Field experiments were conducted in Citra, FL, and Tifton, GA, to evaluate simulated drift of dicamba and 2,4-D on cotton. Drift applications were made at the sixth leaf and first square growth stages using variable and constant carrier volumes and the same herbicide rate. Drift applied using variable carrier volumes were proportionally reduced with the herbicide rate while drift applied at constant carrier volumes were all made at 140 L ha-1, regardless of herbicide rate. At 21 DAT, dicamba applied at variable carrier volumes reduced cotton heights 8% [from nontreated check (NTC)] compared to no change in height with dicamba applied at constant carrier volumes. The same effect was seen with 2,4-D applied at first square where variable carrier volumes decreased cotton heights 18% (from NTC) compared to 2% at 140 L ha-1. Cotton yields were reduced to 70% of NTC when dicamba was applied at sixth leaf at variable carrier volumes compared to 87% at constant carrier volumes. The same response was seen with 2,4-D applied at sixth leaf where variable carrier volumes reduced cotton yields to 19% of NTC compared to 32% at constant carrier volumes. Cotton injury, height, boll production, and yield were all affected by drift carrier volume. When simulating herbicide drift in the future, it is critical to use variable carrier volumes for application as constant carrier volumes have shown to decrease the amount of plant injury observed.

A field study was conducted in 2014 and 2015 in Arkansas, Illinois, Indiana, Ohio, Tennessee, Wisconsin, and Missouri to determine the effects of tillage system and herbicide program on season-long emergence of Amaranthus species in glufosinate-resistant soybean. The tillage systems evaluated were deep tillage (fall moldboard plow followed by (fb) one pass with a field cultivator in the spring), conventional tillage (fall chisel plow fb one pass with a field cultivator in the spring), minimum tillage (one pass of a vertical tillage tool in the spring), and no-tillage (PRE application of paraquat). Each tillage system also received one of two herbicide programs; PRE application of flumioxazin (0.09 kg ai ha–1) fb a POST application of glufosinate (0.59 kg ai ha−1) plus S-metolachlor (1.39 kg ai ha–1), or POST-only applications of glufosinate (0.59 kg ha−1). The deep tillage system resulted in a 62, 67, and 73% reduction in Amaranthus emergence when compared to the conventional, minimum, and no-tillage systems, respectively. The residual herbicide program also resulted in an 87% reduction in Amaranthus species emergence compared to the POST-only program. The deep tillage system, combined with the residual program, resulted in a 97% reduction in Amaranthus species emergence when compared to the minimum tillage system combined with the POST-only program, which had the highest Amaranthus emergence. Soil cores taken prior to planting and herbicide application revealed that only 28% of the Amaranthus seed in the deep tillage system was placed within the top 5-cm of the soil profile compared to 79, 81, and 77% in the conventional, minimum, and no-tillage systems. Overall, the use of deep tillage with a residual herbicide program provided the greatest reduction in Amaranthus species emergence, thus providing a useful tool in managing herbicide-resistant Amaranthus species where appropriate.

The recent interest in cover crops as component of Midwest corn and soybean production systems has led to the need for additional research, including the effects of residual corn and soybean herbicide treatments on fall cover crop establishment. Field studies were conducted in 2013, 2014, and 2015 in Columbia, Missouri to investigate the effects of common residual herbicides applied in corn and soybean on establishment of winter wheat, tillage radish, cereal rye, crimson clover, winter oat, Austrian winter pea, Italian ryegrass, and hairy vetch. Cover crops were evaluated for stand and biomass reduction 28 d after emergence (DAE). Rainfall from herbicide application to cover crop seeding date was much greater in 2014 and 2015, which resulted in less carryover in these years compared to 2013. When averaged across all herbicides evaluated in these experiments, the general order of sensitivity of cover crops to herbicide carryover, from greatest to least was Austrian winter pea=crimson clover>oilseed radish>Italian ryegrass>hairy vetch>wheat >winter oat>cereal rye. Cereal rye had the fewest instances of biomass or stand reduction with only four out of the 27 herbicides adversely effecting establishment. Pyroxasulfone consistently reduced Italian ryegrass and winter oat biomass at least 67% in both the corn and soybean experiments. In the soybean experiment, imazethapyr- and fomesafen-containing products resulted in severe stand and biomass reduction in both years while flumetsulam-containing products resulted in the greatest carryover symptoms in the corn experiment. Results from these experiments suggest that several commonly used corn and soybean herbicides have the potential to hinder cover crop establishment, but the severity of damage will depend on weather, cover crop species, and the specific herbicide combination.

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─1 soybean yield. Averaged across treatments, the PRE fb POST program provided 82% common waterhemp control at soybean harvest, reduced density to 23 plants m─2 at 14 DALPOST, and caused 86% biomass reduction and 1,803 kg ha─1 soybean yield compared with 77% control, 99 plants m─2, 53% biomass reduction, and 1,190 kg ha─1 yield 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.

Two experiments were conducted in 2013 and 2014 in Florida to evaluate the effects of protoporphyrinogen oxidase (PPO)-inhibiting herbicides and single versus sequential applications on Palmer amaranth control and peanut injury. Protoporphyrinogen oxidase-inhibiting herbicides are among the last available herbicides for the POST control of acetolactate synthase (ALS)-resistant Palmer amaranth in peanut. Lactofen (219 g ai ha–1) applied 5 d after the initial application provided the highest level of Palmer amaranth control 7 and 14 d after initial application (DAIT). Delaying sequential applications of lactofen to 15 d resulted in the highest level of Palmer amaranth control 21 and 28 DAIT. Similar to Palmer amaranth control, foliar injury to peanut was often highest from lactofen applications, and by 28 DAIT lactofen treatments were the only treatments that caused foliar injury. Although no statistical difference was observed between yields of plots treated with acifluorfen (280 g ai ha–1), bentazon (560 g ai ha–1), 2,4-DB (280 g ae ha–1) alone or in combination with each other, plots treated with sequential applications of lactofen 5 or 15 DAIT produced the lowest yields. Sequential applications of lactofen applied 15 DAIT controlled Palmer amaranth more effectively than any other treatment but also caused the highest level of peanut injury. The use of sequential applications of lactofen was the most effective method for control of Palmer amaranth in this study, but did reduce peanut yield.

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–2 at 21 d after PRE (DAPRE). Post-only herbicide programs resulted in <70% control and densities of 107 to 215 plants m–2 at 14 d after early-POST (DAEPOST) treatment. PRE fb POST herbicide programs, including saflufenacil plus imazethapyr plus dimethenamid-P, sulfentrazone plus cloransulam, or S-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–2 with PRE fb POST herbicide programs compared with 42% control, and density of 101 plants m–2 with 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.

Field and greenhouse experiments were conducted in Nebraska to (1) confirm the 4-hydroxyphenylpyruvate dioxygenase (HPPD)-inhibiting resistant-waterhemp biotype (HPPD-RW) by quantifying the resistance levels in dose-response studies, and (2) to evaluate efficacy of PRE-only, POST-only, and PRE followed by POST herbicide programs for control of HPPD-RW in corn. Greenhouse dose-response studies confirmed that the suspected waterhemp biotype in Nebraska has evolved resistance to HPPD-inhibiting herbicides with a 2- to 18-fold resistance depending upon the type of HPPD-inhibiting herbicide being sprayed. Under field conditions, at 56 d after treatment, ≥90% control of the HPPD-RW was achieved with PRE-applied mesotrione/atrazine/S-metolachlor+acetochlor, pyroxasulfone (180 and 270 g ai ha−1), pyroxasulfone/fluthiacet-methyl/atrazine, and pyroxasulfone+saflufenacil+atrazine. Among POST-only herbicide programs, glyphosate, a premix of mesotrione/atrazine tank-mixed with diflufenzopyr/dicamba, or metribuzin, or glufosinate provided ≥92% HPPD-RW control. Herbicide combinations of different effective sites of action in mixtures provided ≥86% HPPD-RW control in PRE followed by POST herbicide programs. It is concluded that the suspected waterhemp biotype is resistant to HPPD-inhibiting herbicides and alternative herbicide programs are available for effective control in corn. The occurrence of HPPD-RW in Nebraska is significant because it limits the effectiveness of HPPD-inhibiting herbicides.

Palmer amaranth is the most problematic weed in agronomic crop production fields in the United States. A Palmer amaranth biotype was not controlled with sequential applications of glyphosate in glyphosate-resistant (GR) soybean production field in south-central Nebraska. The seeds of the putative GR Palmer amaranth biotype were collected in the fall of 2015. The objectives of this study were to (1) confirm GR Palmer amaranth and determine the level of resistance in a whole-plant dose-response bioassay, (2) determine the copy number of 5-enolpyruvylshikimate-3-phosphate (EPSPS) gene, the molecular target of glyphosate, and (3) evaluate the response of GR Palmer amaranth biotype to POST corn and soybean herbicides with different modes-of-action. Based on the effective dose required to control 90% of plants (ED90), the putative GR Palmer amaranth biotype was 37- to 40-fold resistant to glyphosate depending on the glyphosate-susceptible (GS) used as a baseline population. EPSPS gene amplification was present in the GR Palmer amaranth biotype with up to 32 to 105 EPSPS copies compared to the known GS biotypes. Response of GR Palmer amaranth to POST corn and soybean herbicides suggest reduced sensitivity to atrazine, hydroxyphenylpyruvate dioxygenase (HPPD)- (mesotrione, tembotrione, and topramezone), acetolactate synthase (ALS)- (halosulfuron-methyl), and protoporphyrinogen oxidase (PPO)- (carfentrazone and lactofen) inhibitors. GR Palmer amaranth was effectively controlled (>90%) with glufosinate applied at 593 g ai ha−1 with ≥95% reduction in biomass. More research is needed to determine whether this biotype exhibits multiple resistant to other group of herbicides and evaluate herbicide programs for effective management in corn and soybean.

Metribuzin will control many problematic weed species in winter wheat in the mid-Atlantic states, including herbicide-resistant biotypes, but it has not been recommended due to crop safety concerns. In a three-year trial, metribuzin was applied at 105 or 210 g ai ha−1 to wheat at the PRE, 2-leaf (Feekes stage 1 to 2), early spring (Feekes stage 3 to 4), and late spring (Feekes stage 4 to 6) growth stages using wheat cultivars sensitive to metribuzin. Early spring applications had the least amount of injury, and injury at this timing was transient and yield was not reduced. Yield loss was observed with the other application timings in at least one out of three years. Rainfall shortly after application appears to increase the risk of wheat injury.

Sheep and hair fescue are perennial, tuft forming grasses that spread by seed and form dense sods in wild blueberry fields. These sods compete with the crop for resources and hinder harvest. Field and greenhouse studies were conducted in 2015 to evaluate 1) the effect of sequential glufosinate and foramsulfuron applications on suppression of fescues in the greenhouse and field, and 2) efficacy of glufosinate and foramsulfuron on fescue seedlings when applied at 2, 4, 6, and 8 wk after seedling emergence in the greenhouse. Glufosinate applications at 750 and 1,005 g ai ha−1 followed by foramsulfuron application at 35 g ai ha−1 reduced fescue leaf number and biomass relative to foramsulfuron application alone in the greenhouse. In the field study, fescue flowering tuft density, tuft inflorescence height, seed production, and seed viability were reduced by foramsulfuron alone, but there was a trend towards lower seed production and tuft height when fescues were treated with glufosinate at 1,005 g ha−1 followed by foramsulfuron. Foramsulfuron caused low seedling mortality at all application timings evaluated, but glufosinate caused >90% mortality in seedlings when applied at 2, 4, 6, or 8 wk after seedling emergence. Our results suggest that sequential applications of these herbicides are less effective under field conditions relative to results obtained in the greenhouse, though burndown glufosinate applications may have a role in reducing fescue seedling recruitment. Additional research should be conducted to determine the effect of early spring and autumn glufosinate applications on fescue seedling recruitment and suppression of established fescue tufts with subsequent foramsulfuron applications.

Poor annual bluegrass control was reported in golf course roughs following treatment with prodiamine (1120 gaiha−1) and glyphosate (840 gaeha−1) during hybrid bermudagrass dormancy. Research was conducted to determine if this annual bluegrass phenotype was resistant to both prodiamine and glyphosate and to determine the efficacy of herbicide mixtures for controlling this phenotype in the field. In PRE or POST dose-response experiments, 9 to 31 times more prodiamine or glyphosate was needed to control (or reduce dry biomass of) this resistant phenotype by 50% compared to an herbicide-susceptible phenotype. Moreover, glyphosate-susceptible plants accumulated 50% more shikimic acid (898 mgkg−1) 6 d after treatment than those resistant to glyphosate (394 mgkg−1). October (fall) applications of herbicide mixtures containing trifloxysulfuron, simazine, S-metolachlor, or mesotrione controlled this resistant annual bluegrass phenotype 84 to 98% in April (spring), with no differences detected among treatments. Our findings document the second instance of annual bluegrass evolving multiple resistance in a managed turfgrass system. However, several herbicide mixtures can be used to effectively manage this resistant phenotype.

Vaseygrass is an invasive, perennial C4-grass commonly found on roadsides in areas with poorly drained soils. Due to its upright growth habit and seedhead production, vaseygrass can impair motorist sightlines and subsequently, require increased management inputs to maintain vegetation at an acceptable height. Two field experiments were conducted from 2012 to 2015 on North Carolina roadsides to evaluate the effect of mowing and mowing timing with respect to applications of various herbicides on vaseygrass control. Both experiments evaluated clethodim (280 g ai ha–1), foramsulfuron+halosulfuron+thiencarbazone-methyl (44+69+22 g ai ha−1), imazapic (140 g ai ha−1), metsulfuron+nicosulfuron (16+59 g ai ha−1), and sulfosulfuron (105 g ai ha−1) with a nonionic surfactant at 0.25% v/v. Experiment one focused on the effect of mowing (routinely mowed or nonmowed) and herbicide application timing (fall-only, fall-plus-spring, or spring-only), while experiment two focused on pre-herbicide application mowing intervals (6, 4, 3, 2, 1, or 0 wk before treatment [WBT]). From experiment one, routine mowing reduced vaseygrass cover in nontreated plots 55% at 52 wk after fall treatment (WAFT), suggesting this cultural practice should be employed where possible. Additionally, routine mowing and herbicide application season affected herbicide efficacy. Treatments providing >70% vaseygrass cover reduction at 52 WAFT included routinely mowed fall-only clethodim and fall-plus-spring imazapic, and fall-plus-spring metsulfuron+nicosulfuron across mowing regimens. Within clethodim, mowing vaseygrass 2 or 1 WBT resulted in the lowest cover at 40 (1 to 2%) and 52 (4 to 6%) wk after treatment (WAT) compared to other intervals, which aligns with current label vegetation height at treatment recommendation. Vaseygrass persisted across all treatments evaluated through 52 WAT, suggesting eradication of this species will require inputs over multiple growing seasons.

Field studies were conducted in 2009 at Clinton, NC and 2014 at Pontotoc, MS to determine the influence of simulated glyphosate drip on sweetpotato yield and quality. Treatments consisted of three glyphosate solution (140 g ae L–1) drip volumes (0.16, 0.32 and 0.48 ml) by four application timings [(4 wk after transplanting (WAP); 6 WAP; 8 WAP; and 4 WAP followed by (fb) 6 WAP fb 8 WAP]. A non-treated check was included for comparison. Visual sweetpotato injury consisted of chlorosis at the shoot tips approximately 1 wk after treatment fb necrosis and stunting. At 6 WAP and 8 WAP, sweetpotato injury following glyphosate applied 4 WAP was 71 and 65%, respectively. Injury from glyphosate applied 4 WAP fb 6 WAP was 78%. Injury from glyphosate applied 6 WAP was 26% at 8 WAP. In 2009, jumbo, no. 1, canner, and marketable yield of the non-treated check were two to three times greater than glyphosate treatments (0.16, 0.32, 0.48 ml). Likewise, yield of the non-treated check was substantially greater than those treated with 0.16 to 0.48 ml glyphosate solution in 2014. In 2009 and 2014, sweetpotato yield of all grades increased as glyphosate application timing was delayed. In 2009, no. 1 yield from glyphosate 8 WAP (8,210 kg ha–1) was similar to the non-treated check. In 2009, there were no cracked storage roots in the non-treated check. However, sweetpotatoes receiving 0.16 to 0.48 ml glyphosate solution displayed 8 to 17%, 11 to 18%, 5 to 13%, and 11 to 16% cracking (by weight) in jumbo, no. 1, canner, and marketable storage roots, respectively. Compared to the non-treated check, glyphosate applied 4 WAP, 6 WAP, or 4 WAP fb 6 WAP fb 8 WAP had a greater percentage of cracked marketable sweetpotato storage roots.

In a repeated multi-year study, mother potato plants were exposed to herbicides at rates that simulated off-target application, such as through tank contamination. Following exposure of mother plants to herbicides, seed from mother plants was planted in the following growing season and crop growth, yield and tuber quality were quantified. Visual injury from herbicides was observed both in the mother plant and daughter tuber growing seasons and occasional impacts on tuber yield were noted. However, an inconsistent relationship was observed for herbicide related injury and tuber yield reductions of mother potato plants with daughter tuber growth and yield. The lack of consistency in the relationship between visual potato injury in the mother plant production and adverse daughter tuber growth and yield in the following year challenges traditional crop scouting as a tool to predict off-target herbicide risk near seed potato production.

Weeds are one of the most significant, and controllable, threats to crop production in North America. Monetary losses because of reduced soybean yield and decreased quality because of weed interference, as well as costs of controlling weeds, have a significant economic impact on net returns to producers. Previous Weed Science Society of America (WSSA) Weed Loss Committee reports, as chaired by Chandler (1984) and Bridges (1992), provided snapshots of the comparative crop yield losses because of weeds across geographic regions and crops within these regions after the implementation of weed control tactics. This manuscript is a second report from the current WSSA Weed Loss Committee on crop yield losses because of weeds, specifically in soybean. Yield loss estimates were determined from comparative observations of soybean yields between the weedy control and plots with greater than 95% weed control in studies conducted from 2007 to 2013. Researchers from each US state and Canadian province provided at least three and up to ten individual comparisons for each year, which were then averaged within a year, and then averaged over the seven years. These percent yield loss values were used to determine total soybean yield loss in t ha−1 and bu acre−1 based on average soybean yields for each state or province as well as current commodity prices for a given year as summarized by USDA-NASS (2014) and Statistics Canada (2015). Averaged across 2007 to 2013, weed interference in soybean caused a 52.1% yield loss. Based on 2012 census data in the US and Canada soybean was grown on 30,798,512 and 1,679,203 hectares with production of 80 million and 5 million tonnes, respectively. Using an average soybean price across 2007 to 2013 of US $389.81 t−1 ($10.61 bu−1), farm gate value would be reduced by US $16.2 billion in the US and $1.0 billion in Canada annually if no weed management tactics were employed.

Tactics that target seedbanks are important components of weed management systems; however, such tactics can be difficult to adopt because consequences of seedbank reduction are often unclear. This study developed model-based software to provide insights on the economic outcomes, in the context of chile pepper production, of additions to tall morningglory seedbanks. Data for the model were derived from this and previous studies. In this study, field experiments were conducted to determine chile pepper yield and harvest efficiency responses to mid-season tall morningglory infestations. The experimental treatments were factorial combinations of herbicide (pendimethalin-treated, nontreated) and tall morningglory density (0, 4, 8, 12, 16, 20 plants 10-m row–1). Treatments were installed 9.5 weeks after crop seeding. Data collected included fresh weight of marketable chile peppers and time required for one individual to harvest 10-m of crop row, which was used to calculate the amount of chile pepper harvested in 1 min (harvest efficiency). Results indicated that crop yield was not influenced by tall morningglory density, pendimethalin treatment and interactions between tall morningglory density and pendimethalin. Harvest efficiency was influenced by tall morningglory density but was not influenced by herbicide treatment or interactions between herbicide treatment and tall morningglory density. Each additional tall morningglory plant decreased the amount of chile pepper harvested in 1 min by 9.7 g. The results of this and previous studies were used to develop model-based software that presents tall morningglory seedbank density effects on: (1) tall morningglory seedling densities after pendimethalin, (2) time requirements for hand-hoeing after pendimethalin, and (3) time requirements for hand-harvesting to acquire yield goals. The model-based software is intended to be used in the instruction of weed seedbank management strategies. By presenting seedbank density effects on weed control outcomes and crop production expenses, the model-based software might promote adoption of seedbank reduction strategies.