PRE Fb POST Research Articles

Herbicide programs for control of waterhemp (Amaranthus tuberculatus) resistant to three distinct herbicide sites of action in corn

AbstractControl of waterhemp is becoming more difficult in Ontario because biotypes have evolved resistance to four herbicide sites of action (SOA), including groups 2, 5, 9, and 14. The objective of this study was to compare PRE, POST, and PRE followed by (fb) POST herbicide programs for their effect on control, density, and biomass of multiple-herbicide–resistant (MHR) waterhemp as well as corn injury and grain yield. Two separate field studies, each consisting of five field trials, were conducted over a 2-yr period (2018 and 2019) in fields where corn was grown in Ontario, Canada. The first experiment evaluated MHR waterhemp control with an inhibitor of 4-hydroxyphenyl-pyruvate dioxygenase (HPPD) applied PRE, PRE fb glufosinate applied POST, and glufosinate applied POST. The second experiment evaluated MHR waterhemp control with a non-HPPD inhibitor applied PRE, then PRE fb a POST application of atrazine + mesotrione, and then atrazine + mesotrione applied POST. Atrazine + isoxaflutole caused 3% to 5% corn injury at environment 1 (E1); no corn injury was observed with PRE and POST herbicide programs at environments E2, E3, E4, and E5. In general, atrazine/bicyclopyrone/mesotrione/S-metolachlor and dimethenamid-P/saflufenacil applied PRE controlled MHR waterhemp ≥95% 12 wk after POST application (WAA). A POST application of glufosinate following atrazine + tolpyralate PRE, and a POST application of atrazine + mesotrione following atrazine/dicamba or atrazine/S-metolachlor PRE, improved control at 4, 8, and 12 WAA in most environments. In general, PRE fb POST applications resulted in better control of MHR waterhemp throughout the growing season than single PRE and POST applications (P < 0.05). We conclude that herbicide programs based on multiple effective SOAs may offer effective control of MHR waterhemp where field corn is grown. It is advisable that when choosing an herbicide application program that excellent control of MHR waterhemp should be the goal given its high fecundity and competitive ability.

Herbicide programs to manage glyphosate/dicamba-resistant kochia (Bassia scoparia) in glyphosate/dicamba-resistant soybean

AbstractEvolution of kochia resistance to glyphosate and dicamba is a concern for growers in the US Great Plains. An increasing use of glyphosate and dicamba with the widespread adoption of glyphosate/dicamba-resistant (GDR) soybean in recent years may warrant greater attention. Long-term stewardship of this new stacked-trait technology will require the implementation of diverse weed control strategies, such as the use of soil-residual herbicides (PRE) aimed at effective control of GDR kochia. Field experiments were conducted in Huntley, MT, in 2017 and 2018, and Hays, KS, in 2018 to determine the effectiveness of various PRE herbicides applied alone or followed by (fb) a POST treatment of glyphosate plus dicamba for controlling GDR kochia in GDR soybean. Among PRE herbicides tested, sulfentrazone provided complete (100%), season-long control of GDR kochia at both sites. In addition, PRE fb POST programs tested in this study brought 71% to 100% control of GDR kochia throughout the season at both sites. Pyroxasulfone applied PRE resulted in 57% to 70% control across sites at 9 to 10 wk after PRE (WAPRE). However, mixing dicamba with pyroxasulfone improved control up to 25% at both sites. Kochia plants surviving pyroxasulfone applied PRE alone produced 2,530 seeds m−2 compared with pyroxasulfone + dicamba (230 seeds m−2) at the Montana site. No differences in soybean grain yields were observed with PRE alone or PRE fb POST treatments at the Montana site; however, dicamba, pyroxasulfone, and pendimethalin + dimethenamid-P applied PRE brought lower grain yield (1,150 kg ha−1) compared to all other tested programs at the Kansas site. In conclusion, effective PRE or PRE fb POST (two-pass) programs tested in this research should be proactively utilized by the growers to manage GDR kochia in GDR soybean.

Control of Atrazine-Resistant Palmer Amaranth (Amaranthus palmeri) in Double-Crop Grain Sorghum

AbstractDouble-crop grain sorghum after winter wheat harvest is a common cropping system in the southern plains region. Palmer amaranth is a troublesome weed in double-crop grain sorghum in Kansas. Populations resistant to various herbicides (e.g., atrazine, glyphosate, metsulfuron, pyrasulfotole) have made Palmer amaranth management even more difficult for producers. To evaluate control of atrazine-resistant and atrazine-susceptible Palmer amaranth in double-crop grain sorghum, we assessed 14 herbicide programs, of which 8 were PRE only and 6 were PRE followed by (fb) POST applications. Visible ratings of Palmer amaranth control were taken at 3 and 8 wk after planting (WAP) grain sorghum. PRE treatments containing very-long-chain fatty acid (VLCFA)–inhibiting herbicides provided 91% control of atrazine-resistant Palmer amaranth 3 WAP, and reduced weed density 8 WAP compared to atrazine-only PRE treatments. PRE fb POST treatments, especially those that included VLCFA-inhibiting herbicides, provided greater control (71% to 93%) of both atrazine-resistant and atrazine-susceptible Palmer amaranth, respectively, at 8 WAP compared to PRE treatments alone (59% to 79%). These results demonstrated the utility of VLCFA-inhibiting herbicides applied PRE and in a layered PRE fb POST approach in controlling atrazine-resistant Palmer amaranth, as well as the importance of an effective POST application following residual PRE herbicides for controlling both atrazine-resistant and atrazine-susceptible Palmer amaranth in double-crop grain sorghum.

Palmer Amaranth (Amaranthus palmeri) and Velvetleaf (Abutilon theophrasti) Control in No-Tillage Conventional (Non–genetically engineered) Soybean Using Overlapping Residual Herbicide Programs

AbstractDue to depressed corn and soybean prices over the last few years in the United States, growers in Nebraska are showing interest in no-tillage (hereafter referred to as no-till) conventional (non–genetically engineered [non-GE]) soybean production. Due to the increasing number of herbicide-resistant weeds in the United States, weed control in no-till non-GE soybean using POST herbicides is a challenge. The objectives of this study were to compare PRE-only, PRE followed by (fb) POST, and PRE fb POST with residual (POST-WR) herbicide programs for Palmer amaranth and velvetleaf control and soybean injury and yield, as well as to estimate the gross profit margins and benefit–cost ratio of herbicide programs. A field experiment was conducted in 2016 and 2017 at Clay Center, NE. The PRE herbicides tested in this study resulted in ≥95% Palmer amaranth and velvetleaf control at 28 d after PRE (DAPRE). Averaged across the programs, the PRE-only program controlled Palmer amaranth 66%, whereas 86% and 97% control was obtained with the PRE fb POST and PRE fb POST-WR programs, respectively, at 28 d after POST (DAPOST). At 28 DAPOST, the PRE fb POST herbicide programs controlled velvetleaf 94%, whereas the PRE-only program resulted in 85% control. Mixing soil-residual herbicides with foliar-active POST programs did not improve velvetleaf control. Averaged across herbicide programs, PRE fb POST programs increased soybean yield by 10% and 41% in 2016 and 2017, respectively, over the PRE-only programs. Moreover, PRE fb POST-WR programs produced 7% and 40% higher soybean yield in 2016 and 2017, respectively, compared with the PRE fb POST programs. The gross profit margin (US$1,184.3 ha−1) was highest under flumioxazin/pyroxasulfone (PRE) fb fluthiacet-methyl plusS-metolachlor/fomesafen (POST-WR) treatment; however, the benefit–cost ratio was highest (6.1) with the PRE-only program of flumioxazin/chlorimuron-ethyl.

Control of Volunteer Horseradish and Palmer Amaranth (Amaranthus palmeri) with Dicamba and Glyphosate

Management of volunteer horseradish is a challenge when it is grown in rotation with other crops, such as corn and soybean. Although volunteer horseradish may not cause yield loss, these plants serve as hosts for various soilborne pathogens that damage subsequent horseradish crops. In addition to volunteer horseradish, glyphosate-resistant Palmer amaranth is becoming difficult to control in southwestern Illinois, as a consequence of the plant’s ability to withstand glyphosate and drought, produce many seeds, and grow rapidly. Field studies were conducted to evaluate the effect of glyphosate and dicamba on volunteer horseradish and Palmer amaranth control in 2014 and 2015. Glyphosate alone (1,265 and 1,893 g ae ha−1) and glyphosate plus dicamba at the high rate (1,680 g ae ha−1) provided the greatest volunteer horseradish control, ranging from 81% to 89% and 90% to 93%, respectively. Measures of root biomass reduction followed similar trends. Glyphosate alone provided the greatest reduction in volunteer horseradish root viability (79% to 100%) but was similar in efficacy to applications of glyphosate plus dicamba in most comparisons. Efficacy of PRE-only applications on Palmer amaranth control ranged from 92% to 99% control in 2014 and 68% to 99% in 2015. However, PRE-only applications were often similar in efficacy to PRE followed by (fb) glyphosate plus dicamba applied POST. Treatments containing flumioxazin did not control Palmer amaranth as well as other treatments. POST applications alone were not effective in managing Palmer amaranth. Many of the PRE fb POST treatment options tested will improve resistance management over PRE-only treatments, provide control of Palmer amaranth, and allow horseradish to be planted the following spring.

Comparison of Herbicide Programs for Season-Long Control of Glyphosate-Resistant Common Waterhemp (Amaranthus rudis) in Soybean

The evolution of glyphosate and acetolactate synthase (ALS) inhibitor-resistant common waterhemp in the Midwestern United States has reduced the number of effective POST herbicide options for management of this problem weed in glyphosate-resistant soybean. Moreover, common waterhemp emerges throughout the crop growing season, justifying the need to evaluate herbicide programs that provide season-long control. The objectives of this study were to compare POST-only and PRE followed by (fb) POST herbicide programs for control of glyphosate-resistant common waterhemp in glyphosate-resistant soybean. Field experiments were conducted in 2013 and 2014 in Dodge County, NE, in a field infested with glyphosate-resistant common waterhemp. Programs containing PRE herbicides resulted in ≥83% control of common waterhemp and densities of ≤35 plantsm–2at 21 d after PRE (DAPRE). Post-only herbicide programs resulted in <70% control and densities of 107 to 215 plants m–2at 14 d after early-POST (DAEPOST) treatment. PRE fb POST herbicide programs, including saflufenacil plus imazethapyr plus dimethenamid-P, sulfentrazone plus cloransulam, orS-metolachlor plus metribuzin, fb fomesafen plus glyphosate;S-metolachlor plus fomesafen fb acifluorfen plus glyphosate resulted in >90% control of glyphosate-resistant common waterhemp throughout the growing season, reduced density to ≤7plantsm–2, ≥92% biomass reduction, and soybean yield >2,200kg ha–1. Averaged across herbicide programs, common waterhemp control was 84%, and density was 15 plants m–2with PRE fb POST herbicide programs compared with 42% control, and density of 101 plants m–2with POST-only herbicide programs at harvest. Results of this study indicated that PRE fb POST herbicide programs with effective modes of action exist for season-long control of glyphosate-resistant common waterhemp in glyphosate-resistant soybean.

Control of Glyphosate-Resistant Common Waterhemp (Amaranthus rudis) in Glufosinate-Resistant Soybean

Glyphosate-resistant (GR) common waterhemp has become a significant problem weed in Nebraska and several Midwestern states. Several populations of GR common waterhemp are also resistant to acetolactate synthase (ALS)-inhibiting herbicides, making them difficult to control with POST herbicides in GR soybean. Glufosinate-resistant (GFR) soybean is an alternate system for controlling GR common waterhemp, justifying the need for evaluating glufosinate-based herbicide programs. The objectives of this study were to compare POST-only herbicide programs (including one-pass and two-pass POST programs) with PRE followed by (fb) POST herbicide programs for control of GR common waterhemp in GFR soybean and their effect on common waterhemp density, biomass, and soybean yield. Field experiments were conducted in 2013 and 2014 near Fremont, NE in a grower’s field infested with GR common waterhemp. Glufosinate applied early- and late-POST provided 76% control of GR common waterhemp at 14 d after late-POST (DALPOST) compared with 93% control with a PRE fb POST program when averaged across treatments. The PRE application of chlorimuron plus thifensulfuron plus flumioxazin,S-metolachlor plus fomesafen or metribuzin, saflufenacil plus dimethenamid-P fb glufosinate provided ≥95% control of common waterhemp throughout the growing season, reduced common waterhemp density to ≤2.0 plants m─2, caused ≥94% biomass reduction, and led to 1,984 to 2,210 kg ha─1soybean yield. Averaged across treatments, the PRE fb POST program provided 82% common waterhemp control at soybean harvest, reduced density to 23 plants m─2at 14 DALPOST, and caused 86% biomass reduction and 1,803 kg ha─1soybean yield compared with 77% control, 99 plants m─2, 53% biomass reduction, and 1,190 kg ha─1yield with POST-only program. It is concluded that PRE fb POST programs with multiple effective modes of action are available for control of GR common waterhemp in GFR soybean.

Integrating Herbicide Programs with Harvest Weed Seed Control and Other Fall Management Practices for the Control of Glyphosate-Resistant Palmer Amaranth (Amaranthus palmeri)

A large-plot field experiment was conducted at Keiser, AR, from fall of 2010 through fall of 2013 to understand to what extent soybean in-crop herbicide programs and postharvest fall management practices impact Palmer amaranth population density and seed production over three growing seasons. The effect of POST-only (glyphosate-only) or PRE followed by (fb) POST (glyphosate or glufosinate) + residual herbicide treatments were evaluated alone and in combination with postharvest management options of soybean residue spreading or soil incorporation, use of cover crops, windrowing with/without burning, and residue removal. Significant differences were observed between fall management practices on Palmer amaranth population density each fall. The use of cover crops and residue collection and removal fb the incorporation of crop residues into soil during the formation of beds were the most effective practices in reducing Palmer amaranth population. In contrast, the effects of fall management practices on Palmer amaranth seed production were inconsistent among years. The inclusion of a PRE herbicide application into the herbicide program significantly reduced Palmer amaranth population density and subsequent seed production each year when compared to the glyphosate-only program. Additionally, the glufosinate-containing residual program was superior to the glyphosate-containing residual program in reducing Palmer amaranth seed production. PRE fb POST herbicides resulted in significant decreases in the Palmer amaranth population density and seed production compared to POST application of glyphosate alone for all fall management practices, including the no-till practice. This study demonstrated that crop residue management such as chaff removal from the field, the use of cover crops, or seed incorporation during bed formation in combination with an effective PRE plus POST residual herbicide program is important for optimizing in-season management of Palmer amaranth and subsequently reducing the population density, which has a profound impact on lessening the risk for herbicide resistance and the consistency and effectiveness of future weed management efforts.

Management of Pigweed (Amaranthusspp.) in Glufosinate-Resistant Soybean in the Midwest and Mid-South

Pigweeds are among the most abundant and troublesome weed species across Midwest and mid-South soybean production systems because of their prolific growth characteristics and ability to rapidly evolve resistance to several herbicide sites of action. This has renewed interest in diversifying weed management strategies by implementing integrated weed management (IWM) programs to efficiently manage weeds, increase soybean light interception, and increase grain yield. Field studies were conducted across 16 site-years to determine the effectiveness of soybean row width, seeding rate, and herbicide strategy as components of IWM in glufosinate-resistant soybean. Sites were grouped according to optimum adaptation zones for soybean maturity groups (MGs). Across all MG regions, pigweed density and height at the POST herbicide timing, and end-of-season pigweed density, height, and fecundity were reduced in IWM programs using a PRE followed by (fb) POST herbicide strategy. Furthermore, a PRE fb POST herbicide strategy treatment increased soybean cumulative intercepted photosynthetically active radiation (CIPAR) and subsequently, soybean grain yield across all MG regions. Soybean row width and seeding rate manipulation effects were highly variable. Narrow row width (≤ 38 cm) and a high seeding rate (470,000 seeds ha−1) reduced end-of-season height and fecundity variably across MG regions compared with wide row width (≥ 76 cm) and moderate to low (322,000 to 173,000 seeds ha−1) seeding rates. However, narrow row widths and high seeding rates did not reduce pigweed density at the POST herbicide application timing or at soybean harvest. Across all MG regions, soybean CIPAR increased as soybean row width decreased and seeding rate increased; however, row width and seeding rate had variable effects on soybean yield. Furthermore, soybean CIPAR was not associated with end-of-season pigweed growth and fecundity. A PRE fb POST herbicide strategy was a necessary component for an IWM program as it simultaneously managed pigweeds, increased soybean CIPAR, and increased grain yield.

Identity-Preserved Soybean Tolerance to Protoporphyrinogen Oxidase-Inhibiting Herbicides

Field experiments were conducted from 2011 to 2013 near Ridgetown and Exeter, Ontario, Canada to evaluate the tolerance of seven identity-preserved (IP) soybean cultivars to protoporphyrinogen oxidase (PPO)-inhibiting herbicides flumioxazin, saflufenacil/dimethenamid-P, and sulfentrazone applied PRE; fomesafen applied POST; as well as PRE followed by (fb) POST application. Ridgetown sites demonstrated excellent tolerance (< 10% injury) to PRE treatments, whereas PRE sulfentrazone caused up to 36% injury at 1 and 2 wk after application (WAA) at Exeter. Of the PRE fb POST treatments evaluated, those containing saflufenacil/dimethenamid-P and sulfentrazone were most injurious to soybean, with cultivar being a further determinant of injury. At Exeter 1 WAA, cultivars ‘S03W4’ and ‘S23T5’ showed 23 to 27 and 45 to 46% injury for saflufenacil/dimethenamid-P and sulfentrazone when followed by POST fomesafen, respectively. Sulfentrazone application (PRE alone or fb fomesafen) consequently reduced S03W4 yield up to 38% and S23T5 up to 25%, whereas saflufenacil/dimethenamid-P fb fomesafen reduced S03W4 by 18%. In general, PRE fb POST caused more injury than PRE treatments; however, injury diminished over time and no significant reduction to soybean yield could be attributed to adding POST fomesafen. On the basis of this study, the injury from PPO herbicides applied PRE is active ingredient, cultivar, and environment specific. Although PPO herbicides have the potential to cause unacceptable crop injury in some IP soybean cultivars, selecting a tolerant cultivar will minimize yield losses.

Integrated Management of Glyphosate-Resistant Giant Ragweed (Ambrosia trifida) with Tillage and Herbicides in Soybean

Giant ragweed is one of the most competitive annual broadleaf weeds in soybean production fields in the midwestern United States and eastern Canada because of its early emergence, rapid growth rate, high plasticity, and resistance to glyphosate and acetolactate synthase inhibitors. Therefore, early-season management of giant ragweed is critical to avoid yield loss. The objectives of this study were to evaluate control of glyphosate-resistant giant ragweed through the integration of preplant tillage or 2,4-D; PRE or early POST (EPOST) followed by (fb) late POST (LPOST) herbicide programs with or without preplant tillage or 2,4-D; and their effect on soybean injury and yield. A field study was conducted in 2013 and 2014 in David City, NE in a field infested with glyphosate-resistant giant ragweed. Preplant tillage or 2,4-D application provided > 90% control of glyphosate-resistant giant ragweed 14 d after preplant treatment. Giant ragweed control and biomass reduction were consistently > 90% with preplant tillage or 2,4-D fb sulfentrazone plus cloransulam PRE or glyphosate plus cloransulam EPOST fb glyphosate plus fomesafen or lactofen LPOST compared with ≤ 86% control with same treatments without preplant tillage or 2,4-D. PRE or EPOST fb LPOST herbicide programs preceded by preplant treatments resulted in giant ragweed density < 2 plants m−2and soybean yield > 2,400 kg ha−1compared with the density of ≥ 2 plants m−2and soybean yield < 1,800 kg ha−1under PRE or EPOST fb LPOST herbicide programs. The contrast analysis also indicated that preplant tillage or 2,4-D fb a PRE or POST program was more effective for giant ragweed management compared with PRE fb POST herbicide programs. Integration of preplant tillage would provide an alternative method for early-season control of giant ragweed; however, a follow up application of herbicides is needed for season-long control in soybean.

Evaluation of Weed Management Programs and Response of FG72 Soybean to HPPD-Inhibiting Herbicides

Field experiments were conducted at two locations in Missouri in 2012 and 2013 to evaluate herbicide programs in 4-hydroxyphenylpyruvate dioxygenase (HPPD)-inhibitor-resistant soybean, referred to as FG72 soybean, and their tolerance to four HPPD-inhibiting herbicides. At the Columbia location, PRE followed by (fb) POST and two-pass POST treatments provided 97% or greater control of all weeds except ivyleaf morningglory. At Moberly in 2012, PRE fb POST treatments provided 95% or greater control and 100% biomass reduction (BR) of glyphosate-resistant (GR) waterhemp, with the exception of isoxaflutole at 0.04 kg ha−1plusS-metolachlor at 0.6 kg ha−1plus metribuzin at 0.2 kg ha−1. In 2013, PRE fb POST treatments provided greater than 89% control and 93% BR. Two-pass POST treatments of isoxaflutole plus glyphosate always provided greater control and BR of GR waterhemp compared with glyphosate fb glyphosate. However, at Columbia, where glyphosate-susceptible weeds were present, there were no differences in control or BR between two-pass POST treatments. In the soybean tolerance experiment, isoxaflutole provided the lowest levels of injury. Applications of tembotrione at the 1× rate resulted in the greatest injury in both years. Topramezone at the 1× rate always provided less injury than tembotrione, but was always similar in BR. The 2× rates increased soybean injury over the 1× rate for the third trifoliate (V3) application, but not for the PRE and first-flower (R1) applications. V3 and R1 applications of isoxaflutole and mesotrione resulted in similar injury, height reduction, and BR to soybean 28 d after application in 2012 and 2013. Overall these results indicate that FG72 soybean could allow the use of HPPD-inhibiting herbicides such as mesotrione PRE along with isoxaflutole PRE and POST to provide an additional herbicide mechanism of action that was not previously available in soybean.

Influence of Soybean Seeding Rate, Row Spacing, and Herbicide Programs on the Control of Resistant Waterhemp in Glufosinate-Resistant Soybean

Field experiments were conducted in 2012 and 2013 to determine the effects of row spacing, seeding rate, and herbicide programs on multiple-resistant waterhemp control and yield in glufosinate-resistant soybean. The two herbicide programs evaluated were: (1) a PRE application of fomesafen plusS-metolachlor followed by early POST application of glufosinate plus acetochlor, referred to as the PRE followed by (fb) POST with residual (w/RES) herbicide program; and (2) an early POST followed by a late POST application of glufosinate, referred to as the two-pass POST herbicide program. Results indicate that the PRE fb POST w/RES program provides greater control of resistant waterhemp compared to the two-pass POST herbicide program. In 2012, the PRE fb POST w/RES program resulted in a 99% waterhemp density reduction and 156 kg ha−1increase in yield compared to the 72% density reduction by the two-pass POST program. In 2013, the two-pass POST program was equally as effective on density reduction and yield as the PRE fb POST w/RES program. Waterhemp control and density reduction was always greatest with 19- and 38- compared to 76-cm rows. In 2012, the PRE fb POST w/RES program provided at least 95% control and greater than 98% density reduction across all row spacings, whereas the two-pass POST program provided 95%, 95%, and 85% control and 87%, 80%, and 50% density reduction in 19-, 38-, and 76-cm rows, respectively. Soybean seeding rate did not affect waterhemp control or density in either year. In both years, 165,000 seeds ha−1yielded lower than the three higher seeding rates. Overall, results from these experiments indicates that the use of a PRE fb POST w/RES program, narrow-row spacing, and seeding rates of 240,000 to 315,000 seeds ha−1or greater provides the greatest waterhemp control, density reduction, and soybean yield when multiple resistant waterhemp is present.

Influence of Herbicide Programs on Weed Management in Soybean with Resistance to Glufosinate and 2,4-D

A field experiment was conducted in Boone and Callaway counties in Missouri in 2010 and 2011 to investigate herbicide programs for the management of summer annual grass and broadleaf weeds in soybean resistant to 2,4-D and glufosinate. Results revealed that the addition of 0.56, 0.84, or 1.12 kg ha−1 2,4-D (amine) to either or both POST applications of glufosinate in a two-pass POST herbicide program increased control of common waterhemp compared to two POST applications of glufosinate alone. Similar levels of common cocklebur, giant foxtail, large crabgrass, and barnyardgrass control were achieved with any of the two-pass POST programs that contained 2,4-D compared to two-pass POST programs containing glufosinate alone. Similar control of these species was also achieved with the inclusion of 2,4-D in either the first or second pass of glufosinate. Two-pass programs resulted in the highest levels of weed control (90%). Annual grass and broadleaf weed control was as much as 59% lower when one-pass POST herbicide programs were compared to PRE followed by (fb) POST or two-pass POST programs. However, one-pass POST programs were sufficient to obtain exceptional control of common cocklebur and giant foxtail. Across all site years, soybean yields ranged from 2,680 to 3,100 kg ha−1 for all herbicide treatments, but did not differ statistically. Overall, results from these experiments indicate that compared to glufosinate alone, PRE fb POST or two-pass POST herbicide programs that incorporate 2,4-D amine with glufosinate in 2,4-D–resistant soybean enhance control of common waterhemp, while providing similar levels of control of other summer annual grass and broadleaf weeds.

Evaluation of Application Program and Timing in Herbicide-Resistant Corn

Field studies were conducted from 2007 to 2009 in East Lansing, MI to evaluate three residual herbicide programs, three POST herbicide application timings, and two POST herbicides in glyphosate- and glufosinate-resistant corn. Herbicide programs included a residual PRE-applied herbicide followed by (fb) POST application (residual fb POST), a residual herbicide tank-mixed with a POST herbicide (residual + POST), and a nonresidual POST. Three POST herbicide application timings included early POST (EP), mid-POST (MP), and late POST (LP) at an average corn growth stage of V3/V4, V4/V5, and V5/V6, respectively. The two POST herbicides evaluated were glyphosate and glufosinate. Control of common lambsquarters and giant foxtail was evaluated 28 d after the LP application. Glyphosate often provided greater weed control than glufosinate. The LP application resulted in greater giant foxtail control compared with the EP application timing, which may be attributed to control of late-emerging weeds. The EP application timing improved common lambsquarters control compared with the LP application timing. The residual + POST program resulted in greater weed control compared with the residual fb POST program in all years. The effect of residual herbicide program, POST herbicide, and POST application timing on corn grain yield varied by year. In 2007, the use of glyphosate resulted in higher grain yield compared with glufosinate. In 2008, corn grain yield was the highest in the PRE fb POST program and with POST applications at EP and MP. To provide the most consistent weed control and minimize the likelihood of grain yield reductions, a PRE fb POST program applied at EP or MP is recommended.

Comparison of Herbicide Tactics to Minimize Species Shifts and Selection Pressure in Glyphosate-Resistant Soybean

There are significant concerns over the long- and short-term implications of continuous glyphosate use and potential problems associated with weed species shifts and the development of glyphosate-resistant weed species. Field research was conducted to determine the effect of herbicide treatment and application timing on weed control in glyphosate-resistant soybean. Ten herbicide treatments were evaluated that represented a range of PPI, PRE, and POST-only application timings. All herbicide treatments included a reduced rate of glyphosate applied POST. PRE herbicides with residual properties followed by (fb) glyphosate POST provides more effective control of broadleaf weed species than POST-only treatments. There was no difference in soybean yield between PRE fb POST and POST-only treatments in 2008. Conversely, PRE fb POST herbicide treatments resulted in greater yield than POST-only treatments in 2009. Using PRE fb POST herbicide tactics improves weed control and reduces the risk for crop yield loss when dealing with both early- and late-emerging annual broadleaf weed species across variable cropping environments.

Glyphosate-Resistant Waterhemp (Amaranthus rudis) Control and Economic Returns with Herbicide Programs in Soybean

Field experiments were conducted in Platte County, Missouri, during 2006 and 2007 to evaluate PRE, POST, and PRE followed by (fb) POST herbicide programs for the control of glyphosate-resistant waterhemp in soybean. All PRE fb POST treatments resulted in at least 66 and 70% control of glyphosate-resistant waterhemp in 2006 and 2007, respectively. Control of glyphosate-resistant waterhemp was less than 23% with lactofen and acifluorfen in 2006, but at least 64% in 2007. Variability in control likely resulted from differences in trial locations and a population of protoporphyrinogen oxidase (PPO)–resistant waterhemp at the Platte County site in 2006 compared with 2007. In both years, glyphosate resulted in less than 23% control of glyphosate-resistant waterhemp and provided the least control of all herbicide programs. Programs containing PRE herbicides resulted in waterhemp densities of less than 5 plants/m2, whereas the POST glyphosate treatment resulted in 38 to 70 plants/m2. Waterhemp seed production was reduced at least 78% in all PRE fb POST programs, from 55 to 71% in POST programs containing lactofen and acifluorfen and by only 21% in the POST glyphosate treatment. Soybean yields corresponded to the level of waterhemp control achieved in both years, with the lowest yields resulting from programs that provided poorest waterhemp control. PRE applications ofS-metolachlor plus metribuzin provided one of the highest net incomes in both years and resulted in $271 to $340/ha greater net income than the glyphosate-only treatment. Collectively, the results from these experiments illustrate the effectiveness of PRE herbicides for the control of glyphosate-resistant waterhemp in glyphosate-resistant soybean and the inconsistency of PPO-inhibiting herbicides or PPO-inhibiting herbicide combinations for the control of waterhemp populations with multiple resistance to glyphosate and PPO-inhibiting herbicides.

Response of Seeded and Transplanted Summer Squash toS-Metolachlor Applied at Planting and Postemergence

Field experiments were conducted in Georgia in 2004 and 2005 to evaluate the effects of S-metolachlor on summer squash fruit yield. Main treatment effects included summer squash cultivar (yellow or zucchini), planting method (seeded or transplanted) and herbicide program (nontreated control, S-metolachlor applied at planting and prior to transplanting [PRE] at 0.5 and 1.0 kg ai/ha, S-metolachlor applied postemergence [POST] 3 wk after planting [WAP] at 0.5 and 1.0 kg/ha, and S-metolachlor applied PRE at 0.5 kg/ha followed by POST at 0.5 kg/ha [PRE fb POST]). Fruit number and weight were measured 12 times during each growing season and the harvests combined into early (harvests 1 to 4), middle (harvests 5 to 8), late (harvests 9 to 12), and cumulative (harvests 1 to 12) yield categories. Mixed-models analyses were used to evaluate the effects of herbicide rate and timing, squash cultivar, and planting method on squash yield for each harvest period. Summer squash cultivar and planting method did not affect squash response to S-metolachlor. Averaged over squash cultivar and planting method, S-metolachlor applied PRE and PRE fb POST reduced fruit number and weight at the early harvest between 35 and 60%, middle harvest between 14 and 30%, and cumulative harvest between 14 and 22%. S-metolachlor applied POST at 0.5 kg/ha did not impact squash yield compared to the nontreated control at any harvest period, whereas 1.0 kg/ha reduced fruit number and weight at the middle harvest 14 and 20%, respectively. We propose that POST applications of S-metolachlor at 0.5 kg/ha or lower can be adopted for use in summer squash production in Georgia.Nomenclature: S-metolachlor, summer squash, Cucurbita pepo L. ‘Enterprise’ and ‘Payroll’

Response of Five Summer-Squash (Cucurbita pepo) Cultivars to Halosulfuron

Response of ‘Dixie’, ‘Lemondrop’, ‘Multipik’, ‘Superpik’, and ‘Seneca Prolific’ summer squash to halosulfuron PRE or POST at 0.036, 0.053, and 0.072 kg ai/ha, or halosulfuron PRE fb halosulfuron POST at 0.018 fb 0.018, 0.027 fb 0.027, and 0.036 fb 0.036 kg/ha was field evaluated in 1997 and 1998. All halosulfuron treatments and rates reduced the height of cultivars 17–19% at 6 WAP (weeks after planting) and summer-squash injury (chlorosis and necrosis of crop foliage) was 6, 14, and 11% from halosulfuron PRE, POST, and PRE fb POST, respectively. Early summer-squash flowering was reduced 32–82% by halosulfuron, resulting in reduced early yields. Dixie was the cultivar most tolerant to halosulfuron. Early flowering of Dixie was reduced 32–36% compared to 32–82% for the other cultivars. Marketable yield of summer squash was reduced 20–30% by all rates of halosulfuron when averaged over all application timings. Marketable yield of Seneca Prolific, Superpik, Dixie, Multipik, and Lemondrop was reduced 0–17% by halosulfuron PRE. Halosulfuron POST or PRE fb POST reduced marketable yield of all summer-squash cultivars by 25–46%. Thus, summer squash was not tolerant of POST halosulfuron; however, Dixie, Multipik, Seneca Prolific, and Superpik exhibited tolerance to halosulfuron PRE.

Effect of Cultural Practices on Weed Control and Crop Response in Imidazolinone-Tolerant Rice

Field research was conducted for 3 yr to evaluate crop response and weed control under conventional and reduced tillage in drill- and water-seeded imidazolinone-tolerant (IT) rice culture. Imazethapyr was applied at 70 g ai/ha PRE followed by (fb) imazethapyr at 70 g/ha applied POST to three- to four-leaf rice or at 105 g/ha PRE fb 70 g/ha POST. In both conventional and reduced tillage systems, imazethapyr applied PRE fb POST at 70 g ai/ha controlled red rice, barnyardgrass, Amazon sprangletop, and rice flatsedge 87 to 99% 35 d after POST treatment (DAT). At 35 DAT, Indian jointvetch control with sequential applications of imazethapyr was as high as 70% in water-seeded rice but no more than 54% in drill-seeded rice. Tillage, seeding method, and imazethapyr rate had no effect on days to 50% heading, seeds per panicle, seed weight per panicle, or percentage of seed harvest. However, a reduction of 27% in days to 50% heading, 80% in seeds per panicle, 84% in seed weight per panicle, and 100% in percentage seed harvest index occurred when imazethapyr was not applied because of weed interference. Culm number was reduced 28%, and culm weight 32% under reduced tillage compared with conventional tillage. With sequential applications of imazethapyr at 70 g/ha, rice yield was 63% greater when rice was water-seeded compared with drill-seeded. No differences in tillage systems for weed control, days to 50% heading, seed number, seed weight per panicle, percent seed, panicle height, lodging, or yield were observed. Results of these experiments demonstrate imazethapyr will effectively control weeds in both water- and drill-seeded rice and that reduced tillage can be used without negatively affecting rice production.