Widespread groundwater contamination has prompted studies on the fate and transport of solute through soil. Large quantities of atrazine and metribuzin are applied annually in the Humid Pampas of Argentina, creating the need to study the fate of these herbicides in soils of the region. The objective of this work was to study the vertical transport of atrazine and metribuzin in packed soil columns for three loam soils representative of the Humid Pampas of Argentina. Bromide was used as a nonreactive tracer. The convection dispersion equation was fitted to chemical breakthrough data to obtain a parameter characterizing chemical transport. Bromide breakthrough curves (BTCs) were similar among soils. BTCs for atrazine and metribuzin revealed significant interaction among soils and herbicides. The average values for the organic carbon (OC) partition coefficients (K_oc) derived from column flow experiments were 119 and 48 ml/g for atrazine and metribuzin, respectively. Metribuzin in leachate was 97.3% of the total recovered, whereas atrazine was 3.5%. This behavior can be explained by their different affinities to OC. The OC contents of the Balcarce, Necochea, and Nueve de Julio soils were 4.1, 3.4, and 1.9%, respectively. The lowest leaching values of herbicides were found in the Balcarce soil, suggesting that OC content was the main factor in controlling herbicide transport in these soils.

Nomenclature: Atrazine; bromide; metribuzin.

Additional index words: GC/MS, groundwater contamination, herbicide transport, soil columns.

Abbreviations: BTC, breakthrough curve; CDE, convection dispersion equation; OC, organic carbon; PV, pore volume; PVC, polyvinyl chloride.

Studies were conducted in 1999, 2000, and 2001 to evaluate cotton response to CGA 362622 applied postemergence with various adjuvants. In field studies, CGA 362622 was applied at 3.8 or 7.5 g ai/ha with nonionic surfactant, crop-oil concentrate (COC), or a urea-based adjuvant. A nontreated control was maintained weed free for comparison. Crop injury over all years at 1 wk after treatment (WAT) was 27 and 34% from 3.8 and 7.5 g/ha CGA 362622, respectively, when rates were pooled over adjuvants. At 4 WAT, injury was 6 to 14% with 3.8 g/ha and 10 to 21% with 7.5 g/ha CGA 362622 during the 3-yr study. Cotton heights at 2 WAT were reduced by 16 to 31% of nontreated cotton heights by CGA 362622. Heights of treated cotton did not differ and were generally equivalent to the nontreated control at 8 WAT. Cotton injury and height reduction were greatest when CGA 362622 was applied with COC. Cotton lint yields and fiber quality were not affected by CGA 362622 rate or adjuvant treatment. Cotton injury from CGA 362622 in the greenhouse was similar to that in the field. Initial cotton injury and subsequent reduction in leaf area or shoot dry weight were generally lowest when CGA 362622 was applied with no adjuvant or UBA in the greenhouse.

Nomenclature: CGA 362622 (proposed common name trifloxysulfuron sodium), N-[(4,6-dimethoxy- 2-pyrimidinyl)carbamoyl]-3-(2,2,2-trifluoroethoxy)-pyridin-2-sulfonamide sodium salt; cotton, Gossypium hirsutum L. ‘SureGrow 125.’

Additional index words: Sulfonylurea herbicide.

Abbreviations: ALS, acetolactate synthase enzyme (EC 4.1.3.18); COC, crop oil concentrate; MSO, methylated seed oil; NA, no adjuvant; NIS, nonionic surfactant; OSS, organosilicone surfactant; POST, postemergence; UBA, urea-based adjuvant; WAT, weeks after treatment.

Field studies were conducted in 1999, 2000, and 2001 to evaluate broadleaf weed control in glyphosate-resistant cotton by glyphosate plus CGA 362622 applied postemergence. Treatments included 560 and 1,120 g ai/ha glyphosate-isopropylamine alone or in mixtures with CGA 362622 at 3.8 and 7.5 g ai/ha, and CGA 362622 at 7.5 g/ha alone. Cotton injury 7 d after treatment (DAT) was 3 to 11% from glyphosate alone and 16 to 24% from glyphosate plus CGA 362622. Injury 28 DAT with CGA 362622 or herbicide mixtures did not exceed 6%. Broadleaf weed control by herbicide mixtures was generally more consistent than control from either herbicide applied alone. Glyphosate plus CGA 362622 controlled common cocklebur and smooth pigweed better than glyphosate alone. In most instances, the mixtures also controlled common ragweed, common lambsquarters, ivyleaf morningglory, pitted morningglory, and tall morningglory better than glyphosate applied alone. Common cocklebur and smooth pigweed were controlled at least 85% by all treatments. CGA 362622 did not control spurred anoda or jimsonweed. Cotton yields generally reflected weed control. According to these results, glyphosate plus CGA 362622 mixtures can consistently control many broadleaf weeds in cotton.

Nomenclature: CGA 362622 (proposed common name trifloxysulfuron sodium), N-[(4,6-dimethoxy- 2-pyrimidinyl)carbamoyl]-3-(2,2,2-trifluoroethoxy)-pyridin-2-sulfonamide sodium salt; glyphosate; common cocklebur, Xanthium strumarium L. #³ XANST; common lambsquarters, Chenopodium album L. # CHEAL; common ragweed, Ambrosia artemisiifolia L. # AMBEL; ivyleaf morningglory, Ipomoea hederacea (L.) Jacq. # IPOHE; jimsonweed, Datura stramonium L. # DATST; pitted morningglory, Ipomoea lacunosa L. # IPOLA; smooth pigweed, Amaranthus hybridus L. # AMACH; spurred anoda, Anoda cristata (L.) Schlecht. # ANVCR; tall morningglory, Ipomoea purpurea (L.) Roth # IPOPU; cotton, Gossypium hirsutum L. ‘PM 1220 RR’, ‘PM 1218 BG/RR’, ‘SG 521 RR’.

Additional index words: Herbicide mixtures, morningglory species.

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

Field experiments were conducted near Chickasha, OK, in 1999 and 2000 and near Perkins, OK, in 2000 to evaluate the effects of Palmer amaranth on harvest efficiency of grain sorghum and full-season competition with that crop. Weed densities were 0 (the weed-free check), 1, 2, 4, 6, 9, 12, and 18 plants/15 m of row. In the harvest efficiency experiments, each additional weed per 15 m of row increased grain moisture before cleaning by 0.7 and 0.2% at Chickasha in 1999 and at Perkins, respectively. After cleaning, it increased moisture by 0.2% in both the experiments for each weed. Foreign material increased 67, 2, and 3 kg/ha at Chickasha in 1999, Chickasha in 2000, and Perkins, respectively. At Chickasha in 2000, sorghum seed loss through the combine increased 11 kg/ha for each additional weed per 15 m of row. Grain grades improved at Perkins at higher weed densities. In the competition experiments, grain yield decreased by 1.8 to 3.5% for each increase of 1 weed/15 m of row. More weeds resulted in higher weed dry weight. Each kilogram of Palmer amaranth dry weight per plot reduced grain yield by 5.3 to 9.1%. In 2000, sorghum seeds per panicle were reduced by 27 to 50 for each weed. Grain grades generally decreased as weed density increased at Chickasha in 2000 but not at Perkins.

Nomenclature: Palmer amaranth, Amaranthus palmeri S. Wats. #³ AMAPA; grain sorghum, Sorghum bicolor (L.) Moench ‘Cherokee’.

Additional index words: Competition effects, foreign material, grain grade, grain moisture, grain yield, harvest efficiency, seed loss during harvest, test weight.

Plant responses to various doses of herbicides usually follow a sigmoid model where the potency is given by the 50% inhibition (I₅₀) value. To assess the potency of a herbicide under a range of environmental conditions, a series of independent bioassays are necessary to account for assay-to-assay variation. Analysis has conventionally been done by separate analysis of the individual bioassays or by simply pooling data. Analyzing the individual bioassays separately throws up relevant information on interassay variation. Such a model becomes too complex because a full set of model parameters is needed for each data set. Pooling data instead, and analyzing the bioassay jointly, inflates parameter uncertainty because of oversimplification. Such a simple model would have too few variables, and the fixed-effect estimates would be more uncertain because they would be explaining the interassay random effects. This means that the underlying statistical model is not realistic. Therefore, we propose a new technique of intermediate complexity that outperforms either technique and provides biologically realistic estimates that allow us to compare herbicide potencies. With this technique, we simultaneously analyze independent experiments by using a combination of nonlinear regression and mixed models. The case study uses a group of independently run bioassays with two photosystem II–inhibiting herbicides, diuron and bentazon, by measuring the oxygen evolution of thylakoid membranes. The introduction of random elements in the nonlinear regression parameters reduces the uncertainty in the parameters of interest. We demonstrate that it is possible to pool data from independent experiments to assess which parameters can be assigned a random element, to conduct hypothesis testing, and to calculate stable confidence limits and thus obtain a more precise interpretation of the biologically relevant parameters, such as I₅₀, compared with the conventional nonlinear regression models of the individual bioassays.

Nomenclature: Bentazon; diuron.

Additional index words: Bioassay, log-logistic analysis, maximum likelihood.

Abbreviation: ML, maximum likelihood.

Field studies were conducted to evaluate red morningglory control with 2,4-D alone and in combination with dicamba, along with other postemergence herbicides applied both over the top and as directed treatments. For red morningglory 30 and 60 cm in height, complete control 14 or 21 d after treatment (DAT) was obtained during 2 yr with 2,4-D at 0.53 kg ai/ha, 2,4-D at 0.4 kg/ ha or more plus dicamba, atrazine at 2.23 kg ai/ha, flumioxazin at 0.10 kg ai/ha, and sulfentrazone at 0.35 kg ai/ha. When red morningglory were 1.8 m, weed control with most herbicides was less consistent than when applied to smaller plants. Red morningglory (1.8 m) was controlled 100% 28 DAT the first year with 2,4-D at 1.06 kg/ha and 78% the second year. In the first year, the 2,4-D plus dicamba prepackaged mixture at 0.8 0.28 kg ai/ha or 2,4-D plus the 2,4-D/dicamba prepackaged mixture (0.53 0.2/0.07, 0.53 0.4/0.14, or 0.79 0.1/0.04 kg/ha) provided control equal to that by 2,4-D alone at 1.06 kg/ha. In the second year, when herbicides were applied 3 wk earlier than the previous year and when weed growth was more vigorous, the 2,4-D plus the 2,4-D/dicamba prepackaged mixture at 0.79 0.1/0.04 kg/ha provided control equal to that by 2,4-D alone at 1.06 kg/ha but was the only 2,4-D plus dicamba treatment to control red morningglory equal to that by 2,4-D at 1.59 kg/ha (87%). Directed applications to the lower 45 cm of 1.8-m red morningglory plants with atrazine at 4.47 kg/ha and sulfentrazone at 0.35 kg/ha controlled weeds at least 96% 28 DAT in 2001, but control was 23 and 30 percentage points less, respectively, the second year.

Nomenclature: Atrazine; 2,4-D; dicamba; flumioxazin; sulfentrazone; red morningglory, Ipomoea coccinea L. #³ IPOCC.

Additional index words: Saccharum interspecific hybrids, sugarcane.

Abbreviations: DAT, days after treatment; POST, postemergence; POST-DIR, postemergence directed; PRE, preemergence.

In 1998, 1,260 soil samples were collected from 63 of 99 Iowa counties to characterize the weed seedbanks in fields under the conservation reserve program (CRP) and adjacent fields under continuous cultivation. Five annual grass and 13 broadleaf weed species were identified in both the CRP and adjacent cultivated fields. Seedbank differences between CRP and adjacent cultivated fields were evident only for foxtails, common lambsquarters, pigweeds, and sweetclover, with the average of 3,288, 10,681, 38, and 1,709 seeds/m², respectively; the corresponding seed population in adjacent CRP fields was 59, 57, 1,924, and 74%, respectively. However, weed species diversity was not significantly different between fields in CRP and continuous cultivation. Only CRP fields in the northwest Iowa crop-reporting district had a higher foxtail species seed population (4,915 seeds/m²) than the adjacent cultivated fields (1,782 seeds/m²). Land under CRP in northern (N), eastern, and southern (S) districts had 58% (4,158 seeds/m²), 6% (312 seeds/m²), and 18% (594 seeds/m²) of the continuously cultivated foxtail species seedbank. Common lambsquarters seed populations were 4,128 and 3,801 seeds/m² in the cultivated fields of the N and central (C) districts, compared with 772 and 252 seeds/m² in adjacent CRP fields, respectively. Pigweed species seeds were more numerous in the cultivated fields than in adjacent CRP fields in the northeast, C, and S Iowa districts. Sweetclover seed population was consistently higher in CRP land because it was included as part of the CRP covers seeding. Overall, broadleaf weed seeds comprised 90% of the seedbanks in both CRP and adjacent cultivated land. A competitive cover crop canopy in CRP probably reduced weed seedbanks by suppression of weeds and seed production. Also, annual seed production, differences in weed biology, and differential herbicide performance in cultivated fields may have contributed to higher seed populations.

Nomenclature: Common lambsquarters, Chenopodium album L. #³ CHEAL; foxtails, Setaria spp. # SETXX; pigweeds, Amaranthus spp. # AMAXX; sweetclover, Melilotus spp. # MEUXX.

Additional index words: Conservation reserve program, seedbank diversity, Shannon's diversity index.

Abbreviations: ANOVA, analysis of variance; C, central; CRP, conservation reserve program; E, eastern; N, northern; NE, northeast; NW, northwest; S, southern; SE, southeast; SW, southwest; W, western.

A 2-yr field study was conducted in Blackville, SC, to assess the potential for using small- grain cover crops and glyphosate as a means of reducing or eliminating the need for atrazine in irrigated Southeastern corn production. Oats, rye, and wheat were no-till, drill seeded each fall and subsequently desiccated in early spring before corn planting. A bareground conventional tilled treatment also was included. Within each cover crop system, the herbicides evaluated included (1) no herbicide; (2) 1.68 kg ai/ha atrazine plus 1.08 kg ai/ha S-metolachlor at corn planting followed by 0.84 kg ae/ha glyphosate; or (3) two applications of 0.84 kg/ha glyphosate alone, applied sequentially. All systems were compared with conventional tillage without a cover crop with 1.68 kg/ha atrazine plus 1.08 kg/ha S-metolachlor at planting followed by 1.12 kg/ha atrazine. Biomass of rye, oats, and wheat at dessication was 497, 369, and 340 g/m², respectively. All cover crops delayed early-season corn growth. Detrimental effects on early-season corn growth from the oats cover crop were still apparent 7 wk after emergence (WAE), with corn height and biomass reduced 8 and 19%, respectively. Weed biomass in nontreated plots was reduced 84, 68, and 21% by oats, rye, and wheat, respectively, 3 WAE. Oats were more inhibitory of corn and weed growth than rye, although rye produced greater surface residue, indicating possible allelopathy affects. In the presence and absence of each cover crop, atrazine plus S-metolachlor followed by glyphosate or sequential glyphosate applications alone were effective in providing season-long control of pitted morningglory, entireleaf morningglory, Palmer amaranth, Florida pusley, large crabgrass, and common bermudagrass. Corn yields and gross profit margins in sequential glyphosate–treated plots were equivalent or superior to the standard atrazine-based program, which indicates that effective and economical alternatives to atrazine are available.

Nomenclature: Atrazine; glyphosate; S-metolachlor; common bermudagrass, Cynodon dactylon (L.) Pers. #³ CYNDA; entireleaf morningglory, Ipomoea hederacea var. integriuscula Gray # IPOHG; Florida pusley, Richardia scabra L. # RCHSC; large crabgrass, Digitaria sanguinalis (L.) Scop. # DIGSA; Palmer amaranth, Amaranthus palmeri S. Wats. # AMAPA; pitted morningglory, Ipomoea lacunosa L. # IPOLA; corn, Zea mays L. ‘DK662 RR’; oats, Avena sativa L. ‘Coker 820’; rye, Secale cereale L. ‘Wrenz’; wheat, Triticum aestivum L. ‘Pioneer 2684’.

Additional index words: Allelopathy, conservation tillage, integrated weed management, strip tillage, transgenic corn.

Abbreviations: POST, postemergence; WAE, weeks after emergence.

Although some herbicides are available for control of broadleaf weeds on rangeland, currently no herbicides are registered for selective control of weedy annual grasses in perennial forage grasses. Field experiments were conducted to determine the tolerance of several perennial forage grass species to preemergence (PRE) and postemergence (POST) applications of imazapic, a herbicide that controls certain weedy annual grasses. In PRE studies, perennial forage grasses were seeded 1 d after spraying with several rates of imazapic. The grass species by herbicide rate by location and the grass species by herbicide rate interactions were not significant for plant height and biomass 395 d after treatment (DAT). Expressed as a percentage of the untreated control, imazapic applied at 18 to 140 g/ha reduced height of all grass species 10 to 18%, whereas 280 g/ha of imazapic reduced height 39%. Imazapic applied at 18 to 70 g/ha reduced biomass 12 to 26%. Biomass was reduced 51 and 63% when imazapic was applied at 140 and 280 g/ha, respectively. Thus, rates of imazapic required to control downy brome likely will excessively injure perennial forage grass seeded 1 DAT. In POST studies, imazapic was applied to 1-yr-old stands of perennial forage grass. A dose– response model provided a good fit for grass species biomass and height data. In year 1, biomass and height of orchardgrass, smooth brome, and meadow brome were reduced 14 to 29% more than those of bluebunch, crested, intermediate, and western wheatgrass as imazapic rate increased, which implies that the wheatgrasses were more tolerant to imazapic. However, in year 2, slope of regression lines did not differ among grass species, implying that all forage grass species responded the same to increasing rates of imazapic. Plant height of all grass species decreased 25 to 56% when compared with the untreated control, and biomass decreased 28 to 59% as imazapic rate increased from 18 to 280 g/ha. As discussed previously, rates of spring-applied imazapic required for downy brome control severely injured perennial forage grasses whether applied PRE or POST. The level of tolerance of perennial forage grasses to imazapic depended on herbicide dose and perhaps environmental differences between years.

Nomenclature: Imazapic; bluebunch wheatgrass, Agropyron spicatum (Pursh) Scribn. & Smith. #³ AGRSP; crested wheatgrass, Agropyron cristatum (L.) Gaertn. # AGRCR; downy brome, Bromus tectorum L. # BROTE; intermediate wheatgrass, Agropyron intermedium (Host) Beauv. # AGRIN; meadow brome, Bromus erectus Huds. # BROER; orchardgrass, Dactylis glomerata L. # DACGL; smooth brome, Bromus inermis Leyss. # BROIN; western wheatgrass, Pascopyrum smithii Rydb. # PASSM.

Additional index words: Biomass, picloram, plant density, tolerance.

Abbreviations: DAT, days after treatment; OM, organic matter; POST, postemergence; PP, preplant; PRE, preemergence.

The effects of pelargonic acid and rainfall on glyphosate activity, absorption, and translocation in trumpetcreeper were investigated. Four- to six-leaf–stage plants raised from rootstocks were treated with glyphosate at 0, 0.42, 0.84, 1.68, and 3.36 kg ae/ha. Glyphosate at 1.68 kg/ha and higher controlled trumpetcreeper >98% and completely inhibited regrowth from rootstocks of treated plants. A simulated rainfall of 2.5 cm water applied at 6 h after glyphosate application (HAA) reduced efficacy by one-fifth compared with no rainfall. Absorption of ¹⁴C-glyphosate in trumpetcreeper increased from 2.3 to 20.2%, whereas translocation increased from 0.4 to 10.5% from 6 to 192 HAA. At 192 HAA, 9.7% of the recovered ¹⁴C-label remained in the treated leaf, 0.6% moved above the treated leaf, and 9.0% moved to fibrous roots and rootstock. The addition of pelargonic acid to glyphosate did not improve glyphosate absorption or translocation or synergize activity in trumpetcreeper compared with glyphosate alone. These results suggest that a 24-h rain-free period and 4 d without disturbance from tillage could maximize glyphosate absorption and translocation in trumpetcreeper.

Nomenclature: Glyphosate; pelargonic acid; trumpetcreeper, Campsis radicans (L.) Seem. ex Bureau #³ CMIRA.

Additional index words: Absorption, interaction, rainfastness, regrowth, translocation, uptake.

Abbreviations: HAA, hours after application; WAT, weeks after treatment.

Tolerance of nine rice varieties to clomazone at 1.12 kg ai/ha was evaluated from 2000 to 2002. Rice injury was 27 to 51% at 14 d after treatment (DAT) and reduced to 5 to 30% at 42 DAT with long-grain ‘Drew’ having less injury compared with all medium-grain varieties. Medium- grain ‘Earl’ and ‘LL-401’ were injured most compared with all other varieties at 42 DAT. Plant height was reduced by clomazone with all varieties except Drew at 34 DAT. Clomazone also reduced plant population of Earl, LL-401, and ‘Wells’, but other varieties were not affected at 34 DAT. However, rice grain yield reduction was only observed with LL-401. These results indicate that differential tolerance to clomazone exists among rice varieties.

Nomenclature: Clomazone; rice, Oryza sativa L. ‘Bengal’, ‘Cocodrie’, ‘Cypress’, ‘Drew’, ‘Earl’, ‘CL-141’, ‘LL-401’, ‘LL-601’, ‘Wells’.

Additional index words: Herbicide tolerance.

Abbreviations: DAT, days after treatment; DPRE, delayed preemergence; PPI, preplant incorporated; PRE, preemergence.

The optimum weeding regime for thorny mimosa control in cassava established at 10,000 plants/ha was studied at Ibadan, Nigeria (7°22½′N, 3°50½′ E), a humid tropical environment. The study compared six weeding regimes, each comprising manual removal of thorny mimosa three times at different intervals within 13 wk after planting (WAP). Cassava vegetative growth recovered from thorny mimosa interference when the first weeding occurred within 5 WAP, but interference for more than 5 WAP reduced storage root yield. Allowing thorny mimosa infestation after 11 WAP had no effect on cassava growth or root yield. Manual removal of thorny mimosa at 4, 7, and 11 WAP consistently gave the highest cassava root yield.

Nomenclature: Thorny mimosa (or giant sensitive plant), Mimosa invisa Mart.; cassava, Manihot esculenta Crantz ‘TMS 30572’.

Additional index words: Canopy closure, cassava vegetative growth, storage root yield, weed interference.

Abbreviations: IAR & T, Institute of Agricultural Research and Training; MAP, months after planting; WAP, weeks after planting.

Volunteer potato is a perennial weed that is difficult to control in crop rotations. Field studies were conducted near Paterson, WA, in 2001 and 2002 to evaluate the control of volunteer potato with carfentrazone-ethyl and dicamba in field corn. When potatoes were not controlled corn yield was reduced 23 and 62% in 2001 and 2002, respectively. Single postemergence (POST) applications of carfentrazone-ethyl at 9 g/ha killed exposed foliage of potato, but new shoots continued to emerge both years and reduced corn yield in 2002. The most effective treatments tested were a single mid-postemergence application of carfentrazone-ethyl plus dicamba (9 280 g/ha), two applications of carfentrazone-ethyl alone at early postemergence and late postemergence, and three POST applications of carfentrazone-ethyl, which controlled volunteer potato 77 to 87% in early June, reduced weight of tubers produced by 76 to 96% compared with nontreated checks, and prevented corn yield loss compared with hand-weeded checks. Herbicide treatments reduced potato tuber weight more than tuber number.

Nomenclature: Carfentrazone-ethyl; dicamba; field corn, Zea mays L.; potato, Solanum tuberosum L. ‘Russet Burbank’.

Additional index words: Groundkeeper (volunteer potato).

Abbreviations: EPOST, early postemergence; LPOST, late postemergence; MPOST, mid-postemergence; POST, postemergence.

Field experiments were conducted to evaluate possible interactions of clethodim with imazapic applied as mixtures or sequentially for control of broadleaf signalgrass, fall panicum, goosegrass, and large crabgrass. Imazapic at 70 g ai/ha alone controlled grass weeds inconsistently, whereas clethodim at 140 g ai/ha alone controlled the same weeds at least 99%. Imazapic did not affect broadleaf signalgrass control by clethodim. Reduced control of fall panicum, goosegrass, and large crabgrass was observed when clethodim and imazapic were applied in mixture. Antagonism of clethodim occurred when clethodim was applied 1 d before or up to 3 d after application of imazapic (fall panicum and large crabgrass). Antagonism of goosegrass control was noted when imazapic was applied 3 d before or up to 7 d after application of clethodim. In other experiments, large crabgrass and Texas panicum control by clethodim (70 and 140 g/ha) applied alone or with imazapic (70 g/ ha) or bentazon (1.1 kg ai/ha) plus 2,4-DB (0.28 kg ai/ha) either with or without ammonium sulfate (2.8 kg/ha) was evaluated. Texas panicum control by clethodim was reduced by imazapic regardless of the ammonium sulfate rate. However, large crabgrass control by imazapic was not affected in these experiments. Control of both grasses by clethodim was reduced substantially by bentazon plus 2,4-DB, although in some instances ammonium sulfate improved control when in mixture. Ammonium sulfate improved control by clethodim in some instances irrespective of the broadleaf–sedge herbicide treatments.

Nomenclature: Bentazon; clethodim; 2,4-DB; imazapic; broadleaf signalgrass, Brachiaria platyphylla (Griseb) Nash #³ BRAPP; fall panicum, Panicum dichotomiflorum L. # PANDI; goosegrass, Eleusine indica L. Gaertn. # ELEIN; large crabgrass, Digitaria sanguinalis L. Scop. # DIGSA; Texas panicum, Panicum texanum Buckl. # PANTE.

Additional index words: Ammonium sulfate, antagonism, herbicide compatibility, herbicide interaction, sequential application.

Soft red winter wheat tolerance to and Italian ryegrass control by a mixture of AE F130060 00 plus AE F115008 00 plus safener applied in water or urea ammonium nitrate (UAN) were evaluated in separate experiments. In the tolerance experiment, wheat responded similarly to AE F130060 00 plus AE F115008 00 at 12.5 plus 2.5 and 25 plus 5 g ai/ha, respectively, applied in water. The herbicides plus nonionic surfactant (NIS) applied in water injured weed-free, five- to seven-tiller wheat 3% or less and did not affect yield. Greater injury occurred with application in UAN, and yield was reduced 11% as result of fewer kernels per spike. NIS added to the herbicides in UAN increased weed-free wheat injury but had no effect on yield. AE F130060 00 plus AE F115008 00 controlled both diclofop-susceptible and -resistant Italian ryegrass. Greater control was obtained with application in UAN as compared with application in water, and NIS increased control. Yield of Italian ryegrass–infested wheat treated with herbicides plus NIS in UAN was similar to or greater than yield when herbicides plus NIS were applied in water.

Nomenclature: AEF 115008 00 (proposed name iodosulfuron-methyl-sodium), 4-iodo-2-[[[[(4-methoxy-6-methyl-1,3,5-triazin-2-yl)amino]carbonyl]amino]sulfonyl]benzoic acid methyl ester; AE F130060 00 (proposed name mesosulfuron-methyl), methyl 2-[[[[(4,6-dimethoxy-2-pyrimidinyl)amino]carbonyl]amino]sulfonyl]-4-[[(methylsulfonyl)amino]methyl] benzoate; Italian ryegrass, Lolium multiflorum Lam. Marshall #³ LOLMU; wheat, Triticum aestivum L. ‘Coker 9704’.

Additional index words: Herbicide carriers, surfactant rates, yield components, LOLMU.

Abbreviations: GS, Feekes growth stage; NIS, nonionic surfactant; UAN, urea ammonium nitrate; WAT, weeks after treatment.

Parthenium is a competitive weed spreading in tropical countries. Field experiments were conducted to compare the effect of hand hoeing, growing a smother crop, and applying a herbicide (2,4-D) on parthenium growth and on yield of grain sorghum in smallholder farming systems in eastern Ethiopia. Hand hoeing twice and a smother crop (cowpea) in combination with hand hoeing once consistently suppressed parthenium at the experimental sites. Application of 2,4-D provided inconsistent control of parthenium, possibly because of reemergence from the soil seed bank after control. Growing cowpea as the smother crop suppressed parthenium, but sorghum grain and stalk yields were reduced even when the smother crop was combined with hoeing. Hand hoeing twice, 4 and 8 wk after emergence of sorghum, consistently resulted in better yields than application of 2,4- D and growing smother crop. It is possible that hoeing, apart from controlling the weed, also created better soil conditions for rain water infiltration.

Nomenclature: 2,4-D, (2,4-dichlorophenoxy)acetic acid; parthenium, Parthenium hysterophorus L.; cowpea, Vigna unguiculata (L.) Walp.; grain sorghum, Sorghum bicolor (L.) Moench.

Additional index words: Hand hoeing, smother crop, Ethiopia.

Lima bean cultivar ‘Improved Kingston’ was evaluated for sensitivity to herbicide treatments in a field experiment, conducted from 1999 to 2001 in Ontario. Crop was evaluated for visual injury at 7, 14, and 28 d after emergence following preemergence (PRE) applications of metolachlor (1,600 and 3,200 g ai/ha) and imazethapyr (75 and 150 g ai/ha). Crop visual injury to postemergence (POST) applications of imazamox fomesafen (25 200 g ai/ha and 50 400 g ai/ha) and quizalopfop-P (72 and 144 g ai/ha) was evaluated at 7, 14, and 28 d after treatment. Plant height and crop yield were also assessed. The imazamox fomesafen mixture caused significant visual injury and tended to decrease lima bean height and yield. Despite some initial injury observed in the metolachlor, imazethapyr, and quizalofop-P treatments, yield was not significantly decreased. Because of their margin of crop safety, metolachlor applied PRE at 1,600 g/ha, imazethapyr applied PRE at 75 g/ha, and quizalofop-P applied POST at 72 g/ha have excellent potential as weed management tools in Ontario lima bean production.

Nomenclature: Fomesafen; imazamox; imazethapyr; metolachlor; quizalopfop-P; lima bean, Phaseolus lunatus L.

Additional index words: Herbicide injury, sensitivity.

Abbreviations: DAE, days after emergence; DAT, days after treatment; OM, organic matter; POST, postemergence; PPI, preplant incorporated; PRE, preemergence; UAN, urea ammonium nitrate.

This study was conducted to evaluate the tolerance of two black bean cultivars, AC Harblack and Midnight Black Turtle, to preplant incorporated (PPI) and preemergence (PRE) applications of S-metolachlor at 1.6 and 3.2 kg ai/ha, imazethapyr at 0.075 and 0.15 kg ai/ha, and S- metolachlor plus imazethapyr at 1.6 plus 0.075 and 3.2 plus 0.15 kg ai/ha, respectively, at Exeter and Ridgetown, Ontario, Canada, in 2001 and 2002. There were generally no differences between the two cultivars in their responses to the herbicide treatments. PPI and PRE applications of S- metolachlor did not reduce black bean growth or yield. The PPI and PRE applications of imazethapyr alone or in tank mixture with S-metolachlor at the low and high rates did not have a significant effect on plant height, dry weight, seed moisture content, or yield at Ridgetown but caused as much as 14% visual crop injury at Exeter and reduced plant height, dry weight, and yield as much as 25, 40, and 49%, respectively. The higher rate of either herbicide alone or in tank mixture generally caused greater crop injury than the lower rate. At sites where there was a significant difference, the PPI application caused less crop injury than the PRE application. On the basis of these results, the PPI and PRE applications of S-metolachlor can be applied safely at the recommended label rate for the control of annual grass in black beans. However, the PPI and the PRE applications of imazethapyr alone and in tank mixture with S-metolachlor require careful application to avoid spray overlaps because there is potential for crop injury and yield reduction under some environmental conditions.

Nomenclature: Imazethapyr; S-metolachlor; black bean, Phaseolus vulgaris L. ‘AC Harblack’, ‘Midnight Black Turtle’.

Additional index words: Crop injury, crop tolerance, maturity, plant dry weight, plant height, yield.

Abbreviations: DAE, days after emergence; PPI, preplant incorporated; PRE, preemergence.

Studies were conducted to evaluate density-dependent effects of tropic croton on weed and peanut growth and peanut yield. Tropic croton remained taller than peanut throughout the growing season, yet tropic croton density did not affect peanut or tropic croton heights. Tropic croton biomass per plant decreased linearly with increasing plant density. Peanut pod weight decreased linearly 4.7 kg/ha with each gram of increase in tropic croton biomass per meter of crop row. The rectangular hyperbola model was used to describe effects of tropic croton density on percent peanut yield loss. Estimated coefficients for a (maximum yield loss) and i (yield loss per unit density as density approaches zero) were 81 and 26 in 1988, 41 and 33 in 1989, and 33 and 61 in 1998, respectively. Although a and i values varied between years, yield loss predictions were stable between years at weed densities below two plants per meter of crop row. Even though the results show that tropic croton is less competitive than many broadleaf weeds in peanut, it has potential to substantially reduce yields and subsequently reduce economic return.

Nomenclature: Tropic croton, Croton glandulosus var. septentrionalis Muell.-Arg. #³ CVNGS; peanut, Arachis hypogaea L. ‘NC 10C’, ‘Florigiant’.

Additional index words: Competition, economic threshold, models, plant height, weed biomass, weed density.

Abbreviations: PRE, preemergence.

Greenhouse and field experiments were conducted to investigate the effect of glyphosate rate and degree of glyphosate spray coverage on pitted morningglory control. Pitted morningglory in the two-, four-, and six-leaf growth stages were treated with the isopropylamine salt of glyphosate at 0.28, 0.56, 0.84, 1.12, 1.40, and 1.68 kg ai/ha. Two- and four-leaf plants were controlled 98% with 1.68 kg/ha glyphosate, whereas six-leaf plants were controlled 68%. Control of two-, four-, and six-leaf plants with the commonly used field rate of 1.12 kg/ha was 68, 60, and 50%, respectively. In a separate greenhouse study, four-leaf pitted morningglory plants with 0, 33, 66, or 100% of their total leaf area exposed to herbicide spray were treated with 0.84, 1.68, or 3.36 kg/ha glyphosate. Increasing glyphosate rate from 0.84 to 3.36 kg/ha increased control from 36 to 88%. In contrast, increasing percent leaf exposure to glyphosate from 0 to 100% increased control from 57 to 75%. Increasing glyphosate rate from 0.84 to 1.68 kg/ha always improved control. However, increasing glyphosate rate from 1.68 to 3.36 kg/ha was beneficial only when no leaves were exposed to the spray solution. In the field, glyphosate spray coverage decreased from 85 to 40% as plant density increased from 1 to 32 plants/m². However, control decreased only 11% (90 to 79%) between the highest and lowest levels of glyphosate spray coverage. These results demonstrated that inadequate control of pitted morningglory with glyphosate was more related to tolerance than glyphosate spray coverage. Glyphosate rates higher than 1.68 kg/ha may be beneficial when spray coverage is severely limited or when plants are beyond the four-leaf growth stage.

Nomenclature: Glyphosate; pitted morningglory, Ipomoea lacunosa L. #³ IPOLA.

Additional index words: Biomass reduction, herbicide efficacy, herbicide tolerance, plant density, plant population.

Abbreviations: LAI, leaf area index; PAR, photosynthetically active radiation; WAT, weeks after treatment.

A field study was conducted from 1999 through 2001 at Stoneville, MS, to determine the effects of bromoxynil-resistant (BR) and glyphosate-resistant (GR) cotton rotation systems under ultranarrow- (25-cm spacing) and wide- (102-cm spacing) row planting on weed control, weed density and shift, and cotton yield. The four rotations during 3 yr included BR–BR–BR, GR–GR–GR, BR–GR–BR, and GR–BR–GR, all with bromoxynil or glyphosate postemergence (POST) only or following fluometuron plus pendimethalin preemergence (PRE). Control of hemp sesbania, pitted morningglory, prickly sida, and hyssop spurge was ≥97% regardless of row width, rotation, and herbicide program. Control of common purslane, sicklepod, and smooth pigweed was higher with glyphosate POST in GR cotton than with bromoxynil POST in BR cotton. Broadleaf and yellow nutsedge weed biomass were higher with bromoxynil POST in BR cotton than with glyphosate POST in GR cotton. Continuous BR cotton system resulted in higher densities of common purslane, sicklepod, and yellow nutsedge (15.3, 1.5, and 373 plants/m², respectively) compared with continuous GR cotton (0.7, 0.1, and 1.0 plant/m², respectively). Seed cotton yield was consistently higher in wide- than in ultranarrow-row cotton. Seed cotton yield was lower in continuous BR cotton than in the other three rotation systems, and yields greatly improved when BR cotton was rotated with GR cotton. During a 3-yr period, seed cotton yields with glyphosate POST only (4,000 to 4,890 kg/ha) or after PRE herbicides (4,480 to 4,860 kg/ha) were similar in GR cotton, whereas in BR cotton, bromoxynil POST only (1,390 to 4,280 kg/ha) resulted in lower yield than did bromoxynil POST after PRE herbicides (2,550 to 4,480 kg/ha). The results indicated that the shift in spectrum of weeds toward more tolerant species and yield decline in continuous BR cotton can be prevented by rotating BR with GR cotton.

Nomenclature: Bromoxynil; fluometuron; glyphosate; pendimethalin; common purslane, Portulaca oleracea L. #³ POROL; hemp sesbania, Sesbania exaltata (Raf.) Rydb. ex A.W. Hill # SEBEX; hyssop spurge, Euphorbia hyssopifolia L. # EPHHS; pitted morningglory, Ipomoea lacunosa L. # IPOLA; prickly sida, Sida spinosa L. # SIDSP; sicklepod, Senna obtusifolia (L.) Irwin & Barneby # CASOB; smooth pigweed, Amaranthus hybridus L. # AMACH; yellow nutsedge, Cyperus esculentus L. # CYPES; cotton, Gossypium hirsutum L. ‘BXN 47’, ‘DP 436 RR’.

Additional index words: Bromoxynil, fluometuron, glyphosate, pendimethalin, rotation, transgenic cotton, weed density, weed shift.

Abbreviations: BR, bromoxynil resistant; GR, glyphosate resistant; POST, postemergence; PRE, preemergence; WAA, weeks after second POST application.

Field experiments were conducted in 1999 and 2000 at the Rice Research and Extension Center at Stuttgart, AR, on a DeWitt silt loam and at the Pine Tree Branch Experiment Station near Colt, AR, on a Calloway silt loam to evaluate the tolerance of 14 rice cultivars and four experimental cultivar lines to clomazone at 0.34 and 0.67 kg ai/ha applied preemergence. Early-season chlorosis 14 days after emergence (DAE) ranged from 1 to 21% in 1999 and 3 to 40% in 2000 when averaged over herbicide rate. The experimental cultivar line RU961096 was slightly more susceptible to clomazone than other cultivars and lines. However, RU9701041 was more tolerant to clomazone at 0.67 kg/ha 14 DAE than other cultivars and lines. By 28 DAE, all cultivars had <13% chlorosis in 1999 and <8% in 2000. The experimental cultivar line RU9601096 and cultivar Koshihikari did not recover as quickly as the other 16 cultivars and cultivar lines. Early-season chlorosis had no effect on days to 50% heading or yield for any of the cultivars evaluated.

Nomenclature: Clomazone; rice, Oryza sativa L. ‘Ahrent’, ‘Bengal’, ‘Cocodrie’, ‘Cypress’, ‘Drew’, ‘Jefferson’, ‘Kaybonnet’, ‘Koshihikari’, ‘Lemont’, ‘LaGrue’, ‘Madison’, ‘Mars’, ‘Priscilla’, ‘RU9601096’, ‘RU9701041’, ‘RU9801081’, ‘RU9801148’, and ‘Wells’.

Additional index words: Rice cultivar tolerance, rice injury.

Abbreviations: DAE, days after emergence; PPI, preplant incorporated; PRE, preemergence.

Greenhouse studies were conducted to evaluate shoot number, shoot weight, rhizome weight, and root weight reduction of green and false-green kyllinga at three placement levels (soil applied, foliar applied, and soil foliar applied) and five herbicide treatments (CGA-362622, halosulfuron, imazaquin, MSMA, and imazaquin MSMA). Averaged over herbicide and placement level, false-green kyllinga shoot number 30 d after treatment (DAT) and rhizome weight 60 DAT were reduced more than those of green kyllinga. Furthermore, imazaquin, MSMA, and imazaquin MSMA, averaged across placement levels, as well as CGA-362622 and halosulfuron, both foliar and soil applied, reduced false-green kyllinga shoot number greater than that of green kyllinga 60 DAT. Halosulfuron reduced false-green kyllinga shoot weight greater than that of green kyllinga 60 DAT; however, MSMA reduced green kyllinga greater. In general, foliar- and soil foliar–applied treatments reduced shoot number (30 DAT), rhizome weight, and root weight of both kyllinga species greater than soil-applied treatments, whereas soil foliar–applied treatments were more effective in reducing shoot weight 60 DAT. CGA-362622 and halosulfuron reduced kyllinga species shoot number (30 DAT), false-green kyllinga shoot weight (60 DAT), and root weight of both species greater than all other herbicides. However, CGA-362622 reduced green kyllinga shoot weight (60 DAT) and rhizome weight of both species greater than all other herbicides.

Nomenclature: CGA-362622, N-[(4,6-dimethoxy-2-pyrimidinyl)carbamoyl]-3-(2,2,2-trifluroethoxy)- pyridin-2-sulfonamide sodium salt; halosulfuron; imazaquin; MSMA; false-green kyllinga, Kyllinga gracillima L.; green kyllinga, Kyllinga brevifolia Rottb. #³ KYLBR.

Additional index words: Foliar absorption, root absorption.

Abbreviations: ALS, acetolactate synthase; DAP, days after planting; DAT, days after treatment; NIS, nonionic surfactant; POST, postemergence.

Field trials were carried out in organic soils to determine the effects of different phosphorus (P) fertilization programs and common lambsquarters duration of interference on lettuce. Phosphorus was either banded (125 kg/ha) or broadcast (250 kg/ha) before lettuce planting. A common lambsquarters population density of four plants per 6 m row interfered with ‘South Bay’ lettuce for 2, 4, 6, or 8 wk after lettuce emergence, along with a weed-free control. For banded P, lettuce fresh weight declined linearly (y = 14.82 − 0.97x; r² = 0.96) as duration of common lambsquarters interference increased. The effect of broadcast P over common lambsquarters duration of interference fit a quadratic equation (y = 10.67 − 1.69x 0.12x²; r² = 0.98). The difference in the regression model intercepts for both P fertilization programs showed that in the absence of common lambsquarters interference, marketable lettuce fresh weight was higher by banding P than by broadcasting P. Based on predicted values, this difference is approximately 28% ( 4.15 kg per 6 m row) in favor of banded P. Therefore, banding P at 125 kg/ha proved to be beneficial in raising lettuce fresh weight regardless of the duration of interference. For banded P, the model predicts that 10% yield reduction would be observed at 1.53 wk (10.5 d) of common lambsquarters interference. However, with broadcast P, this period declined to 0.67 wk (4.7 d).

Nomenclature: Common lambsquarters, Chenopodium album L. #³ CHEAL; lettuce, Lactuca sativa L.

Additional index words: Competition, cultural weed control, integrated weed management, nutrients.

Dryland rotations are changing in the semiarid Great Plains because of no-till systems. Producers now rotate summer annual crops such as corn with winter wheat and fallow, which can disrupt weed population growth because of diverse life cycles among crops. This study estimated changes in weed populations as affected by rotation design, with the goal of suggesting crop sequences that lower weed community density. We used an empirical life-cycle simulation based on demographics of jointed goatgrass and green foxtail to compare various rotations consisting of winter wheat, corn, proso millet, and fallow across a 12-yr period. The simulation indicated that designing rotations to include a 2-yr interval when seed production of either jointed goatgrass or green foxtail is prevented will drastically reduce weed populations. Arranging four different crops in sequences of two cool-season crops, followed by two warm-season crops was the most beneficial for weed management. Fallow, if used, serves in either life-cycle category. However, if the same crop is grown 2 yr in a row, such as winter wheat, the benefit of rotation design on weed density is reduced considerably. Impact of rotation design on weed density was enhanced by improving crop competitiveness with cultural practices. Rotations with balanced life-cycle intervals not only reduce weed density but enable producers to use alternative weed management strategies, improve effectiveness of herbicides used, and minimize herbicide resistance.

Nomenclature: Green foxtail, Setaria viridis (L.) Beauv. #³ SETVI; jointed goatgrass, Aegilops cylindrica Host. # AEGCY; corn, Zea mays L.; proso millet, Panicum miliaceum L.; winter wheat, Triticum aestivum L.

Additional index words: AEGCY, frequency of seed production, life-cycle simulation, seed bank, SETVI, weed community density.

Abbreviations: C, corn crop rotation; F, fallow crop rotation; M, proso millet crop rotation; W, winter wheat crop rotation.

Corn and soybean were planted in narrow and wide row spacings to determine the effect of glyphosate application timing and row spacing on crop yield. Glyphosate was applied when average weed canopy height reached 5, 10, 15, 23, and 30 cm. Weeds present in these studies included velvetleaf, redroot pigweed, common ragweed, common lambsquarters, jimsonweed, barnyardgrass, fall panicum, giant foxtail, yellow foxtail, green foxtail, and eastern black nightshade. Under highly competitive growing conditions (below normal rainfall and high weed density), corn yield was first reduced when weeds reached 10 and 15 cm in height with corn planted in 38- and 76-cm rows, respectively. Under similar conditions, soybean yield was first reduced when weeds reached 15 and 23 cm with soybean planted in 19- and 38-cm rows, respectively. Yield losses occurred only in the untreated control when soybean was planted in 76-cm rows. When growing conditions were less competitive (adequate rainfall and lower weed density), yield losses occurred only when weeds reached 30 cm or more in corn and only in the untreated control in soybean. Corn and soybean yields were higher when planted in narrow rows in three of 4 yr but were more susceptible to early- season weed interference than corn and soybean in wide rows. Corn yield was affected more by weed interference than was soybean yield. The product of weed height by weed density, as the independent variable, resulted in the best linear fit for both corn and soybean yields. High weed densities increase the risk of yield loss and must be considered when determining the appropriate timing for total postemergence herbicide applications such as glyphosate. Sequential glyphosate applications in corn did not increase yield.

Nomenclature: Glyphosate; barnyardgrass, Echinochloa crus-galli (L.) Beauv. #³ ECHCG; common lambsquarters, Chenopodium album L. # CHEAL; common ragweed, Ambrosia artemisiifolia L. # AMBEL; eastern black nightshade, Solanum ptycanthum Dun. # SOLPT; fall panicum, Panicum dichotomilflorum Michx. # PANDI; giant foxtail, Setaria faberi Herrm. # SETFA; green foxtail, Setaria viridis (L.) Beauv. # SETVI; jimsonweed, Datura stramonium L. # DATST; redroot pigweed, Amaranthus retroflexus L. # AMARE; velvetleaf, Abutilon theophroasti Medik. # ABUTH; yellow foxtail, Setaria glauca (L.) Beauv. # SETLU; corn, Zea mays L. ‘DK 493RR’; soybean, Glycine max (L.) Merr. ‘Pioneer 92B71’.

Additional index words: Application timing, competitive load, narrow-row corn, narrow-row soybean, weed competition, weed interference.

Abbreviations: DAE, days after crop emergence; GDDAE, growing degree days after emergence; POST, postemergence; SAS, Statistical Analysis Systems.

Corn and soybean were planted in narrow- and wide-row spacings to study the effects of glyphosate application timing and row spacing on light interception and subsequent weed growth. Corn planted in narrow rows (38 cm) had greater light interception than corn planted in wide rows (76 cm) from 35 to 55 d after crop emergence. Soybean planted in narrow rows (both 19 and 38 cm) had greater light interception throughout the growing season than soybean in 76-cm rows. At maximum canopy closure, narrow-row soybean (both 19 and 38 cm) intercepted more light than narrow-row corn. Biomass of weeds emerging after glyphosate application was greater when soybean was planted in 76-cm than in 19- or 38-cm rows. However, weed biomass was generally similar in both row spacings of corn. Sequential glyphosate applications reduced weed biomass in corn each year compared with a single glyphosate application at the 5-cm weed height. Sequential glyphosate applications that followed initial glyphosate application to 10- or 15-cm-tall weeds did not reduce weed biomass compared with a single application.

Nomenclature: Glyphosate; corn, Zea mays L. ‘DK 493RR’; soybean, Glycine max (L.) Merr. ‘Pioneer 92B71’.

Additional index words: Light intensity, narrow-row corn, narrow-row soybean, shading, weed control, weed interference.

Abbreviations: DAE, days after crop emergence; POST, postemergence.

The difficulty of measuring fecundity of weeds with shatter-prone seeds can be overcome by postanthesis application of nontoxic glues to seedheads. Such glues reduced seed losses of redroot pigweed to <5% of total seed production compared with 12 to 14% for nontreated plants.

Nomenclature: Redroot pigweed, Amaranthus retroflexus L. #³ AMARE.

Additional index words: Amaranthus retroflexus, AMARE, fecundity, seed dispersal, seed production, seed shed.

Abbreviation: PVAC, polyvinyl acetate copolymer.

Maintaining crop residues on the soil surface has changed cropping practices in the Central Great Plains. Where previously winter wheat–fallow was the prevalent rotation, producers now grow warm-season crops in sequence with winter wheat and fallow. Controlling weeds during fallow with herbicides eliminates the need for tillage, thus conserving more crop residues. However, producers are considering subsurface tillage as an option to manage herbicide-resistant weeds. We reviewed the impact of subsurface tillage with the sweep plow on weed dynamics and crop growth compared with no-till systems. Cropping systems studies show that rotations can be designed to reduce weed community density severalfold; tillage lessens this rotational effect by burying weed seeds and prolonging their survival in soil. Crop residues on the soil surface reduce weed seedling establishment in no-till systems, but tillage eliminates this effect. Crops also yield less after tillage compared with no-till in this semiarid climate. Tillage may help in managing herbicide resistance, but it also may increase weed density as well as reduce crop yield.

Nomenclature: Winter wheat, Triticum aestivum L.

Additional index words: Crop residues, rotation design, seed bank dynamics, sweep plow.

Abbreviations: W-C-CP, winter wheat–corn–chickpea; W-CP, winter wheat–chickpea rotation; W-F, winter wheat–fallow.