A survey of Florida highway rights-of-way was conducted during 1984–85 to determine the occurrence and severity of cogongrass infestation. Florida Department of Transportation district maintenance engineers surveyed 8,200 km of limited-access and other major highways (22% of highway system). Cogongrass was distributed widely from the north central region southward through the central Florida ridge north of Lake Okeechobee. Highest frequencies were in counties where cogongrass was used for forage and soil stabilization during the 1950s. The large, widely scattered cogongrass infestations probably were established during extensive roadway construction and routine maintenance which used rhizome-contaminated fill soil.

Field research was conducted for 2 yr to evaluate response of corn and rice to simulated drift rates of a commercial premix of imazethapyr plus imazapyr [3:1 (w/w)]. Drift rates of the imazethapyr plus imazapyr premix represented 0.8, 1.6, 3.2, 6.3, and 12.5% of the usage rate of 63 g ai/ha (0.5, 1, 2, 4, and 7.9 g/ha, respectively). The imazethapyr plus imazapyr premix applied to six-leaf corn at 7.9 g/ha reduced height 11% compared with the nontreated control 7 days after treatment (DAT) but did not affect corn height 14 and 28 DAT. Corn yield was equivalent regardless of imazethapyr plus imazapyr rate and ranged from 10,200 to 11,500 kg/ha. At 28 DAT, rice height was reduced 12% when 7.9 g/ha of the imazethapyr plus imazapyr premix was applied early postemergence (EPOST) at two- to three-leaf and 14 and 5% when the imazethapyr plus imazapyr premix at 7.9 and 4 g/ha, respectively, was applied late postemergence (LPOST) at panicle differentiation. Reductions in mature rice height of 11 and 6% were observed when the imazethapyr plus imazapyr premix was applied LPOST at 7.9 and 4 g/ha, respectively, and a 5% reduction was observed for 7.9 g/ha of the imazethapyr plus imazapyr premix applied EPOST. Application of the imazethapyr plus imazapyr premix EPOST at 7.9 g/ha delayed heading in only 1 yr, but heading was delayed both years when applied LPOST. Rice yield was reduced 39 and 16% when the imazethapyr plus imazapyr premix was applied LPOST at 7.9 and 4 g/ha, respectively, compared with a 9% yield reduction for 7.9 g/ha applied EPOST.

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.

Herbicide-resistant Echinochloa spp. pose a significant threat to U.S. rice production. Two surveys were conducted to characterize Echinochloa resistance to common rice herbicides and provide important demographic information on the populations in Arkansas: one was the Echinochloa Herbicide Resistance Confirmation Survey conducted annually since 2006; the other was the Echinochloa Herbicide Resistance Demographics Survey conducted since 2010. The Resistance Confirmation Survey showed that resistance to propanil (50%) was most prevalent, followed by quinclorac (23%), imazethapyr (13%), and cyhalofop (3%). Multiple resistance increased with time, with 27% of accessions being multiple-resistant, mostly to propanil+quinclorac (12%). The parallel Resistance Demographics Survey tested resistance by species. Of the 264 accessions collected, 73% were junglerice, 14% were rough barnyardgrass, and 11% were barnyardgrass. Overall, this survey also showed resistance to propanil (53%) and quinclorac (28%) being most prevalent, with low frequencies of resistance to cyhalofop (12%) and imazethapyr (6%). Resistance to herbicides was less frequent with barnyardgrass (54%) and rough barnyardgrass (28%) than with junglerice (73%). Multiple resistance was most frequent with junglerice (33%) and least frequent with rough barnyardgrass (8%). Across both surveys, the resistance cases were clustered in the northeast and Grand Prairie regions of the state. Herbicide resistance among Echinochloa populations in rice fields is continuing to increase in frequency and complexity. This is a consequence of sequential selection with different major herbicide sites of action, starting with propanil followed by quinclorac and others.

Sethoxydim, tralkoxydim, imazethapyr, quinclorac, propanil, glyphosate, and glufosinate were tested at rates below those recommended by the manufacturers with Pyricularia setariae Niskada under greenhouse conditions for control of green foxtail. At one-tenth of the recommended rate in a 100 L/ha carrier volume, only the sethoxydim–P. setariae combination achieved more effective green foxtail control when compared with the herbicide or pathogen alone. Selected herbicides at one-tenth, one-fourth, and one-half of the recommended rates showed variable interactions with the pathogen on plants with three and five leaves. Propanil (recommended rate 0.99 kg ai/ha) was more synergistic at higher rates, especially on larger plants, for which the combined treatment increased green foxtail mortality from zero in the herbicide alone to 100%. Quinclorac (recommended rate 0.10 kg ai/ha) acted similarly to propanil with slightly lower synergy effects. Sethoxydim (recommended rate 0.15 kg ai/ha) at one-tenth or one-quarter of the rate plus P. setariae often enhanced green foxtail control on larger plants. On smaller plants, the herbicide and pathogen alone were highly efficacious. Compared with tank mixes with P. setariae, propanil, quinclorac, or sethoxydim applied 6 h before the pathogen or earlier generally showed greater efficacy. Delaying a tank mix application for up to 2 h had little negative effect, but longer than 4 h often reduced efficacy. When combining the pathogen at different doses with propanil, quinclorac, or sethoxydim at one-tenth, one-quarter, and one-half of the rate, both fungal dose and herbicide rate affected the efficacy. Coapplying any of the herbicides at the one-quarter rate with the pathogen at the sublethal dose of 2 × 107 spores/ml achieved complete control of green foxtail.

Cotton gins in Arkansas, western Tennessee, and western Mississippi were sampled for weed seed contamination of gin trash in fall 2007. A total of 473 samples were collected, with 453 samples from Arkansas. The objectives of this research were to determine the weed species most frequently found in gin trash and determine what means gin operators are using to dispose of gin trash. There were 25 weed species found in the gin trash samples—11 grass and 14 broadleaf weeds. Grass and broadleaf weeds were present in 41.4 and 8.5% of the samples, respectively. The most frequently found species were large crabgrass (19.0%), barnyardgrass (14.0%), goosegrass (12.9%), red sprangletop (8.2%) and Palmer amaranth (4.2%). Viable seeds of barnyardgrass, large crabgrass, Palmer amaranth, and prickly sida were present in the surface layer (0- to 25-cm depth) of gin trash piles after 1 yr of composting. Viable Palmer amaranth seeds were present in the surface layer of gin trash piles after 2 yr of composting, but no germinable seeds were found deeper than 25 cm following 1 yr of composting. Gin trash disposal involved application of the material to crop fields during the fall or winter months; composting followed by application of the compost as mulch or a soil amendment to gardens, flower beds, or crop fields; use as cattle feed; and coverage for landfills to reduce erosion and encourage growth of vegetation. Because of the demonstrated potential for weed seed dispersal via gin trash, including composted material, development of technologies to ensure disposal of a gin-trash product free of viable weed seed are justified.

Rhizome johnsongrass control with sethoxydim and clethodim was evaluated with the addition of three adjuvants: Agrioil3, BAS8153, and Li7003. Johnsongrass control was similar with the three adjuvants when either sethoxydim or clethodim was applied alone, but when bentazon was added, johnsongrass control declined with both sethoxydim and clethodim. Control generally declined less with BAS815 than with Agrioil or Li700, but the addition of BAS815 did not completely reverse the effects of adding bentazon to sethoxydim or clethodim. Johnsongrass control was not affected by adjusting pH when sethoxydim or clethodim was applied with bentazon and Agrioil. Johnsongrass control over a pH range of 3.5 to 6.5 via the addition of glacial acetic acid and calcium hydroxide or ammonium hydroxide indicated a lack of pH dependence for these herbicides. The addition of ammonium hydroxide resulted in slight, but inconsistent increases in johnsongrass control.

Field experiments were conducted to determine factors that predispose winter wheat to injury by sulfonylurea herbicides. Wheat was injured occasionally when herbicides were applied postemergence in November or when tank mixed with malathion. CGA 131036 at 28 or 56 g ha-1 was less injurious than chlorsulfuron or DPX-G8311 at 26 or 53 g ha-1, and preemergence treatments of chlorsulfuron and DPX-G8311 were less injurious than preplant incorporated or postemergence treatments. Wheat growth stage, minimum post-treatment daily temperature, and summed diurnal temperature fluctuations after treatment and after first post-treatment rainfall were correlated with wheat injury. Grazing and cultivar selection did not affect injury.

Cultural practices, such as delayed crop seeding, tillage, black fallow, crop rotation, hand weeding, and competitive crops, when used to replace herbicides, provide an opportunity to reduce the selection pressure that causes weeds to become resistant to herbicides. Herbicides on the other hand reduce the selection pressure that causes weeds to resist cultural practices. Rotation of the two systems should then delay resistance to both systems. Growers consider many factors in addition to weed resistance in selecting herbicide or cultural weed control, from associated convenience to economic potential. Rotation of different types of weed control practices would delay resistance, compared to a continuous single practice. The extent of the delay depends upon genetics of resistance, weed reproduction characteristics, weed seed survival, and fitness of resistant weed plants. An understanding of the basic aspects of weeds and herbicides, as well as their interaction with the environment, would help in predicting the delay in resistance to an herbicide from use of cultural practices in the rotation. A grower's final choice of a weed control practice will involve available equipment, time, markets, and soil erosion in addition to potential weed resistance. Weeds that develop resistance to a control practice still allow for reversion to preresistance practices, an important component of a grower's decision. The rate of resistance development is dependent upon the removal of susceptible genes from the population and fitness of the resistant plants. Resistance might be delayed for many years or be manageable when the resistance is genetically recessive or resistant weeds are poorly fit. Growers may not want to accept alternative cultural practices as long as there is the potential for development of another herbicide or reversion to cultural control after resistance occurs.

In the spring of 2007, a widespread freeze occurred that led to the replanting of about 81,000 hectares of corn in Tennessee. Limited research was available on effective herbicide options to control failed stands of glyphosate-tolerant corn where replanting to corn was desired. Therefore, in 2007 on three failed freeze-damaged corn stands (27,000 plants/ha) and in 2008 on two non–freeze-damaged corn stands (81,000 plants/ha), studies were initiated to determine how to control the failed stand while not harming subsequent replant corn. The results from this research clearly show that producers have several effective options to manage undesirable glyphosate-tolerant corn before replanting to corn. Clethodim at 0.14 kg/ha, paraquat at 0.84 kg/ha, or paraquat at 0.70 kg/ha plus a photosystem II (PSII) inhibitor provided very good control of the original corn stand without affecting the yield of the replanted corn. Moreover, the addition of a PSII inhibitor could provide residual weed control in the replanted corn. Paraquat should be applied at rates of 0.70 kg/ha or greater to obtain consistent control of an undesirable corn stand.

Field experiments were established in 1989 and 1990 on silty clay and sandy loam soils to evaluate selective herbicides in combination with non-selective weed control measures in conventional and stale seedbed soybean production. Metribuzin PRE followed by chlorimuron POST controlled sicklepod better with paraquat than with glyphosate. A POST application of imazaquin increased sicklepod and pitted morningglory control by imazaquin PRE alone in a stale seedbed or tillage program. Pitted morningglory control with imazaquin PRE was lower with tillage than with glyphosate or paraquat combinations in a stale seedbed program. All metribuzin plus chlorimuron PRE treatments, whether conventional tillage or stale seedbed, controlled pitted morningglory more than 75%. Hemp sesbania control was above 80% with all metribuzin followed by chlorimuron or metribuzin plus chlorimuron PRE combinations, and less than 70% with all treatments containing imazaquin. Selective herbicides increased yield in stale seedbed when glyphosate or paraquat was added. Imazaquin PRE, imazaquin PRE followed by imazaquin POST, and metribuzin PRE followed by chlorimuron POST tank mixed with glyphosate or paraquat in a stale seedbed program increased yield compared with the same treatments used with tillage.

Time requirements, weed control efficacy, and yield effects of three labor-saving weed technologies were tested against hand weeding during three seasons in 2012 and 2013. The technologies included two hand-operated mechanical weeders, the straight-spike and the twisted-spike floating weeder, and the PRE application of oxadiazon. The straight-spike floating weeder reduced weeding time by 32 to 49%, the twisted-spike floating weeder reduced weeding time by 32 to 56%, and the application of herbicide required 88 to 97% less time than hand weeding. Herbicide application provided the best weed control in two of the three seasons. No differences in weed control efficacy were observed between mechanical and hand weeding. Yield differences were only observed in season 3 with higher rice yields after PRE application of oxadiazon compared with other weed management treatments.

The present work aimed to evaluate plant injury caused by residues in the soil of the formulated mixture of imazethapyr and imazapic to a nontolerant genotype of rice (IRGA 417) drilled at 371 and 705 d after herbicide application (DAA). Herbicide carryover reduced up to 55% of the grain yield of the IRGA 417 drilled at 371 DAA, and plant injury was still evident at 705 DAA but without grain yield reduction.

Field experiments were conducted in 1982 through 1986 to evaluate the effect of six postemergence grass herbicides, clethodim, the methyl ester of diclofop, the butyl ester of fluazifop-P, the methyl ester of haloxyfop, the methyl ester of quizalofop, and sethoxydim, on 31 seedling grass species. All herbicides except diclofop controlled most of the species tested. None of the herbicides controlled rattail fescue or red fescue. Only clethodim and haloxyfop controlled annual bluegrass.

Glyphosate [N-(phosphonomethyl)glycine] toxicity to volunteer wheat (Triticum aestivum L.) and large crabgrass [Digitaria sanguinalis (L.) Scop. # DIGSA] was influenced more by carrier volume than application method. Glyphosate phytotoxicity increased when carrier volume was reduced with both application methods. When compared at equal carrier volumes, there was little difference in volunteer wheat or large crabgrass control when glyphosate was applied with a controlled droplet applicator or flat fan nozzles. Volunteer wheat and large crabgrass control with controlled droplet applicators was influenced by application factors. At a given carrier volume, application factors which yielded larger droplets generally resulted in superior vegetation control.

Early canopy closure and manipulation of crop row spacing or density can reduce the amount and frequency of herbicide use in corn. Field studies were conducted at Woodstock, ON from 1996 to 1999 to evaluate the effect of corn row spacing, plant density, and frequency of glufosinate application on weed biomass and corn yield in glufosinate-resistant corn. Treatments included row width, corn density, and herbicide. The effect of row width and corn density on weed biomass was variable among years. In a wet year (1996), narrow (38 cm) rows provided greater weed suppression than wide (76 cm) rows regardless of crop density. In a dry year (1998), narrow-row high-density (100,000 plants/ha) corn had the lowest weed biomass. In other years, either narrow row or high density was equally successful in suppressing weeds. Effectiveness of herbicides in reducing weed biomass was not influenced by row width or corn density. Corn yield was influenced by row width or corn density. Although weed biomass was lowered by two applications of glufosinate in comparison with a single application, corn grain yields were similar between the two treatments. Planting corn at higher densities may help in reducing early-season weed competition, whereas narrow rows may help in controlling later-emerging species.

Greenhouse and laboratory studies were conducted to evaluate the effects of rye residues on germination and growth of three triazine-resistant weed species, barnyardgrass, willowherb, and horseweed. Soil containing rye roots was used to determine rye residue influence on seed germination. Willowherb and horseweed germination was inhibited up to 50% with the highest rate of rye, but there was no inhibition of barnyardgrass germination. Aqueous extracts of rye shoot tissue inhibited incubator germination of horseweed and willowherb, but did not significantly influence barnyardgrass germination. Although there were no germination differences between the susceptible and resistant biotypes, both were totally inhibited when a 5% w/v solution was used. Growth inhibition of the selected weeds was evaluated using rye root residues in soil. All combinations of biotypes, species, and rye seeding rates showed significant growth inhibition.

Field experiments were conducted to compare efficacy of BAS 514, bromoxynil, fenoxaprop, sethoxydim, and triclopyr with standard herbicides for barnyardgrass and bearded sprangletop control in rice at four locations. Sequential applications of BAS 514 and fenoxaprop, or propanil and sethoxydim consistently controlled barnyardgrass and bearded sprangletop as well as or better than standard treatments of propanil, thiobencarb, or pendimethalin alone or combined; and rice so treated produced high yields. BAS 514 applied postemergence alone, BAS 514 applied sequentially with sethoxydim, or BAS 514 tank mixed with propanil controlled barnyardgrass but not bearded sprangletop. Bromoxynil or triclopyr tank mixed with fenoxaprop or sethoxydim antagonized activity on barnyardgrass and bearded sprangletop. Combinations of fenoxaprop with BAS 514 or propanil consistently provided barnyardgrass and bearded sprangletop control and high yields.

Annual grass weed control and switchgrass cultivar response to PRE-applied pendimethalin and POST-applied mesotrione and quinclorac was evaluated in 2005 and 2006 near Paterson, WA, in both newly seeded and 1-yr-old established switchgrass. Pendimethalin applied to newly planted switchgrass at 1.1 kg ai ha−1 at the one-leaf stage in 2005 or at 0.67 kg ha−1 PRE in 2006 severely injured and greatly reduced switchgrass stands. Mesotrione applied POST at 0.07 kg ai ha−1 injured newly planted switchgrass, reduced switchgrass height for several weeks after treatment, and reduced final switchgrass biomass by 54% both years. ‘Kanlow’ and ‘Cave-in-Rock’ cultivars were injured less by mesotrione than ‘Shawnee’ in 2005, whereas in 2006, Kanlow was injured less than Shawnee and Cave-in-Rock. Quinclorac applied POST at 0.56 kg ai ha−1 injured newly planted switchgrass less than mesotrione and pendimethalin but reduced final switchgrass biomass by 33% both years compared with treatment with atrazine alone. All three herbicide treatments controlled large crabgrass in the year of establishment. Green foxtail counts were reduced 93% or more by pendimethalin and quinclorac compared with nontreated controls, but mesotrione failed to control green foxtail. Pendimethalin applied PRE at 1.1 kg ha−1 did not injure 1-yr-old established switchgrass or reduce switchgrass biomass. Quinclorac applied POST at 0.56 kg ha−1 to established switchgrass reduced switchgrass biomass of the first harvest by 16% in 1 of 2 yr. Mesotrione applied POST at 0.07 kg ha−1 injured established switchgrass and reduced biomass of the first harvest by 33 and 17% in 2005 and 2006, respectively. Kanlow was injured the least by mesotrione in both years. Established switchgrass suppressed late-emerging annual grass weeds sufficiently to avoid the need for a grass-specific herbicide application.

Field studies were conducted to measure the response of sorghum [Sorghum bicolor (L.) Moench. ‘Conlee's Top Hand’] to foliar applications of haloxyfop {2-[4-[[3-chloro-5-(trifluoromethyl)-2-pyridinyl] oxy] phenoxy] propanoic acid} or fluazifop-P {(R)-2-[4-[[5-(trifluoromethyl)-2-pyridinyl] oxy] phenoxy] propanoic acid} plus either bentazon [3-(1-methylethyl)-(1H)-2,1,3-benzothiadiazin-4(3H)-one 2,2-dioxide] at 0.84 kg ai/ha imazaquin {2-[4,5-dihydro-4-methyl-4-(1-methylethyl)-5-oxo-1H-imidazol-2-yl]-3-quinolinecarboxylic acid} at 0.21 kg ae/ha, or chlorimuron {2-[[[[(4-chloro-6-methoxy-2-pyrimidinyl) amino] carbonyl] amino] sulfonyl] benzoic acid} at 12 g ae/ha. Tank-mixing imazaquin or chlorimuron with haloxyfop or fluazifop-P reduced phytotoxicity to sorghum 12 days after treatment (DAT) by 14 to 64% compared to applications of haloxyfop or fluazifop-P alone. Much of this antagonism was reduced 24 DAT as imazaquin and chlorimuron apparently delayed the activity of haloxyfop and fluazifop-P when losses in activity ranged from 13 to 21%. Increases in haloxyfop and fluazifop-P rates tended to reduce the severity of the antagonism. Tank-mixing bentazon with haloxyfop or fluazifop-P did not reduce phytotoxicity to sorghum compared to applications of haloxyfop and fluazifop-P alone.