Waterhemp Control Research Articles

Influence of Tillage, Cover Crop, and Preemergence Herbicides on Weed Control and Pumpkin Yield

ABSTRACT Many Midwestern and Eastern U.S. jack-o-lantern pumpkin (Cucurbita pepo L.) growers are interested in using no-tillage production practices. However, improved weed management practices must be developed before no-tillage will be readily adopted for pumpkin production. A field study was conducted to determine effects of tillage, winter rye (Secale cereale L.), and preemergence herbicides on weed control and pumpkin yield. Grass and broadleaf weed control was greater in no-tillage compared to conventional tillage, and cover crop use in a no-tillage system further improved the level of weed control. Early- and late-season control of giant foxtail (Setaria faberi Herrm.), common cocklebur (Xanthium strumarium L.), redroot pigweed (Amaranthus retroflexus L.), and common waterhemp (A. rudis Sauer) were highly correlated (0.48 ≥ r ≤ 0.86, P ≤ 0.01) with pumpkin yield and fruit size. Although applying clomazone+ethalfluralin to winter rye residues improved broadleaf weed control compared with no herbicide, the level of control for all broadleaf species was unacceptable (∼86%) by 60 days after herbicide treatment (DAT), regardless of tillage treatment. However, addition of halosulfuron or imazamox to clomazone+ethafluralin generally improved redroot pigweed and common waterhemp control in no-tillage and conventional tillage compared with clomazone+ethafluralin alone; common cocklebur control was improved with addition of halosulfuron only to clomazone+ethafluralin. Although pumpkin fruit numbers in the weed-free control did not differ (P > 0.05) from tank mixtures of clomazone+ethalfluralin with halosulfuron or imazamox, reductions in pumpkin fruit size (30–35%) and yields (18–20%) were observed for these herbicide treatments compared with the weed-free control. This research indicates that cover crops, such as winter rye, can be successfully integrated in no-tillage pumpkin production systems along with herbicides to improve weed management.

Influence of Fall and Early Spring Herbicide Applications on Winter and Summer Annual Weed Populations in No-Till Corn

Field trials were initiated at three Missouri locations in the fall of 2004 and 2005 to compare the efficacy of fall and early spring herbicide applications on winter and summer annual weed populations in no-till soybean. All three experiments received applications of chlorimuron plus sulfentrazone plus 2,4-D; chlorimuron plus tribenuron plus 2,4-D; and glyphosate plus 2,4-D in the fall, 60 d prior to planting (60 PP), 30 days prior to planting (30 PP), and 7 d prior to planting (7 PP). Good control of all winter annual weeds, except for common chickweed, was observed at planting from fall applications of chlorimuron plus sulfentrazone plus 2,4-D and chlorimuron plus tribenuron plus 2,4-D. Fall, 60 PP, and 30 PP applications of chlorimuron plus sulfentrazone plus 2,4-D provided relatively poor control of common chickweed. Glyphosate plus 2,4-D applied in the fall and 60 PP provided adequate control of early fall germinating winter annuals, such as common chickweed and henbit. However, these treatments provided poor control of winter annual species that exhibited some degree of spring germination. Control of summer annual weed species 7 wk after planting (WAP) was highly variable. Relatively poor control of common waterhemp was obtained from all treatments except chlorimuron plus sulfentrazone plus 2,4-D applied at all three spring timings. Giant foxtail control was highly variable ranging from 0 to 95%. Measurements of weed biomass collected 7 WAP revealed larger amounts of biomass present in the fall application timing of each herbicide treatment. Emergence of winter and summer annual weed species from early March to 7 WAP varied by herbicide treatment. Emergence of total winter and summer annual weed species was reduced with both residual herbicide treatments when compared with glyphosate plus 2,4-D and the untreated control. Nomenclature: Chlorimuron; 2,4-D; glyphosate; sulfentrazone; tribenuron; common chickweed, Stellaria media (L.) Vill. STEME; common waterhemp, Amaranthus rudis Sauer AMATA; giant foxtail, Setaria faberi Herrm. SETFA; henbit, Lamium amplexicaule L. LAMAM; soybean, Glycine max (L.) Merr. ‘Dekalb 38-52’.

Influence of Fall and Early Spring Herbicide Applications on Winter and Summer Annual Weed Populations in No-till Soybean

Field trials were initiated at three Missouri locations in the fall of 2004 and 2005 to compare the efficacy of fall and early spring herbicide applications on winter and summer annual weed populations in no-till soybean. All three experiments received applications of chlorimuron plus sulfentrazone plus 2,4-D; chlorimuron plus tribenuron plus 2,4-D; and glyphosate plus 2,4-D in the fall, 60 d prior to planting (60 PP), 30 days prior to planting (30 PP), and 7 d prior to planting (7 PP). Good control of all winter annual weeds, except for common chickweed, was observed at planting from fall applications of chlorimuron plus sulfentrazone plus 2,4-D and chlorimuron plus tribenuron plus 2,4-D. Fall, 60 PP, and 30 PP applications of chlorimuron plus sulfentrazone plus 2,4-D provided relatively poor control of common chickweed. Glyphosate plus 2,4-D applied in the fall and 60 PP provided adequate control of early fall germinating winter annuals, such as common chickweed and henbit. However, these treatments provided poor control of winter annual species that exhibited some degree of spring germination. Control of summer annual weed species 7 wk after planting (WAP) was highly variable. Relatively poor control of common waterhemp was obtained from all treatments except chlorimuron plus sulfentrazone plus 2,4-D applied at all three spring timings. Giant foxtail control was highly variable ranging from 0 to 95%. Measurements of weed biomass collected 7 WAP revealed larger amounts of biomass present in the fall application timing of each herbicide treatment. Emergence of winter and summer annual weed species from early March to 7 WAP varied by herbicide treatment. Emergence of total winter and summer annual weed species was reduced with both residual herbicide treatments when compared with glyphosate plus 2,4-D and the untreated control.Nomenclature: Chlorimuron, 2,4-D, glyphosate, sulfentrazone, tribenuron, common chickweed, Stellaria media (L.) Vill. STEME, common waterhemp, Amaranthus rudis Sauer AMATA, giant foxtail, Setaria faberi Herrm. SETFA, henbit, Lamium amplexicaule L. LAMAM, soybean, Glycine max (L.) Merr. ‘Dekalb 38-52’

Control of herbicide-resistant common waterhemp (Amaranthus tuberculatus var. rudis) with pre- and post-emergence herbicides in soybean

Common waterhemp was first identified in Ontario in 2002. Eight trials were conducted ove r a 3-yr period (2003-2005) at Cottam and Petrolia, Ontario to determine the efficacy of various preemergence and postemergence herbicides for the control of herbicideresistant waterhemp in soybean [ Glycine max (L.) Merr.]. Waterhemp was resistant to the a cetolactate synthase-inhibiting herbicides at both locations and to the photosystem II-inhibiting herbicides at Petrolia; therefore, results varied by location. Results at Petrolia also varied across years. Waterhemp interference resulted in soybean yield losses of up to 73% in weedy versus weed-free checks. Preemergence-applied metolachlor, dimethenamid, and linuron provided greater than 80% control and metolachlor plus metribuzin 94% or greater control of waterhemp 70 d after emergence at both locations. Acifluorfen, fomesafen, imazamox plus fomesafen, and glyphosate applied postemergence resulted in 87% or greater waterhemp control 70 d after application at Cottam in 2003 and 2004 and at Petrolia in 2004; however, waterhemp control with these herbicides was only 60 to 66% at Petrolia in 2005 due to prolonged weed emergence. Multiple herbicide applications per season may be required for effective waterhemp control in soybean. Key words:

Protox-resistant common waterhemp (Amaranthus rudis) response to herbicides applied at different growth stages

Greenhouse and field studies were conducted with a population of common waterhemp resistant to POST protoporphyrinogen oxidase (protox)-inhibiting herbicides to compare its response to PRE and POST applications of selected herbicides. In the greenhouse, a dose–response study of PRE applications of acifluorfen, fomesafen, or lactofen was conducted on protox-susceptible and -resistant common waterhemp. The protox-resistant biotype was approximately 6.3, 2.5, and 2.6 times more resistant than the susceptible biotype to acifluorfen, fomesafen, and lactofen, respectively. In a separate study under field conditions, protox-resistant common waterhemp were treated with PRE and POST applications of acifluorfen, azafenidin, flumioxazin, fomesafen, lactofen, oxyfluorfen, or sulfentrazone. At 14 and 28 d after POST treatment (DAPT) in 2002 and 2004, all PRE applications of herbicides gave greater control than did POST applications. At 14 DAPT, oxyfluorfen had the greatest difference in PRE and POST control, with 85 and 10% control in 2002, respectively. An additional field study was conducted to determine the stage of growth at which resistance to protox-inhibiting herbicides becomes most prevalent. Protox-resistant common waterhemp were treated with herbicides at the 2-leaf, 4- to 6-leaf, and 8- to 10-leaf growth stage. Acifluorfen and fomesafen at 420 g ha−1gave greater than 90% control at the 2-leaf stage and 4- to 6-leaf stage, except in 2003 when control was 85% with acifluorfen. In 2003 and 2004, common waterhemp control at the 8- to 10-leaf stage ranged between 54 and 75% with acifluorfen or fomesafen. Results indicate that common waterhemp resistance to customary rates of POST protox-inhibiting herbicides becomes prevalent after the 4- to 6-leaf growth stage.

Climatic Constraints to the Potential of Microsphaeropsis amaranthi as a Bioherbicide for Common Waterhemp

ABSTRACT The fungal plant pathogen Microsphaeropsis amaranthi is a candidate bioherbicide for the control of weedy Amaranthus species since it grows and sporulates readily in culture, is a pathogen of a number of important weed species, and is host-restricted to the family Amaranthaceae. This study was designed to determine the optimum and limiting environmental conditions for the efficacy of foliar applications of M. amaranthi for the control of common waterhemp. The greatest disease severity and the greatest plant biomass reductions were found when a prolonged leaf wetness period of >/= 12 h at 18 to 24 degrees C was provided after application. Disease severity and plant biomass reductions were greater when plants were treated at an earlier stage of development. Disease severity and impact were limited when shorter leaf wetness periods were provided, with negligible disease observed on plants provided with </=6 h leaf wetness. When common waterhemp seedlings were sprayed in the field weekly through the summer of 2004 with conidial suspensions of M. amaranthi, only 7 of 14 applications resulted in disease and only 3 applications resulted in more than slight disease flecks. Moderate levels of disease were observed when applications had been made on days with moderate temperatures and leaf wetness duration in excess of 12 h following application. We confirm the pathogenicity of M. amaranthi against common waterhemp and demonstrate that the unformulated pathogen can cause disease in the field. Nonetheless, the weather conditions of the Midwestern United States, where common waterhemp control would be desired, are infrequently conducive to infection. The continued candidacy of M. amaranthi as a bioherbicide for the control of common waterhemp in the Midwest-ern United States will require that its efficacy under suboptimal environmental conditions be improved.

Incorporating Foramsulfuron into Annual Weed Control Systems for Corn1

Field studies were conducted in 2000, 2001, and 2002 at Brownstown, DeKalb, Perry, and Urbana, IL, to evaluate weed control and corn tolerance from postemergence (POST) applications of foramsulfuron in sequential and total-POST herbicide programs. Foramsulfuron applied alone controlled giant foxtail, fall panicum, and redroot pigweed 88, 99, and 99%, respectively, 28 d after treatment (DAT), which was comparable with the standard treatment of nicosulfuron. However, control of common cocklebur, velvetleaf, and common lambsquarters was significantly higher with foramsulfuron compared with nicosulfuron. Sequential herbicide programs of atrazine, S-metolachlor, or isoxaflutole applied preemergence (PRE) followed by a POST application of foramsulfuron provided greater than 85% control of giant foxtail, fall panicum, common cocklebur, velvetleaf, common waterhemp, and redroot pigweed. Of the sequential herbicide treatments, atrazine applied PRE followed by a POST application of foramsulfuron provided the greatest Pennsylvania smartweed control. A PRE application of either atrazine or isoxaflutole was needed before a POST application of foramsulfuron to control common lambsquarters. POST tank mixtures of foramsulfuron with atrazine, dicamba, and dicamba plus diflufenzopyr improved control of Pennsylvania smartweed, common cocklebur, velvetleaf, common lambsquarters, and common waterhemp when compared with foramsulfuron applied alone. The tank mixture of foramsulfuron with mesotrione improved control of all species, except Pennsylvania smartweed, common lambsquarters, and common cocklebur. Foramsulfuron tank mixtures with carfentrazone did not improve control of any weed species to commercial acceptance. Adjuvant selection was important for POST tank mixtures. Control of giant foxtail and fall panicum was reduced when atrazine was tank mixed with foramsulfuron and crop oil concentrate (COC). However, when methylated seed oil (MSO) was added to the atrazine–foramsulfuron tank mixture instead of COC, giant foxtail and fall panicum control were similar to foramsulfuron applied alone.Nomenclature: Atrazine; carfentrazone; dicamba; diflufenzopyr, 2-[1-[[[(3,5-difluorophenyl)amino]carbonyl]hydrazono]ethyl]-3-pyridinecarboxylic acid; foramsulfuron, 2-[[[[(4,6-dimethoxy-2-pyrimidiny)amino]carbonyl]amino]sulfonyl]-4-(formylamino)-N,N-dimethylbenzamide; isoxaflutole; mesotrione; S-metolachlor; common cocklebur, Xanthium strumarium L. #3 XANST; common lambsquarters, Chenopodium album L. # CHEAL; common waterhemp, Amaranthus rudis Sauer. # AMATA; fall panicum, Panicum dichotomiflorum Michx. # PANDI; giant foxtail, Setaria faberi Herm. # SETFA; Pennsylvania smartweed, Polygonum pennsylvanicum L. # POLPY; redroot pigweed, Amaranthus retroflexus L. # AMARE; velvetleaf, Abutilon theophrasti Medicus # ABUTH; corn, Zea mays L.Additional index words: Antagonism, tank mixture, total-POST.Abbreviations: ALS, acetolactate synthase (EC 4.1.3.18); COC, crop oil concentrate; DAT, days after treatment; MSO, methylated seed oil; NIS, nonionic surfactant; POST, postemergence; PRE, preemergence; UAN, urea ammonium nitrate.

A waterhemp (Amaranthus tuberculatus) biotype with multiple resistance across three herbicide sites of action

A population of waterhemp was identified in Adams County, Illinois, that survived treatment of several acetolactate synthase (ALS) inhibitors and a postemergence (POST) application of lactofen, a protoporphyrinogen oxidase (PPO)–inhibiting herbicide. Greenhouse studies were conducted to quantify the responses of this waterhemp population, designated ACR, to multiple PPO inhibitors and various other herbicides with different sites of action. Resistance ratios were obtained by comparing herbicide dose–response curves between the ACR population and a herbicide-susceptible waterhemp population. The ACR population was resistant to lactofen (23-fold) and to five other PPO-inhibiting herbicides (ranging from 2.2- to 6.2-fold). Furthermore, the ACR waterhemp population was 17,000-fold and 18,000-fold resistant to imazamox and thifensulfuron, respectively, two ALS-inhibiting herbicides. In response to atrazine, a Photosystem II inhibitor, the ACR population was 38-fold resistant. Plants within the ACR waterhemp population survived treatment of a herbicide mixture containing lactofen at 175 g ai ha−1, imazamox at 44 g ae ha−1, and atrazine at 1,000 g ai ha−1. Thus, individual plants—not just the population as a whole—displayed multiple herbicide resistance. The ACR population was not resistant to glyphosate or paraquat. This is the first reported weed population from the United States with resistances to herbicides inhibiting three unique sites of action. Furthermore, this research identifies a significant reduction in the number of POST herbicide options available for waterhemp control in soybean production.

Control of Protoporphyrinogen Oxidase Inhibitor–Resistant Common Waterhemp (Amaranthus rudis) in Corn and Soybean

Field experiments were conducted in 2001 and 2002 to evaluate the efficacy of herbicides on protoporphyrinogen oxidase (protox, EC 1.3.3.4) inhibitor–resistant common waterhemp in corn and soybean. All corn herbicides tested gave greater than 90% common waterhemp control by 8 wk after postemergence herbicide treatment (WAPT). In soybean, common waterhemp control was less than 40% by 8 WAPT with postemergence protox-inhibiting herbicides lactofen and acifluorfen. However, preemergence protox-inhibiting herbicides sulfentrazone and flumioxazin gave greater than 85% common waterhemp control in both years. The greatest common waterhemp control in soybean was with glyphosate alone, alachlor + metribuzin, alachlor followed by (fb) glyphosate, and S-metolachlor + metribuzin fb glyphosate.

Waterhemp Control in Transgenic and Conventional Corn Varieties

Waterhemp Control in Transgenic and Conventional Corn Varieties

Weed Management in Strip Tillage Corn

Field studies were conducted in 2000 and 2001 to determine if differences exist in weed emergence patterns, weed control, corn (Zea mays L.) plant population, and grain yield in strip tillage, conventional tillage, and no‐tillage production. Weed emergence peaked at 2 and 4 wk after planting (WAP) in 2000 and 2001, respectively. Weed emergence at 2 WAP in 2000 was greater in conventional tillage compared with no‐tillage and strip tillage. Tillage system did not affect total weed species emergence in 2001. In both years, lower control of giant foxtail (Setaria faberi Herrm.) and common waterhemp (Amaranthus rudis Sauer) occurred following acetochlor {[2‐chloro‐N‐(ethoxymethyl)‐N‐(2‐ethyl‐6‐methylphenyl)acetamide]} plus atrazine {[6‐chloro‐N‐ethyl‐N′‐(1‐methylethyl)‐1,3,5‐triazine‐2,4‐diamine]} applied pre‐emergence in no‐tillage compared with conventional tillage. Control of common waterhemp in 2001 and giant foxtail in both years was similar with glyphosate [N‐(phosphonomethyl)glycine] applied postemergence regardless of tillage system. Corn population was greater in conventional tillage compared with no‐tillage in 2000 and greater than the population in no‐tillage and strip tillage in 2001. When averaged across weed management strategies, corn yield was greatest in strip tillage in 2000 and conventional tillage in 2001.

Weed Management and Economic Returns in No-Tillage Herbicide-Resistant Corn (Zea mays)1

Field studies were conducted to evaluate corn vigor reduction, weed control, corn yield, and economic returns in a no-till system with various herbicide strategies using full and reduced rates of acetochlor and atrazine with glyphosate, glufosinate, or imazethapyr + imazapyr in their respective type of herbicide-resistant, no-tillage corn. Crop vigor reduction due to herbicide injury was 10% or less with all treatments. A burndown plus a full label rate of a residual herbicide applied early preplant (EPP) generally provided less than 80% control of giant foxtail, common waterhemp, and common cocklebur but usually greater than 85% control of common ragweed and common lambsquarters. Two-pass strategies generally provided greater than 85% control of all species evaluated. Early postemergence, mid-postemergence (MPOST), and late postemergence strategies generally provided inconsistent and poor overall weed control. EPP–MPOST strategies generally provided lower weed control than strategies using acetochlor or ...

Sulfentrazone Enhances Weed Control of Glyphosate in Glyphosate-Resistant Soybean (Glycine max)1

Field studies were conducted from 1998 to 2000 at Belleville, IL, to evaluate tolerance and weed control in glyphosate-resistant soybean with sulfentrazone application. Sulfentrazone alone caused 14 to 16% height reduction 14 d after treatment (DAT), and sulfentrazone plus chlorimuron caused 26% height reduction 14 DAT. Minimal height reduction (0 to 11%) was observed 56 DAT. Sulfentrazone alone controlled giant foxtail 97 to 100%, yellow nutsedge 96 to 98%, common waterhemp 97 to 98%, common cocklebur 91 to 94%, and ivyleaf morningglory 100%. Sulfentrazone alone controlled common ragweed 63 to 89% and giant ragweed 50 to 72%. Sulfentrazone plus chlorimuron or cloransulam increased control of common and giant ragweed to 95% or greater. Sulfentrazone followed by glyphosate increased control of yellow nutsedge, common waterhemp, and ivyleaf morningglory compared with a single application of glyphosate. Sequential applications of glyphosate controlled weeds 93 to 100%. Sulfentrazone plus chlorimuron or clora...

Influence of Diphenylether Herbicide Application Rate and Timing on Common Waterhemp (Amaranthus rudis) Control in Soybean (Glycine max)1

Diphenylether herbicides may be viable options for postemergence (POST) control of common waterhemp in soybean. A 2-yr field research project was conducted to determine whether common waterhemp control is influenced by application timing and rate of acifluorfen, fomesafen, and lactofen. Common waterhemp control was 9, 9, and 8% greater 7, 14, and 21 d after treatment, respectively, after the early postemergence (EPOST) application timing compared with the POST application timing. Lactofen provided greater common waterhemp control than did acifluorfen or fomesafen, and only the highest application of lactofen provided greater than 85% common waterhemp control 21 d after POST application. No significant differences in common waterhemp dry weight were determined among the three rates of acifluorfen, fomesafen, and lactofen applied EPOST. The highest application rates of fomesafen and lactofen reduced common waterhemp dry weight more than did the lowest application rates applied POST. The highest application rate of fomesafen also reduced common waterhemp dry weight more than did the intermediate application rate. Single degree of freedom contrasts indicated that all diphenylether herbicides reduced common waterhemp dry weight more than did imazethapyr.Nomenclature: Acifluorfen; fomesafen; imazethapyr; lactofen; common waterhemp, Amaranthus rudis Sauer #3 AMATA; soybean, Glycine max (L.) Merr.Additional index words: Postemergence, herbicide application rate, herbicide application timing, weed control.Abbreviations: ALS, acetolactate synthase; COC, crop oil concentrate; DAT, days after treatment; EPOST, early postemergence; NIS, nonionic surfactant; POST, postemergence.

Common Waterhemp (Amaranthus rudis) Control in Corn (Zea mays) with Single Preemergence and Sequential Applications of Residual Herbicides1

Control of common waterhemp in corn with chloroacetamide and dinitroaniline herbicides can be inconsistent. Common waterhemp control by dimethenamid, S-metalochlor, pendimethalin, and three formulations of acetochlor, applied alone or with atrazine as single preemergence (PRE) or sequential PRE followed by postemergence (POST) treatment, was determined. The manufacturer's suggested use rate (1 time) of PRE herbicides was compared with sequential applications (0.66 time PRE followed by 0.34 time POST) of herbicides. POST applications included dicamba to control emerged common waterhemp. Single and sequential herbicide applications controlled common waterhemp at least 98%, 28 d after planting (DAP). But herbicides applied sequentially were more effective than PRE treatments by 56 DAP. Encapsulated acetochlor formulations controlled common waterhemp at least 85% by 56 DAP regardless of application method. Sequential applications of S-metolachlor controlled common waterhemp greater than 83%. Atrazine improved common waterhemp control regardless of herbicide or application method 56 DAP. Sequential applications of dimethenamid or S-metolachlor alone or with atrazine were more effective than single PRE applications of these herbicides.Nomenclature: Acetochlor; atrazine; chloroacetamide; dimethenamid; dinitroaniline; pendimethalin; S-metalochlor; common waterhemp, Amaranthus rudis Sauer #3 AMATA; corn, Zea mays L. ‘Pioneer 34R07’.Additional index words: Chloroacetamide, dinitroaniline, herbicide application timing, sequential applications.Abbreviations: DAP, days after planting; POST, postemergence; PRE, preemergence.

Common waterhemp ( Amaranthus rudis Sauer) management with soil-applied herbicides in soybean ( Glycine max (L.) Merr.)

Amaranthus rudis can exhibit multiple emergence events during a crop growing season. Control of this species in soybean with soil-applied herbicides can be variable. A 2-yr field research project was conducted to examine the influence of herbicide application timing and dose on efficacy of six soil-applied herbicides for common waterhemp control in soybean. Four weeks after soybean planting, herbicides applied preemergence provided 18% greater common waterhemp control than when herbicides were applied 5 weeks early preplant. Herbicide dose did not significantly influence common waterhemp control, but differences among the six herbicides were evident. Sulfentrazone controlled common waterhemp better and reduced its density more than other herbicides. All herbicides reduced the weed density and dry weight more than the nontreated control.

Efficacy and Economic Return on Investment for Conventional and Herbicide-Resistant Soybean (Glycine max)1

Field studies were conducted in 1999 and 2000 at Belleville, IL, to evaluate herbicide efficacy, grain yield, and economic return on investment (EROI) from various herbicide systems in conventional, sulfonylurea-tolerant, glufosinate-resistant, and glyphosate-resistant soybean varieties. Several grower-oriented herbicide systems were developed within each soybean variety using current weed control options. These herbicide systems included herbicides with soil-residual, herbicides with different modes of action, and combinations of preemergence (PRE) followed by (fb) postemergence (POST) herbicides. Soybean injury ranged from 1 to 16% 28 d after (DA) PRE applications of sulfentrazone plus chlorimuron plus pendimethalin. Injury 7 DA POST treatments ranged from 2 to 39%, with the greatest injury occurring from the conventional herbicide combination of fomesafen plus fenoxaprop plus fluazifop-P. No injury symptoms persisted through soybean harvest. Control of giant foxtail was the most consistent (97% or greater) in the PRE broadleaf fb POST herbicide system 56 DA planting. Common waterhemp control was at least 85% for all treatments, with the exception of the single POST herbicide systems of chlorimuron plus thifensulfuron plus quizalofop-P, and glufosinate and the standard sequential herbicide system of pendimethalin fb chlorimuron plus thifensulfuron. All treatments provided at least 87% control of common ragweed and ivyleaf morningglory. Soybean grain yield was similar between varieties in the weed-free control plots in 1999. In 2000, grain yield of the glyphosate- and glufosinate-resistant soybean varieties was greater than for the conventional and sulfonylurea-tolerant varieties. Overall, soybean grain yield and EROI were less for some treatments because of poor control of common waterhemp. Weed control had a greater influence than herbicide-related costs on EROI.Nomenclature: Chlorimuron; fenoxaprop; fluazifop-P; fomesafen; glufosinate; glyphosate; pendimethalin; quizalofop-P; sulfentrazone; sulfonylurea; thifensulfuron; common ragweed, Ambrosia artemisiifolia L. # AMBEL; common waterhemp, Amaranthus rudis Sauer. #3 AMATA; giant foxtail, Setaria faberi Herrm. # SETFA; ivyleaf morningglory, Ipomoea hederacea (L.) Jacq. # IPOHE; soybean, Glycine max (L.) Merr.Additional index words: Glufosinate-resistant, glyphosate-resistant, sulfonylurea-tolerant.Abbreviations: ALS, acetolactate synthase; DA, days after; DAT, days after treatment; EROI, economic return on investment; fb, followed by; POST, postemergence; PRE, preemergence.

Glufosinate Efficacy on Amaranthus Species in Glufosinate-Resistant Soybean (Glycine max)1

Field studies were conducted in 1998 and 1999 to evaluate the efficacy of glufosinate on Palmer amaranth, redroot pigweed, and common waterhemp at different growth stages in soybean planted at early, normal, and late dates. At 2, 4, and 8 wk after treatment, common waterhemp control was greater than Palmer amaranth and redroot pigweed control with single glufosinate applications of 410 g ai/ha at 2- to 5-, 7- to 10-, or 15- to 18-cm Amaranthus height or with two sequential applications of 293 g/ha at 2- to 5-cm height and 2 wk later. Only the sequential applications of 410 and 293 g/ha resulted in more than 80% control of Palmer amaranth and redroot pigweed, but all five treatments controlled common waterhemp more than 80%. All glufosinate treatments reduced the dry weight of all Amaranthus species by more than 65%. However, the sequential applications resulted in the greatest dry weight reductions.Nomenclature: Glufosinate; common waterhemp, Amaranthus rudis Sauer #3 AMATA; Palmer amaranth, Amaranthus palmeri S.Wats. # AMAPA; redroot pigweed, Amaranthus retroflexus L. # AMARE; soybean, Glycine max (L.) Merr.Additional index words: Environmental conditions, herbicide-tolerant soybean, postemergence herbicide.Abbreviations: ALS, Acetolactate synthase (EC 4.1.3.18); OM, organic matter; RH, relative humidity; WAT, weeks after treatment.

Weed Management in Narrow- and Wide-Row Glyphosate-Resistant Soybean (Glycine max)1

Field experiments were conducted over 3 yr at three locations in Illinois to evaluate the efficacy of glyphosate in glyphosate-resistant soybean planted in rows spaced 19, 38, and 76 cm. Minimal soybean injury (less than 10%) was observed from any glyphosate treatment. Glyphosate treatments controlled 82 to 99% of giant foxtail. Common waterhemp control was increased as soybean row spacing was decreased. Applying sequential glyphosate applications or increasing the glyphosate rate from 420 g ae/ha to 840 g/ha frequently increased common waterhemp control in 76-cm rows. Velvetleaf control with glyphosate was variable, ranging from 48 to 99%. Decreasing soybean row spacing, utilizing sequential glyphosate applications, or increasing the glyphosate rate improved velvetleaf control in at least four of eight site-years. Glyphosate treatments generally resulted in weed control and soybean yield equal to or greater than the standard herbicide treatments. However, glyphosate treatments yielded less than the hand-weeded control in four of eight site-years, suggesting that weed control from glyphosate treatments was sometimes inadequate.Nomenclature: Glyphosate; common waterhemp, Amaranthus rudis Sauer #3 AMATA; giant foxtail, Setaria faberi Herrm. # SETFA; velvetleaf, Abutilon theophrasti Medik. # ABUTH; soybean, Glycine max (L.) Merr.Additional index words: Herbicide-resistant crops.Abbreviations: EPOST, early postemergence; LPOST, late postemergence; MPOST, mid-postemergence; NIS, nonionic surfactant; UAN, 28% urea ammonium nitrate; WAT, weeks after treatment.

Efficacy and Economics of Weed Management in Glyphosate-Resistant Corn (Zea mays)1

Field experiments were conducted in 1997 and 1998 near Columbia and Novelty, MO, and Urbana, IL, to evaluate crop injury, weed control, corn yield, and net economic returns provided by weed control programs in glyphosate-resistant corn. The herbicide programs evaluated included acetochlor preemergence (PRE) followed by (fb) glyphosate with or without atrazine postemergence (POST) and total POST programs consisting of single and sequential applications of glyphosate alone and tank-mixed with actochlor, atrazine, or both. Metolachlor PRE fb dicamba plus atrazine POST and metolachlor plus atrazine PRE were included for comparison. In the total POST treatments, mid-post (MPOST) applications provided better control than early-post (EPOST) applications on weeds that germinated throughout the growing season such as shattercane and common cocklebur, but also resulted in yield reductions of up to 23% caused by early-season weed competition. The addition of atrazine to glyphosate POST generally increased control of common cocklebur, morningglory species, and common waterhemp. EPOST or PRE fb EPOST applications generally provided higher yields than MPOST treatments, although MPOST treatments often provided equal or greater weed control at midseason. Treatments including two herbicide applications tended to provide greater weed control, yield, and profit than those with a single application. Input costs for glyphosate-resistant corn are slightly higher than nontransgenic hybrids. However, net economic returns are similar and the use of glyphosate POST allows greater flexibility in POST weed management decisions.Nomenclature: Acetochlor, 2-chloro-N-(ethoxymethyl)-N-(2-ethyl-6-methylphenyl)acetamide; atrazine, 6-chloro-N-ethyl-N′-(1-methylethyl)-1,3,5-triazine-2,4-diamine; dicamba, 3,6-dichloro-2-methoxybenzoic acid; glyphosate, N-(phosphonomethyl)glycine; metolachlor, 2-chloro-N-(2-ethyl-6-methylphenyl)-N-(2-methoxy-1-methylethyl)acetamide; common cocklebur, Xanthium strumarium L.#3 XANST; common waterhemp, Amaranthus rudis Sauer # AMATA; ivyleaf morningglory, Ipomoea hederaceae (L.) Jacq. # IPOHE; pitted morningglory, Ipomoea lacunosa L. # IPOLA; shattercane, Sorghum bicolor (L.) Moench. # SORVU; corn Zea mays L. # ZEAMX ‘MON802’.Additional index words: Corn yield, acetochlor, atrazine, dicamba, economic returns, metolachlor, AMATA, ABUTH, CHEAL, IPOHE, IPOLA, POLPY, SETFA, SORVU, XANST, ZEAMX.Abbreviations: ALS, autolactate synthase; EPOST, early postemergence; fb, followed by; MPOST, mid-postemergence; OM, organic matter; POST, postemergence; PRE, preemergence; WAT, weeks after treatment.