Field studies assessed weed control and potato injury with rimsulfuron applied postemergence at various rates in combination with various adjuvants. Weed control was influenced by choice of adjuvant and rimsulfuron rate. Rimsulfuron at 0, 9, 18, 26, and 35 g ai/ha was applied with nonionic surfactant (NIS), crop oil concentrate (COC), methylated seed oil (MSO), or silicone-polyether copolymer (SIL). Potato injury was less than 5% for all rimsulfuron rates and adjuvant combinations. Redroot pigweed was controlled greater than or equal to 93% by all treatments except rimsulfuron at 9 g/ha SIL. Except for redroot pigweed, rimsulfuron treatments with SIL controlled kochia, hairy nightshade, common lambsquarters, and volunteer oats less than with other adjuvants. At lower rimsulfuron rates, weed control with rimsulfuron MSO tended to be greater than with rimsulfuron NIS or rimsulfuron COC. Common lambsquarters control was 75% or less regardless of rimsulfuron rate or adjuvant. Tuber yield generally increased with increasing rimsulfuron rates. Depending on rimsulfuron rate, tuber yield was 10 to 15% lower with rimsulfuron NIS or rimsulfuron COC compared to rimsulfuron MSO, while tuber yield was 18 to 37% lower with rimsulfuron SIL compared to rimsulfuron NIS, rimsulfuron COC, or rimsulfuron MSO.

Nomenclature: Rimsulfuron; common lambsquarters, Chenopodium album L. #³ CHEAL; hairy nightshade, Solanum sarrachoides # SOLSA; kochia, Kochia scoparia L. Shrad. # KCHSC; potato, Solanum tuberosum L. ‘Russet Burbank’; redroot pigweed, Amaranthus retroflexus L. # AMARE; volunteer oat, Avena sativa L. # AVESA.

Additional index words: Crop oil concentrate, methylated seed oil, nonionic surfactant, organosilicone.

Abbreviations: COC, crop oil concentrate; MSO, methylated seed oil; NIS, nonionic surfactant; PNW, Pacific Northwest; POST, postemergence; PRE, preemergence; SIL, silicone-polyether copolymer; WAT, weeks after treatment.

Experiments were conducted in 1995, 1996, and 1999 to evaluate clomazone preemergence in flue-cured tobacco seedbeds. The seedbeds were fumigated with methyl bromide to kill all the weeds and other pests, and then the subplots designated as grassy were sown with a mixture of equal mass of crowfootgrass, goosegrass, and rhodesgrass. Clomazone rates were 0, 0.14, 0.28, 0.42, and 0.56 kg ai/ha for 1995 and 0, 0.42, 0.47, 0.52, and 0.94 kg ai/ha for 1996 and 1999. The rates of clomazone in 1996 and 1999 were based on the 1995 results where 0.42 kg/ha decreased the dry mass of grasses by 98% compared with the nontreated control, while clomazone at 0.28 kg/ha decreased the dry mass of grasses by 75%, and this was not considered adequate. In 1995, 0.56 kg clomazone/ha decreased the tobacco seedling number but had no effect on the growth of tobacco plants. In 1996 and 1999 clomazone had no effect on the number or dry mass of tobacco seedlings despite causing slight discoloration 4 wk after sowing at all rates in 1996. This discoloration had disappeared by transplanting time. The effect of grass competition on the emergence and growth of tobacco was severe in the nontreated subplots. More grasses emerged in 1999 than in 1996, possibly due to higher temperatures; however, few grasses survived until transplanting time in clomazone-treated areas. All the clomazone rates tested in 1996 and 1999 were satisfactory as there was no crop injury or detrimental effect of the grasses on tobacco in the clomazone-treated grassy subplots. Therefore, the 0.42-kg clomazone/ha rate is considered optimal.

Nomenclature: Clomazone; crowfootgrass, Dactyloctenium aegyptium (L.) Beauv. #³ DDTAE; goosegrass, Eleusine indica (L.) Gaertn. # ELEIN; rhodesgrass, Chloris gayana Kunth # CHRGA; tobacco, Nicotiana tabacum L. ‘Kutsaga Mammoth 10’.

Additional index words: Fluazifop-p-butyl, haloxyfop-R-methyl, methyl bromide, preemergence herbicide.

Abbreviations: ANOVA, analysis of variance; PRE, preemergence.

Field and greenhouse dose-response experiments were conducted to investigate the potential for resistance in a johnsongrass biotype from New Kent County, VA that survived repeated applications of quizalofop and quizalofop-P. During 1996 and 1997, foliar injury (30 to 60%) was initially observed on the johnsongrass at the New Kent field site, but this biotype eventually recovered and survived applications of fluazifop-P, quizalofop-P, and sethoxydim at twice the recommended field use rates. However, applications of clethodim at twice the recommended field use rates during 1997 provided essentially complete control of the New Kent johnsongrass biotype. In greenhouse dose-response experiments, the amount of quizalofop-P required to inhibit shoot growth by 50% (GR₅₀) was 13.6 g/ha in the New Kent johnsongrass biotype and 0.8 g/ha in the susceptible johnsongrass biotype. In response to sethoxydim, the GR₅₀ for the New Kent biotype was 122.5 g/ha while that of the susceptible biotype was 21.6 g/ha. Additionally, the New Kent biotype was least sensitive to fluazifop-P, which provided a GR₅₀ value of 148.7 g/ha for the New Kent biotype and 5.1 g/ha for the susceptible biotype. As in the field trials, the New Kent biotype was sensitive to clethodim, which provided a GR₅₀ value of 49.8 g/ha and 69.1 g/ha for the susceptible biotype. These values indicate that the New Kent biotype was 17 times more resistant to quizalofop-P, 5.7 times more resistant to sethoxydim, and 29.5 times more resistant to fluazifop-P than the susceptible biotype, and that the New Kent and susceptible johnsongrass biotypes are equally sensitive to clethodim. These GR₅₀ values for the New Kent johnsongrass biotype are inconsistent with the much higher GR₅₀ values most commonly observed in graminicide-resistant weed biotypes, and suggest a mechanism of resistance other than an insensitive ACCase in the New Kent johnsongrass biotype.

Nomenclature: Clethodim; fluazifop-P; quizalofop-P; sethoxydim; johnsongrass, Sorghum halepense (L.) Pers. #³ SORHA; soybean, Glycine max (L.) Merr.

Additional index words: Cross-resistance, herbicide resistance, ACCase inhibitors, graminicides.

Abbreviations: ACCase, acetyl coenzyme-A carboxylase; APP, aryloxyphenoxypropionate; CHD, cyclohexanedione; DAT, days after treatment; GR₅₀, dosage giving 50% reduction in shoot dry weight; WAT, weeks after treatment.

The effect of glyphosate on aromatic amino acid metabolism in purple nutsedge sprouted tubers and shoots was investigated. Glyphosate at 33.5 mM caused inhibition of bud elongation, increased total free amino acid concentration, and caused rapid accumulation of shikimic acid in sprouted tubers. However, only one aromatic amino acid, tryptophan, decreased quickly to 22% of control 3 d after treatment (DAT) and remained low afterwards. This suggests that the inhibition of bud elongation is due to the rapid accumulation of shikimic acid and the repression of tryptophan synthesis. Foliar application of glyphosate at 14.5 mM to purple nutsedge shoots resulted in the rapid accumulation of glyphosate which was rapidly converted to its metabolite, aminomethylphosphoric acid. Free amino acids in leaves were also increased by glyphosate 3 DAT. The reduction in soluble protein 5 DAT and increased acid protease activity 3 DAT suggests that the late accumulation of free amino acids partially resulted from protein hydrolysis. Shikimic acid accumulated in glyphosate-treated leaves 5 DAT, but the concentration of the three aromatic amino acids was not reduced. This suggests that glyphosate toxicity in purple nutsedge shoots was associated with the rapid accumulation of glyphosate, followed by large accumulation of shikimic acid. Aromatic amino acids deficiency was apparently not a factor in toxicity.

Nomenclature: Glyphosate; purple nutsedge, Cyperus rotundus L. #³ CYPRO.

Additional index words: Bud growth.

Abbreviations: AMPA, aminomethyl phosphoric acid; DAT, days after treatment; ELISA, enzyme-linked immunosorbent assay; EPSPS, 5-enol-pyruvyl-shikimate-3-phosphate synthase (EC 2.5.1.19); HPLC, high-performance liquid chromatography; OPA, ortho-phthaldialdehyde.

This study documents the first case of triazine resistance in wild radish and the resistance mechanism involved. The high survival (57 to 97%) of the resistant (R) biotype progeny plants treated at a rate four times higher than the commonly recommended rate of simazine or atrazine clearly established that the R biotype plants were resistant to triazines. All the plants of the susceptible (S) biotype plants were killed when treated at half the commonly recommended rate of atrazine (0.5 kg/ha) or simazine (0.25 kg/ha). The dry weight of the S biotype was reduced by 89 to 96% at the commonly recommended rate of atrazine or simazine, while the dry weight of the R biotype plants was reduced by only 36 to 54% even when treated at a rate four times higher than the commonly recommended rate of atrazine or simazine. The growth-reduction–ratio values indicated that the R biotype progeny plants were 105 and 159 times more resistant to atrazine and simazine, respectively, than the S biotype plants. Leaf chlorophyll fluorescence yield was reduced by 97% in the S biotype 24 h after application of triazine compared with only 9% reduction in the R biotype, indicating that the resistance mechanism involved is target-site based. The R biotype was effectively controlled by herbicides of different modes of action.

Nomenclature: Atrazine; simazine; triazine; wild radish, Raphanus raphanistrum L. #³ RAPRA.

Additional index words: Dry weight, leaf chlorophyll fluorescence.

Abbreviations: ALS, acetolactate synthase; DAE, days after emergence; GR₅₀ ratio, the ratio of the herbicide rate required to inhibit the growth of resistant biotype progeny plants by 50% to the rate required to inhibit the growth of S biotype plants by 50%; PS II, photosystem II; R, resistant (biotype); S, susceptible (biotype); SE, standard error; TT, triazine tolerant; WA, Western Australia.

Greenhouse studies were conducted to determine the responses of ripgut brome and foxtail brome to MON 37500 applied at rates up to 80 g/ha. At 30 d after treatment MON 37500 at 20 g/ha reduced fresh weight of ripgut brome by 89% when applied at the three-leaf stage and 57% when applied at the tillering stage. MON 37500 at 10 g/ha reduced fresh weight of foxtail brome by 80% when applied at the three-leaf stage and 61% when applied at the tillering stage. Rates above 20 g/ha did not increase control of either species. In field experiments during 1998 to 1999 and 1999 to 2000 in Settat, Morocco, bread wheat tolerated MON 37500 at 67 g/ha. Ripgut brome control in the field was 78% in the first growing season and 90% in the second season when MON 37500 was applied at 27 g/ha to brome plants at the one- to four-leaf stage. A similar rate controlled tillering ripgut brome by 69% during 1998 to 1999 and 62% during 1999 to 2000. MON 37500 at 20 to 30 and 10 to 20 g/ha applied before tillering controlled ripgut brome and foxtail brome, respectively, ≥ 70%.

Nomenclature: MON 37500,1-(2-ethylsulfonylimidazo[1,2-a]pyridin-3-ylsulfonyl)-3-(4,6-dimethoxypyrimidin-2-yl)urea; foxtail brome, Bromus rubens L. #³ BRORU; ripgut brome, Bromus rigidus Roth #BRORI; wheat, Triticum aestivum L. ‘Tilila’.

Additional index words: Herbicide, weed control efficacy, rainfed wheat.

Abbreviations: DAT, days after treatment; NIS, nonionic surfactant; POST, postemergence; PRE, preemergence.

Field experiments were conducted in 1996, 1997, and 1998 at Ste. Anne de Bellevue, Quebec, Canada, and in 1996 at Ottawa, Ontario, Canada, to quantify the impact of corn hybrids, differing in canopy architecture and plant spacing (plant population density and row spacing), on biomass production by transplanted and naturally occurring weeds. The treatments consisted of a factorial combination of corn type (leafy reduced stature [LRS], late-maturing big leaf [LMBL], a conventional Pioneer 3979 [P3979], and, as a control, a corn-free condition [weed monoculture]), two weed levels (low density [transplanted weeds: common lambsquarters and redroot pigweed] and high density [weedy: plots with naturally occurring weeds]), two corn population densities (normal and high), and row spacings (38 and 76 cm). At all site-years under both weed levels, the decrease in biomass production by both transplanted and naturally occurring weeds was greater due to the narrow row spacing than due to the high plant population density. The combination of narrower rows and higher population densities increased corn canopy light interception by 3 to 5%. Biomass produced by both transplanted and naturally occurring weeds was five to eight times less under the corn canopy than in the weed monoculture treatment. Generally, weed biomass production was reduced more by early-maturing hybrids (LRS and P3979) than by LMBL. Thus, hybrid selection and plant spacing could be used as important components of integrated pest management (weed control) for sustainable agriculture.

Nomenclature: Common lambsquarters, Chenopodium album L. #³ CHEAL; corn, Zea mays L.; redroot pigweed, Amaranthus retroflexus L. # AMARE.

Additional index words: Competitivness, early maturity, weed management.

Abbreviations: LAI, leaf area index; Lfy, leafy; LMBL, late-maturing big leaf; LRS, Leafy reduced stature; P3979, Pioneer 3979; rd1, reduced stature.

Field studies were conducted near Painter, VA, in 1995 and 1996 to investigate the effects of herbicides and cultivation on weed control, yield, and net returns in potato. Potato injury from herbicides and/or cultivation was not observed in 1995 and was less than 12% in 1996. Metribuzin plus metolachlor preemergence controlled yellow nutsedge by at least 81% regardless of the number of cultivations in 1995 and 1996. Yellow nutsedge control with metribuzin plus rimsulfuron postemergence (POST) plus three cultivations was as high as 70% in 1995 and 88% in 1996. Metribuzin plus rimsulfuron POST controlled common lambsquarters by at least 95% and common ragweed by at least 83% regardless of the number of cultivations in 1995 and 1996. A-size tuber production and net returns from potato receiving herbicides were not improved with two or three cultivations in 1995 or 1996. However, when averaged over all weed control systems (herbicide and cultivation-only systems) multiple cultivations significantly increased control of all weed species, A-size tuber production, and net returns.

Nomenclature: Metolachlor; metribuzin; rimsulfuron; common lambsquarters, Chenopodium album L. #³ CHEAL; common ragweed, Ambrosia artemisiifolia L. # AMBEL; potato, Solanum tuberosum L., ‘Superior’; yellow nutsedge, Cyperus esculentus L. # CYPES.

Additional index word: A-size potato.

Abbreviations: POST, postemergence; PRE, preemergence; WAP, weeks after planting; WAT, weeks after treatment.

A 2-yr field study was conducted during 1998 and 1999 at Stoneville, MS, on a Dundee silt loam to determine weed control, yield, and net return associated with winter cover crops in soybean. Cover crop systems included Italian ryegrass, oat, rye, wheat, hairy vetch, crimson clover, subterranean clover, no-cover crop conventional tillage (CT), and no-cover crop no-tillage (NT), all with standard preemergence (PRE), postemergence (POST), PRE POST, and no-herbicide weed management. Oat (11.1 Mg/ha) had highest dry biomass compared to all other cover crops (6.0 to 7.6 Mg/ha) at soybean planting. Biomass decreased 9 wk after planting (WAP) compared to the respective biomass at soybean planting in all cover crops. Italian ryegrass and rye biomass decay was slow and about two-thirds of plant residue persisted at 9 WAP. Cover crops had no effect on densities of barnyardgrass, prickly sida, and yellow nutsedge, but altered the density of browntop millet. Total weed biomass was higher in rye, wheat, and subterranean clover than in Italian ryegrass cover crop systems, and higher with the PRE-only vs. POST-only or PRE POST programs at 10 WAP soybean. Soybean yield decreased in the order of no-cover crop NT ≥ no-cover crop CT ≥ hairy vetch ≥ crimson clover ≥ rye ≥ oat ≥ wheat ≥ subterranean clover > Italian ryegrass. None of the cover crop systems gave soybean yield higher than the no-cover crop CT system in the absence of herbicides. Under a PRE-only program, all cover crop systems had lower yield compared to the no-cover crop CT system. When late-emerged weeds were controlled with POST applications (POST-only or PRE POST programs), all cover crops, except Italian ryegrass, had no detrimental effect on soybean yields, which were not different from no-cover crop CT plots. In cover crops, input costs were high due to additional cost of seeds, planting, and desiccation. Net return was highest in no-cover crop NT ($105/ha) followed by no-cover crop CT ($76/ha) system. Net returns were negative for all cover crops and losses were highest in crimson clover (−$62/ha) and subterranean clover (−$161/ha).

Nomenclature: Barnyardgrass, Echinochloa crus-galli (L.) Beauv. #³ ECHCG; browntop millet, Brachiaria ramosa (L.) Stapf # PANRA; prickly sida, Sida spinosa L. # SIDSP; yellow nutsedge, Cyperus esculentus L. # CYPES; crimson clover, Trifolium incarnatum L. ‘Dixie’; hairy vetch, Vicia villosa Roth; Italian ryegrass, Lolium multiflorum Lam. ‘Gulf’; oat, Avena sativa L. ‘Bob’; rye, Secale cereale L. ‘Elbon’; soybean, Glycine max (L.) Merr. ‘DP 3588’; subterranean clover, Trifolium subterraneum L. ‘Mount Barker’; wheat, Triticum aestivum L. ‘Cocker 9803’.

Additional index words: Allelopathy, conventional tillage, herbicide, integrated weed management, mulch, net return, no-tillage, weed emergence, weed biomass.

Abbreviations: ANOVA, analysis of variance; CT, conventional tillage; NT, no-tillage; POST, postemergence; PRE, preemergence; WAP, weeks after planting soybean.

Research was conducted to compare the results of an enzyme-linked immunosorbent assay (ELISA) to high-performance liquid chromatography (HPLC) for detecting fluometuron in the environment. A linear relationship for HPLC (R² > 0.90) and ELISA (R² > 0.66) analysis was observed between the natural logarithm of the detected fluometuron concentrations regressed against time in soil collected from a cropped area, a grass filter strip, and a riparian forest. Both methods detected the same initial fluometuron concentration (y-intercept) for two of the three soils evaluated. The ELISA and HPLC measurements of fluometuron concentrations compared favorably with r values from 0.83 to 0.98. Predicted fluometuron half-lives determined from HPLC and ELISA measurements were: 110 and 112 d in the cropped watershed, 28 and 29 d in the riparian area, and 11 and 11 d in the grass filter strip, respectively. Results from both techniques indicated shorter half-lives in soil from the grass filter strip and riparian area than in cropped area soil. There was an inverse correlation between predicted half-lives and soil organic matter, pH, clay, and cation exchange capacity.

Additional index word: Herbicide degradation.

Abbreviations: BMPs, best management practices; CEC, cation-exchange capacity; DAT, days after treatment; DT₅₀, 50% disappearance time; ELISA, enzyme-linked immunosorbent assay; HPLC, high-performance liquid chromatography; IC₅₀, analyte concentration that decreases chromogen activity by 50%; LDD, least detectable dose; MD-MSEA, Mississippi Delta Management Systems Evaluation Area; OM, organic matter.

Studies were conducted in 1997 and 1998 to evaluate the efficacy and economics of glyphosate-resistant and nontransgenic soybean systems. The three highest yielding glyphosate-resistant and nontransgenic soybean cultivars were chosen each year for three Mississippi locations based on Mississippi Soybean Variety Trials. Treatments within each cultivar/herbicide system included nontreated, low input (one-half of the labeled rate), medium input (labeled rate), and high input level (labeled rate plus an additional postemergence application). In 1997, all systems controlled hemp sesbania by more than 80% but nontransgenic systems controlled hemp sesbania more than the glyphosate-resistant systems in most instances in 1998. High input levels usually controlled pitted morningglory more than low or medium inputs in 1997. In 1998, both systems controlled pitted morningglory by 90% or more at Shelby; however, at other locations control was less than 85%. Soybean yield in 1997 at Shelby was more with the glyphosate-resistant system than with the nontransgenic systems at medium and high input levels, primarily because of early-season injury to a metribuzin-sensitive cultivar in the nontransgenic system. In 1998, soybean yield at Shelby was more with the nontransgenic system than the glyphosate-resistant system, regardless of input level, due to poor late-season hemp sesbania control with glyphosate. Net returns were often more with the glyphosate-resistant system at Shelby in 1997. Within the glyphosate-resistant system, there were no differences in net return between input levels. Within the nontransgenic system, low input level net returns were higher compared to medium and high input levels due to higher soybean yield and less herbicide cost. At Brooksville, using high input levels, the glyphosate-resistant systems net returns were $55.00/ha more than the nontransgenic system. Net returns were higher with the nontransgenic system compared to the glyphosate-resistant system at Shelby in 1998, regardless of input level.

Nomenclature: Glyphosate; metribuzin; hemp sesbania, Sesbania exaltata (Raf.) Rydb. ex. A. W. Hill #³ SEBEX; pitted morningglory, Ipomoea lacunosa L. # IPOLA; soybean, Glycine max (L.) Merr.

Additional index words: Economics, reduced rates, glyphosate-resistant soybean systems.

Abbreviations: MSBG, Mississippi state budget generator; POST, postemergence.

Field experiments were conducted in 1997 and 1998 to evaluate the effects of gypsum (GYP), anionic polyacrylamide (PAM), and dolomitic lime (DL) on atrazine and acetochlor efficacy and leaching. Surface-applied treatments of PAM (20 kg/ha), GYP (1,000 kg/ha), PAM GYP (20 1,000 kg/ha), and DL (1,000 kg/ha), plus an unamended control, were evaluated at two locations, i.e., West Lafayette, IN (sites A and B) and Bourbon, IN (sites C and D). Sites A and C were evaluated in 1997 and 1998, while sites B and D were evaluated only in 1998. West Lafayette experiments were on a Raub silt loam (fine-silty, mixed, mesic, aquic Arguidolls) soil, and Bourbon experiments were on a Rensselaer silt loam (fine-silty, mixed, mesic, Typic Arguidolls) soil. Atrazine and acetochlor efficacy was reduced on GYP-amended plots at site A in 1997 where the total weed density was significantly higher (P = 0.10) compared with all other treatments. The results were not significant at this site in 1998; however, weed density tended to be higher with the GYP treatment. Weed density was significantly higher with DL amendments than with the other treatments at site B in 1998. Weed control was not reduced with any treatment at site C or site D in 1997 and 1998. The very strong weed pressures at these sites may have hidden potential differences. Rainfall within hours of amendment and herbicide application resulted in increased atrazine and acetochlor leaching below 15 cm under GYP-amended plots compared with all the treatments at site C in 1997. Atrazine and acetochlor concentrations in the soil were similar among treatments at all other sites, sampling times, and in both years. The application of GYP increased herbicide leaching only when heavy rainfall occurred immediately after application.

Nomenclature: Acetochlor; atrazine.

Additional index word: Zea mays.

Abbreviations: ANOVA, analysis of variance; DL, dolomitic lime; GYP, gypsum; LSD, least significant difference; PAM, anionic polyacrylamide.

A rapid method for determining the response of cereals to glyphosate is described. This method detects the differential responses of plants in 4 d, allowing for the rapid selection of glyphosate-tolerance response. Two types of tests determined the efficacy of this rapid method: differential response to different dosages of herbicide in a coleoptile growth test and in sprayed plants. In seedling assay, barley cultivars showed a higher level of tolerance to glyphosate (the dose that causes 50% of the total effect [I₅₀] = 0.066 and I₅₀ = 0.060 mM) than wheat cultivars (I₅₀ = 0.018 and I₅₀ = 0.014 mM). The response in seedling assay is well correlated (r² = 0.95 and r² = 0.98) with the response in plant-sprayed assays. The method was employed to verify the tolerance level of a sensitive barley cultivar and the four tolerant and sensitive mutant lines derived from it.

Nomenclature: Glyphosate; barley, Hordeum vulgare L.; wheat, Triticum aestivum L.

Additional index words: Herbicide response.

Abbreviations: AN, Amaji Nijo; ANOVA, analysis of variance; CS, Chinese Spring; DAT, days after treatment; I₅₀, dose that causes 50% of the total effect.

The role of preplant glyphosate applications and residual herbicides in the efficacy of glyphosate for weed management in double-crop no-till glyphosate-resistant soybean (GRS) was investigated in the coastal plains of Mid-Atlantic United States. The experiment had a two- by two- by five-factorial treatment structure laid out in three or four randomized complete blocks at research centers in Delaware and New Jersey. The factors investigated were preplant weed management: preplant or no preplant glyphosate applications; postemergence (POST) herbicide treatments: 0.8 kg ae/ha glyphosate alone or 0.8 kg/ha glyphosate tank-mixed with 0.6 kg ai/ha clomazone plus 0.07 kg ai/ha imazethapyr; and GRS growth stage at herbicide application which ranged from cracking, 5 to 8 d after planting, (DAP) to the V6 stage (35 DAP). Preplant glyphosate applications did not influence the efficacy of POST glyphosate applications alone or with the residual herbicides. Glyphosate alone or with clomazone plus imazethapyr provided excellent control of horseweed and fall panicum irrespective of the time of herbicide application from GRS at cracking to the V6 stage. With other weed species, residual herbicide influence varied with year, weed species, and GRS growth stage at herbicide application. Generally, glyphosate alone was most effective when applied at the V2 to V6 stages (16 to 35 DAP). A tank-mix of glyphosate with clomazone plus imazethapyr extended this window to include applications at GRS cracking and the V1 stage. Herbicide treatments were safe on GRS at all stages of application up to the V6 stage (35 DAP).

Nomenclature: Clomazone, glyphosate, imazethapyr, horseweed, Conyza (= Erigeron) canadensis L. #³ ERICA; fall panicum, Panicum dichotomiflorum Michx. # PANDI.

Additional index words: Preplant glyphosate applications, critical weed period, integrated weed management, Amaranthus hybridus, Ambrosia artemisiifolia, Chenopodium album, Ipomea hederacea, Xanthium strumarium, AMACH, AMBEL, CHEAL, IPOHE, SETFA, XANST.

Abbreviations: DAP, days after planting; GRS, glyphosate-resistant soybean; POST, postemergence; RAREC, Rutgers Agricultural Research and Extension Center; UD-REC, University of Delaware Research and Education Center.

The efficacy of glyphosate applied alone or in combination with residual herbicides in full-season no-till glyphosate-resistant soybean (GRS) was investigated in New Jersey and Delaware on sandy drought-prone soils. Treatments were in a two- by two- by five-factorial arrangement laid out in three or four randomized complete blocks. The factors investigated were—two preplant glyphosate applications: preplant glyphosate applications or no preplant glyphosate applications; two herbicide treatments: 0.8 kg ae/ha glyphosate alone or 0.8 kg/ha glyphosate tank-mixed with 0.6 kg ai/ha clomazone plus 0.07 kg ai/ha imazethapyr; and herbicide application at five GRS growth stages: at cracking or one of the four times between the V1 and V7 stages. Preplant glyphosate application for the control of emerged weeds was essential for satisfactory control of common annual weeds with glyphosate alone or glyphosate combined with residual herbicides when rainfall was high (avg. 120 mm/mo), but less important when rainfall was low (avg. 72 mm/mo). Compared to glyphosate alone, glyphosate plus residual herbicides improved the control of common lambsquarters, fall panicum, and common ragweed, when applied at cracking or at the V1 stage and preceded by preplant glyphosate applications. At all stages of application, satisfactory full-season control of ivyleaf morningglory was achieved only with glyphosate plus residual herbicides. Horseweed, large crabgrass, giant foxtail, or smooth pigweed control varied from good to excellent (80 to 100%) at all stages of application of glyphosate alone or with residual herbicides. Glyphosate applied alone or with residual herbicides was safe on GRS regardless of time of application up to the V7 stage. The highest soybean yield was consistently achieved with preplant glyphosate applications followed by glyphosate alone at the V2 to V4 stages or a preplant glyphosate application followed by glyphosate plus residual herbicides applied from crop emergence to the V4 stage.

Nomenclature: Common lambsquarters, Chenopodium album L. #³ CHEAL; fall panicum, Panicum dichotomiflorum Michx. # PANDI; common ragweed, Ambrosia artemisiifolia L. # AMBEL; ivyleaf morningglory, Ipomoea hederacea (L.) Jacq. # IPOHE; horseweed, Conyza (= Erigeron) canadensis L. # ERICA; large crabgrass, Digitaria sanguinalis (L.) Scop. # DIGSA; giant foxtail, Setaria faberi Herrm. # SETFA; smooth pigweed, Amaranthus hybridus L. # AMACH; soybean, Glycine max (L.) Merr. # GLYMA.

Additional index words: Critical weed removal, integrated weed management, preplant glyphosate applications.

Abbreviation: DAP, days after planting; DAT, days after treatment; GRS, glyphosate-resistant soybean; POST, postemergence; RAREC, Rutgers Agricultural Research and Extension Center; UD-REC, University of Delaware Research and Education Center.

The cause of differential susceptibility of barnyardgrass, hemp sesbania, pitted morningglory, and prickly sida to glyphosate was examined by measuring the absorption of ¹⁴C-glyphosate, quantifying the amount of epicuticular wax, and observing the wettability of leaf surfaces. In greenhouse experiments, the biomass of barnyardgrass and prickly sida was reduced by 95% by Roundup Ultra®. Hemp sesbania and pitted morningglory showed more tolerance, with 66 and 51% average biomass reduction, respectively. Absorption of ¹⁴C-glyphosate in a controlled environment did not follow the trend in species susceptibility with barnyardgrass, 30%; prickly sida, 18%; hemp sesbania, 52%; and pitted morningglory, 6%; absorption. The high tolerance of pitted morningglory to glyphosate can be attributed mostly to limited absorption, but the tolerance of hemp sesbania is due to other mechanisms. The addition of nonionic surfactant (NIS) to a low rate of Roundup Ultra® reduced absorption of ¹⁴C-glyphosate by barnyardgrass and hemp sesbania, but had no effect on the herbicidal activity. Glyphosate absorption in the four weed species was not correlated with quantity of chloroform-extracted wax or leaf wettability. Pitted morningglory and prickly sida, which contained the least leaf wax, also had smaller contact angles or higher leaf wettability than the species with more waxy leaves. The adjuvant in Roundup Ultra® reduced contact angles of the four species compared to contact angles obtained using deionized water alone. The addition of 0.25% v/v NIS alone to water reduced contact angles more than did the adjuvant in Roundup Ultra® solution.

Nomenclature: 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.

Additional index words: Droplet contact angle, epicuticular wax, ¹⁴C-glyphosate, glyphosate absorption, glyphosate uptake.

Abbreviations: CMC, critical micelle concentration; DAT, days after treatment; EPSPS, 5-enolpyruvylshikimate-3-phosphate synthase; HAT, hours after treatment; NIS, nonionic surfactant.

Torpedograss is a serious problem in southern turfgrass, especially along the U.S. gulf coast. Studies were conducted during 1998, 1999, and 2000 to evaluate quinclorac for torpedograss control in bermudagrass turf. Three applications of quinclorac at 0.6 kg/ha spaced 21 d apart provided better torpedograss control (88%) than two applications at 0.8 kg/ha (69%) or one application at 1.7 kg/ha (69%). Two applications of quinclorac (0.8 kg/ha) plus diclofop (0.8 kg/ha) provided better torpedograss control (82%) than either herbicide applied alone when evaluated after a single season of application. Increasing the mowing interval prior to quinclorac application to allow for more foliage to be present did not improve control. Nitrogen application prior to quinclorac treatment did not improve torpedograss control. Long-term control will most likely require quinclorac applications for more than one season.

Nomenclature: Diclofop; quinclorac; bermudagrass, Cynodon dactylon (L.) Pers. × C. transvaalensis Burtt-Davey ‘Tifway’; torpedograss, Panicum repens L. #³ PANRE.

Additional index words: Application frequency, cultural practices, mowing interval, nitrogen fertility, turfgrass.

Abbreviations: LSD, least significant difference; POST, postemergence; WAIT, weeks after initial treatment.

The critical period of weed control for crops grown under conventional tillage systems has been well studied, and the results generated by these studies have been proven to be very useful in developing ecologically and economically sound weed management practices. However, these management systems may not be directly applicable under no-till situations because the species composition, total amount, and temporal pattern of seedling emergence change substantially with tillage. The objective of this study was to identify the critical period of weed control for soybean and corn in fields that had been under no-till management for 1 yr. Although estimates of the critical period for a crop vary from year to year and site to site, some interesting comparisons can be made between no-till and conventional tillage. The start of the critical period in no-till corn was stable, usually beginning at the six-leaf stage. The end of the critical period was more variable ranging from the 9- to 13-leaf stage. The critical period for corn under no-till conditions tended to start and end earlier than under conventional tillage practices. In soybean, we were unable to identify a critical period at one of the sites. At the other location (sandy loam soil), the critical period was estimated to begin at the first or second node developmental stage, whereas the end was determined to be at the R1 stage (early flowering). The critical period in soybean was longer than that observed under conventional tillage.

Nomenclature: Corn, Zea mays L. ‘Pioneer 3573’; soybean, Glycine max (L.) Merr. ‘Northrup King 2492’.

Additional index words: Weed interference, crop yield loss.

Abbreviation: DAE, days after emergence.

Field studies were conducted from 1998 to 2000 to evaluate the effects of the trimethylsulfonium (Tms) salt of glyphosate on glyphosate-resistant soybean at Belleville, IL. Glyphosate-Tms and glyphosate-isopropylamine (Ipa) at 1,120, 1,680, 2,240, 3,360, and 4,480 g ai/ha were applied at the V4 and R1 growth stages of glyphosate-resistant soybean. Glyphosate-Tms and glyphosate-Ipa caused greater chlorosis when applied at the R1 growth stage when compared with the V4 growth stage, and chlorosis increased with rate. Chlorosis ranged from 0 to 20% depending on the year. In 1998, glyphosate-Ipa at 2,240 and 3,360 g/ha applied at the R1 growth stage caused 4 to 5% more chlorosis than glyphosate-Tms at the same rates. In addition to chlorosis, glyphosate-Tms caused bleaching (white speckling) of soybean leaves, with bleaching increasing as glyphosate-Tms rate increased. Glyphosate-Ipa caused no bleaching, regardless of rate. Glyphosate-Tms and -Ipa caused no visible height reduction at 14 and 28 d after treatment in any year. In 1998 and 1999, glyphosate-Tms and -Ipa, at the highest rate applied at the R1 growth stage, increased days to maturity of soybean. Despite the injury and delay in maturity caused by glyphosate-Tms and -Ipa, there was no difference in grain yield across years because of glyphosate salt, rate, or application timing.

Nomenclature: Glyphosate; soybean, Glycine max (L.) Merr. ‘Pioneer 94B01 RR’.

Additional index words: Herbicide injury, postemergence, transgenic soybean.

Abbreviations: DAT, days after treatment; EPSPS, 5-enolpyruvylshikimate-3-phosphate synthase; Ipa, isopropylamine; Tms, trimethylsulfonium.

Spotted knapweed is an important weed of rangeland in the northwestern United States. A study was conducted near Corvallis, MT, during 1992 to 1994 in order to assess the relationship among the growth attributes of spotted knapweed to identify a minimum set of measurable plant characteristics that are representative of spotted knapweed vigor. Spotted knapweed growth attributes that were examined included plant age, root diameter, plant height, number of stems per plant, aboveground biomass, number of capitula (seed heads) per plant, and number of capitula per stem. Spotted knapweed age was positively correlated with root diameter, number of stems per plant, aboveground biomass, and proportion of bolted plants. Most spotted knapweed plants did not bolt until the third or fourth year. Although plant age is not measured easily in the field, it may be useful as a covariate in an analysis of experiments involving plant competition or nonlethal biological control agents. Root diameter can be used as a nondestructive measure of approximate plant age, especially for the first 5 yr of growth. Root diameter was also highly correlated with many growth measurements, including number of capitula per plant and aboveground biomass, which are most relevant to assessing overall plant vigor. Plant height was positively correlated with aboveground biomass, number of capitula per plant, and mean number of capitula per stem. Number of stems per plant was positively correlated with plant height, aboveground biomass, and number of capitula per plant. Aboveground biomass was positively correlated to number of capitula per plant and mean number of capitula per stem. Measurements of root diameter, plant height, and number of stems are easy to perform and should provide a good indication of plant vigor.

Nomenclature: Spotted knapweed, Centaurea maculosa Lam. #³ CENMA.

Additional index words: ANCOVA, analysis of covariance; growth–attribute relationships.

A mesorhizotron and scanning system was modified to study the development of Russian thistle root systems during the 1996 and 1997 growing seasons at Lind, WA. Our imaging equipment combined the full profile images afforded by conventional rhizotrons with the portability of cylinder-based minirhizotron systems at a fraction of the cost of either system. Root development of Russian thistle in early spring was rapid and extensive compared with shoot growth. In 1996, 30 d after planting (DAP) Russian thistle roots were at least five times as long as the corresponding plant's shoots. During the next 20 d, shoots grew a maximum of 20 cm, whereas roots grew a maximum of 120-cm deep. Maximum root elongation rate reached 2 to 3 mm/cm²/d at the 70- to 120-cm depths 30 to 50 DAP in 1996 and 55 to 70 DAP in 1997. More than one (multiaxial grouping) Russian thistle root was often observed growing through the same soil channels. After the rapid early season growth, roots began to shrink or die back until shoots were clipped to simulate wheat harvest. Within 7 d after harvest, roots regenerated in old root channels. Our mesorhizotron system is a promising inexpensive tool for monitoring root morphological development of Russian thistle under field conditions.

Nomenclature: Russian thistle, Salsola iberica Sennen and Pau #³ SASKR; wheat, Triticum aestivum L.

Additional index words: Root development in situ.

Abbreviations: DAP, days after planting; RER, root elongation rates.

Field studies in 1998 and 1999 determined the response of grain sorghum and sunflower to soil residues of MKH 6561 and MON 37500 in a failed winter wheat recropping situation. Averaged across years, MON 37500 at 30 g ai/ha reduced sunflower density by 39%. MKH 6561 at 30 or 45 g/ha did not reduce sunflower density, growth, or late-season biomass, whereas MON 37500 reduced late-season biomass in both years. Neither herbicide affected grain sorghum plant density in 1998, but MON 37500 decreased sorghum density 80% in 1999. Sorghum growth, biomass, and yield were limited more severely by MON 37500 in 1999 than in 1998. MKH 6561 had little or no effect on grain sorghum during either year. Differences in crop response to MON 37500 between years was likely because of wheat plant size at the time of application and greater interception and metabolism of the herbicide in 1998 crop year.

Nomenclature: MKH 6561, methyl 2-({[(4-methyl-5-oxo-3-propoxy-4,5-dihydro-1H-1,2,4-triazol-1-yl)carbonyl]amino}sulfonyl)benzoate sodium salt; MON 37500, 1-(4,6-dimethoxypyrimidin-2-yl)-3-(2-ethanesulfonyl-imidazo[1,2-a]pyridine-3-yl)sulfonylurea; grain sorghum, Sorghum bicolor (L.) Moench ‘Dekalb DK51’; sunflower, Helianthus annuus L. ‘Cargill SF187A’; winter wheat, Triticum aestivum.

Additional index words: Herbicide carryover, emergency plant back, recrop, rotation.

Abbreviations: POST, postemergence; PRE, preemergence.

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

Nomenclature: Bahiagrass, Paspalum notatum Fluegge #³ PASNO; Florida beggarweed, Desmodium tortuosum (Sw.) DC. # DEDTO; large crabgrass, Digitaria sanguinalis L. # DIGSA; morningglory species, Ipomoea spp.; nutsedge species, Cyperus spp.; pigweed species, Amaranthus spp.; sicklepod, Senna obtusifolia (L.) Irwin and Barnaby # CASOB; Texas panicum, Panicum texanum Buckl. # PANTE; tropic croton, Croton glandulosus var. septentrionalis Muell.-Arg. # CVNGS; wild radish, Raphanus raphanistrum L. # RAPSN; corn, Zea mays L.; cotton, Gossypium hirsutum L.; peanut, Arachis hypogaea L.; soybean, Glycine max (L.) Merr.; tobacco, Nicotiana tabacum L.

Additional index words: Economically important weeds, weed distributions, weed population shifts, weed survey.

Abbreviations: C, Central district; EC, East-Central district; NC, North-Central district; NE, Northeastern district; NW, Northwestern district; SC, South-Central district; SE, Southeastern district; SW, Southwestern district; SWSS, Southern Weed Science Society; WC, West-Central district.

Allelopathy has been suggested as a mechanism of interference in several weed species. Allelochemicals released from certain weed species influence the growth and yield of crop species. Several laboratory studies present circumstantial evidence of the occurrence of allelopathy as a causative agent in weed–crop agroecosystems. Field evidence, however, is still lacking. In this paper, the significance of field studies is argued in terms of a multifaceted approach to allelopathy, and mugwort is used as an example. Previous research demonstrated the allelopathic potential of mugwort; however, experiments were not carried out in a natural environment. Inderjit and Foy (1999) have demonstrated that chemical characteristics (pH, inorganic ions, and phenolics) of soil amended with mugwort leaf leachate were altered when compared to unamended soil. We have analyzed the mugwort-infested field soil and compared its chemical characteristics with those of amended soils. No definite trend, in terms of influence of mugwort on soil chemistry, was observed. Results indicate the importance of field studies in order to obtain ecologically relevant data from laboratory studies. Field situations are often complex in terms of the presence of interfering flora. Cyanobacteria, for example, play an important role in weed–crop interactions in rice paddy soils. Allelochemicals released from weed species present in the paddy field may influence nitrogen-fixing cyanobacteria in addition to their phytotoxic effects to the paddy crop. Significance of phytotoxic effects of weed species on crop growth, and N₂-fixing potential of cyanobacteria in paddy soils is discussed.

Nomenclature: Cyanobacteria; mugwort, Artemisia vulgaris L. #³ ARTVU; rice, Orzya sativa L.

Additional index words: Phenolics, soil characteristics, Trifolium pratense.

The involvement of allelopathy in various crop–weed competition studies has been suggested by several authors, but its significance has been demonstrated with varying success. It is extremely difficult to unambiguously demonstrate allelopathy in nature because of the complexity of plant interference and its relationship to soil chemistry. However, with an increased understanding of the chemical processes occurring in the agroecosystem, genetic mapping of quantitative traits, and the ability to identify allelochemicals, an effort should now be directed toward understanding the mechanisms for allelopathy, as well as trying to optimize an allelopathic effect to produce more competitive crops. The approach used for rice allelopathy research can be used as a general framework for understanding how genetically encoded traits affect the competitive ability of plants. This framework requires the involvement of a range of scientists from multidisciplinary research areas with the overall objective of optimizing competitive ability in crops. Such research efforts could reduce dependency on herbicides and thus increase the sustainability of weed management practices. This paper aims to illustrate the importance of allelopathy for crop competitive ability and to identify a framework suitable for result-oriented collaborative research toward breeding for competitive ability in crops.

Nomenclature: DNA, deoxyribonucleic acid; IRRI, International Rice Research Institute; QTL, quantitative trait loci; rice, Oryza sativa L.

Additional index words: Competition, plant interference, allelopathic cultivar, weeds.

The interaction between wheat and perennial ryegrass seed density and seedlings of different ages was studied under controlled conditions. Root length of perennial ryegrass, after sowing, was suppressed by wheat and was dependent on the density of wheat seeds. Shoot growth of perennial ryegrass, however, was unaffected by the presence of wheat. Perennial ryegrass density had no effect on the first 2 wk of wheat seedling growth. The age of wheat seedlings had no appreciable influence on either root or shoot growth of perennial ryegrass. The present study highlights the need for an unbiased two-way experimental design to identify the dominant competitor.

Nomenclature: Perennial ryegrass, Lolium perenne L. #³ LOLPE; wheat, Triticum aestivum L.

Additional index words: Allelopathy, competitive ability.

Abbreviation: ANOVA, analysis of variance.

The root exudates produced by sorghums contain a biologically active constituent known as sorgoleone. Seven sorghum accessions were evaluated for their exudate components. Except for johnsongrass, which yielded 14.8 mg root exudate/g fresh root wt, sorghum accessions consistently yielded approximately 2 mg root exudate/g fresh root wt. Exudates contained four to six major components, with sorgoleone being the major component (> 85%). Three-dimensional structure analysis was performed to further characterize sorgoleone's mode of action. These studies indicated that sorgoleone required about half the amount of free energy (493.8 kcal/mol) compared to plastoquinone (895.3 kcal/mol) to dock into the Q_B-binding site of the photosystem II complex of higher plants. Light, cryo-scanning, and transmission electron microscopy were utilized in an attempt to identify the cellular location of root exudate production. From the ultrastructure analysis, it is clear that exudate is being produced in the root hairs and being deposited between the plasmalemma and cell wall. The exact manufacturing and transport mechanism of the root exudate is still unclear. Studies were also conducted on sorgoleone's soil persistence and soil activity. Soil impregnated with sorgoleone had activity against a number of plant species. Recovery rates of sorgoleone-impregnated soil ranged from 85% after 1 h to 45% after 24 h. Growth reduction of 9 14-d-old weed species was observed with foliar applications of sorgoleone.

Nomenclature: Sorgoleone (2-hydroxy-5-methoxy-3-[(8′Z,11′Z)-8′,11′,14′-pentadecatriene]-p-hydroquinone); common purslane, Portulaca oleracea L. POROL; common ragweed, Ambrosia artemisiifolia L. AMBEL; cress, Lepidium sativum L. ;ns³ LEPSA; giant foxtail, Setaria faberi Herrm. SETFA; johnsongrass, Sorghum halepense (L.) Pers. SORHA; lambsquarters, Chenopodium album L. CHEAL; large crabgrass, Digitaria sanguinalis (L.) Scop. DIGSA; lettuce, Lactuca sativa L.; nightshade, Solanum spp.; purple photosynthetic bacterium, Rhodopseudomonas viridis; redroot pigweed, Amaranthus retroflexus L. AMARE; sicklepod, Cassia obtusifolia L. CASOB; spinach, Spinacea oleracea; shattercane, Sorghum bicolor (L.) Moensch SORVU; sorghum, S. bicolor (L.) Moensch SORVU; sudex, S. bicolor × Sorghum sudanense; sweet sorghum, S. bicolor ‘Della’; SX-15 and SX-17, S. bicolor × S. sudanense; 8446 and 855-F, S. bicolor (L.) Moensch SORVU; tomato, Lycopersicon esculentum L.; velvetleaf, Abutilon theophrasti Medicus ABUTH.

Additional index words: Sorgoleone, root hairs, SORVU, SORHA.

Abbreviations: ER, endoplasmic reticulum; HPLC, high-pressure liquid chromatography; PSII, photosystem II; Q_B, quinone binding; SEM, scanning electron microscopy; TEM, transmission electron microscopy; TLC, thin-layer chromatography; 3D, three dimensional; UV, ultraviolet.

Crop allelopathy has seldom been used effectively by farmers in weed management. Traditional breeding methods have not been successful in producing highly allelopathic crops with good yields. Genetic engineering may have the potential for overcoming this impasse. Crops have been made resistant to insects, pathogens, and herbicides with transgenes, but biotechnology has not produced crops that control weeds with allelochemicals. The strategies for producing allelopathic crops by biotechnology are relatively complex, usually involving multiple genes. One can choose to enhance production of allelochemicals already present in a crop or to impart the production of new compounds. The first strategy involves identification of the allelochemical(s), determination of the enzymes and genes encoding them, and the use of genetic engineering to enhance their production. The latter strategy employs altering existing biochemical pathways by insertions of transgenes to produce new allelochemicals. With either strategy, there are potential problems with tissue-specific promoters, autotoxicity, metabolic imbalances, and proper movement of the allelopathic compound to the rhizosphere.

Nomenclature: Glufosinate; barley, Hordeum vulgare L.; celery, Apium graveoens L. var. Dulce (Miller) Pers.; cucumber, Cucumis sativa L.; diffuse knapweed, Centaurea diffusa Lam #³ CENDI.; maize, Zea mays L.; potato, Solanum tuberosum L.; rice, Oryza sativa L.: sorghum, Sorghum bicolor (L.) Moench # SORVU; sudangrass, Sorghum sudanese (Piper) Stapf; tomato, Lypersicon esculentum L.; wheat, Triticum aestivum L.

Additional index words: Allelochemical, genetic engineering, phytotoxin, transgene.

Abbreviations: DIMBOA, 2,4-dihydroxy-7-methoxy-1, 4-benzoxazin-3-one; PCR, polymerase chain reaction.

The initiative to use plant pathogens and allelochemicals from pathogens and other microorganisms as biological weed control agents (bioherbicides) began about 30 yr ago. Since then, numerous plant pathogens (bacteria and fungi) and microbial allelochemicals have been isolated, identified, and tested for their bioherbicidal potential. Pathogens (and in some cases microbial phytotoxins) may be used directly on target weed species, or such allelochemicals may provide unique chemical templates for the synthesis of new herbicide classes with novel molecular modes of action. To date, the most successful microbial products that have led to the development of commercial herbicides are bialaphos (commercially available in Japan) and glufosinate (marketed worldwide). Glufosinate is the ammonium salt of phosphinothricin, which is the active ingredient of bialaphos derived from a nonphytopathogenic Streptomyces species. This overview will examine selected advances in the isolation and identification of novel plant pathogens that have weed hosts, and some microbial allelochemicals with phytotoxic properties. Perspectives on the use of these bioherbicides in weed control, relative to their allelopathic interactions with plants will be discussed.

Nomenclature: Bialaphos; glufosinate.

Additional index words: Mode of action, rhizobacteria, mycoherbicide, phytopathogen, allelopathy, phytotoxin, natural product, enzyme inhibitor.

Abbreviations: AAL toxin, toxin produced by Alternaria alternata (Fr.:Fr.) Keissl. f.sp. lycopersici; AAT, aspartate amino transferase; AOA, aminooxyacetate; DRB, deleterious rhizobacteria; FB₁, fumonisin B₁; GS, glutamine synthetase; HMG-CoA, 3-hydroxy-3-methylglutaryl-coenzyme A; PGPR, plant growth–promoting rhizobacteria; PPT, phosphinothricin.

Kalmia angustifolia is a problematic understory shrub in boreal forests of eastern Canada. In this article, we discuss the nature of its interference with conifer regeneration, and control measures using herbicides. Although laboratory studies suggest the leaching of water-soluble phenolic compounds from Kalmia, allelopathic interference cannot be invoked as a cause of conifer regeneration failure because of lack of field evidences. Because of severe inhibitory effects of Kalmia on conifer growth and its role in long-term habitat degradation, the development of strategies for Kalmia management is a priority research area. Neither prescribed burning, plowing, scarification, nor 2,4-D application was proven effective in Kalmia control. Two studies were carried out using transplanted Kalmia to determine the efficacy of several herbicides and a herbicide plus burning treatment to control Kalmia. In one experiment, eight treatments were applied. They were control, glyphosate (3.36 kg/ha), glyphosate (2.88 kg/ha) plus surfactant Tween 20 (0.2%), glyphosate (2.88 kg/ha) plus surfactant Triton XR (0.2%), fosamine ammonium (5.38 kg/ha), triclopyr (4 kg/ha), hexazinone (2.2 kg/ha), and hexazinone (5 kg/ha). Triclopyr (4 kg/ha) provided the best control with significant reduction in the number and dry weight of sprouts, rhizomes, and current-year foliage of Kalmia compared with control. In another experiment, triclopyr (4 kg/ha), hexazinone (5 kg/ha), and hexazinone (2.2 kg/ha) plus burning treatments were applied. To determine the response of black spruce seedlings to the herbicide treatments, black spruce seedlings were planted in all the Kalmia pots except the ones that received the hexazinone (2.2 kg/ha) plus burning treatment. Triclopyr (4 kg/ha) caused the maximum damage to Kalmia, affecting all the above- and below-ground components of the plant. We recommend that an experimental field trial be established using triclopyr to test its efficacy for Kalmia control under field conditions.

Nomenclature: Glyphosate; fosamine ammonium; triclopyr; hexazinone; Triton XR; Tween 20; Kalmia, Kalmia angustifolia (L.); black spruce, Picea mariana (Mill.) BSP.

Additional index words: Allelopathy, regeneration failure, surfactants.

Pollen allelopathy results when pollen releases toxins that inhibit seed germination, seedling emergence, sporophytic growth, or sexual reproduction. Of the six known pollen-allelopathic species, two are crops (timothy and corn and four are weeds (orange hawkweed, parthenium, yellow hawkweed, and yellow-devil hawkweed). Allelopathic pollen in weeds could pose threats to crops, especially if both are wind pollinated. Even if it is the crop that is pollen-allelopathic, other crops could be threatened, or more likely, weeds might adapt to pollen allelopathy and pose a greater problem. Nonetheless, pollen allelopathy could be a useful approach to biological control because allelochemicals are packaged in a natural targeting system (pollen grains) and are biologically active at low doses (<10 grains/mm² on stigmas). If it is to be an effective biological control agent, pollen allelopathy must be examined within the wider context of farming systems management and used as one method of varying selection pressures to prevent weeds from adapting to any one particular management technique or suite of techniques.

Nomenclature: Corn, Zea mays L. var. chalquiñocónico Hernández; orange hawkweed, Hieracium aurantiacum L. #³ HIEAU; yellow hawkweed, Hieracium pratense Tausch # HIECA; yellow-devil hawkweed, Hieracium floribundum W. et G. # HIEFL; parthenium, Parthenium hysterophorus L. # PTNHY; timothy, Phleum pratense L. # PHLPR.

Additional index words: Biological control, farming systems, heterospecific pollen transfer, improper pollen transfer, integrated weed management, pollination, Elytrigia repens, Sonchus oleraceus, AGRRE, SONOL.

Several weed species have been reported to have allelopathic activities. However, most of these studies indicate the probable involvement of allelochemicals but are not conducted in field settings. In addition to their adverse effects on growth and yield of many crop species, many troublesome weeds such as mugwort and lantana influence biodiversity. More studies on the ecological, physiological, and molecular aspects of weed allelopathy should be conducted in order to better understand community structure and declining biodiversity.

Nomenclature: Lantana, Lantana camara L. #³ LANCA; mugwort, Artemisia vulgaris L. # ARTVU.

Additional index words: Allelochemicals, plant diversity, allelopathy, phytotoxicity.

Field studies were conducted from 1997 to 1999 in Westminster, MD, to evaluate a variety of preemergence (PRE) and postemergence (POST) herbicide programs on crop injury and control of triazine-resistant common lambsquarters (TR-CHEAL) in no-till corn. In 1997 PRE studies, combinations of metolachlor with flumetsulam or halosulfuron and high rates of rimsulfuron thifensulfuron (0.02 0.009 kg ai/ha) injured corn most 4 weeks after treatment (WAT), averaging 11 to 15%. In 1998 and 1999, metolachlor plus a high rate of halosulfuron (0.07 kg/ha) injured corn most 4 WAT, averaging 13 and 10%, respectively. High rates of rimsulfuron thifensulfuron also provided a higher level of corn injury in 1998 and 1999 in comparison with many of the other treatments. However, for all three years of the study, no injury was observed from any PRE treatment 8 WAT. In 1997 and 1998, at 8 WAT, combinations of metolachlor with flumetsulam or halosulfuron provided greater TR-CHEAL control than many of the other treatments, averaging 98 and 100%, respectively. In 1999, however, control of TR-CHEAL with these same treatments did not vary in comparison with most of the other treatments. At 8 WAT, there was a trend for increased TR-CHEAL control as the rates of RPA-201772 and rimsulfuron thifensulfuron increased. Control of TR-CHEAL with metolachlor atrazine pendimethalin varied across years 8 WAT. Similar observations were made 16 WAT. In 1997, POST applications of dicamba, SAN 1269H at 0.3 lb ai/ha, primisulfuron dicamba, and primisulfuron CGA 152005 dicamba provided the highest level of TR-CHEAL control 8 WAT, averaging 93, 93, 95, and 93%, respectively. In 1998, with the exception of carfentrazone atrazine, all POST treatments provided 90% control of TR-CHEAL or better 8 WAT. In 1999, POST applications of SAN 1269H at 0.3 kg/ha, pyridate atrazine, and primisulfuron CGA 152005 pyridate provided the highest level of TR-CHEAL control, averaging 80, 90, and 96%, respectively, 8 WAT. With the exception of carfentrazone atrazine, control of TR-CHEAL with the other POST treatments varied in 1999 from 60 to 74% 8 WAT. Carfentrazone atrazine applied POST provided the lowest level of TR-CHEAL control 8 WAT averaging 28, 37, and 17% for 1997 to 1999, respectively.

Nomenclature: Atrazine; carfentrazone; dicamba; flumetsulam; halosulfuron; RPA-201772 (proposed name, isoxaflutole) [5-cyclopropyl-4-(2-methylsulfonyl-4-trifluoromethyl-benzoyl)isoxazole]; metolachlor; pendimethalin; CGA 152005 (proposed name, prosulfuron) {1-(4-methoxy-6-methyltriazin-2-yl)-3-[2-(3,3,3-trifluoropropyl)-phenylsulfonyl] urea}; rimsulfuron; SAN 1269H (formerly BAS 662H), a mixture of SAN 835H {2-[1-[[[(3,5-difluorophenyl)amino]carbonyl]hydrozono]ethyl]-3-pyridinecarboxylic acid} and dicamba in a 1:2.5 ratio; thifensulfuron methyl; common lambsquarters, Chenopodium album L. #³ CHEAL; corn, Zea mays L. Pioneer 3140.

Additional index words: Herbicide-resistant weeds.

Abbreviations: PRE, preemergence; POST, postemergence; TR-AMACH, triazine-resistant smooth pigweed; TR-CHEAL, triaizine-resistant common lambsquarters; TS-CHEAL, triazine-susceptible common lambsquarters; WAT, weeks after treatment.

The competitive ability of five cool-season grasses relative to Dalmatian toadflax, musk thistle, and downy brome was assessed in two field studies. In 1994, Bozoisky Russian wildrye and four wheatgrass varieties (Critana thickspike, Hycrest crested, Luna pubescent, and Sodar streambank wheatgrass) were seeded into populations of downy brome and musk thistle at Riverside, WY. The same grasses were seeded into populations of Dalmatian toadflax at Cheyenne, WY, in 1995. In 1997 and 1998, weed populations at both study sites were reduced in areas seeded with the five grasses relative to unseeded controls. Hycrest crested and Luna pubescent wheatgrasses were the most competitive against the three weed species. Bozoisky Russian wildrye was more competitive against Dalmatian toadflax than against the other weeds. Sodar streambank wheatgrass suppressed musk thistle and downy brome but was not competitive against Dalmatian toadflax. Seeded grasses, such as Hycrest crested and Luna pubescent wheatgrass, appeared to limit the re-establishment of these weeds. Economic model predictions of the net present values and the internal rates of return suggest that Hycrest crested and Luna pubescent wheatgrass can provide financially feasible long-term weed control only if desired grass yields are maintained for more than 15 yr.

Nomenclature: Crested wheatgrass, Agropyron cristatum (L.) Gaertn. × Agropyron desertorum Gaertn. var. ‘Hycrest’; Dalmatian toadflax, Linaria genistifolia spp. dalmatica (L.) Maire and Petitmengi #³ LINDA; downy brome, Bromus tectorum L. # BROTE; musk thistle, Carduus nutans L. # CRUNU; pubescent wheatgrass, Thinopyrum intermedium (Host. Barkworth & Dewey) Nevski var. ‘Luna’; Russian wildrye, Psathyrostachys juncea (Fisch) Nevski var. ‘Bozoisky’; streambank wheatgrass, Elymus lanceolatus (Scribn. & J. G. Smith) Gould var. ‘Sodar’; thickspike wheatgrass, Elymus macrourus (Turcz.) Tzvelev var. ‘Critana’.

Additional index words: Competition, economic analysis, integrated controls, perennial grass.

Abbreviations: ANOVA, analysis of variance; IRR, internal rate of return; LSD, least significant difference; NPV, net present value.