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).

Field and greenhouse studies examined glyphosate and several ACCase-inhibitor herbicides for control of wirestem muhly. A field study evaluated application rate and timing for control in glyphosate-resistant soybean. Herbicides were applied at two rates, 4 and 6 wk after planting (WAP) of soybean when wirestem muhly was about 30 and 50 cm tall, respectively. A complimentary greenhouse experiment with the same herbicides examined wirestem muhly rhizome viability 30 days after herbicide application. A second field study examined optimum glyphosate application timing for control of wirestem muhly during a fallow rotation period. Glyphosate was applied at 14-day intervals from mid-May through early October. In the first field study, glyphosate, fluazifop, and clethodim provided better control of wirestem muhly than sethoxydim and quizalofop. Clethodim, fluazifop, and glyphosate provided 74 to 92% control over both rates and application timings, whereas quizalofop and sethoxydim were the least effective, providing only 52 to 64% control regardless of rate or timing. In the greenhouse, glyphosate was the most effective herbicide, providing at least 97% control and an 87% reduction in shoot biomass; sethoxydim and fluazifop were the least effective herbicides. All of the herbicides reduced rhizome viability compared to nontreated plants up to 71%, but no difference between herbicides was observed. In the second field study, end-of-season ratings revealed that glyphosate provided 95 to 100% control of wirestem muhly from applications made between late June and early September. The year following application, glyphosate provided 91 to 99% control of wirestem muhly from applications made between mid-June and early September. Applications earlier than mid-June or after early September resulted in less than 90% control. Overall, results show that (1) glyphosate was the most effective herbicide; (2) the ACCase-inhibitor herbicides differ in their effectiveness; and (3) rate and application timing can impact control.

Field-studies were conducted over two years to determine the area of influence and duration of interference of wild poinsettia in soybean. Soybean canopy width averaged across years was reduced approximately 10% beginning at 6 wk of interference for both the 0- to 10-and 10-to 20-cm distances from the weed. Soybean dry weights decreased from 14 to 38% within 20 cm of the weed for 12 through 18 wk of interference. In 1990, soybean yield within 10 cm of wild poinsettia was similar to distances of 10 to 20 and 20 to 40 cm, but was less than that for distances greater than 40 cm from the weed. Weed interference resulted in a 9.5% yield reduction in the 0- to 10-cm distance when compared with the 80-to 100-cm distance. In contrast, yield of soybean in 1991 growing within 10 cm of the weed was less than at greater distances, corresponding to an 18% yield reduction when compared with 80 to 100 cm. Differences in wild poinsettia dry weights when growing alone and when growing within the soybean row occurred after 6 and 8 wk of interference in 1990 and 1991, respectively. In 1991 when rainfall during the growing season was twice that of the previous year, canopy width and dry weight of wild poinsettia growing within the soybean row were reduced an average of 57 and 82%, respectively, compared with weeds growing alone.

Diflufenzopyr is an auxin-transport inhibitor that can increase the phytotoxicity of certain auxin-mimicking herbicides such as dicamba on broadleaf species. Dicamba is commonly used alone and in combination with other auxin herbicides for broadleaf weed control in various species of turfgrass. Dicamba efficacy applied over a series of rates either alone or as an admixture with either 20 or 40% by weight of diflufenzopyr relative to the weight of dicamba was evaluated on purple cudweed and common lespedeza. The 20% admixture reduced the LD50 of dicamba on purple cudweed from 23 to 20 g/ha. Similarly, LD50 on common lespedeza was reduced from 36 and 27 g/ha. The 20% admixture was 13 and 25% more active than dicamba alone for these two weed species, respectively. However, the synergistic benefit was limited to a relatively narrow range of rates that are below the minimal registered rate of dicamba. Turfgrass injury, as expressed by the suppression of foliage growth, was similar whether dicamba was applied alone or with diflufenzopyr for all species evaluated except St. Augustinegrass. The admixture was less injurious than dicamba alone in St. Augustinegrass. The synergistic benefit with respect to weed control was obtained without a corresponding increase in injury on the turfgrasses.

Adoption of conservation tillage practices has been slow in organic vegetable production, partially due to producers’ concerns regarding weed management. Integrating cover crops into a conservation tillage program may provide organic producers a viable weed management option enabling growers to practice conservation tillage. A four-year study was conducted to evaluate the influence of different tillage methods (two conventional and two conservation practices) jointly with a mixed winter cover crop for weed suppression, time required for hand weeding, and crop yield in organically managed eggplant (2012 and 2014) and sweet corn (2013 and 2015) production systems. Tillage treatments were conventional tillage without surface mulch (CT-BG) and with black polyethylene (plastic) mulch (CT-BP), strip-tillage (ST), and no-tillage (NT) with cover crop residue. At 2 and 7 WAT/P (weeks after transplanting/planting), intra-row weed density was higher in CT-BG and ST, and inter-row weed density was higher in CT-BG and CT-BP treatments. Time required for hand-weeding was greatest in CT-BG and least in CT-BP and NT treatments. Eggplant yield was lowest in NT treatment in 2012 but similar among treatments in 2014. Sweet corn yield was similar among treatments in 2013 but highest in ST in 2015. Though both CT-BP and NT treatments showed greater potential for weed suppression, production input was highest in CT-BP but least in NT. Implications of these findings suggest that there is a potential to use strip tillage integrating with stale seedbed tactic for weed management in organic vegetables, which reduces herbicide use, hand-labor, and overall weed management cost while maintaining high yield potential.

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.

Field experiments were established near Casselton and Fargo, ND, to evaluate the effect of aminopyralid soil residue on alfalfa, corn, soybean, and sunflower planted one or two growing seasons after treatment. At Fargo, ND, aminopyralid caused no injury or yield reduction to alfalfa, corn, and sunflower seeded 20 or 23 mo after treatment (MAT) in a silty clay soil. However, soybean yield was reduced when aminopyralid at 120 or 240 g ae ha−1 was fall- or spring-applied 20 or 23 mo before seeding. At Casselton, ND, aminopyralid injured alfalfa, soybean, and sunflower planted 8 and 11 MAT. Injury and yield reduction were less from treatments spring-applied than from those that were fall-applied. For example, aminopyralid at 120 g ha−1 applied in September caused 95, 94, and 100% injury to alfalfa, sunflower, and soybean, respectively, 8 MAT, whereas the same treatment applied in June caused 10, 8, and 44% injury 11 MAT. Aminopyralid at 120 g ha−1 continued to reduce soybean yield by an average of 45% at 20 MAT (fall-applied), but yield was similar to the control when aminopyralid was applied 23 mo before seeding (spring-applied). Warm soil with moderate moisture during the summer months appeared to be very important for degradation of aminopyralid. Corn was not affected by aminopyralid when seeded 8 or 11 MAT and appeared to be the best cropping option for land recently treated with aminopyralid. Aminopyralid applied at spot-treatment rates of 240 g ha−1 had long-term soil activity similar to picloram at 560 g ha−1.

Studies were conducted under greenhouse conditions at Michigan State University and Texas Tech University to investigate the tolerance of Miscanthus × giganteus and Miscanthus sinensis to POST herbicides. Miscanthus sinensis and M. × giganteus were treated with 10 and 18 POST herbicide treatments, respectively. Plants were evaluated for injury as well as dry aboveground and belowground biomass production 28 days after treatment. Imazethapyr at 0.069 kg ai ha−1 caused 5% injury to M. sinensis, which was greater than the nontreated check. Imazethapyr, imazamox at 0.044 kg ai ha−1, and rimsulfuron at 0.017 kg ai ha−1 reduced aboveground biomass of M. sinensis compared with the nontreated check. Dicamba at 0.56 kg ai ha−1 and halosulfuron at 0.035 kg ai ha−1 resulted in M. sinensis aboveground biomass similar to the nontreated check. Injury exhibited by M. × giganteus was greater than the nontreated check with glyphosate at 0.84 kg ae ha−1 (54%), foramsulfuron at 0.037 kg ai ha−1 (32%), nicosulfuron at 0.035 kg ai ha−1 (28%), and imazamox at 0.044 kg ai ha−1 (10%). These treatments also yielded the lowest aboveground biomass values. The results of this study demonstrate that M. sinensis is more tolerant of the POST herbicides tested here than M.×x. giganteus. Several herbicide options may be available for weed management in M. sinensis and M. × giganteus stands following additional field trials to validate initial findings.

This study characterized seedling emergence of downy brome from August to early December over a 6-yr period. Seedlings were counted weekly in quadrats established in winter wheat stubble at Akron, CO. Seedling emergence varied among years, which was caused by erratic seasonal precipitation. Producers delay planting of winter wheat to reduce downy brome density in the crop, but in only 1 yr out of 6 would producers have benefited from this control strategy. Furthermore, delayed planting has negative crop consequences: less grain yield and more susceptibility to plant diseases and wind erosion because of less fall plant growth. Because fall precipitation is erratic in the semiarid Great Plains, other control strategies, such as nitrogen placement and increased seeding rates of winter wheat, would be more effective for downy brome management, yet not detrimental to winter wheat production.

Wide-spread planting of glyphosate-resistant (GR) crops in the United States and glyphosate patent expiration has led to a proliferation of glyphosate products. Growers have questioned their advisors on efficacy and crop tolerance of the many products available. Field experiments were conducted to evaluate eight glyphosate products in 2002 and ten in 2003, representing isopropylamine, diammonium, and potassium salts, applied POST and postemergence-directed (PDIR) at 630 and 1,680 g ae/ha for GR corn and GR cotton tolerance and weed control. There were no differences among products for control of six annual grass and 10 annual broadleaf weed species. No injury to corn from any glyphosate product at 630 or 1,680 g/ha or to cotton from 630 g/ha was noted at any of seven locations. ClearOut 41 Plus™, an isopropylamine salt of glyphosate, and Roundup WEATHERMAX™, a potassium salt of glyphosate, applied POST at 1,680 g/ha injured cotton 27 to 30% and 10 to 17%, respectively, at 3 of 7 locations. No cotton injury was noted with Glyfos®, Glyfos® X-TRA, Glyphomax™, Gly Star™ Original, Roundup Original™, Roundup UltraMAX®, Touchdown®, or Touchdown Total™. No differences were noted among glyphosate products or between rates for corn or cotton yield or cotton fiber properties.

Herbicide treatments (4:1 ratio of 2,4-D amine:picloram) at 0.7 and 1.4 kg ae/ha at early postemergence (10- to 15-cm horsenettle height), midpostemergence (early flower), and late postemergence (fruit initiation) applied both early and late in the growing season provided >80% horsenettle control. Horsenettle density at season's end in all treated plots was less than 0.25 stems/m2, whereas untreated plots contained about 5 stems/m2. Horsenettle control the next spring was between 47 and 66% for all rates and application timings, and horsenettle density in treated plots was less than 3 stems/m2 as opposed to about 6 stems/m2 in the untreated plots. Clover drilled into the treated area the year after herbicide application was injured, indicating clover establishment the season after application of this package mixture would be difficult.

Efficacy of KIH-2023 applied POST at rates of 15 to 40 g ai/ha was evaluated in dry-seeded and water-seeded rice production in 1994 and 1995. Purple ammannia, ducksalad, and Indian jointvetch control with KIH-2023 was similar, and alligatorweed control was greater with KIH-2023 compared with propanil or tank mixtures of propanil plus thiobencarb in water-seeded rice. Barnyardgrass was controlled similarly with KIH-2023 and sequential applications of propanil in dry-seeded rice. Rice yields generally reflected differences in weed control, with yields of rice treated with KIH-2023 similar to sequential propanil applications or propanil plus thiobencarb in dry-, and water-seeded rice, respectively. Minor rice foliar and root injury were noted with KIH-2023.

Experimental controlled release starch granules (CRSG) containing 5.3% a.i. (w/w) norflurazon or 6% a.i. (w/w) simazine retarded the leaching of both herbicides in loamy sand soil columns when compared to commercial formulations of norflurazon [80% (w/w) dry flowable] or simazine [90% (w/w) water dispersible granule]. Barley bioassays indicated norflurazon and simazine remained in the surface 0 to 2.5 cm of soil when applied as CRSG formulations and moved to a depth of 15 cm when applied as commercial dry formulations and leached with 6 cm of water. CRSG placed on pre-wetted soil columns began to release norflurazon by 7 d at 25 C or 14 d at 15 C, and subsequent leaching moved norflurazon beyond the top 2.5 cm of the soil column.

Synthetic auxin herbicides are widely used because of their effective control of broadleaf weeds and safety in many turfgrass species. However, two synthetic auxin herbicides, triclopyr and aminocyclopyrachlor (AMCP; DPX-KJM44), are known to injure warm-season turfgrasses. Our objective was to quantify this injury through evaluations of turfgrass quality and turfgrass green cover in response to herbicide treatment. The results of this study indicate that relative to the labeled use rates of triclopyr (0.56 to 1.12 kg ae ha−1) and AMCP (0.053 kg ai ha−1), zoysiagrass is the only turfgrass tested with sufficient tolerance to the respective compounds for their use as weed-control agents. Bermudagrass and centipedegrass may be injured by triclopyr and AMCP at labeled rates, characterized by a reduction in turfgrass quality and green cover. St. Augustinegrass is not tolerant of either triclopyr or AMCP at labeled rates.

Surface and subsurface soils are complex biological, chemical, and physical environments and to understand the fate of pesticides in the soil environment is a formidable task. To determine the environmental fate of pesticides requires a diverse array of techniques and procedures. Microbiological approaches range from applied to basic, laboratory to field, qualitative to quantitative, and from low to high technology. In the arena of biodegradation, teams of scientists are needed to develop predictive models for the behavior of pesticides in the soil environment. From our perspectives, we have documented the existing status of the microbiology of environmental fate studies with pesticides. Verification of data from laboratory studies to the field environment is needed. On the other hand, efforts to design better field studies to assess microbial processes are essential to advance our understanding of environmental fate studies with pesticides.

Genetically modified, herbicide-resistant crop (HRC) cultivars, which allow for simplified weed control decisions compared with conventional cultivars, have considerable potential in Latin America. The number of herbicide applications can be reduced in HRCs, and otherwise difficult-to-control species, including red rice in rice or herbicide-resistant weeds, can potentially be managed. The American tropics include the centers of origin of several crops, such as corn and potato, and natural or agrestal floras containing wild near relatives of introduced crops, including rice and cotton, for which HRCs could be used. Potential direct impacts of HRC adoption on biodiversity in Latin America include changes in the genetic diversity of crops, increased volunteer crop problems, and invasion by resistant cultivars of natural areas beyond the farm boundary. Additionally, there is a risk associated with the escape of transgenes from HRCs, involving introgression to weedy relatives on a field scale leading to amplification effects of existing weeds or modification of gene pools of crop progenitors in the centers of origin or diversity. Possible indirect effects of HRCs include the potential expansion of agriculture into uncleared wild areas made economically attractive by the more efficient cropping system, and adverse effects on nontarget organisms and ecosystem processes. The occurrence of spontaneous hybrids of crop–weed or near-relative complexes is known in the United States for rice and sunflower, and the development of HRC–weed hybrids would provide a particularly difficult set of weed management problems. For crops in which HRCs are currently in use or under development, the greatest risks in Latin America appear to be with corn, cotton, and potato. However, some of the genetic and geographical barriers reduce the risk of hybridization between these crops and their wild relatives. Furthermore, unlike the case of feral rapeseed on road verges in Europe, there is no known example of a conventionally bred crop or interfertile hybrid with a near relative becoming established outside of cultivation in Latin America. Soybean, the most widely adopted HRC in Latin America to date, is an exotic beyond the range of its wild relatives. With the exception of O. glumaepatula, there appears to be little threat to endemic wild species from HRC rice as strong infertility barriers should prevent transgene flow, but possible transgene movement to the conspecific red rice may become an issue. Beyond the field boundary, a key question concerns the possible persistence of HRC x wild relative hybrids and the manner in which resistance traits affect their overall fitness. Designing practical risk assessment protocols on which to base HRC release approval is a considerable challenge, and it is vital that Latin American states continue to build and maintain functional biosafety regulatory structures.

Field experiments were conducted at five sites in Nebraska in 2000 and 2001 to determine the effect of planting depth and isoxaflutole rate on the response of an isoxaflutole-sensitive corn hybrid, ‘Pioneer 33-G’ across variable environments. Corn was planted at depths of 2.5 and 5.0 cm, and isoxaflutole was applied PRE at the recommended (1×) and twice the recommended (2×) rate. The effects of planting depth and herbicide rate on injury varied considerably across site–years. When injury was evident, it was generally greater at the high rate of isoxaflutole (2×) and at the shallow planting depth (2.5 cm). In most site–years, corn recovered from early season injury, and yields were not reduced, except at Scottsbluff, NE, and North Platte, NE, where soils were lower in organic matter and higher in pH. Isoxaflutole rates should be carefully selected for soils with low organic matter and high pH.

Experiments were conducted in 2004 and 2005 in Virginia to evaluate KIH-485 for shattercane control. KIH-485 was applied to glyphosate-resistant corn at three rates (166, 209, and 250 g ai/ha) and two timings (preemergence [PRE] and postemergence [POST]) and compared to standard rates of S-metolachlor, acetochlor, and pendimethalin. PRE treatments were applied alone, while POST treatments were combined with 1,121 g ai/ha of glyphosate. A single POST application of glyphosate, a nontreated weedy control (WC), and a weed-free control (WFC) were also evaluated. In 2004, at 1 mo after planting (MAP), shattercane was controlled 73, 86, and 93% with the three PRE treatments of KIH-485 compared to only 50, 49, and 48% control with S-metolachlor, acetochlor, and pendimethalin, respectively. Similar levels of shattercane control were also observed with these treatments at 1 MAP in 2005. In both years at 4 MAP, KIH-485 combined with glyphosate applied POST controlled shattercane 96% or greater. However, when glyphosate was combined with S-metolachlor, acetochlor, or pendimethalin, shattercane control was less than 88% in either year. Corn yields with POST applications of KIH-485 plus glyphosate were equivalent to yield from the WFC and greater than yield from any PRE treatment. Early-season shattercane control was promising with KIH-485; however, season-long control only occurred when KIH-485 was applied POST in combination with glyphosate.

Uncontrolled kochia plants that regrow after small-grain harvest can produce substantial numbers of seeds. An average of 4,100 seeds per plant were produced between harvest (late July to mid August) and the first killing frost (late September) at three locations in Montana. Field experiments were conducted to determine the optimal timing of postharvest herbicide applications to prevent kochia from producing viable seeds. Herbicide treatments were applied at three timings from late August to mid September. The most effective treatments were glyphosate (631 g/ha) and paraquat (701 g/ha) applied at the second application timing (late August to early September). These treatments reduced kochia seed production by 92% or greater at each site. Kochia regrowth by this time had sufficient leaf area for herbicide absorption, but few viable seed had been produced. Herbicide treatments at the first and third application timings were generally less effective and more variable in reducing kochia seed production. Sulfentrazone (157 g/ha) and 2,4-D (561 g/ha) were not as effective at reducing seed production as other herbicide treatments.

Two experiments were conducted over a two-year period for large crabgrass control in centipedegrass. The experiments were: (a) sequential applications of PRE and POST herbicides, and (b) POST herbicides applied as sequential or tank-mixed applications. Large crabgrass control was greater than 80% throughout the spring and summer when dithiopyr was applied PRE at 0.14 kg/ha late in February or when tank mixed at the same rate with sethoxydim at 0.11 kg/ha and applied POST in late May. Control was similar from a sequential program of dithiopyr applied PRE at 0.14 kg/ha in February and followed either by dithiopyr at 0.14 kg/ha or sethoxydim at 0.11 kg/ha as POST in early to mid-summer. Dithiopyr at 0.28 kg/ha as a POST application in late May did not control large crabgrass consistently. Control was > 80% for 6 to 8 wk when the POST dithiopyr treatment was repeated at the same rate 1 mo later. Sethoxydim applied POST controlled large crabgrass greater than 80% throughout the spring and summer when applied once at 0.22 kg/ha or twice at 0.11 kg/ha in each of two applications. Dithiopyr applied alone or with sethoxydim as sequential or tank-mixed treatments caused slight to moderate centipedegrass injury but was commercially acceptable (< 30%). However, injury from tank-mix herbicides was similar to either applied alone. Quinclorac severely injured (> 30%) centipedegrass, and 8 to 10 wk were required for recovery.