A field study was conducted from 1999 through 2001 at Stoneville, MS, to determine the effects of bromoxynil-resistant (BR) and glyphosate-resistant (GR) cotton rotation systems under ultranarrow- (25-cm spacing) and wide- (102-cm spacing) row planting on weed control, weed density and shift, and cotton yield. The four rotations during 3 yr included BR–BR–BR, GR–GR–GR, BR–GR–BR, and GR–BR–GR, all with bromoxynil or glyphosate postemergence (POST) only or following fluometuron plus pendimethalin preemergence (PRE). Control of hemp sesbania, pitted morningglory, prickly sida, and hyssop spurge was ≥97% regardless of row width, rotation, and herbicide program. Control of common purslane, sicklepod, and smooth pigweed was higher with glyphosate POST in GR cotton than with bromoxynil POST in BR cotton. Broadleaf and yellow nutsedge weed biomass were higher with bromoxynil POST in BR cotton than with glyphosate POST in GR cotton. Continuous BR cotton system resulted in higher densities of common purslane, sicklepod, and yellow nutsedge (15.3, 1.5, and 373 plants/m2, respectively) compared with continuous GR cotton (0.7, 0.1, and 1.0 plant/m2, respectively). Seed cotton yield was consistently higher in wide- than in ultranarrow-row cotton. Seed cotton yield was lower in continuous BR cotton than in the other three rotation systems, and yields greatly improved when BR cotton was rotated with GR cotton. During a 3-yr period, seed cotton yields with glyphosate POST only (4,000 to 4,890 kg/ha) or after PRE herbicides (4,480 to 4,860 kg/ha) were similar in GR cotton, whereas in BR cotton, bromoxynil POST only (1,390 to 4,280 kg/ha) resulted in lower yield than did bromoxynil POST after PRE herbicides (2,550 to 4,480 kg/ha). The results indicated that the shift in spectrum of weeds toward more tolerant species and yield decline in continuous BR cotton can be prevented by rotating BR with GR cotton.

The effect of planting system and cover crop residues on weed emergence in irrigated vegetable row crops was studied in field experiments from 1995 through 1997. Vegetable crops were either no-till planted (NTP) through cover crop residues or conventionally planted (CP) into soil with cover crop residues incorporated. NTP reduced emergence of hairy nightshade by 77 to 99% and Powell amaranth emergence by 50 to 87% compared with CP. Cover crop treatments were much less important than planting system in regulating weed emergence. Tillage in the spring did not increase the number of viable seeds near the soil surface. Hairy nightshade emergence ranged from 0.6 to 9.8% of the intact seeds in CP compared with 0 to 0.1% emergence of the seeds in the NTP plots. Powell amaranth emergence ranged from 4.9 to 6.5% of the intact seeds in CP contrasted with only 0.4 to 0.9% emergence of the seeds in NTP plots.

Field experiments were conducted in 2001 and 2002 to evaluate the efficacy of herbicides on protoporphyrinogen oxidase (protox, EC 1.3.3.4) inhibitor–resistant common waterhemp in corn and soybean. All corn herbicides tested gave greater than 90% common waterhemp control by 8 wk after postemergence herbicide treatment (WAPT). In soybean, common waterhemp control was less than 40% by 8 WAPT with postemergence protox-inhibiting herbicides lactofen and acifluorfen. However, preemergence protox-inhibiting herbicides sulfentrazone and flumioxazin gave greater than 85% common waterhemp control in both years. The greatest common waterhemp control in soybean was with glyphosate alone, alachlor + metribuzin, alachlor followed by (fb) glyphosate, and S-metolachlor + metribuzin fb glyphosate.

Field studies were conducted in 2000 and 2001 in Plains, GA, to determine peanut and weed response to the residual herbicides sulfentrazone, imazapic, diclosulam, and flumioxazin. Herbicide treatments included sulfentrazone applied preemergence (PRE) or preplant incorporated (PPI) at 112, 168, 224, and 280 g ai/ha, imazapic postemergence (POST) at 71 g ai/ha, diclosulam PPI at 26 g ai/ha, and flumioxazin PRE at 88 g ai/ha. Peanut exhibited early-season injury from all herbicide treatments, ranging from 0 to 10% for sulfentrazone PPI or PRE, 10% for imazapic, 3 to 23% for flumioxazin, and 1 to 7% for diclosulam. Yields were similar for sulfentrazone PPI- or PRE-treated and flumioxazin-, imazapic-, and diclosulam-treated peanut. Yellow nutsedge control was 83% or greater with all rates of sulfentrazone PRE or PPI, 83 and 90% with diclosulam, and 96 and 99% with imazapic, respectively. Flumioxazin did not control yellow nutsedge or wild poinsettia. Tall morningglory control was 82% or greater with imazapic, diclosulam, flumioxazin, and sulfentrazone PPI or PRE at 168 g/ha or higher. Florida beggarweed control was 88% or greater with diclosulam, flumioxazin, and sulfentrazone PRE at 224 and 280 g/ha. Overall, peanut tolerance to sulfentrazone at 112 to 280 g/ha PPI and PRE was high and yield was equivalent to the currently registered peanut residual herbicides.

Selective placement studies were conducted under greenhouse conditions to determine the relative importance of root vs. foliar absorption of postemergence-applied quinclorac by torpedograss. Foliar + soil and soil-only applications were more effective than foliar-only in reducing torpedograss foliage at 4 wk after treatment (WAT). However, foliar-only and foliar + soil were more effective than soil-only in suppressing regrowth at 10 WAT. Quinclorac foliar absorption by torpedograss and subsequent translocation, as determined with radiotracer techniques, was minimal. After 72 h, only 26% of the applied quinclorac had been absorbed, and 13.7% of the amount applied remained within the treated leaf. Only 0.3% of applied was recovered in the roots, and none was detected in the developing rhizomes. Quinclorac was readily root absorbed and translocated. After 6 h, a 26.7 μg/plant dose of quinclorac had been absorbed, and 54% of this quantity remained in the roots; the remaining 46% having been translocated throughout the plant. The youngest leaf and the immature rhizomes accumulated 5 and 9% of the amount absorbed, respectively. Quinclorac was not readily soil sorbed as determined by soil solution experiments. Quinclorac was displaced nearly concomitant with the wetting front in soil chromatography. Soil solution concentration and soil mobility were greater at pH 6.7 than at 5.7. Results establish that consistent control of torpedograss with quinclorac is dependent on soil entry and root absorption. Unfortunately, the propensity of quinclorac to be water displaced could negatively affect this control.

Field trials were conducted to compare the effect of various soil fumigants along with in-bed pebulate and row-middle metribuzin applications on purple nutsedge control and on tomato and bell pepper growth and yield. Treatments consisted of combinations of soil fumigants, pebulate, and metribuzin. Fumigants levels were (1) untreated control, (2) methyl bromide (MBr) + chloropicrin (Pic) (67 + 33%, respectively), (3) Pic, (4) metham, (5) dazomet, and (6) 1,3-dichloropropene (1,3-D) + Pic (83 + 17%, respectively). Pebulate levels were either applied in-bed or not applied. Row middles were either sprayed with metribuzin or untreated. In both crops, purple nutsedge populations were independently influenced by fumigants and pebulate applications, with the highest number of purple nutsedge plants in the untreated control. The addition of pebulate reduced purple nutsedge populations in all treatments. In tomato trials, the yield was affected by fumigants, with the highest losses (53 and 50% reductions in fruit number and weight) observed in the nonfumigated control. In pepper trials, fruit number and weight were individually influenced by fumigants and metribuzin sprayings. Application of metribuzin to row middles increased yields 10% relative to nontreated plots.

Efficacy of trifloxysulfuron with and without surfactant was evaluated against balsamapple, cat's claw vine, Florida beggarweed, hairy beggarticks, ivyleaf morningglory, johnsongrass, prickly sida, redroot pigweed, sicklepod, strangler vine, tall morningglory, and yellow nutsedge at 21, 42, and 63 g ai/ha applied at the four- or six-leaf stages and compared with glyphosate at 280, 560, and 840 g ae/ha. Delayed application from the four- to six-leaf stage significantly reduced trifloxysulfuron efficacy; reduction was less with glyphosate. Trifloxysulfuron plus 0.25% X-77 was more effective on the four-leaf stage than on the six-leaf stage plants of redroot pigweed, johnsongrass, hairy beggarticks, strangler vine, and prickly sida; effect was similar on yellow nutsedge, sicklepod, Florida beggarweed, balsamapple, ivyleaf morningglory, and tall morningglory. Trifloxysulfuron at 63 g/ha plus surfactant reduced the fresh weight of all test plants more than 80% compared with control, except prickly sida, strangler vine, and cat's claw vine. Glyphosate was less effective than trifloxysulfuron plus surfactant against tall morningglory, sicklepod, ivyleaf morningglory, and yellow nutsedge but was significantly better against balsamapple, prickly sida, and cat's claw vine. None of the herbicides provided satisfactory control of cat's claw vine, strangler vine, and prickly sida.

Field experiments were conducted in 1999 and 2000 at the Rice Research and Extension Center at Stuttgart, AR, on a DeWitt silt loam and at the Pine Tree Branch Experiment Station near Colt, AR, on a Calloway silt loam to evaluate the tolerance of 14 rice cultivars and four experimental cultivar lines to clomazone at 0.34 and 0.67 kg ai/ha applied preemergence. Early-season chlorosis 14 days after emergence (DAE) ranged from 1 to 21% in 1999 and 3 to 40% in 2000 when averaged over herbicide rate. The experimental cultivar line RU961096 was slightly more susceptible to clomazone than other cultivars and lines. However, RU9701041 was more tolerant to clomazone at 0.67 kg/ha 14 DAE than other cultivars and lines. By 28 DAE, all cultivars had <13% chlorosis in 1999 and <8% in 2000. The experimental cultivar line RU9601096 and cultivar Koshihikari did not recover as quickly as the other 16 cultivars and cultivar lines. Early-season chlorosis had no effect on days to 50% heading or yield for any of the cultivars evaluated.

The efficacy of imazethapyr in drill-seeded imidazolinone-resistant rice was evaluated in Louisiana in 2000 and 2001. Imazethapyr was applied preemergence (PRE), or no PRE, followed by a postemergence (POST) application of imazethapyr alone, or in a mixture with bensulfuron, bentazon plus aciflurofen, carfentrazone, halosulfuron, propanil plus molinate, triclopyr, or V-10029. Imazethapyr applied PRE followed by a POST application of imazethapyr controlled barnyardgrass equivalent to or higher than other treatments evaluated. Red rice control at 35 days after postemergence treatment (DAT) was 66 to 81% with imazethapyr applied PRE followed by any POST application, but a reduction in control was observed with a POST application of imazethapyr. Although alligatorweed control increased with POST applications, these treatments suggested only suppression. Hemp sesbania control never exceeded 10% with imazethapyr-only treatments and at 35 DAT, all POST applications, except bensulfuron, increased control above 84%. Rice yield increased with treatments receiving a PRE application of imazethapyr compared with no imazethapyr applied PRE.

Field studies were conducted to evaluate foramsulfuron for postemergence control of goosegrass in bermudagrass turf as a possible replacement for MSMA. In field trials on a golf course and sports fields planted to bermudagrass, mature goosegrass was controlled effectively (>85% goosegrass dead on the basis of canopy area) with two applications of foramsulfuron plus metribuzin. The herbicide rates that were effective varied among studies, e.g., foramsulfuron at 0.029 or 0.044 kg ai/ha mixed with metribuzin at 0.105 to 0.210 kg ai/ha. Goosegrass was often controlled with MSMA at 2.24 kg ai/ha plus metribuzin at 0.105 to 0.210 kg/ha, but foramsulfuron was always as effective, or more effective, than MSMA, in controlling mature goosegrass, at the same rate of metribuzin. Bermudagrass phytotoxicity of foramsulfuron plus metribuzin was temporary and not different from MSMA plus metribuzin. In one location there was noticeable phytotoxicity 4 wk after initial treatment.

Greenhouse studies were conducted to evaluate shoot number, shoot weight, rhizome weight, and root weight reduction of green and false-green kyllinga at three placement levels (soil applied, foliar applied, and soil + foliar applied) and five herbicide treatments (CGA-362622, halosulfuron, imazaquin, MSMA, and imazaquin + MSMA). Averaged over herbicide and placement level, false-green kyllinga shoot number 30 d after treatment (DAT) and rhizome weight 60 DAT were reduced more than those of green kyllinga. Furthermore, imazaquin, MSMA, and imazaquin + MSMA, averaged across placement levels, as well as CGA-362622 and halosulfuron, both foliar and soil applied, reduced false-green kyllinga shoot number greater than that of green kyllinga 60 DAT. Halosulfuron reduced false-green kyllinga shoot weight greater than that of green kyllinga 60 DAT; however, MSMA reduced green kyllinga greater. In general, foliar- and soil + foliar–applied treatments reduced shoot number (30 DAT), rhizome weight, and root weight of both kyllinga species greater than soil-applied treatments, whereas soil + foliar–applied treatments were more effective in reducing shoot weight 60 DAT. CGA-362622 and halosulfuron reduced kyllinga species shoot number (30 DAT), false-green kyllinga shoot weight (60 DAT), and root weight of both species greater than all other herbicides. However, CGA-362622 reduced green kyllinga shoot weight (60 DAT) and rhizome weight of both species greater than all other herbicides.

Field studies were conducted from 1998 to 2000 in Tennessee, North Carolina, Arkansas, and Oklahoma to determine the effects of sulfentrazone carryover to a cotton rotational crop from sulfentrazone applied the previous year. Sulfentrazone applied the previous year at 400 g/ha caused no yield loss in Tennessee, >30% yield reduction in Oklahoma, and 20% yield loss in Arkansas and North Carolina. In most experiments in this study, visual evaluations of injury closely correlated with final cotton lint yield (r2 =0.84).

Field trials were carried out in organic soils to determine the effects of different phosphorus (P) fertilization programs and common lambsquarters duration of interference on lettuce. Phosphorus was either banded (125 kg/ha) or broadcast (250 kg/ha) before lettuce planting. A common lambsquarters population density of four plants per 6 m row interfered with ‘South Bay’ lettuce for 2, 4, 6, or 8 wk after lettuce emergence, along with a weed-free control. For banded P, lettuce fresh weight declined linearly (y = 14.82 − 0.97x; r 2 = 0.96) as duration of common lambsquarters interference increased. The effect of broadcast P over common lambsquarters duration of interference fit a quadratic equation (y = 10.67 − 1.69x + 0.12x2; r 2 = 0.98). The difference in the regression model intercepts for both P fertilization programs showed that in the absence of common lambsquarters interference, marketable lettuce fresh weight was higher by banding P than by broadcasting P. Based on predicted values, this difference is approximately 28% (+4.15 kg per 6 m row) in favor of banded P. Therefore, banding P at 125 kg/ha proved to be beneficial in raising lettuce fresh weight regardless of the duration of interference. For banded P, the model predicts that 10% yield reduction would be observed at 1.53 wk (10.5 d) of common lambsquarters interference. However, with broadcast P, this period declined to 0.67 wk (4.7 d).

Field experiments were conducted at Kalispell, MT, and Corvallis, OR, to determine the optimum rate and application timing of imazamox for downy brome control in winter wheat. Crop injury occurred as a reduction in plant height and was minimal at Kalispell, never exceeding 10%. Crop injury at Corvallis was more severe and was dependant on application timing. No injury was observed with spring applications, but fall applications resulted in as much as 33% injury at the highest rate of imazamox. Fall applications generally provided more consistent control of downy brome, as evidenced by the lower dosage required to reduce downy brome dry weight by 50% (lower I50 values). Nonetheless, spring applications generally provided control comparable with that of fall applications when imazamox was applied at the highest rate. The one exception was at Corvallis during 1997 to 1998, where spring applications failed to provide adequate control of downy brome even at the highest rate applied. Although imazamox generally provided excellent control of downy brome, wheat yield response to downy brome interference was negligible, declining by less than 10% in the absence of imazamox. The absence of a yield response to downy brome interference was attributed to the lack of competition for soil moisture from downy brome under the high-rainfall conditions of the experiment.

Experiments were conducted to evaluate safety and effectiveness of herbicides during establishment of seeded centipedegrass. Centipedegrass tolerance to herbicides was evaluated at seeding and early postemergence. Imazapic at 105 g ai/ha, sulfometuron at 53 g ai/ha, or metsulfuron at 21 or 42 g ai/ha applied at seeding reduced centipedegrass ground cover compared with the nontreated. Imazapic at 18 or 35 g/ha or applications of atrazine or simazine at seeding did not reduce centipedegrass ground cover compared with the nontreated. Applications of chlorsulfuron plus mefluidide (7 + 140 g ai/ha) or metsulfuron at 21 or 42 g/ha applied 6 wk after seeding (WAS) centipedegrass (one-leaf to one-tiller growth stage) caused 20, 16, and 83% phytotoxicity, respectively, 56 d after treatment (DAT). Imazapic, sulfometuron, atrazine, or simazine applied 6 WAS caused <15% phytotoxicity 56 DAT. When large crabgrass and centipedegrass were seeded together, large crabgrass emergence was reduced 41% when atrazine (1,100 g ai/ha) was applied at seeding. Centipedegrass tiller production was reduced with increasing amounts of crabgrass. However, centipedegrass tiller production and ground cover were higher when atrazine was applied because of reduced interspecific interference from large crabgrass. These data indicate that centipedegrass can be established more quickly if appropriate herbicides are used at seeding or shortly after seeding.

Effects of cogongrass foliage and rhizome plus root residues on germination and shoot and root growth of barnyardgrass, bermudagrass, browntop millet, hemp sesbania, Italian ryegrass, and prickly sida were investigated in greenhouse experiments. Ground residues of dried cogongrass foliage and rhizomes plus roots were mixed separately with sterilized sand to obtain residue concentrations of 0, 0.25, 0.5, 1, 2, 4, and 8%. These residue concentrations were investigated on bermudagrass and Italian ryegrass, and the 8% residue concentrations were also evaluated on hemp sesbania, prickly sida, barnyardgrass, and browntop millet. Foliage and rhizome plus root residues at concentrations as low as 0.25% inhibited seed germination and shoot and root growth of all species except hemp sesbania. Germination of bermudagrass and Italian ryegrass was reduced by as much as 97% and shoot and root growth by as much as 94% at the highest residue concentrations. Rhizome plus root residues reduced germination and shoot and root growth of bermudagrass and Italian ryegrass more than foliage residues. Foliage and rhizome plus root residues reduced germination and shoot and root biomass of prickly sida, barnyardgrass, and browntop millet at similar levels. Results indicate that cogongrass tissue may contain allelochemicals that contribute to its invasiveness and extreme competitiveness.

Dryland rotations are changing in the semiarid Great Plains because of no-till systems. Producers now rotate summer annual crops such as corn with winter wheat and fallow, which can disrupt weed population growth because of diverse life cycles among crops. This study estimated changes in weed populations as affected by rotation design, with the goal of suggesting crop sequences that lower weed community density. We used an empirical life-cycle simulation based on demographics of jointed goatgrass and green foxtail to compare various rotations consisting of winter wheat, corn, proso millet, and fallow across a 12-yr period. The simulation indicated that designing rotations to include a 2-yr interval when seed production of either jointed goatgrass or green foxtail is prevented will drastically reduce weed populations. Arranging four different crops in sequences of two cool-season crops, followed by two warm-season crops was the most beneficial for weed management. Fallow, if used, serves in either life-cycle category. However, if the same crop is grown 2 yr in a row, such as winter wheat, the benefit of rotation design on weed density is reduced considerably. Impact of rotation design on weed density was enhanced by improving crop competitiveness with cultural practices. Rotations with balanced life-cycle intervals not only reduce weed density but enable producers to use alternative weed management strategies, improve effectiveness of herbicides used, and minimize herbicide resistance.

Glyphosate and paraquat herbicide drift injury to crops may substantially reduce growth or yield. Determining the type and degree of injury is of importance to a producer. This research was conducted to determine whether remote sensing could be used to identify and quantify herbicide injury to crops. Soybean and corn plants were grown in 3.8-L pots to the five- to seven-leaf stage, at which time, applications of nonselective herbicides were made. Visual injury estimates were made, and hyperspectral reflectance data were recorded 1, 4, and 7 d after application (DAA). Several analysis techniques including multiple indices, signature amplitude (SA) with spectral bands as features, and wavelet analysis were used to distinguish between herbicide-treated and nontreated plants. Classification accuracy using SA analysis of paraquat injury on soybean was better than 75% for both 1/2- and 1/8× rates at 1, 4, and 7 DAA. Classification accuracy of paraquat injury on corn was better than 72% for the 1/2× rate at 1, 4, and 7 DAA. These data suggest that hyperspectral reflectance may be used to distinguish between healthy plants and injured plants to which herbicides have been applied; however, the classification accuracies remained at 75% or higher only when the higher rates of herbicide were applied. Applications of a 1/2× rate of glyphosate produced 55 to 81% soybean injury and 20 to 50% corn injury 4 and 7 DAA, respectively. However, using SA analysis, the moderately injured plants were indistinguishable from the uninjured controls, as represented by the low classification accuracies at the 1/8-, 1/32-, and 1/64× rates. The most promising technique for identifying drift injury was wavelet analysis, which successfully distinguished between corn plants treated with either the 1/8- or the 1/2× rates of paraquat compared with the nontreated corn plants better than 92% 1, 4, and 7 DAA. These analysis techniques, once tested and validated on field scale data, may help determine the extent and the degree of herbicide drift for making appropriate and, more importantly, timely management decisions.

To address grower concerns that repeated use of dichlobenil could negatively affect cranberry productivity, field studies were conducted at two commercial farms in either high weed density (HW) or low weed density (LW) areas. Data from 4 yr of repeat annual applications of 0, 1.8, and 4.5 kg ai/ha dichlobenil indicated minimal negative impact on cranberry vines. Herbicide application did not affect upright productivity, leaf biomass production, percent fruit set, or other yield parameters adversely; in addition, no improvement in these parameters was noted. Although the interaction of herbicide application with weed density on cranberry root length varied with sampling date, no consistent trend (adverse or positive) was seen. The presence of weeds, rather than herbicide application, was the important determinant of yield. Vines in LW areas produced more marketable fruit and had higher percentage of fruit set than vines growing in HW areas. Repeat annual applications of dichlobenil on commercial cranberry beds may be considered as part of a viable integrated weed management program with no adverse effect on crop growth or yield.

Three field studies were conducted at Lewiston Woodville, NC, in 2001 and 2002 to evaluate crop tolerance, weed control, grain yield, and net returns in glyphosate-resistant corn with various herbicide systems. Crop injury, weed control, and grain yield were not influenced by glyphosate formulation. Atrazine preemergence (PRE) and atrazine plus metolachlor PRE, averaged over postemergence (POST) systems, controlled Texas panicum at least 80 and 87%, respectively. Sequential glyphosate applications (early postemergence [EPOST] followed by [fb] POST) provided at least 99% control of Texas panicum compared with at least 86 and 88% control with glyphosate EPOST and glyphosate plus halosulfuron EPOST, respectively. Atrazine plus metolachlor PRE fb any glyphosate system controlled large crabgrass and goosegrass 89 to 100% and 94 to 100%, respectively. Sequential glyphosate treatments controlled large crabgrass and goosegrass at least 99 and 95%, respectively. Regardless of PRE system, glyphosate plus halosulfuron EPOST and sequential applications of glyphosate controlled common ragweed and common lambsquarters at least 99%, whereas glyphosate EPOST alone provided at least 88 and 96% control, respectively. Glyphosate plus halosulfuron EPOST and glyphosate sequentially controlled yellow nutsedge similarly and more consistently than glyphosate EPOST. Regardless of PRE treatment, sequential glyphosate applications provided at least 98% control of entireleaf and pitted morningglory, whereas glyphosate EPOST controlled at least 64 and 62%, respectively. Glyphosate EPOST and the sequential glyphosate EPOST fb POST systems yielded similarly at all three locations. Net returns were highest at all three locations with the glyphosate sequential system, with similar net returns obtained with glyphosate EPOST and glyphosate plus halosulfuron EPOST at two and one locations, respectively.