Multilocational characterization of selected herbaceous and shrub legumes was carried out over a 3-yr period at lowland and midlatitude sites in the moist savanna agroecological zone in Nigeria where a south–north gradient exists for rainfall, length of growing season, and insolation. One study site each was located in coastal–derived savanna (DS), southern Guinea savanna (SGS), and northern Guinea savanna (NGS). A fourth site was located in the plateau of the NGS. Only one legume cover crop, lablab, produced adequate ground cover and good weed suppression in three locations irrespective of rainfall amount, duration, and distribution. Velvetbean was superior to other legume cover crops in the lowland savanna locations where rainfall exceeded 1,100 mm/yr but not in the plateau of the NGS where rainfall was less than 1,000 mm. Pigeonpea grew luxuriantly and produced canopy cover that effectively suppressed weeds in the DS where rainfall was high. Centurion grew well and suppressed weeds effectively only in the SGS site where rainfall of 1,120 mm was well distributed over a 5-mo period. Sunnhemp grew well and suppressed weeds only in the NGS study sites where rainfall duration was 4 mo and the length of the growing period was short. Weed density was negatively correlated (P < 0.05) with percentage ground cover of those legume cover crops that were effective in weed suppression in all locations. Redundancy analysis revealed significant associations between weed species and cover crops at each of the locations.

Nomenclature: Centurion, Centrosema pascuorum Mart. Ex Benth. #³ CENPA; lablab, Lablab purpureus (L.) Sweet # LABPU; pigeonpea, Cajanus cajan (L.) Millsp. # CAJCA; sunnhemp, Crotalaria ochroleuca G. Don # CROOC; velvetbean, Mucuna pruriens (L.) DC. var. utilis (Wright) Burck # MUCPR.

Additional index words: Legume ground cover, weed density, weed species composition.

Abbreviation: WAP, weeks after planting; DS, coastal–derived savanna; SGS, southern Guinea savanna; NGS, northern Guinea savanna; RDA, redundancy analysis.

Diphenylether herbicides may be viable options for postemergence (POST) control of common waterhemp in soybean. A 2-yr field research project was conducted to determine whether common waterhemp control is influenced by application timing and rate of acifluorfen, fomesafen, and lactofen. Common waterhemp control was 9, 9, and 8% greater 7, 14, and 21 d after treatment, respectively, after the early postemergence (EPOST) application timing compared with the POST application timing. Lactofen provided greater common waterhemp control than did acifluorfen or fomesafen, and only the highest application of lactofen provided greater than 85% common waterhemp control 21 d after POST application. No significant differences in common waterhemp dry weight were determined among the three rates of acifluorfen, fomesafen, and lactofen applied EPOST. The highest application rates of fomesafen and lactofen reduced common waterhemp dry weight more than did the lowest application rates applied POST. The highest application rate of fomesafen also reduced common waterhemp dry weight more than did the intermediate application rate. Single degree of freedom contrasts indicated that all diphenylether herbicides reduced common waterhemp dry weight more than did imazethapyr.

Nomenclature: Acifluorfen; fomesafen; imazethapyr; lactofen; common waterhemp, Amaranthus rudis Sauer #³ AMATA; soybean, Glycine max (L.) Merr.

Additional index words: Postemergence, herbicide application rate, herbicide application timing, weed control.

Abbreviations: ALS, acetolactate synthase; COC, crop oil concentrate; DAT, days after treatment; EPOST, early postemergence; NIS, nonionic surfactant; POST, postemergence.

Field research conducted for 3 yr evaluated weed control in glyphosate-resistant soybean with glyphosate applied alone and in mixtures with reduced rates of the broadleaf herbicides acifluorfen, CGA-277476 (proposed name: oxasulfuron), chlorimuron, fomesafen, or lactofen. Barnyardgrass was controlled at least 95% with glyphosate at 840 and 1,120 g ai/ha, and control was not antagonized with any of the mixtures. At 14 d after treatment (DAT), wild poinsettia and prickly sida were controlled at least 90% with all glyphosate treatments. Control remained greater than 90% 28 DAT in one of the 2 yr, but in the other year, wild poinsettia was controlled 80%, and prickly sida was controlled 43% with the high rate of glyphosate. Acifluorfen or fomesafen (210 and 315 g ai/ha), or lactofen (112 and 168 g ai/ha) applied with glyphosate increased wild poinsettia control to 91 to 95% and prickly sida to 60 to 83%. At 14 DAT, pitted morningglory control improved in most cases when acifluorfen, fomesafen, or lactofen was applied with glyphosate. Hemp sesbania control was increased when glyphosate was applied with these same herbicides as well as CGA-277476 (39 and 59 g ai/ha) and chlorimuron (4.5 and 6.7 g ai/ha). But by 28 DAT, improvement in weed control with glyphosate and broadleaf herbicide mixtures was noted only in 1998, when pitted morningglory and hemp sesbania were larger at time of application (15 to 18 cm). Glyphosate alone in 1998 controlled these weeds no more than 30%, and only for hemp sesbania was control consistently more than 80% with the addition of acifluorfen or lactofen. At 28 DAT in 1999 and 2000, when weeds were no more than 10 cm, pitted morningglory was controlled 91 and 98%, respectively, and hemp sesbania 88 and 100%, respectively, with glyphosate alone at 1,120 g/ha. Soybean injury 28 DAT, consisting of foliar chlorosis and height reduction, was as much as 14, 13, and 23% for the acifluorfen, chlorimuron, and lactofen treatments, respectively, but no more than 6% for the CGA-277476 and fomesafen treatments. When weed control with glyphosate alone was at least 85%, addition of broadleaf herbicides did not improve soybean yield.

Nomenclature: Acifluorfen; CGA-277476 (2-[[[[(4,6-dimethyl-2-pyrimidinyl)amino]carbonyl]amino]sulfonyl]-3-oxetanyl ester); chlorimuron; fomesafen; glyphosate; lactofen; barnyardgrass, Echinochloa crus-galli (L.) Beauv. #³ ECHCG; hemp sesbania, Sesbania exaltata (Raf.) Rydb. ex. A. W. Hill # SEBEX; pitted morningglory, Ipomoea lacunosa L. # IPOLA; prickly sida, Sida spinosa L. # SIDSP; wild poinsettia, Euphorbia heterophylla L. # EPHHL; soybean, Glycine max (L.) Merr. ‘Asgrow 5901 RR’.

Additional index words: Herbicide mixtures.

Abbreviations: DAT, days after treatment.

A field study was conducted during 1999, 2000, and 2001 at Stoneville, MS, on a Dundee silty clay loam to determine the impact of a rye cover crop with one or two postemergence (POST) herbicide applications on weed control, yield, and net return in narrow-row glyphosate-resistant, glufosinate-resistant, and conventional soybean systems. Cover crop systems included no–cover crop conventional tillage (CT), no–cover crop no-tillage (NT), and rye NT, all with early POST (EPOST), EPOST followed by late POST (LPOST), and no-herbicide weed management. Weed control and net return among glyphosate-resistant, glufosinate-resistant, and conventional soybean systems were similar. One POST ($111/ha) application of herbicides was more profitable than two POST ($79/ha) applications regardless of soybean cultivar and cover crop system. Rye residue reduced total weed density by 9 and 27% and biomass by 19 and 38% compared with no–cover crop CT and NT, respectively. In the rye cover crop, input costs were higher because of the additional cost of seed, planting, and rye desiccation. The additional cost resulted in a lower net return with the rye cover crop ($29/ha) compared with the no–cover crop CT ($84/ha) or NT ($87/ha) system, even though soybean yield in the rye cover crop system was comparable to that from the no–cover crop CT and NT systems. These results showed that because of additional cost, rye cover crop–based soybean production was less profitable compared with existing no–cover crop–based production systems.

Nomenclature: Acifluorfen; bentazon; chlorimuron; clethodim; glufosinate; glyphosate; barnyardgrass, Echinochloa crus-galli (L.) Beauv. #³ ECHCG; browntop millet, Brachiaria ramosa (L.) Stapf # PANRA; hemp sesbania, Sesbania exaltata (Raf.) Rydb. ex A. W. Hill # SEBEX; pitted morningglory, Ipomoea lacunosa L. # IPOLA; prickly sida, Sida spinosa L. # SIDSP; sicklepod, Senna obtusifolia (L.) Irwin and Barneby # CASOB; smooth pigweed, Amaranthus hybridus L. # AMACH; rye, Secale cereale L. ‘Elbon’; soybean, Glycine max (L.) Merr. ‘DP 3588’, ‘DP 5806 RR’, ‘A 5547 LL’.

Additional Index words: Conventional tillage, herbicide, integrated weed management, mulch, net return, no-tillage, transgenic soybean, weed biomass, weed density.

Abbreviations: CT, conventional tillage; EPOST, early postemergence; LPOST, late postemergence; NT, no-tillage; POST, postemergence; WAP, weeks after planting.

Greenhouse and field experiments were conducted to evaluate the efficacy of nicosulfuron, primisulfuron, glyphosate, glufosinate, imazethapyr plus imazapyr, and quizalofop on johnsongrass biomass reduction, rhizome total nonstructural carbohydrate (TNC) content, and subsequent regrowth from rhizomes. In the greenhouse, johnsongrass plants originating from rhizome segments were controlled 88 to 97% with quizalofop, glyphosate, imazethapyr plus imazapyr, nicosulfuron, and primisulfuron and 56% with glufosinate 3 wk after treatment (WAT). Johnsongrass treated with quizalofop, glyphosate, and nicosulfuron did not regrow 6 WAT, whereas plants treated with primisulfuron, imazethapyr plus imazapyr, and glufosinate regrew from the rhizome of the treated plant. Rhizome TNC levels 3 WAT were not reduced by glufosinate or nicosulfuron, but they were reduced 64% by quizalofop, 32% by primisulfuron, 61% by glyphosate, and 29% by imazethapyr plus imazapyr. When rhizome TNC was reduced by 60% or more compared with nontreated plants, johnsongrass did not regrow from the treated rhizomes. In field experiments, nicosulfuron and glyphosate controlled johnsongrass 94 and 99%, respectively, whereas imazethapyr plus imazapyr (79%) and glufosinate (85%) provided less control 6 WAT.

Nomenclature: Glufosinate; glyphosate; imazapyr; imazethapyr; nicosulfuron; primisulfuron; quizalofop; johnsongrass, Sorghum halepense (L.) Pers. #³ SORHA.

Additional index words: Biomass reduction, carbohydrate analysis, corn yield, fresh weight.

Abbreviations: DAT, days after treatment; POST, postemergence; TNC, total nonstructural carbohydrate; WAT, weeks after treatment.

Evaluations of the effects of minimum tillage vs. conventional tillage and the effects of organic amendments (cover crops and compost) vs. no organic amendments were conducted in a California vegetable field. Weed densities were monitored, and soil samples were taken to measure the effects of the treatments on weed seedbanks and microbial biomass over a 24-mo period. Reduced tillage increased the density of shepherd's-purse in the upper soil layer (0 to 15 cm) of the soil seedbank compared with conventional tillage. Evidence is presented that suggests relationships between organic amendments, weed population reductions, and increases in soil microbial biomass: (1) shepherd's-purse emergence and seedbank densities were lower in the organic amendment plots, (2) microbial biomass was nearly always higher in the organic amendment plots, and (3) significant negative correlations between microbial biomass and burning nettle and shepherd's-purse emergence densities were found. These results suggest that organic matter addition may lead to reduced weed emergence.

Nomenclature: Burning nettle, Urtica urens L. #³ URTUR; shepherd's-purse, Capsella bursa-pastoris L. Medik. # CAPBP.

Additional index words: Alternative weed management, compost, cover crop, microbial biomass carbon, soil amendments, CAPBP, URTUR.

Seeds were collected from field-grown glufosinate-resistant rice treated at various growth stages from the one-leaf through the boot stage with glufosinate at 0.84 kg ai/ha. The collected seeds were evaluated for 100-count seed weights, germination, and seedling vigor. Germination of the long-grain transformant CPRS PB-13 at five temperatures was unaffected by glufosinate treatment or temperature. At 14 d after initiation, germination of the medium-grain transformant BNGL HC-11/62 was reduced at 22 C when treated at the preboot timing. One hundred–count seed weights and seedling vigor were not affected for either glufosinate-resistant line.

Nomenclature: Glufosinate; rice, Oryza sativa L., ‘BNGL HC-11/62’, ‘CPRS PB-13’.

Additional index word: Liberty-Link rice.

Abbreviations: AOSA, Association of Official Seed Analysts; DAI, days after initiation; POST, postemergence.

Field experiments were conducted in north-central Oklahoma to evaluate the effects of MON 37500 at 35, 70, and 140 g ai/ha applied to wheat on rotational crops seeded no-till 16 to 29 mo after treatment (MAT). Wheat yields were not reduced by MON 37500 at site 1 but at site 2 were decreased 6, 11, and 24% by 35, 70, and 140 g/ha, respectively. Wheat yield reductions at site 2 were attributed to late seeding, small wheat growth stage, and cool, wet weather during the month after application. Corn and soybean seeded approximately 16 MAT were not visibly injured by residual MON 37500 at any rate at either site. At site 1 grain sorghum seeded 17 MAT was visibly injured by residual MON 37500 applied at 70 and 140 g/ha (soil pH value 6.4), and grain yield was reduced 58% by residual MON 37500 applied at 140 g/ha. Grain sorghum seeded 17 MAT was not visibly injured at site 2 (soil pH value 5.0), and that seeded 29 MAT was not injured at either site. Sunflower seeded 17 MAT at site 1 was visibly injured by residual MON 37500 at 140 g/ha, and yield was reduced 17% but at site 2 was not affected by MON 37500. Sunflower seeded 29 MAT was not injured at either site.

Nomenclature: MON 37500, 1-(4,6-dimethoxypyrimidin-2-yl)-3-(2-ethanesulfonyl-imidazo[1,2-a]pyridine-3-yl)sulfonylurea; corn, Zea mays L. ‘Dekalb 566 RR’; grain sorghum, Sorghum bicolor (L.) Moench ‘Dekalb 36’, ‘Pioneer 8500’; soybean, Glycine max L. ‘Asgrow 3701 RR’; sunflower, Helianthus annuus L. ‘Dekalb 3872’, ‘Pioneer 6451’; winter wheat, Triticum aestivum L. ‘2163’, ‘Tonkawa’.

Additional index words: Crop injury, precipitation, soil pH, sulfosulfuron.

Abbreviations: MAT, months after treatment.

Field experiments were conducted in 1996 and 1997 to evaluate the tolerance of imidazolinone-resistant (IR) and non-IR corn cultivars to preemergence (PRE) and postemergence (POST) treatments of diclosulam. Crop injury was evaluated early- (5 to 6 wk after planting [WAP]), mid- (10 to 11 WAP), and late-season (13 to 15 WAP). Early-season injury of IR corn was no more than 12% in systems that included diclosulam PRE or POST at 18, 27, or 36 g ai/ha. Early-season injury of non-IR corn ranged from 85 to 89% in systems that included diclosulam PRE at any rate. At the mid-season evaluation, crop injury to IR corn was 1% or less. Non-IR corn was injured 73 to 94% in systems that included diclosulam PRE, while systems that included diclosulam POST caused 45 to 58% injury at mid-season. At the late-season evaluation, non-IR corn was injured 56, 88, and 96% with diclosulam PRE at 18, 27, and 36 g/ha, respectively, whereas systems that included diclosulam POST had 11 to 14% injury. Injury to IR corn from diclosulam PRE or POST was not apparent at the late-season evaluation. Weed-free yield of IR corn treated with diclosulam was 6,490 to 6,850 kg/ha and was equivalent to or better than yield from IR corn treated only with atrazine plus metolachlor PRE. Yield from non-IR corn treated with any diclosulam-containing system did not exceed 3,770 kg/ha.

Nomenclature: Atrazine; diclosulam; metolachlor; corn, Zea mays L. ‘Pioneer 3242’, ‘Pioneer 3223’.

Additional index words: Crop injury, crop tolerance, discoloration, stand reduction, stunting.

Abbreviations: ALS, acetolactate synthase [EC 4.1.3.18]; IR, imidazolinone-resistant; POST, postemergence; PRE, preemergence; WAP, weeks after planting.

Field and greenhouse studies were conducted near Painter, VA, in 1999, 2000, and 2001 to evaluate mesotrione postemergence for control of horsenettle in corn. Mesotrione at 105 g ai/ha controlled horsenettle at least 80% in all studies, and in 2001, after two consecutive annual applications, mesotrione controlled horsenettle up to 91%. Additions of primisulfuron, dicamba, and 2,4-D to mesotrione did not increase horsenettle control. Occasionally, combinations of dicamba with mesotrione controlled horsenettle less than did mesotrione alone, and primisulfuron combinations with mesotrione delayed or reduced development of bleaching symptoms associated with mesotrione. Initial horsenettle response to mesotrione was increased by addition of 280 g ai/ha atrazine; however, late-season horsenettle control was not improved by atrazine. Two consecutive annual applications of mesotrione alone decreased horsenettle biomass > 89%. Treatments of primisulfuron plus dicamba, primisulfuron plus CGA 152005 plus dicamba, and 2,4-D plus dicamba provided similar horsenettle control and biomass reductions as did 105 g/ha mesotrione alone.

Nomenclature: Atrazine; CGA 152005 [1-(4-methoxy-6-methyl-triazin-2-yl)-3-[2-(3,3,3-trifluoropropyl)-phenylsulfonyl]-urea]; dicamba; halosulfuron; mesotrione; primisulfuron; 2,4-D; horsenettle, Solanum carolinense L. #³ SOLCA; corn, Zea mays L.

Additional index words: Bleaching herbicides, perennial weeds, triketone herbicides.

Abbreviations: COC, crop oil concentrate; POST, postemergence; PRE, preemergence; UAN, urea ammonium nitrate; WAT, weeks after treatment; YAT, years after treatment.

Some herbicides applied to corn or soybean can carry over and injure vegetable crops grown the following year. Injury and yield effects on vegetable crops from carryover of nicosulfuron and flumetsulam 1 yr after application to corn and from carryover of imazethapyr and imazamox 1 yr after application to soybean were evaluated. Experiments were conducted at Oakes, ND, on sandy loam soils with pH 7.7 in 1994 and pH 7.3 in 1996. Nicosulfuron and flumetsulam were applied to corn, and imazethapyr was applied to soybean at a standard herbicide rate (1×) (35, 56, and 70 g ai/ha, respectively) or 2× rates in 1994 and at 1×, 2×, or 4× rates in 1996. Imazamox was also applied to soybean at 1× (35 g ai/ha), 2×, or 4× rates in 1996. Cabbage, carrot, potato, onion, acorn squash, and tomato were planted in 1995, 1997, and 1998. In 1995, residual imazethapyr delayed tomato maturity but did not reduce tomato yield. Other vegetable crops were not injured by herbicide residues. In 1997, flumetsulam carryover injured plants and reduced yields of cabbage and squash; nicosulfuron carryover injured cabbage and onion plants and reduced onion yield; and imazethapyr carryover injured cabbage, onion, and tomato plants and reduced tomato yield. Most crop injury occurred only at the 2× or 4× herbicide rates. Potato and carrot were not injured by herbicide carryover. Herbicides applied to corn or soybean in 1996 did not carry over to injure any vegetable planted in 1998. The low rate of injury to vegetable crops in this study was mostly due to adequate soil moisture and warm summer soil temperatures, which enhanced microbial degradation of these herbicides and hastened their dissipation. Low clay and moderately low soil organic matter, and slightly alkaline soil pH, also promoted dissipation of some herbicides.

Nomenclature: Flumetsulam; imazamox; imazethapyr; nicosulfuron; cabbage, Brassica oleracea L. var. capitata (L.); carrot, Daucus carota L.; corn, Zea mays L.; onion, Allium cepa L.; potato, Solanum tuberosum L.; soybean, Glycine max (L.) Merr.; squash, Cucurbita maxima L.; tomato, Lycopersicon esculentum Mill.

Additional index words: Crop rotation, herbicide carryover, herbicide dissipation in soils, herbicide residues.

Abbreviations: OM, soil organic matter; ×, standard herbicide rate.

Growth and growth rate characteristics of diffuse knapweed and bluebunch wheatgrass seedlings were evaluated under two temperature regimes, 10 and 16 C, and two moisture regimes, − 0.01 and − 0.03 MPa, in an environmental chamber. Intra- and interspecific interference were evaluated at 10 C and at either − 0.01 or − 0.03 MPa. Root–leaf ratios for bluebunch wheatgrass were greater than for diffuse knapweed at − 0.01 MPa. Under moist conditions, root penetration was greater for diffuse knapweed than for bluebunch wheatgrass roots; penetration was equal in warm or dry conditions. Wheatgrass root length was greater than knapweed length under dry conditions. Intraspecific interference was greater than interspecific interference for both species. Leaf area data and root–leaf ratios suggest that diffuse knapweed is the faster-growing species under moist conditions.

Nomenclature: Bluebunch wheatgrass, Pseudoroegneria spicata (Pursh) Scribn. & Smith; diffuse knapweed, Centaurea diffusa Lam. #³ CENDI.

Additional index words: Competition, growth rate.

Common cocklebur is an adaptable and competitive weed with variable morphology. To learn how the rate of net photosynthesis (Pn) by common cocklebur relates to traits that may influence competitiveness, we compared Pn, accumulation of biomass (shoot mass, root mass, and total plant mass), and total leaf area among six accessions of common cocklebur grown in a greenhouse. There were highly significant (P ≤ 0.01) differences among accessions for all measured traits. Correlations of each measure of biomass with total leaf area were positive and highly significant, but correlations of Pn with each biomass measure and with total leaf area were negative. The negative correlations were largely a result of relatively low biomass accumulation by the two accessions with the highest Pn. This contrasted with results from a previous experiment on field-grown common cocklebur plants, in which the correlation of Pn with shoot mass was positive (r = 0.64). Despite the negative Pn-to-biomass correlations in the greenhouse study, the rank among accessions for Pn in the greenhouse was nearly the same as with the field-grown plants (Spearman rank correlation r = 0.89). We conclude that the relative Pn rates among common cocklebur accessions grown in the greenhouse may be used to predict their relative Pn rates in the field, but relationships of Pn with biomass or leaf area observed in the greenhouse may not be a reliable indicator of relationships in the field.

Nomenclature: Common cocklebur, Xanthium strumarium L. #³ XANTH.

Additional index words: Biotype, competitiveness, leaf area, photosynthesis, stomatal density.

Abbreviations: CDT, Central Daylight Time; Pn, net photosynthetic rate; PPFD, photosynthetic photon flux density; TOD, time of day.

Field studies were conducted in 1999 and 2000 at Marianna and Gainesville, FL, to evaluate the response of three runner-type peanut cultivars, ‘Georgia Green’, ‘C-99R’, and ‘MDR-98’, to flumioxazin applied preemergence at 0, 71, 105, and 211 g/ha in a weed-free environment. Peanut exhibited excellent tolerance to flumioxazin, regardless of flumioxazin rate or peanut cultivar, at Gainesville in 1999 and both locations in 2000. In 1999, at Marianna, flumioxazin caused early-season stunting and season-long reduction in peanut canopy width. Peanut response was independent of cultivar and did not exceed 25%, with an increase in stunting with higher flumioxazin rates. Peanut stunting was associated with cool and extremely wet growing conditions during the first 2 mo after planting in 1999 at Marianna. Peanut yield and grade parameters, in both years, were not affected by flumioxazin treatment.

Nomenclature: Flumioxazin; peanut, Arachis hypogaea L. ‘Georgia Green’, ‘C-99R’, ‘MDR-98’.

Additional index words: Cultivar, extra large kernels, grade analysis, other kernels, sound mature kernels, sound splits, total sound mature kernels, yield.

Abbreviations: DAP, days after planting; ELK, extra large kernels; OK, other kernels; PPI, preplant incorporated; PPO, protoporphyrinogen oxidase; PRE, preemergence; SMK, sound mature kernels; SS, sound splits; TSMK, total sound mature kernels.

Yellow starthistle, spotted knapweed, and meadow hawkweed are three competitive noxious weeds that collectively infest about 640,000 ha of range and noncropland in Idaho. The purpose of this study was to determine the control of these three noxious weeds and undesirable annual grasses with imazapic applied at different rates and growth stages. Imazapic applied fall preemergence, spring postemergence (POST), or fall plus spring POST and picloram applied fall and spring POST reduced yellow starthistle plant density 75 to 100% compared with the untreated control, at about 100 d after spring treatment (DAST). However, by 440 DAST, yellow starthistle plant density did not differ from the untreated control in plots treated with imazapic, whereas picloram reduced plant density 82 to 99%. Imazapic did not control meadow hawkweed or spotted knapweed, whereas picloram reduced plant density of both species 91 to 100% 60 DAST compared with the untreated control and 71 to 100% 440 DAST. Imazapic did not affect weedy annual grass plant density compared with the untreated control at the yellow starthistle or the spotted knapweed sites. However, at the meadow hawkweed sites, imazapic applied spring POST at 210 g/ha and fall plus spring POST at 70 plus 140 g/ha reduced grass plant density 77 to 84% 60 DAST. All treatments that contained spring-applied imazapic reduced grass plant density 46 to 92%, 440 DAST at the meadow hawkweed site. At all locations, picloram increased weedy annual grass plant density two- to eightfold 440 DAST compared with the untreated control. Imazapic offers only moderate control of weedy annual grasses for a brief period after application and does not provide control of these three major noxious weed species in the West. Unfortunately, it eliminates a fairly new rangeland herbicide from the list of potential tank mix partners for control of auxinic herbicide–resistant weed species.

Nomenclature: Imazapic; picloram; meadow hawkweed, Hieracium caespitosum Dumort. #³ HIECA; spotted knapweed, Centaurea maculosa Lam. # CENMA; yellow starthistle, Centaurea solstitialis L. # CENSO.

Additional index words: Biomass, grass species, plant density.

Abbreviations: DAST, days after spring treatment; DAT, days after treatment; POST, postemergence; PRE, preemergence.

Effective, long-term wild oat management requires an integrated approach that uses management techniques beyond simple herbicide application. A 5-yr (1996 to 2000) zero tillage study was conducted to assess the influence of barley harvest timing on wild oat densities in subsequent years at Lacombe, AB, Canada and Melfort, SK, Canada. Harvest timings included barley harvested 1 wk after heading (early), approximately 14 to 16 d later at the soft dough stage (normal), and at maturity (grain). In the absence of herbicides, wild oat densities decreased in silage plots harvested early and increased in grain plots. Reductions were more distinct at Lacombe where barley phenological differences and whole plant moisture contents between early and normal silage harvests were greater than at Melfort. Half rates of wild oat herbicides (ICIA 0604 and imazamethabenz) did not augment reductions in wild oat densities after early silage harvest, but did improve wild oat management after normal silage harvest, and in grain production. At Lacombe, early silage harvest reduced wild oat densities more than did herbicides in grain production. Similar trends were apparent at Melfort but were not statistically significant. Early barley silage harvests may be an effective integrated weed management tool for wild oat.

Nomenclature: ICIA 0604 (proposed common name: tralkoxydim), 2-[1-(ethoxyimino)propyl]-3-hydroxy-5-(2,4,6-trimethylphenyl)cyclohex-2-enone; imazamethabenz; wild oat, Avena fatua L. #³ AVEFA; barley, Hordeum vulgare L. (‘AC Lacombe’).

Additional index words: Cultural control, green forage, integrated weed management, seed rain, timing.

Field studies were conducted in 1999, 2000, and 2001 to determine the effectiveness of mesotrione applied preemergence (PRE) or postemergence (POST) in no-till corn. Also, a proposed prepackage mix of mesotrione plus acetochlor (1:11 ratio of mesotrione–acetochlor) in combinations with the trimethylsulfonium salt of glyphosate (glyphosate-TMS), paraquat, and 2,4-D was investigated. Mesotrione PRE at 235 g ai/ha or greater controlled common lambsquarters, smooth pigweed, and common ragweed at least 80%. POST mesotrione at 35 g/ha and higher controlled common lambsquarters 91% or greater. Mesotrione applied POST at 140 g/ha controlled smooth pigweed greater than 97%. Common ragweed control from POST mesotrione was inconsistent, ranging from 56 to 97%. PRE and POST applications of mesotrione did not adequately control goosegrass, giant foxtail, fall panicum, johnsongrass, or cutleaf eveningprimrose. The mesotrione plus acetochlor prepackage mix plus glyphosate-TMS or paraquat controlled field pansy and ivyleaf morningglory similar to or better than did the prepackage mixture of the isopropylamine salt of glyphosate (glyphosate-IPA) plus atrazine plus acetochlor. But common ragweed control by mesotrione plus acetochlor plus glyphosate-TMS or paraquat was occasionally lower than control by the prepackage mixture of glyphosate-IPA plus atrazine plus acetochlor. Corn injury was generally less than 10% with PRE and POST mesotrione applications.

Nomenclature: Acetochlor; atrazine; 2,4-D; glyphosate-Ipa (isopropylamine salt of glyphosate); glyphosate-TMS (trimethylsulfonium salt of glyphosate); mesotrione; paraquat; common lambsquarters, Chenopodium album L. #³ CHEAL; common ragweed, Ambrosia artemisiifolia L.# AMBEL; cutleaf eveningprimrose, Oenothera laciniata Hill # OEOLA; fall panicum, Panicum dichotomiflorum Michx. # PANDI; field pansy, Viola arvensis Murr. # VIOAR; giant foxtail, Setaria faberi Herrm. # SETFA; goosegrass, Eleusine indica (L.) Gaertn # ELEIN; ivyleaf morningglory, Ipomoea hederacea (L.) Jacq. # IPOHE; johnsongrass, Sorghum halepense (L.) Pers. # SORHA; smooth pigweed, Amaranthus hybridus L. # AMACH; corn, Zea mays L.

Additional index words: Bleaching herbicides, burndown, nonselective herbicides, triketone herbicides.

Abbreviations: COC, crop-oil concentrate; DAT, days after treatment; fb, followed by; PRE, preemergence; POST, postemergence; UAN, urea ammonium nitrate; WAT, weeks after treatment.

Studies on weed management systems with bromoxynil-resistant, glyphosate-resistant, and nontransgenic cotton cultivars in a no-tillage environment were evaluated in North Carolina and Tennessee in 1997 and 1998. All weed management systems in nontransgenic and bromoxynil-resistant cotton controlled sicklepod 72 to 78%, whereas sicklepod was controlled at least 94% in glyphosate-resistant cotton. Entireleaf and pitted morningglory, smooth pigweed, and common cocklebur were controlled at least 92% with all management systems. Fiber quality was also measured in Tennessee. Micronaire and fiber strength ranged from 4.35 to 4.55 units and 28.6 to 30.1 g/tex, respectively, in bromoxynil-resistant and glyphosate-resistant cultivars that received postemergence over-the-top (POT) herbicides. Harvest trash content ranged from 0.80 to 1.15% in systems where POT and late postemergence–directed (LAYBY) herbicides were used and was 1.65% where multiple glyphosate applications were used with no LAYBY herbicides. Cotton lint yield and net returns were generally highest in systems that included bromoxynil or pyrithiobac POT followed by cyanazine plus MSMA LAYBY or multiple glyphosate treatments where no LAYBY herbicides were used.

Nomenclature: Bromoxynil; cyanazine; glyphosate; MSMA; pyrithiobac; common cocklebur, Xanthium strumarium L. #³ XANST; entireleaf morningglory, Ipomoea hederacea var. integriuscula Gray # IPOHG; pitted morningglory, Ipomoea lacunosa L. # IPOLA; sicklepod, Senna obtusifolia (L.) Irwin and Barnaby # CASOB; smooth pigweed, Amaranthus hybridus L. # AMACH; cotton, Gossypium hirsutum L. ‘Stoneville 474’, ‘Stoneville BXN 47’, ‘Deltapine 5415RR’, ‘Paymaster 1220RR’.

Additional index words: Color grade, extraneous material, fiber length, fiber strength, fiber uniformity, harvest trash, high-volume instrumentation, gin turn out, leaf grade, micronaire, staple length.

Abbreviations: ASN, as needed; LAYBY, late postemergence–directed; fb; followed by; PDS, postemergence-directed spray; PRE, preemergence; POT, postemergence over-the-top.

Eight sweet corn cultivars were evaluated for tolerance to AE F130360 in five field experiments conducted over 2 yr in Ontario. AE F130360 was applied postemergence at 70 and 140 g ai/ha, the latter rate representing a spray overlap in the field. Response to AE F130360 varied with respect to sweet corn cultivar and herbicide rate. Minimal crop injury was observed in seven of the eight cultivars. One cultivar, ‘DelMonte 2038’, was extremely sensitive, showing 94% or more injury for all site-years. Plant height of most cultivars was not negatively affected by AE F130360 treatments, with the exception of DelMonte 2038, which sustained height reductions of 78% or greater compared with the untreated control. The crop injury and height reductions that were observed in DelMonte 2038 were reflected in the marketable yields, which were reduced by 92% or more as a result of the AE F130360 treatments. Marketable yields also tended to be reduced in ‘Calico Belle’ and ‘Rival’. On the basis of marketable yields, it was concluded that ‘CNS 710’, ‘GG 222’, ‘GG 246’, ‘GH 2684’, and ‘Reveille’ have full tolerance to AE F130360, Calico Belle and Rival tend to have moderate tolerance, and DelMonte 2038 has zero tolerance to AE F130360.

Nomenclature: AE F130360 (proposed common name, foramsulfuron), N,N-dimethyl-2-[3-(4,6-dimethoxypyrimidin-2-yl)ureidosulfonyl]-4-formylaminobenzamide; sweet corn, Zea mays L.

Additional index words: Cultivar, herbicide injury, sensitivity, sulfonylurea.

Abbreviations: DAT, days after treatment; POST, postemergence; SAS, Statistical Analysis Systems; SU, sulfonylurea.

There are few efficient and cost-effective methods for controlling weeds in processing tomatoes. Sulfosulfuron is a sulfonylurea herbicide developed for controlling weeds in wheat. In previous studies, we have demonstrated the efficacy of sulfosulfuron in selectively controlling Orobanche aegyptiaca in tomato. The objective of the present study was to elucidate the potential of sulfosulfuron to selectively control troublesome, nonparasitic weeds in tomato. In the greenhouse, sulfosulfuron efficacy at 37.5, 75.0, and 112.5 g ai/ha applied preplant incorporated (PPI), preemergence (PRE), and postemergence (POST) was tested. Sulfosulfuron when applied PPI and POST was highly selective in controlling weeds without causing injury to tomato. The weeds that were efficiently controlled, even at low rates of application, included purple nutsedge, black nightshade, mustard, pigweed, and bindweed. PRE application resulted in the most efficient weed control but was phytotoxic to tomato at high rates.

Nomenclature: Sulfosulfuron; black nightshade, Solanum nigrum L. #³ SOLNI; field bindweed, Convolvulus arvensis L., # CONAR; purple nutsedge, Cyperus rotundus L. # CYPRO; redroot pigweed, Amaranthus retroflexus L. # AMARE; tomato, Lycopersicon esculentum Mill. # LYPES; wheat, Triticum aestivum L.; wild mustard, Sinapis arvensis L. # SINAR.

Additional index words: Acetolactate synthase inhibitors, sulfonylurea herbicides, weed management.

Abbreviations: ALS, acetolactate synthase; DAA, days after application; DAP, days after planting; POST, postemergence; PPI, preplant incorporated; PRE, preemergence.

Model simulations predict that lowering herbicide efficacy by reducing the application rate would slow the rate of enrichment of herbicide-resistant individuals in a weed population, but the resulting increase in density of susceptible plants would reduce crop yield and increase the weed seed bank. A study was conducted at three sites in Saskatchewan, Canada, from 1997 to 2000 to examine the implication of reduced rates of acetyl-CoA carboxylase (ACCase) inhibitors in a diverse 4-yr crop rotation, in conjunction with variable crop seeding rates, on the enrichment of resistant wild oat in a mixed (resistant and susceptible) population. Main-plot treatments were crop (barley, canola, field pea, and spring wheat), subplot treatments were crop seeding rate (recommended and high), and sub-subplot treatments were ACCase inhibitor rate (0, 0.33, 0.67, and 1.0 times the recommended rate). Herbicide rate frequently interacted with seeding rate in affecting wild oat seedling density, seed return, the viable fraction of the weed seed bank, and crop seed yield. As simulation models predict, reduced herbicide efficacy decreased the proportion of resistant individuals in the population. The high crop seeding rate compensated for a one-third reduction in herbicide rate by limiting total wild oat seed return and by reducing the number of resistant seedlings recruited from the seed bank. The level of resistance in the seed bank can be reduced without increasing the total (resistant plus susceptible) seed bank population by manipulating agronomic practices to increase crop competitiveness against wild oat when ACCase inhibitor rates are reduced to a maximum of two-thirds of that recommended.

Nomenclature: Wild oat, Avena fatua L. #³ AVEFA; barley, Hordeum vulgare L. ‘AC Oxbow’; canola, Brassica napus L. ‘AC Excel’; field pea, Pisum sativum L. ‘Grande’; spring wheat, Triticum aestivum L. ‘AC Barrie’.

Additional index words: Crop seeding rate, herbicide resistance, ICIA 0604, integrated weed management, sethoxydim.

Abbreviations: ACCase, acetyl-CoA carboxylase (EC 6.4.1.2); ALS, acetolactate synthase (EC 4.1.3.18); HR, herbicide resistant; HS, herbicide susceptible; OM, organic matter.

Field experiments were conducted at Lacombe, Lethbridge, and Vegreville, Alberta, Canada and Kalispell, MO, over several years to determine the influence of recommended (minimum label) and lower-than-recommended tralkoxydim rates on wild oat seed production, spring wheat yield, and economic return. Wild oat seed production as a function of tralkoxydim rate varied considerably among locations and years. For example, at the recommended rate, wild oat seed production varied from none at both Lethbridge and Vegreville in 1994 to over 800 seeds/m² at Vegreville in 1995. At 50% of recommended rate, seed production varied from none at Lethbridge in 1994 to over 3,000 seeds/m² at Vegreville in 1995. In most cases, wheat yield response to tralkoxydim rate was curvilinear. Yields generally increased exponentially as rates increased up to about 40 or 50% of the recommended rate, but then plateaued as rates were increased further. In some cases economic returns tended to plateau or decrease at rates higher than this, but reductions in economic returns at the recommended herbicide rates were, in most cases, relatively slight. In contrast, at Lethbridge in 1993 and 1995 and at Kalispell in 1994 and 1996, yield and economic returns generally increased as herbicide rate increased, and there was an economic disadvantage to reducing the tralkoxydim rate below that recommended. In view of the variable effects on wild oat seed production, and the questionable economic benefit, our study suggests that reducing the rate of tralkoxydim below that recommended may not be without risk. Further studies are necessary to determine the long-term implications of returning relatively large amounts of wild oat seed to the soil seedbank at reduced herbicide rates.

Nomenclature: Tralkoxydim; wild oat, Avena fatua L. #³ AVEFA; wheat, Triticum aestivum L. ‘Hi-line’, ‘Katepwa’, ‘Newana’, ‘Roblin’.

Additional index words: Reduced herbicide rates.

At 14 d after treatment (DAT), glufosinate-resistant CPRS PB-13 rice was injured with early- and late-season glufosinate applications in 1998 and 1999. At 35 DAT, injury was less than 5%. Glufosinate delayed CPRS PB-13 heading by 7 to 15 d and reduced plant height at harvest but did not affect grain moisture and yield compared with the nontreated CPRS PB-13. But yield was reduced compared with conventional Cypress. At 14 DAT, glufosinate-resistant BNGL HC-11/62 rice was injured with early- and late-season applications. Treated BNGL HC-11/62 reached 50% heading 3 to 5 d later than did the nontreated. An application of glufosinate reduced plant height and increased grain moisture compared with the nontreated. Rice treated at the three- to five-leaf, preboot, and boot timings resulted in reduced yields compared with the nontreated BNGL HC-11/62.

Nomenclature: Glufosinate; rice, Oryza sativa L. ‘BNGL HC-11/62’, ‘Cypress’, ‘CPRS PB-13’.

Additional index words: Application intervals, injury, tolerance.

Abbreviations: DAT, days after treatment; POST, postemergence.

Imazamox is an imidazolinone herbicide being developed for weed control in imidazolinone-resistant wheat (IMI-wheat) cultivars and various legume crops. In a series of studies conducted under a range of dryland cropping environments in the Pacific Northwest United States, imazamox applied to IMI-wheat or pea injured barley and canola grown 1 yr after imazamox treatment in low-rainfall, low–soil pH locations of Oregon. Injury was not observed in higher rainfall locations near Pullman, WA. Non–herbicide-resistant wheat planted 1 yr after IMI-wheat treated with imazamox was not injured. Of particular concern for imazamox carryover are low-rainfall areas with low-pH soils. Reduced soil moisture appears to limit imazamox degradation. Imazamox sorption is reduced in low-pH soils, which increases its bioavailability, thereby increasing the potential for injury to rotational crops such as barley, canola, and spring wheat.

Nomenclature: Imazamox; barley, Hordeum vulgare L.; canola, Brassica napa L.; pea, Pisum sativum L.; wheat, Triticum aestivum L.

Additional index words: Barley, canola, carryover, Clearfield™, herbicide-resistant wheat, pea.

Abbreviations: IMI-wheat, imidazolinone-resistant wheat; PNW, Pacific Northwest.

At locations in Argentina and the United States, solaria (miniature, portable, plastic greenhouses or a plastic sheet approximately 1 m²) were placed on field soils in autumn or late winter in an attempt to predict summer annual weed densities. Initial emergence of summer annual weeds covered by solaria commenced weeks before that of weeds in exposed seedbeds. Cumulative emergence of many species in solaria reached asymptotes before crops were sown. At asymptotic cumulative emergence, densities of dominant weeds in solaria (common lambsquarters, green foxtail, and large crabgrass) were correlated with weed densities occurring 4 wk after sowing, the typical time for making postemergence weed control decisions. These results indicate that solaria may supplement seedbank-sampling techniques for predicting weed densities in crops.

Nomenclature: Common lambsquarters, Chenopodium album L. #³ CHEAL; green foxtail, Setaria viridis (L.) Beauv. # SETVI; large crabgrass, Digitaria sanguinalis (L.) Scop. # DIGSA.

Additional index words: Decision making, emergence, postemergence control, seedlings, solarization.

Abbreviations: CWD, in-crop weed density; OM, organic matter; PPI, preplant incorporated; PRE, preemergence; SWD, weeds inside solaria; TWD; threshold weed density.

Greenhouse studies were conducted to evaluate halosulfuron tolerance of several squash and cucumber cultivars commonly grown in Georgia. There was an inverse linear relationship between squash plant biomass and rate of halosulfuron (r² = 0.70 to 0.92). With the exception of ‘Supersett’, the slopes from regression of all squash cultivars were equivalent. The estimated amount of halosulfuron required to reduce growth by 20%, based on regression, ranged from 8.2 to 45 g ai/ha (for Supersett and ‘Dixie’, respectively). Squash plant height was also reduced by halosulfuron, though plants began to recover from the injury by the end of the study. There was no effect of halosulfuron rate on cucumber plant biomass or height. Cucumber cultivars appeared to be more tolerant to halosulfuron than did squash cultivars.

Nomenclature: Halosulfuron; cucumber, Cucumis sativus L.; squash, Cucurbita pepo L.

Additional index words: Cyperus esculentus, Cyperus rotundus, methyl bromide alternatives, MON 12037, purple nutsedge, vegetable weed management, yellow nutsedge.

Abbreviations: DAP, days after planting; GR₂₀, herbicide rate required to reduce crop growth 20%; POST, postemergence; PRE, preemergence; PRE–POST, half rate PRE followed by half rate POST.

Corn injury from isoxaflutole has been reported in sites across the corn belt, and variation in tolerance to isoxaflutole has been observed among corn hybrids. Greenhouse trials were conducted to study the variation and the inheritance of isoxaflutole tolerance in corn. Inbred lines from two families were treated with isoxaflutole applied preemergence, at four times the typical use rate, to study plant response. Plant height of corn inbreds treated with isoxaflutole compared with the untreated control ranged from 47 to 96% among eight lines within the B73 family and from 54 to 97% among six lines within the C103 family. One tolerant and one sensitive inbred line were selected from each family. The inbred lines were crossed in the field to produce four F₁ hybrids. The four hybrids were treated with isoxaflutole in the greenhouse, and the tolerance of each hybrid was compared with the parent inbred lines. Data indicate that sensitivity to isoxaflutole is a heritable trait that is mediated by nuclear genes. Tolerance of corn hybrids to isoxaflutole can be predicted from the response of the parent inbreds.

Nomenclature: Isoxaflutole (5-cyclopropyl-4-(2-methylsulfonyl-4-trifluoromethylbenzoyl) isoxazole); corn, Zea mays L.

Common ragweed is an annual weed problem after winter wheat harvest in southwest Michigan. Although an interseeded cover crop of red clover is known to reduce weed populations in winter wheat stubble, the most effective rates and cultivars for weed suppression under Michigan conditions have not been identified. Three red clover cultivars were planted in March at three seeding rates in established winter wheat; after wheat harvest, a section of each plot was mowed to mimic forage harvest of clover. The experiment was repeated in 2 yr. Mowing significantly reduced common ragweed biomass each year. All cultivars and seeding rates were equally effective at significantly reducing common ragweed biomass in each year, despite the variation among years, cultivars, seeding rates, and mowing treatments in production of clover biomass.

Nomenclature: Common ragweed, Ambrosia artemisiifolia L. ;ns³ AMBEL; red clover, Trifolium pratense L.; winter wheat, Triticum aestivum L.

Additional index words: Cover crops, weed management.

Field trials were conducted in Georgia and South Carolina in 2000 and 2001 to evaluate the effects of glyphosate, diflubenzuron, and boron combinations on crop injury and soybean yield. No interactions among glyphosate, diflubenzuron, and boron were observed. Both glyphosate and boron caused significant foliar injury (chlorosis and leaf burn) ranging from 7 to 10% in one of three locations. Diflubenzuron did not cause significant soybean injury at any location. Soybean injury symptoms observed in these experiments were transient and did not negatively affect yield.

Nomenclature: Boron; diflubenzuron, N-[[(4-chlorophenyl)amino]carbonyl]-2,6-difluorobenzamide; glyphosate; soybean, Glycine max (L.) Merr. ‘Hartz 7550RR’.

Additional index words: Compatibility, crop tolerance, glyphosate-resistant soybean, pesticide combinations, tank-mixtures, transgenic soybean.

Abbreviations: DAT, days after treatment; POST, postemergence.

Two field experiments were conducted to evaluate the reduction of torpedograss canopy by multiple split applications of quinclorac applied postemergence (POST) to bermudagrass golf course roughs in Florida. In one experiment, quinclorac treatments were reapplied for a second year to the same plots, followed by biomass harvest, to evaluate the reduction of torpedograss rhizomes. Quinclorac sprayed at 1.68 kg/ha/yr visually reduced torpedograss canopy to a varying extent, depending on the number of split applications. The most effective treatment, 0.42 kg/ha quinclorac applied four times each year for 2 yr, reduced torpedograss canopy to 10% compared with 86% torpedograss canopy in the untreated plots, and reduced torpedograss dry weight to 1,570 kg/ha compared with 8,010 kg/ha in the untreated plots. After reapplication for 2 yr with the commercially labeled treatment, quinclorac at 0.84 kg/ha applied twice per year, torpedograss canopy was reduced to 45% and dry weight to 4,640 kg/ha. Visual evaluation of canopy was too optimistic in representing the herbicidal control of torpedograss by quinclorac because torpedograss regrew from rhizomes, and canopy was a relatively small part of the plant. In plots not chemically treated, pachymorph rhizomes were 63%, leptomorph rhizomes were 24%, and leaves were only 13% of the total dry weight of torpedograss.

Nomenclature: Quinclorac; hybrid bermudagrass, Cynodon transvaalensis × C. dactylon; torpedograss, Panicum repens L. #³ PANRE.

Additional index words: Golf course, turf.

Abbreviations: MSO, methylated seed oil; OM, organic matter; POST, postemergence; TPC, Tournament Players Club; WAIT, weeks after initial treatment.

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

Nomenclature: Common cocklebur, Xanthium strumarium L. #³ XANST; morningglory species, Ipomoea spp.; Palmer amaranth, Amaranthus palmeri S. Wats. # AMAPA; sicklepod, Senna obtusifolia L. Irwin and Barneby # CASOB; Texas panicum, Panicum texanum Buckl. # PANTE; yellow nutsedge, Cyperus esculentus L. # CYPES; soybean, Glycine max (L.) Merr.

Additional index words: Glyphosate-resistant soybean, on-farm survey, transgenic soybean.

Over a decade ago, the Weed Science Society of America sponsored a symposium on the then emerging technology of herbicide-tolerant crops (HTCs). The symposium and subsequent proceedings addressed potential benefits and concerns about that new technology to control weeds. Technological, biological, and ethical questions were addressed that were likely to emerge from the widespread adoption of HTCs. It was suggested at that time that if such questions were answered, HTC development would proceed on a more solid foundation, whereas continued uncertainty and criticism would probably result if the questions were not answered. We now review developments in HTC technology. Questions and concerns posed one decade ago are still pertinent, but current knowledge is still insufficient to address them. Adoption of HTC has risen dramatically since their commercial introduction, but there is still no evidence of associated production cost reductions or enhanced yields.

Additional index words: Biotechnology, herbicide-tolerant crops, weeds.

Abbreviations: HTC, herbicide-tolerant crops.