Field studies were conducted to evaluate soybean injury and yield reduction from foliar applications of mesotrione. Mesotrione was applied at 1.1, 3.2, 11, 35, and 105 g ai/ha to ‘BT 386C’ soybeans at the V1 stage of growth. All rates of mesotrione resulted in visual injury to soybean at 7 and 14 d after treatment (DAT). Overall soybean injury from mesotrione was greatest at 14 DAT, with 25 to 78% injury observed. By 28 DAT, soybean injury was 31 and 66% from mesotrione at 35 and 105 g/ha, respectively, and less than 10% from mesotrione at 1.1, 3.2, and 11 g/ha. Soybean yield was reduced 11 and 22% by mesotrione at 35 and 105 g/ha, respectively. No reduction in soybean yield was observed from mesotrione at rates up to 11 g/ha. Regression analysis indicated that soybean injury from mesotrione at 28 DAT was the best predictor of yield loss (r 2 = 0.77), compared with injury evaluations at 7, 14, and 56 DAT. Greenhouse studies were conducted to determine whether soybean injury from mesotrione was affected by soybean growth stage and variety. Soybean varieties BT 386C, ‘Asgrow 4602RR’, ‘Pioneer 94B01’, and ‘LS 930375’ were more sensitive to mesotrione at the VC growth stage than at the V1 and V2 stages. At the V2 stage, Asgrow 4602RR was three to five times more sensitive to mesotrione than the other three varieties.

Field experiments were conducted in 1994 and 1995 at Vegreville, Legal, and Lacombe, AB, to determine the effects of a preharvest application of glyphosate on seedling emergence and growth of field pea. Glyphosate was applied at 0.9 kg ai/ha at each of the three crop development stages, as determined by seed moisture content (SMC), to determinate (‘Ascona’ and ‘Radley’) and indeterminate (‘Miko’ and ‘Trapper’) cultivars. Applying glyphosate when the SMC was less than 30% had little to no effect on seedling emergence but reduced seedling shoot fresh weight in two of six experiments. Applying glyphosate at SMC above 40% reduced seedling emergence and shoot fresh weight in two and three of the six experiments, respectively. Reductions in seedling emergence and shoot fresh weight were greater from seeds collected from the top than from seeds collected from the bottom one-third of sprayed plants. Differences in response between determinate and indeterminate cultivars occurred, but there was no consistent trend. Given the variable maturity in most fields and on individual pea plants, applications of preharvest glyphosate to peas destined for seed production may decrease seed germination and biomass accumulation.

Row spacing affects the time of canopy closure, thus influencing the growth and development of both crop and weeds. Field studies were conducted in 1999, 2000, and 2001 at Mead, NE, and 2000 and 2001 at Concord in eastern Nebraska to determine the effects of three row spacings (19, 38, and 76 cm) on the critical time for weed removal (CTWR) in dryland soybean. A three-parameter logistic equation was fit to data relating relative crop yield to increasing duration of weed presence. In general, earliest CTWR occurred in the 76-cm rows, and coincided with the first trifoliate stage of soybean. Latest CTWR occurred in the 19-cm rows and coincided with the third trifoliate. The CTWR in 38-cm rows occurred at the second trifoliate. Practical implications are that planting soybean in wide rows reduces early-season crop tolerance to weeds requiring earlier weed management programs than in narrower rows.

Field studies were conducted in 1999, 2000, and 2001 to evaluate mesotrione at 105 and 210 g ai/ha alone and in mixtures with imazethapyr at 70 g ai/ha and the prepackage mixture of imazethapyr at 47 g/ha plus imazapyr at 16 g ai/ha postemergence. Mixtures of mesotrione with imazethapyr and imazethapyr plus imazapyr controlled common ragweed, common lambsquarters, and morningglory species better than imazethapyr or imazethapyr plus imazapyr alone. Similarly, mixtures of mesotrione with these imidazolinone herbicides improved the control of giant foxtail over that by mesotrione alone. Crop injury did not exceed 11% with all treatments and appeared as transient stunting. Yields of corn treated with mixtures of mesotrione plus imidazolinone herbicides were highest in 2000 and 2001, when rainfall was higher than in 1999.

Field studies were conducted in 1999, 2000, and 2001 to investigate weed control and glyphosate-resistant corn tolerance to postemergence applications of mesotrione at 70, 105, and 140 g ai/ha applied with and without glyphosate at 560 g ai/ha. Mesotrione alone and mixed with glyphosate controlled smooth pigweed greater than 97% and common lambsquarters 93 to 99%. Control of common ragweed and morningglory species was variable. Common ragweed control was generally best when mesotrione was applied at 105 or 140 g/ha, and control increased only in 2000 with the addition of glyphosate. Giant foxtail control was below 25% with all rates of mesotrione, but mixtures of mesotrione plus glyphosate controlled giant foxtail 65 to 75%. Mesotrione injured glyphosate-resistant corn 4 to 24% when averaged over glyphosate rates, and injury was usually increased by higher mesotrione rates, with rainfall after herbicide applications, and in mixtures with glyphosate. Injury was transient and did not reduce corn yields. Mesotrione injury on glyphosate-resistant corn was confirmed in the greenhouse, where all mesotrione treatments reduced glyphosate-resistant corn biomass 9 to 23% compared with the nontreated check.

Studies were initiated at two different planting dates and conducted at two different locations in 2001 to determine the critical weed-free period for certain populations of weeds in organically produced ‘Beauregard’ sweetpotato. Naturally occurring weed populations were used, and they included sicklepod, redroot pigweed, and yellow nutsedge. Treatments included allowing weeds to grow for 2, 4, 6, or 8 wk after transplanting (WAT) sweetpotato before weed removal and maintaining the sweetpotato weed-free for 2, 4, 6, or 8 WAT. Weedy and weed-free checks were also included in the study. These treatments were used to determine the length of time weeds can compete with sweetpotato without reducing yield and the length of time sweetpotato must grow before yield is no longer affected by newly emerging weeds. Yield of number one grade sweetpotato roots best fit a quadratic plateau curve for the grow-back treatments and a logistic curve for the removal treatments. Yields in weed-free plots of sweetpotato were higher at the early planting date, whereas yields in plots of weedy sweetpotato were higher at the late planting date. Weed biomass was lower in the grow-back treatments at the late planting date. Data indicate that sweetpotato may gain a competitive advantage over weeds when planted at a later date. At both planting dates, a critical weed-free period of 2 to 6 WAT was observed.

Field experiments were conducted in 1999 at Stoneville, MS, to determine the potential of multispectral imagery for late-season discrimination of weed-infested and weed-free soybean. Plant canopy composition for soybean and weeds was estimated after soybean or weed canopy closure. Weed canopy estimates ranged from 30 to 36% for all weed-infested soybean plots, and weeds present were browntop millet, barnyardgrass, and large crabgrass. In each experiment, data were collected for the green, red, and near-infrared (NIR) spectrums four times after canopy closure. The red and NIR bands were used to develop a normalized difference vegetation index (NDVI) for each plot, and all spectral bands and NDVI were used as classification features to discriminate between weed-infested and weed-free soybean. Spectral response for all bands and NDVI were often higher in weed-infested soybean than in weed-free soybean. Weed infestations were discriminated from weed-free soybean with at least 90% accuracy. Discriminant analysis models formed from one image were 78 to 90% accurate in discriminating weed infestations for other images obtained from the same and other experiments. Multispectral imagery has the potential for discriminating late-season weed infestations across a range of crop growth stages by using discriminant models developed from other imagery data sets.

Dioscorea oppositifolia (Chinese yam) is an exotic perennial vine invading natural areas in the temperate regions of the eastern United States. Rapid early-season growth of D. oppositifolia is facilitated by an extensive tuber system. Plants can reach heights greater than 370 cm, as the plants climb trees and other vegetation. Shoot length increased 3.6 cm/d from late May to mid-August under field conditions, and primary and secondary tuber length increased 0.28 and 0.2 cm/d, respectively. This indicated rapid vegetative growth and substantial food reserves to form new plants in subsequent years. Dioscorea oppositifolia plants also formed aerial bulbils of 0.8- to 1.2-cm diameter, which are important in dissemination of the species over geographical areas. A field study indicated incomplete control from manual removal, clipping by hand, or glyphosate (2% v/v) application to control D. oppositifolia, although glyphosate was the most effective. Additionally, the use of herbicides was more efficient from a time-utilization perspective than either manual removal or clipping. In a separate study, glyphosate application at flowering was more effective in reducing D. oppositifolia growth the year after application as compared with glyphosate applications soon after emergence. Under greenhouse conditions, however, glyphosate at 0.84 kg ae/ha provided <15% control. The ester formulation of triclopyr at 2.5 kg ai/ha provided >90% D. oppositifolia control. Metsulfuron provided 31% control, and mesotrione provided 36% control and at higher rates may reduce D. oppositifolia growth. Several other herbicides having diverse modes of action provided minimal control of D. oppositifolia.

Site-specific weed management can increase crop production efficiency by minimizing herbicide input costs without compromising crop yields. A reduction in herbicide inputs resulting from site-specific weed management may also decrease the probability level of nonpoint pollution compared with conventional herbicide applications. A 4.5-ha field was selected to compare site-specific and conventional weed management techniques in 1997 and 1998 at Knoxville, TN. Variable rate applications (VRAs) of atrazine preemergence (PRE) followed by dicamba postemergence (POST) were investigated for the reduction of herbicide inputs and their resulting impact on weed control and corn yield. VRAs of atrazine were on the basis of weed density data collected in 1996. VRAs of dicamba were according to common cocklebur density evaluations within the field. Compared with conventional applications, atrazine usage was decreased by 43 and 32% in the site-specific application treatments in 1997 and 1998, respectively. VRAs of dicamba reduced herbicide inputs by greater than 45% for 1997 and 1998. Corn yields were similar for the conventional and site-specific treatments in both years. On the basis of these data, site-specific herbicide applications have the greatest potential and least risk for managing weeds when POST or PRE + POST variable rate herbicide applications are used.

Field trials were conducted in Virginia during 2000 and 2001 to evaluate long-term trumpetcreeper control in corn with dicamba, BAS 654 plus dicamba, 2,4-D, CGA 152005 plus primisulfuron, halosulfuron, primisulfuron, and mesotrione. Each of these herbicides was applied alone as a single postemergence (POST) treatment or as a component of a POST herbicide combination. Trumpetcreeper suppression ratings 3 mo after treatment (MAT) revealed a general trend toward higher levels of suppression with combinations of dicamba, BAS 654 plus dicamba, or 2,4-D with any of the sulfonylurea herbicides and lower levels of suppression with applications of any of the sulfonylurea herbicides alone. Combinations of dicamba, BAS 654 plus dicamba, or 2,4-D with mesotrione also provided some of the highest levels of trumpetcreeper suppression 3 MAT in both years. At 1 yr after treatment (YAT), 2,4-D alone, BAS 654 plus dicamba, CGA 152005 plus primisulfuron plus 280 g ai/ha dicamba, primisulfuron plus 280 g/ha dicamba, primisulfuron plus 2,4-D, mesotrione plus BAS 654 plus dicamba, and mesotrione plus 2,4-D reduced trumpetcreeper stem density by at least 52% when compared with that of the nontreated control. These herbicide treatments were the only ones that provided reductions in trumpetcreeper stem density 1 YAT when compared with that of the nontreated control. In 2000 and 2001, there were few differences in corn yield among the treatments evaluated in these trials, and no treatment resulted in corn yields that were lower than the nontreated control. Acceptable trumpetcreeper suppression may be achieved during the season of treatment with any of these herbicide combinations, but only a few treatments will provide long-term trumpetcreeper control.

The objective of this 2-yr study was to determine the optimal length of time between stale-seedbed preparation and planting that maximized weed control along with growth, development, and yield of cucumbers, compared with conventional seedbeds. Stale-seedbeds were prepared 40, 30, 20, and 10 d before planting (DBP), with an additional treatment of 40-DBP seedbed that received an application of glyphosate at 0.9 kg ae/ha, 20 DBP (40 and 20 DBP). The control (0 DBP) was prepared at planting. Glyphosate plus glufosinate ammonium at 1.26 and 0.042 kg ae/ha were applied after cucumber seeding to kill any emerged weeds. The experiment was a split-plot design in which one half of the main plots were treated with a preemergence application of clomazone at 0.42 kg ai/ha after cucumber seeding. Management of the stale-seedbed influenced the level of weed control and final crop yield. Generally, the 40-DBP seedbed had the highest weed biomass at planting and the lowest at harvest. Cucumber density, leaf number, and vine length were reduced in this treatment, and flowering was delayed because of the high weed biomass present during seedling emergence. All stale-seedbeds, with the exception of the 40-DBP stale-seedbed, had greater yields compared with the control (0 DBP) seedbed. The optimal timing of stale-seedbed preparation was 20 to 30 DBP. Seedbed preparation could be expanded to 40 DBP; however, an application of glyphosate at 20 DBP would be required to optimize yield. The stale-seedbed in combination with herbicides was a superior integrated weed management tool compared with conventional weed management practices.

Five herbicides were tested in green pea, and their residual effects on several rotational crops were investigated in northwestern Washington from 1998 through 2000. In both years, imazamox applied postemergence caused 21 and 28% early-season injury at 0.036 and 0.045 kg/ha, respectively, but only in 1999 did early-season injury result in yield loss compared with nontreated, weedy pea. Trifluralin, clomazone, and sulfentrazone caused 15 to 19% injury to pea in 1998 but not in 1999. Although pea treated with sulfentrazone produced more harvestable pods than nontreated pea (5.0 and 4.1 pods/plant, respectively), pod numbers were similar to peas treated with clomazone, pendimethalin, pendimethalin plus imazamox, or trifluralin. All rotationally grown crops were tolerant to herbicides used in green pea, except for strawberry in 1999, in which leaf area was reduced 23% in plots treated with 0.045 kg/ha imazamox compared with nontreated plots. Intensive tillage combined with favorable soil and climatic conditions in this study indicate that western Washington green pea producers may have greater flexibility in their choice of herbicides and rotational crop alternatives than previously believed.

Field studies were conducted in Arizona and California to evaluate the performance of glyphosate-tolerant lettuce and to determine the critical time of weed removal. Glyphosate was applied as a single or as a sequential application at 840 g ae/ha. Single glyphosate applications were made to lettuce at the two-, four-, six-, and eight-leaf stages. Sequential applications were made to lettuce at the two- or four-leaf stage followed by (fb) a second application 14 d after the first. Weed control efficacy, weeding times, and lettuce yield were all measured. Overall, glyphosate applied postemergence (POST) provided better weed control than the commercial standards bensulide or pronamide applied preemergence. Single glyphosate applications at the four-leaf stage and sequential applications at the two-leaf stage fb a second application 14 d later provided excellent control of most weeds, including redroot pigweed. Estimates of the critical time of weed removal were 26 to 29 d after emergence. Glyphosate treatments caused no adverse effects on lettuce. Lettuce head fresh weights in the glyphosate treatments were equal to or higher than those in bensulide or pronamide treatments. For crops such as lettuce, with few effective herbicides, the development of glyphosate-tolerant lettuce offers the opportunity to develop effective POST weed control programs.

Field studies evaluated the effect of brown patch control on preemergence herbicide efficacy in tall fescue. Pendimethalin (1.7 followed by [fb] 1.7; 3.4 kg ai/ha), prodiamine (0.7 fb 0.6; 1.3 kg ai/ha), and oxadiazon (2.2 fb 2.2; 4.5 kg/ha), applied sequentially and as a single application, were evaluated for smooth crabgrass control with and without the use of azoxystrobin, a fungicide that controls brown patch. Azoxystrobin suppressed brown patch and increased smooth crabgrass control with pendimethalin in both years. This enhanced efficacy with azoxystrobin was attributed to improved tall fescue turf density and thus increased competition between this turf species and smooth crabgrass. Longer soil-residual herbicides such as oxadiazon and prodiamine provided high levels of smooth crabgrass control (often >90%). With the exception of oxadiazon at 4.5 kg ai/ha in 2000, smooth crabgrass control with oxadiazon and prodiamine was unaffected by the use of azoxystrobin.

Field studies were conducted in the spring of 1997 and 1998 to quantify the effect of season-long yellow nutsedge interference on watermelon yield. The competitive ability of watermelon with yellow nutsedge was compared in two establishment methods (watermelon transplanted and direct seeded). Critical yellow nutsedge densities and the biological threshold (BT) were used to characterize the competitive ability of watermelon. The critical density in both direct-seeded and transplanted watermelons was 2 yellow nutsedge plants/m2. The BT of yellow nutsedge in seeded watermelons was 37 yellow nutsedge plants/m2, whereas the BT in transplanted watermelons was 25 plants/m2. Transplanting watermelons did not improve their competitive ability with yellow nutsedge. Percent yield loss was similar for both establishment methods at the respective yellow nutsedge densities. Over 40% yield loss was incurred with 12 yellow nutsedge plants/m2 for both establishment methods. Furthermore, it was concluded that watermelons are poor competitors with yellow nutsedge.

The effect of season-long interference by bands of weeds growing only between rows (BR) on field corn yields has not been reported before or compared with weedy and weed-free (i.e., weeded) plots or bands of weeds growing only in row (IR). The null hypothesis of this research was that field corn yields would be ranked as weed-free > BR weedy only > IR weedy only > weedy (IR + BR weedy) in response to season-long weed interference by these four treatments. Weeds growing as bands closest to field corn were expected to reduce field corn yields more than those growing as bands further away between field corn rows. Field corn yield response to these four weed interference treatments was studied in Missouri for 4 yr. In late summer, most weed ground cover consisted of giant foxtail, the chief weed present, and common waterhemp, a lesser weed. Observed field corn yields averaged for 4 yr were ranked as weed-free > IR weedy only > BR weedy only > weedy. Field corn yields of the IR weedy only, BR weedy only, and weedy treatments averaged 76, 63, and 41%, respectively, of the weed-free treatment (=7,820 kg/ha). In two of the 4 yr, field corn yield of the IR weedy treatment exceeded that of the BR weedy treatment, whereas these treatments could not be statistically distinguished from one another in the other 2 yr. These research results refute the null hypothesis and demonstrate that it may be more critical to control BR than IR weeds, although controlling both BR and IR weeds maximized field corn yields.

Common management alternatives were compared in a factorial arrangement for 2 yr to determine their effects on green foxtail and yellow foxtail seed production in spring wheat in the Northern Great Plains of the United States. Seed production was measured twice, at wheat harvest (in August) and postharvest (after first lethal frost in autumn). Management alternatives were early, middle, and late crop-sowing dates; no-till, chisel, and moldboard plow tillage systems; and broadleaf herbicide only and broadleaf herbicide plus fenoxaprop applications. Fenoxaprop reduced foxtail seed production at wheat harvest but not at postharvest. Early sowing also decreased seed production at wheat harvest but increased postharvest seed production. Tillage system had no consistent effects on foxtail seed production. Postharvest seed production often was greater than or equal to that at wheat harvest regardless of management system. These results indicate that in-crop management alternatives, such as postemergence grass herbicide and early crop sowing, may lower the number of foxtail seeds at harvest substantially, but they must be accompanied by postharvest weed control to reduce overall seed production.

The postemergence herbicide ethofumesate and the plant growth regulator paclobutrazol were evaluated for annual bluegrass control in creeping bentgrass turf managed as golf fairways. Both products were applied under several different timing regimes relative to the time of the year. Paclobutrazol treatments provided significantly greater annual bluegrass control than ethofumesate. There were no differences between rates of paclobutrazol (0.28 and 0.14 kg ai/ha) when applied from spring through summer. Annual bluegrass control after spring and summer applications of paclobutrazol was 85% or more. Clipping weight data indicated that paclobutrazol suppressed growth in annual bluegrass longer than in creeping bentgrass. It was concluded that prolonged suppression of annual bluegrass by paclobutrazol resulted in creeping bentgrass dominance and subsequent annual bluegrass control. Additionally, applications of ethofumesate in autumn–winter, followed by paclobutrazol applied in spring–summer, provided significant control of annual bluegrass in 1 yr of the study.

Nonfungicidal effects of agricultural fungicides on crop plants have been reported previously; however, there are few reports of nontarget effects of fungicides on weedy species. Field research trials in Oregon demonstrated that the growth of several broadleaf weeds was reduced after multiple applications of the fungicide propiconazole. Greenhouse experiments confirmed that preemergence applications of propiconazole reduced the biomass accumulation of several common broadleaf and grass weeds 15 to 63%. Laboratory experiments were performed on redroot pigweed, the most sensitive species, to examine the effects of propiconazole on germination and early seedling growth. Redroot pigweed germination and total seedling length (root plus shoot) were reduced at propiconazole concentrations above 37 and 0.36 mg/L, respectively. Growth-regulating effects of fungicides such as propiconazole on the germination and early growth of weeds may contribute to integrated weed management, especially when adequate moisture ensures the presence of germinating seeds and small seedlings throughout the growing season.

Diminishing availability and increasing costs of herbicides cause strawberry growers to seek both chemical and nonchemical alternatives, especially for within-row weed control soon after strawberries are transplanted. Several weed control treatments for strawberry establishment were examined during 2 yr in Minnesota. Treatments included: woolen landscaping fabric centered over the crop row; as above, but 2-ply fabric; spring canola incorporated into soil when 30 cm tall; as above, but canola killed with burndown herbicide and left as mulch; standard herbicide, DCPA; hand weeded; and no weed control. Areas between all strawberry rows were cultivated. Measurements included weed densities and weights, numbers of strawberry daughter plants, and fruit yield 1 yr after transplantation. The best alternative treatment was the 1-ply woolen fabric. It nearly eliminated weeds from rows, promoted daughter plant rooting, and allowed maximum fruit yields, equivalent to those of the DCPA and hand-weeded treatments. Canola mulch controlled weeds inconsistently and achieved only modest to low production of daughter plants and fruit. Weed control and fruit yield with incorporated canola were similar to the weedy check treatment.