current formulation. Conversely, this assumption has no basis within liverwort biology or quinoclamine behavior.

Nomenclature: Liverwort, Marchantia polymorpha L; quinoclamine.

Transgenic, herbicide-resistant cultivars and equipment to spindle-pick 38-cm rows has renewed interest in narrow-row cotton production. Field experiments were conducted at four locations in North Carolina during 2004 and 2005 to evaluate weed management systems in glufosinate-resistant cotton planted in 38- and 97-cm rows. Weeds included broadleaf signalgrass, goosegrass, fall panicum, large crabgrass, Palmer amaranth, smooth pigweed, pitted morningglory, and tall morningglory. Greater than 90% control of annual grasses and Amaranthus spp. in 2004 and Ipomoea spp. in both years was obtained in narrow-row cotton with glufosinate applied early POST (EPOST) and mid-POST (MPOST) to two- and six-leaf cotton, respectively. With good early-season control by glufosinate and rapid canopy closure, there was little benefit from pendimethalin, fluometuron, or pyrithiobac applied PRE, S-metolachlor or pyrithiobac mixed with glufosinate applied MPOST, or trifloxysulfuron applied late POST (LPOST) to 11-leaf cotton in 2004. In 2005, with larger weeds at initial application, glufosinate EPOST and MPOST did not adequately control annual grasses and Amaranthus spp. Pendimethalin PRE increased control to greater than 90% and increased yields 59 to 75%. Pendimethalin PRE followed by S-metolachlor or pyrithiobac mixed with glufosinate at MPOST was no more effective than pendimethalin alone. Without PRE herbicides, trifloxysulfuron applied LPOST increased Amaranthus but not annual grass control. Cotton row spacing had no effect on cotton yield and little effect on weed control.

Nomenclature: Fluometuron; S-metolachlor; MSMA; pendimethalin; prometryn; pyrithiobac; trifloxysulfuron; broadleaf signalgrass, Brachiaria platyphylla (Griseb.) Nash BRAPP; fall panicum, Panicum dichotomiflorum Michx. PANDI; goosegrass, Eleusine indica (L.) Gaertn. ELEIN; large crabgrass, Digitaria sanguinalis (L.) Scop. DIGSA; Palmer amaranth, Amaranthus palmeri (S.) Wats. AMAPA; pitted morningglory, Ipomoea lacunosa L. IPOLA; smooth pigweed, Amaranthus hybridus L. AMACH; tall morningglory, Ipomoea purpurea (L.) Roth PHBPU; cotton, Gossypium hirsutum L. ‘FM 958LL’.

Field studies were conducted in 2002 and 2005 to evaluate autumn vegetable tolerance to residual herbicides applied the previous spring under low-density polyethylene (LDPE) mulch. Spring applications of 1.12 kg/ha S-metolachlor, 0.027 kg/ha halosulfuron, 0.28 kg/ha sulfentrazone, and 1.12 kg/ha S-metolachlor plus 0.027 kg/ha halosulfuron were made under LDPE mulch in March of each year and included a nontreated control. After removal of the spring crop, vegetables were planted the following August. Seeded and transplanted squash, seeded cucumber, transplanted eggplant, and transplanted cabbage were evaluated. Injury to eggplant, cucumber, and transplanted and seeded squash ranged from 8 to 16% for halosulfuron, sulfentrazone, and S-metolachlor plus halosulfuron in 2002, but no injury was observed in 2005. Cabbage injury was less than 5% for any herbicide treatment either year. There were no differences for cabbage biomass for three harvests for any herbicide treatment relative to the nontreated control. Vine length for cucumber and transplanted squash was significantly reduced by sulfentrazone relative to the nontreated control. Eggplant yield for the first harvest was significantly reduced by sulfentrazone as compared with the nontreated control in 2002 but not in 2005. To avoid injury to rotational crops, growers should read all herbicide labels when considering spring herbicide applications under LDPE mulch when autumn vegetable plantings are part of their production scheme to ensure successful crop production.

Nomenclature: Halosulfuron; S-metolachlor; sulfentrazone; cabbage, Brassica oleracea L; cucumber, Cucumis sativus L; eggplant, Solanum melongena L; squash, Cucurbita pepo L.

Viable horseradish roots of various sizes remain in the soil after harvest and can develop into volunteer plants in subsequent crops. Experiments were conducted to evaluate POST herbicides on volunteer horseradish control and to determine if efficacy is dependent upon horseradish root segment size, herbicide rate, horseradish cultivar, or horseradish shoot size at application. In the greenhouse, horseradish root segment size did not affect herbicide efficacy. Chlorimuron, cloransulam, imazamox, (2,4-dichlorophenoxy)acetic acid (2,4-D) amine, halosulfuron, and imazethapyr plus imazapyr provided greater than 95% foliar control of volunteer horseradish. Chlorimuron, halosulfuron, and 2,4-D amine were also among the herbicides that provided the greatest reduction in horseradish root biomass (69% or greater). Glyphosate provided little foliar control (76%) and root biomass reduction (57%) after one application. The efficacy of 2,4-D amine on horseradish foliage and root biomass increased with increasing herbicide rate; however, the response of horseradish to halosulfuron was similar for all rates evaluated. Root biomass reduction of the horseradish cultivar ‘1573’ was less responsive to 2,4-D amine and halosulfuron applications compared with ‘1038’ and ‘1722’. However, foliar injury from 2,4-D amine and halosulfuron was less for the horseradish cultivar ‘1038’ compared with ‘1573’ and ‘1722’. In field studies, 2,4-D amine applied to 15- and 30-cm-tall horseradish and halosulfuron applied to 15-cm-tall horseradish resulted in the greatest foliar and root biomass reduction. This study indicated that in-season control of volunteer horseradish in rotational crops may be achieved through proper herbicide selection.

Nomenclature: Chlorimuron; clopyralid; cloransulam-methyl; 2,4-D amine; dicamba; diflufenzopyr; flumetsulam; glyphosate; halosulfuron; imazamox; imazapyr; imazethapyr; mesotrione; primisulfuron; corn, Zea mays L. ‘Pioneer 33P69LL’; horseradish, Armoracia rusticana (Gaertn., Mey., Scherb.); soybean, Glycine max (L.) Merr.

Conservation tillage systems, used successfully by cotton producers on the Texas southern High Plains, have facilitated the development of new weed problems including horseweed (Conyza canadensis) and Russian thistle (Salsola iberica). Studies were conducted in 2004 and 2005 near Lubbock, TX to evaluate winter weed control with (2,4-dichlorophenoxy)acetic acid (2,4-D), dicamba, and diflufenzopyr plus dicamba. All of these herbicides have current registration restrictions limiting their use in cotton. Cotton response studies were initiated in 2003 and repeated in 2004 and 2005 to evaluate cotton injury and yield from dicamba (0.14 and 0.28 kg ai/ha), diflufenzopyr plus dicamba (0.10 and 0.20 kg/ha), and 2,4-D (0.56 and 1.12 kg/ha) applied 4, 2, and 1 wk before planting (WBP); and to determine the minimum interval between application of these herbicides and cotton planting without affecting yield. 2,4-D controlled both horseweed and Russian thistle, and could be applied as close as 2 WBP without injuring cotton. However, dicamba as well as diflufenzopyr plus dicamba were less effective on horseweed and Russian thistle as size increased and both injured cotton regardless of interval between application and planting.

Nomenclature: Dicamba; diflufenzopyr plus dicamba; 2,4-D; horseweed, Conyza canadensis (L.) Cronq. ERICA; Russian thistle, Salsola iberica Sennen & Pau SASKR; cotton, Gossypium hirsutum, L.

In previous research, use of PRE soil residual herbicides was reduced 50% in no-till corn and soybean by banding herbicides over crop rows followed by mowing weeds growing between rows two times. The research goals were (1) to determine whether such between-row mowing systems adequately controlled weeds and prevented grain yield loss in other competitive field crops, such as no-till grain sorghum, and (2) to compare broadcast herbicide treatments with between-row mowing systems. PRE atrazine plus dimethenamid at relative rates of 0.75× and 1× (where 1×  =  1.7 plus 1.3 kg ai/ha, respectively) were band-applied over rows shortly after planting followed by two between-row mowings close to the soil surface. In 2 of 3 yr in Missouri, this system controlled giant foxtail and common waterhemp as well as broadcast herbicides in no-till sorghum. In 2 of 3 yr, between-row mowing systems also prevented yield loss in no-till sorghum as well as both broadcast herbicides at the same rates and the weed-free check.

Nomenclature: Atrazine; dimethenamid; giant foxtail, Setaria faberii (L.) Beauv. SETVI; common waterhemp, Amaranthus rudis Sauer. AMATA; corn, Zea mays L. ZEAMX; sorghum, Sorghum bicolor (L.) Moench SORVU ‘Northup King GS10’and ‘Pioneer 84G62’; soybean, Glycine max (L.) Merr.

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.

Nomenclature: Tetrazolium, 2,3,5 triphenyl tetrazolium chloride; giant foxtail, Setaria faberi Herrm. SETFA; green foxtail, Setaria viridis (L.) Beauv. SETVI; yellow foxtail, Setaria glauca (L.) Beauv. SETLU.

Guar production in the United States is limited to a relatively small region in the semiarid southern Great Plains of Texas and Oklahoma. The lack of POST broadleaf herbicides is a potential limiting factor to increased production. A greenhouse study was initiated in 2001 at the Texas A&M Research Center near Vernon, TX to evaluate guar tolerance to 10 POST herbicides typically used in soybean or cotton. Guar seedlings were grown in pots, and herbicides with appropriate adjuvants were applied to 3-wk-old seedlings at the registered rate (1×) and twice (2×) the registered rate for soybean or cotton. The study was repeated twice, with six replications in each run. Twenty-eight d after treatment (DAT), visual injury and aboveground dry weight of viable biomass were recorded for each plant. Significant differences (P  =  0.05) were noted among herbicides for visual injury and viable biomass. Little or no differences in visual injury and aboveground dry weight were observed between the control (no herbicide applied) and the 1× rate of 4-(2,4-dichlorophenoxy)butanoic acid, bentazon, or imazethapyr 28 DAT. A 1× application rate of acifluorfen, imazamox, thifensulfuron, or bromoxynil caused minor visual injury of 7 to 9% and a reduction in dry weight of 8 to 23%. Pyrithiobac and chlorimuron caused 38 and 47% visible injury and a 35 and 58% reduction in dry weight, respectively. Guar was most sensitive to lactofen, with the 1× rate causing 100% visual injury and no recoverable aboveground biomass. This greenhouse study identified three POST herbicide candidates with potential to control broadleaf weeds in guar without noticeable plant injury, and offers data to support herbicide registrations for this minor crop.

Nomenclature: Acifluorfen; bentazon; bromoxynil; chlorimuron; imazamox; imazethapyr; lactofen; pyrithiobac; thifensulfuron; 2,4-DB; cotton, Gossypium hirsutum L; guar, Cyamopsis tetragonoloba (L.) Taub ‘Kinman’; soybean, Glycine max (L.) Merr.

Field surveys were conducted to evaluate the prevalence of stalk-boring insects in giant ragweed in Indiana and Michigan soybean fields. Greenhouse studies were also conducted to determine whether stalk-boring insects had a negative impact on control of giant ragweed with glyphosate. In the June 2005 field surveys, 18 to 30% of all giant ragweed plants sampled contained stalk-boring insects or insect tunnels. Languriidae, Noctuidae, Pyralidae, and Tortricidae families were found most often at the time glyphosate was being applied to soybean fields to control giant ragweed. Cerambycidae and Curculionidae families were typically found later in the season after herbicide applications were completed. In the August field surveys in Indiana, 28 to 62% of the giant ragweed plants that showed evidence of stalk-boring insects were not controlled by POST herbicide applications suggesting that control was compromised by the presence of stalk-boring insects. In greenhouse studies, glyphosate efficacy on 15-cm-tall giant ragweed was enhanced by the presence of stalk-boring insects; however, glyphosate efficacy on 45-cm plants was reduced by the presence of stalk-boring insects. Overall, this research suggests that there is a possibility that stalk-boring insects could reduce glyphosate efficacy on giant ragweed.

Nomenclature: Giant ragweed, Ambrosia trifida L. AMBTR; soybean, Glycine max (L.) Merr.; Lepidoptera: Noctuidae; Lepidoptera: Tortricideae; Lepidoptera: Pyralidae; Coleoptera: Cerambycidae; Coleoptera: Curculionidae; Coleoptera: Languriidae; European corn borer, Ostrinia nubilalis (Hubner).

Growers and certified crop advisors (CCAs) across Indiana were surveyed during the winter of 2003 to 2004 to assess their perceptions about soybean cyst nematode (SCN) and use of SCN management practices. Most farmers (57%) and CCAs (72%) surveyed reported a moderate to high level of concern regarding SCN and its potential impact on soybean yield. The majority of those surveyed were also aware that some winter annual weeds can serve as hosts for SCN. Crop management practices specifically aimed at managing the impact of SCN were employed by 55 and 78% of growers and CCAs, respectively. However, only 21% percent of growers said that they had sampled a field for nematodes within the last two years. Growers from eastern and southern Indiana were less likely to be concerned about SCN, to implement SCN management strategies, and to have the soil tested for SCN than growers throughout the rest of the state. In addition, smaller farmers appear to be less concerned and knowledgeable about SCN than those who operate larger farms. The results of this survey suggest that the majority of Indiana growers would likely adopt winter weed control to manage SCN. Also, with respect to winter weed control, future Extension efforts should be focused on southern Indiana where both the risk for SCN reproduction on winter annuals and the need for education on SCN appear to be highest.

Nomenclature: Soybean, Glycine max (L.) Merr; soybean cyst nematode, Heterodera glycines Ichinohe.

Field trials were conducted in 2004 and 2005 to identify sulfonylurea (SU) herbicides that would provide improved weed control, minimal soil residual, and crop safety to SU-resistant chicory. SU-resistant chicory had previously been selected in vitro for resistance to chlorsulfuron. Our research evaluated three commercial, nonresistant and three breeding lines of SU-resistant chicory. Each of the cultivars was treated POST at the two true-leaf growth stage with either foramsulfuron, rimsulfuron, rimsulfuron plus thifensulfuron, tribenuron, thifensulfuron, thifensulfuron plus tribenuron, triflusulfuron, flumetsulam, or imazamox at normal use rates. Established plant densities and root yields of SU-resistant chicory breeding lines were greater than or equal to the densities and root yields of commercial cultivars. The plant density of commercial chicory cultivars was reduced by rimsulfuron, rimsulfuron plus thifensulfuron, tribenuron, and thifensulfuron plus tribenuron, but SU-herbicides did not reduce the density of SU-resistant breeding lines. SU-resistant chicory differed in cross-resistance to SU-herbicides, with tribenuron causing the most crop injury and thifensulfuron, the least. Weed control varied between the SU-herbicides. The greatest reduction in weed biomass occurred with tribenuron, thifensulfuron plus tribenuron, and rimsulfuron plus thifensulfuron; the least reduction occurred with triflusulfuron, foramsulfuron, and rimsulfuron. Chicory root yields comparable to the hand-weeded treatment were achieved with rimsulfuron plus thifensulfuron and thifensulfuron plus tribenuron treatments. The SU herbicides that met the initial project objectives of crop tolerance and improved weed control were combinations of rimsulfuron plus thifensulfuron and thifensulfuron plus tribenuron.

Nomenclature: Imazamox; flumetsulam; foramsulfuron; rimsulfuron; thifensulfuron; tribenuron; triflusulfuron; chicory, Cichorium intybus L. ‘Beryl’, ‘Orchies’, ‘Turquoise’, ‘FD 28’, ‘FD 29’, ‘FD 30’.

Corn and soybean growers across Indiana were surveyed to assess their perceptions about the importance of preplant and POST weed control timing, focusing mainly on soybean production. Despite studies demonstrating the importance of planting into a clean field, almost a third of Indiana growers do not think it is important to plant into a weed-free seedbed and 74% do not use residual herbicides in glyphosate-resistant soybean production systems. Growers who farmed less than 200 ha were more likely to overestimate the ability of soybean to tolerate weed interference than growers who farmed more hectares. Growers who manage smaller farms were also more likely to use a one-pass weed control program than larger growers. This suggests that yield losses to weed interference may be greater for smaller farms than for larger farms. Weed size and density were the most common criteria used by growers to decide when to apply herbicides. This suggests that field scouting plays an important role in the decision-making process of growers. However, a substantial proportion of growers apply POST herbicides to large common lambsquarters and giant ragweed in an attempt to minimize the number of trips across the field for weed control. Delayed control of these species likely contributes to reduced crop yields, higher application rates, and to the survival of treated plants. Opportunities to improve control and increase yields through more optimal herbicide use appear possible for Indiana corn and soybean growers.

Nomenclature: Glyphosate; corn, Zea mays L; soybean, Glycine max (L.) Merr.

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.

Nomenclature: Glyphosate; corn, Zea mays L; soybean, Glycine max (L.) Merr.

The vast majority of crop hectares in the United States are treated with chemical herbicides annually. The adoption of herbicides for weed control was rapid in the 1950s and 1960s. Herbicides replaced the use of millions of workers to pull and hoe weeds by hand and greatly reduced the use of tillage for weed control. Costs of production were reduced and crop yields increased because herbicides were cheaper and more effective than hand weeding and cultivation. Organic crop growers cite weed control as their greatest difficulty in crop production because they are not permitted the use of chemical herbicides. They substitute hand weeding and cultivation for herbicides at a greatly increased cost and with reduced effectiveness. Aggregate studies that estimate the value of herbicides assume that growers would substitute a certain amount of hand weeding and tillage if chemicals were not used, which would not be sufficient to prevent yield losses totaling about 20% of U.S. crop production.