Postemergence Glyphosate Research Articles

Deep Tillage and Glyphosate-Reduced Redvine (Brunnichia ovata) and Trumpetcreeper (Campsis radicans) Populations in Glyphosate-Resistant Soybean

Field studies were conducted during the years 2000 to 2003 at Stoneville, MS, to determine the efficacy of fall deep tillage and glyphosate applications on redvine and trumpetcreeper populations and soybean yield in glyphosate-resistant soybean. Fall deep (≈45 cm) tillage for 1, 2, and 3 yr reduced redvine density by 95, 88, and 97%, respectively, compared with shallow (≈15 cm) tillage, but deep tillage did not reduce trumpetcreeper density. Glyphosate applied preplant reduced trumpetcreeper density (25 to 44%), but not redvine density, compared to that with no glyphosate. Glyphosate early postemergence (EPOST) either alone (45 to 67%) or followed by (fb) late postemergence (LPOST; 59 to 83%) reduced density of trumpetcreeper, but not of redvine, compared to that with no herbicide. However, dry biomass of both vines was reduced with glyphosate EPOST or LPOST compared to that with no herbicide. Soybean yields were higher with deep tillage vs. shallow tillage, glyphosate preplant application vs. no glyphosate, and glyphosate EPOST either alone or fb LPOST vs. no herbicide. Redvine did not reestablish in 2003, which was after skipping fall deep tillage for 1 yr following three consecutive years of deep tillage compared with shallow tillage. It is possible to manage redvine infestations with fall deep tillage and trumpetcreeper infestations with glyphosate preplant and postemergence (POST) in-crop applications. Integration of fall deep tillage and glyphosate POST applications could be an effective strategy to manage combined infestations of these vines in glyphosate-resistant soybean.

Weed Management Systems in Glyphosate-Resistant Cotton

Studies were conducted at Clayton, Lewiston-Woodville, and Rocky Mount, NC, to evaluate weed and cotton response to herbicide systems in glyphosate-resistant cotton in 1995 and 1997. Herbicide systems evaluated included various combinations of soil-applied (trifluralin and fluometuron) and postemergence (POST) (glyphosate or pyrithiobac) herbicides with or without late postemergence-directed (LAYBY) treatments of cyanazine plus MSMA. Glyphosate-resistant cotton injury was less than 5% with all herbicide treatments. Glyphosate POST systems were as efficacious in weed control as other herbicide systems. Depending on location, glyphosate and pyrithiobac POST systems usually required cyanazine plus MSMA LAYBY for season-long control of common lambsquarters, goosegrass, large crabgrass, pitted morningglory, prickly sida, and Texas panicum. Glyphosate POST applied as needed provided weed control equivalent to soil-applied plus POST herbicides, although lint yield was slightly reduced depending on location. Herbicide systems that included soil-applied herbicides required one to two treatments of glyphosate POST and post-directed for season-long weed control and high cotton lint yields, whereas the same herbicide systems without soil-applied herbicides required two to three glyphosate treatments. In all herbicide systems, a residual soil-applied or LAYBY herbicide treatment increased yield compared with glyphosate POST only systems. Location influenced weed control and cotton yield. Generally, as herbicide inputs increased, yield increased.

Preemergence Herbicide and Glyphosate Effects on Seedling Diseases in Glyphosate-Resistant Cotton

Field experiments were conducted to evaluate the influence of preemergence (PRE) herbicides metolachlor at 1,700 g ai/ha, pyrithiobac at 70 g ai/ha, or pendimethalin at 840 g ai/ha applied alone or with fluometuron at 1,300 g ai/ha and glyphosate postemergence (POST) at 840 g ai/ha on seedling diseases in glyphosate-resistant cotton. Hypocotyl disease severity both years averaged across PRE herbicide treatments was greater after glyphosate application to four-leaf cotton than cotyledon cotton. The PRE herbicide treatments, particularly those including fluometuron, increased root and hypocotyl disease ratings compared with a nontreated control, and a sequential application of glyphosate did not further increase disease severity. Greenhouse experiments using soil infested withRhizoctonia solaniconfirmed findings from the field study showing that PRE herbicides can predispose cotton to greater seedling disease injury with no increased seedling disease severity associated with application of glyphosate. In the field study, glyphosate applied at cotyledon or four-leaf growth stages decreased disease severity on cotton hypocotyls both years. This inhibitory effect of glyphosate was less evident in the greenhouse study and may have been related to species of fungi present, infestation level, and differences in environmental conditions when compared with the field.

Effect of Glyphosate and MSMA Application Timing on Weed Control, Fruiting Patterns, and Yield in Glyphosate-Resistant Cotton

The limited window of opportunity for glyphosate postemergence (POST) over-the-top applications in glyphosate-resistant cotton poses a problem for growers where a midseason salvage weed control remedy is necessary. The objectives of these experiments were to compare glyphosate and MSMA for midseason weed control and their subsequent effect on cotton fruiting characteristics and yield. Glyphosate at 0.85 kg ai/ha was more effective than MSMA at 1.7 kg ai/ha for POST control of sicklepod, redweed, and pitted morningglory. Single glyphosate treatments applied at the 8-, 10-, or 12-leaf cotton stage resulted in less-effective weed control than when applied at the four-leaf cotton stage. Glyphosate applied at the four-leaf cotton stage followed by a sequential POST-directed application at 6-, 8-, 10-, or 12-leaf cotton stage increased season-long weed control and yield compared with a single application at the four-leaf stage. Both glyphosate and MSMA controlled Florida beggarweed, regardless of POST application timing. Generally, cotton was more tolerant to glyphosate than MSMA when applied over-the-top. Glyphosate applied POST over-the-top to weed-free 12-leaf cotton resulted in a 19 and 14% yield loss compared with the weed-free nontreated cotton in 1997 and 1999. MSMA reduced yield by 58 and 36% in 1997 and 1999, respectively. Glyphosate did not affect weed-free cotton fruit development or yield when applied over-the-top to four-leaf cotton or when a POST-directed application was followed at the 12-leaf stage.

Control of volunteer glyphosate-tolerant maize ( Zea mays) in glyphosate-tolerant soybean ( Glycine max)

Glyphosate-tolerant volunteer maize (Zea mays L.) has become a problem when glyphosate-tolerant soybean [ Glycine max (L.) Merr.] follows glyphosate-tolerant maize in a crop rotation. A total of four field trials were conducted at Exeter, Ontario over a 2-year period (2003 and 2004) to evaluate the control of glyphosate-tolerant volunteer maize in glyphosate-tolerant soybean. Treatments consisted of postemergence (POST) glyphosate at 900 g ae/ha alone (control) and in tank-mix with clethodim (at 15, 22.5 and 30 g ai/ha), fenoxaprop- p-ethyl (at 27, 40.5 and 54 g ai/ha), fluazifop- p-butyl (at 37.5, 56.2 and 75 g ai/ha), quizalofop- p-ethyl (at 18, 27 and 36 g ai/ha), or sethoxydim (at 75, 112.5 and 150 g ai/ha). Glyphosate tank-mixed with clethodim, fenoxaprop- p-ethyl, fluazifop- p-butyl, quizalofop- p-ethyl, and sethoxydim did not injure the soybean and controlled volunteer maize. Glyphosate-tolerant volunteer maize density and dry weight were reduced with clethodim, fenoxaprop- p-ethyl, fluazifop- p-butyl, quizalofop- p-ethyl, and sethoxydim, but sethoxydim did not provide control of glyphosate-tolerant volunteer maize equivalent to the other herbicides evaluated. There was a decrease in control as the dose of clethodim, fenoxaprop- p-ethyl, fluazifop- p-butyl, and sethoxydim were reduced. There was no dose response with quizalofop- p-ethyl. Soybean yields closely reflected the level of glyphosate-tolerant volunteer maize control achieved. Based on these results, clethodim, fenoxaprop- p-ethyl, fluazifop- p-butyl, and quizalofop- p-ethyl at the recommended dose can be used to effectively control glyphosate-tolerant volunteer maize in glyphosate-tolerant soybean under Ontario growing conditions.

Broadleaf signalgrass (Brachiaria platyphylla) interference in corn

Abstract Field research was conducted in Tennessee at two locations over 3 yr under no-tillage conditions to examine the interaction of broadleaf signalgrass and corn yield. Broadleaf signalgrass was killed at various time intervals during the growing season by applying postemergent glyphosate. Corn injury was avoided by the use of a glyphosate-tolerant variety. Yield reductions because of broadleaf signalgrass interference occurred with weed densities > 150 plants m−2 and when the corn and weeds emerged simultaneously. Corn had the ability to withstand broadleaf signalgrass presence up to 28 d after planting with no yield loss at all locations. Nomenclature: Glyphosate; broadleaf signalgrass, Brachiaria platyphylla (Griseb.) Nash BRAPP; corn, Zea mays.

Preemergence Weed Control in Soybean with Cloransulam

Field experiments were conducted from 1999 to 2001 to evaluate preemergence (PRE) activity of cloransulam on broadleaf weed species and to determine the effectiveness of cloransulam as a PRE herbicide in glyphosate-resistant soybean weed management systems. Cloransulam PRE controlled prickly sida, velvetleaf, and morningglory species even at reduced rates (recommended rate 36 g ai/ha) but only suppressed growth of Palmer amaranth, hemp sesbania, and sicklepod. Cloransulam applied PRE provided initial control or suppression of most weeds, but late-season control declined appreciably. Adding metribuzin to cloransulam PRE generally improved control of hemp sesbania, Palmer amaranth, annual grasses, and morningglory species, leading to soybean yield increases. Control of weeds was greater on a silt loam soil compared with a silty clay soil. Delayed herbicide activation by rainfall or irrigation reduced control of hemp sesbania and prickly sida and affected efficacy more than soil texture. Single postemergence (POST) applications of glyphosate or fomesafen plus fluazifop-P provided 90% or less control of most weed species. When glyphosate POST or fomesafen plus fluazifop-P POST followed PRE applications of cloransulam or cloransulam plus metribuzin PRE, control of all weeds was generally greater than 85%. The highest soybean yields were recorded from treatments that contained sequential PRE followed by (fb) POST herbicide applications. Composition of weed flora determined the effect of herbicide program on soybean seed yield. No yield benefit was gained from the sequential program when the dominant species was Palmer amaranth, which was controlled by glyphosate. When hemp sesbania was the dominant species, PRE herbicides fb glyphosate POST increased yield compared with total POST glyphosate.

Weed Management in Glyphosate-Resistant Corn with Glyphosate, Halosulfuron, and Mesotrione

Four field studies were conducted at the Peanut Belt Research Station near Lewiston Woodville, NC, in 2000, 2001, and 2002 to evaluate crop tolerance, weed control, grain yield, and net returns in glyphosate-resistant corn with various herbicide systems. Preemergence (PRE) treatment options included no herbicide, atrazine at 1.12 kg ai/ha, or atrazine plus metolachlor at 1.68 kg ai/ha. Postemergence (POST) treatment options included glyphosate at 1.12 kg ai/ha as either the isopropylamine salt or the diammonium salt, either alone or in mixtures with mesotrione at 105 g ai/ha plus crop oil concentrate at 1% (v/v) or halosulfuron at 53 g ai/ha plus 0.25% (v/v) nonionic surfactant. All response variables were independent of glyphosate formulation. Addition of metolachlor to atrazine PRE improved large crabgrass and goosegrass control but did not always improve Texas panicum control. POST control of these annual grasses was similar with glyphosate alone or in mixture with halosulfuron or mesotrione. Glyphosate POST controlled common lambsquarters and common ragweed 89 and 93%, respectively. Glyphosate plus halosulfuron POST provided more effective yellow nutsedge control than glyphosate POST. Atrazine PRE or atrazine plus metolachlor PRE followed by any glyphosate POST treatment controlledIpomoeaspp. at least 93%. Glyphosate plus mesotrione in total POST systems always provided greater control ofIpomoeaspp. than glyphosate alone. The highest yielding treatments always included glyphosate POST, either with or without a PRE herbicide treatment. Similarly, systems that included any glyphosate POST treatment had the highest net returns.

Weed Management with Fomesafen Preemergence in Glyphosate-Resistant Cotton

Field studies were conducted in Alabama in 1998 and 1999 to evaluate fomesafen preemergence (PRE) in glyphosate-resistant cotton. Fomesafen (0.3 and 0.4 kg ai/ha), fluometuron (1.4 kg ai/ha), and pyrithiobac (0.05 kg ai/ha) were applied alone or in tank mixtures. Glyphosate (0.4 kg ae/ha) was applied postemergence over-the-top (POT) and postemergence directed (PD). Regardless of PRE treatment, POT followed by (fb) PD applications of glyphosate were necessary for greater than 82% sicklepod control at midseason. In the absence of glyphosate, fomesafen, and fomesafen-containing tank mixtures controlled common cocklebur andIpomoeaspecies 77 and 72%, respectively, 14 d after PD application. However, two applications of glyphosate were needed for >94% season-long control of common cocklebur andIpomoeaspecies because of continued germination throughout the growing season. Postemergence applications of glyphosate added a 1,000 kg/ha seed cotton yield increase to all PRE treatments at both locations. Common cocklebur,Ipomoeaspecies, and sicklepod control was not significantly increased by the addition of fomesafen PRE tank mixtures fb glyphosate postemergence as compared with glyphosate postemergence only.

Live and Killed Hairy Vetch Cover Crop Effects on Weeds and Yield in Glyphosate-Resistant Corn

A 2-yr field study was conducted from 2002 to 2003 on a Dundee silt loam soil at the Southern Weed Science Research Unit Farm, Stoneville, MS (33°26′N latitude), to examine the effects of hairy vetch cover crop (hairy vetch killed at corn planting [HV-K], hairy vetch killed in a 38-cm-wide band centered over the crop row at corn planting [HV-B], hairy vetch left alive [HV-L], and no hairy vetch [NHV]) and glyphosate postemergence (broadcast, banded, and no herbicide) application on weed control and yield in glyphosate-resistant corn. Two applications of glyphosate at 0.84 kg ae/ha were applied 3 and 5 wk after planting (WAP) corn. Hairy vetch dry biomass was higher in HV-L (4,420 kg/ha) and HV-B (4,180 kg/ha) than in HV-K (1,960 kg/ha) plots at 7 WAP. Hairy vetch reduced densities of pitted morningglory, prickly sida, and yellow nutsedge in HV-B and HV-L compared with NHV plots, but hairy vetch had no effect on densities of barnyardgrass, johnsongrass, and large crabgrass at 7 WAP regardless of desiccation. Total weed dry biomass at 7 WAP was lower in HV-B and HV-L than in HV-K and NHV plots. Corn yield was higher in HV-K (10,280 kg/ha) than in HV-B (9,440 kg/ha) and HV-L (9,100 kg/ha), and yields were similar between HV-K and NHV (9,960 kg/ha). Glyphosate applied broadcast resulted in the highest corn yield (11,300 kg/ha) compared with a banded application (10,160 kg/ha). These findings indicate that hairy vetch cover crop has the potential for reducing the density of certain weed species in glyphosate-resistant corn production systems; however, optimum weed control and higher yield were obtained when glyphosate was used.

Weed Management in Nonirrigated Glyphosate‐Resistant and Non‐Resistant Soybean following Deep and Shallow Fall Tillage

Management inputs that maximize economic return from the early plantings of soybean [Glycine max (L.) Merr.] in the midsouthern USA have not been evaluated fully. The objective was to compare perennial weed control in and yields and economic returns from plantings of maturity group (MG) IV and V soybean cultivars grown in the field under different weed management systems (WMS) following shallow (ST) and deep (DT) fall tillage. Adjacent experiments were conducted on Tunica clay (clayey over loamy, smectitic, nonacid, thermic Vertic Haplaquept) near Stoneville, MS (lat. 33°26′N). Weed management systems were (i) glyphosate [N‐(phosphonomethyl)glycine]‐resistant (GR) cultivars with preemergent (PRE) nonglyphosate herbicides followed by postemergent (POST) glyphosate; (ii) GR cultivars with POST glyphosate; (iii) non‐GR cultivars with PRE plus POST nonglyphosate herbicides; and (iv) non‐GR cultivars with POST nonglyphosate herbicides. Control of perennial redvine [Brunnichia ovata (Walt.) Shinners] declined in the ST environment when non‐GR cultivars were used, but this did not result in a yield decline. Control of perennial johnsongrass [Sorghum halepense (L.) Pers.] at the end of the study period averaged <40% when non‐GR cultivars were used and >93% when GR cultivars were used regardless of tillage treatment, and this was associated with lower yield. Use of PRE + POST vs. POST‐only weed management sometimes resulted in lower profits regardless of fall tillage treatment. The fall tillage treatment × WMS interaction was not significant for yield or net return, which indicates that use of DT for perennial weed management is not economical.

Weed Control and Species Shift in Bromoxynil- and Glyphosate-Resistant Cotton (Gossypium hirsutum) Rotation Systems1

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.

Planted green ash ( Fraxinus pennsylvanica Marsh.) and herbaceous vegetation responses to initial competition control during the first 3 years of afforestation

Formerly row-cropped bottomland sites that have been planted with hardwoods through afforestation programs are typically associated with low tree stocking, slow tree growth, and an enduring influence of agricultural weeds in the herbaceous stratum. Consequently, many of the intended benefits of a forestry land use are delayed or forfeited. This study evaluates the growth of planted green ash and the development of volunteer herbaceous communities on a bottomland site in southern Illinois, USA in response to two different operational herbicide (pre-emergence sulfometuron and post-emergence glyphosate) and tillage treatments, applied individually and in combination. Both herbicide treatments increased green ash height and diameter growth while tillage produced no response. At the end of three growing seasons, the post-emergence glyphosate treatment increased forb cover. Total grass cover was not impacted by herbicide treatments after the second growing season. During the second and third growing seasons, the pre-emergence sulfometuron treatment increased broomsedge ( Andropogon virginicus L.) cover and decreased goldenrod ( Solidago nemoralis Ait.) cover relative to post-emergence glyphosate and no herbicide treatments. Johnsongrass ( Sorghum halepense L.) cover decreased dramatically following the first growing season in all treatments. Horseweed ( Conyza canadensis L.) exhibited a complex response to silvicultural treatments through the course of the study. Commonly used silvicultural treatments that yielded similar and positive tree growth responses differed in their impact on herbaceous community composition through the first 3 years of stand development. These results suggest that treatment choice may have long-term implications for overstory canopy development on these and similar afforested bottomland sites characterized by presence of agricultural weeds and low tree stocking.

Efficacy and Economics of Weed Control Programs in Glyphosate-Resistant Potato (Solanum tuberosum)1

Field studies were conducted to evaluate weed control, tuber yield, gross return, economic return on investment (EROI), and net return in glyphosate-resistant ‘Ranger Russet’ potato in 2000 and 2001 at the University of Idaho Aberdeen Research and Extension Center near Aberdeen, ID. Three types of weed control programs were evaluated: a total glyphosate program of single or sequential applications (TGLY), tank mixtures of glyphosate and residual herbicides applied early postemergence (GLY + RES EPOST), and residual preemergence herbicides followed by (fb) a late postemergence glyphosate application (RES PRE fb LPOST GLY). A standard rimsulfuron + metribuzin + nonionic surfactant EPOST treatment was included for comparison. The standard EPOST treatment and all glyphosate-containing treatments controlled hairy nightshade 88 to 99%. RES PRE fb LPOST GLY treatments improved hairy nightshade control compared with the RES PRE components applied alone. All herbicide treatments controlled kochia 87 to 99% and gre...

Effect of Postemergence Glyphosate Application Timing on Weed Control and Grain Yield in Glyphosate-Resistant Corn: Results of a 2-Yr Multistate Study1

Field studies were conducted at 35 sites throughout the north-central United States in 1998 and 1999 to determine the effect of postemergence glyphosate application timing on weed control and grain yield in glyphosate-resistant corn. Glyphosate was applied at various timings based on the height of the most dominant weed species. Weed control and corn grain yields were considerably more variable when glyphosate was applied only once. The most effective and consistent season-long annual grass and broadleaf weed control occurred when a single glyphosate application was delayed until weeds were 15 cm or taller. Two glyphosate applications provided more consistent weed control when weeds were 10 cm tall or less and higher corn grain yields when weeds were 5 cm tall or less, compared with a single application. Weed control averaged at least 94 and 97% across all sites in 1998 and 1999, respectively, with two glyphosate applications but was occasionally less than 70% because of late emergence of annual grass and...

Cultural Practices and Glyphosate Applications for Sicklepod (Senna obtusifolia) Control in Soybean (Glycine max)1

A study was conducted in 1995 and 1996 to determine the effect of soybean planting date, tillage level, and glyphosate application on sicklepod control and seed production. Soybean was planted in April, May, June, and July into conventional and no-till seedbeds. Herbicide programs evaluated were metribuzin plus chlorimuron preemergence (PRE) followed by (fb) chlorimuron postemergence (POST) (standard program [STD]), metribuzin plus chlorimuron PRE fb glyphosate POST as-needed, and glyphosate POST as-needed. Control was similar across planting dates in both years with slight variations due to weather. The July planting date had the lowest total sicklepod seed production over the 2-yr study. Sicklepod control was better in conventional tillage, but soybean yields were greater in no-till. Herbicide programs that included glyphosate provided greater sicklepod control, lower sicklepod seed production, and higher soybean yields than the STD. Use of glyphosate in combination with later planting dates, especially...

Influence of Early‐Season Nitrogen and Weed Management on Irrigated and Nonirrigated Glyphosate‐Resistant and Susceptible Soybean

Field studies were conducted on Sharkey clay soil (very‐fine, smectitic, thermic Chromic Epiaquert) at Stoneville, MS (33°26′ N lat). The objectives were to determine the effect of application of 0 and 35 kg N ha−1 applied early in the growing season to glyphosate‐resistant (GR) and non‐GR soybean [Glycine max (L.) Merr.] cultivars using two weed management systems in irrigated and nonirrigated environments. Weed management systems were (i) pre‐emergent followed by postemergent weed management using nonglyphosate herbicides applied to both GR and non‐GR cultivars (PRE + POST) and (ii) postemergent weed management using glyphosate on GR cultivars and nonglyphosate herbicides on non‐GR cultivars (POST). Applied N had no effect on weed management in or yield from soybean and lowered average net returns by $28 to $50 ha−1. Average seed yields from the highest‐yielding GR cultivar in 1999 and 2000 were 135 and 270 kg ha−1 more than 1999 and 2000 yields from a non‐GR cultivar in the nonirrigated environment (all net returns were negative and yields <1500 kg ha−1). In the irrigated environment, use of a non‐GR cultivar compared with a GR cultivar resulted in a significant 200 and 250 kg ha−1 greater yield and greater profits in 2 of 3 yr. Use of PRE + POST compared with POST‐only was not necessary for achieving greatest yield or net return with either non‐GR or GR cultivars. Use of postemergent glyphosate always resulted in the cheapest weed control ($43 to $81 ha−1), even with the greater cost for seed of GR cultivars included. There was no measured effect of glyphosate compared with nonglyphosate herbicides on GR cultivar yield.

Influence of Early-Season Nitrogen and Weed Management on Irrigated and Nonirrigated Glyphosate-Resistant and Susceptible Soybean

Field studies were conducted on Sharkey clay soil (very-fine, smectitic, thermic Chromic Epiaquert) at Stoneville, MS (33°26′ N lat). The objectives were to determine the effect of application of 0 and 35 kg N ha−1 applied early in the growing season to glyphosate-resistant (GR) and non-GR soybean [Glycine max (L.) Merr.] cultivars using two weed management systems in irrigated and nonirrigated environments. Weed management systems were (i) pre-emergent followed by postemergent weed management using nonglyphosate herbicides applied to both GR and non-GR cultivars (PRE + POST) and (ii) postemergent weed management using glyphosate on GR cultivars and nonglyphosate herbicides on non-GR cultivars (POST). Applied N had no effect on weed management in or yield from soybean and lowered average net returns by $28 to $50 ha−1. Average seed yields from the highest-yielding GR cultivar in 1999 and 2000 were 135 and 270 kg ha−1 more than 1999 and 2000 yields from a non-GR cultivar in the nonirrigated environment (all net returns were negative and yields <1500 kg ha−1). In the irrigated environment, use of a non-GR cultivar compared with a GR cultivar resulted in a significant 200 and 250 kg ha−1 greater yield and greater profits in 2 of 3 yr. Use of PRE + POST compared with POST-only was not necessary for achieving greatest yield or net return with either non-GR or GR cultivars. Use of postemergent glyphosate always resulted in the cheapest weed control ($43 to $81 ha−1), even with the greater cost for seed of GR cultivars included. There was no measured effect of glyphosate compared with nonglyphosate herbicides on GR cultivar yield.

Comparison of Glyphosate with Other Herbicides for Weed Control in Corn (Zea mays): Efficacy and Economics1

Abstract: Glyphosate was compared with other commonly used corn herbicides at three locations in Kentucky in 1998 and 1999. Sequential glyphosate treatments provided greater than 87% control of common cocklebur, ivyleaf morningglory, common lambsquarters, and giant ragweed. Control of these species with glyphosate treatments was similar to the control with atrazine plus other postemergence (POST) herbicides. Generally, treatments containing s-triazines and chloracetamides applied to the soil surface were not as effective as sequential glyphosate or atrazine plus POST herbicides. Corn yield was not significantly different at any location or for any year, whereas differences in net return occurred at one location in 1998. Nomenclature: Atrazine; chloracetamides; glyphosate; s-triazines; common cocklebur, Xanthium strumarium L. #3 XANST; common lambsquarters, Chenopodium album L. # CHEAL; giant ragweed, Ambrosia trifida L. # AMBTR; ivyleaf morningglory, Ipomoea hederacea L. Jacq. # IPOHE; corn, Zea mays L. ‘...

Glyphosate in Double-Crop No-Till Glyphosate-Resistant Soybean: Role of Preplant Applications and Residual Herbicides1

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.