Studies were conducted to observe the effect of full and reduced rates of postemergence (POST) herbicides on seedling and rhizome johnsongrass (Sorghum halepense) at different growth stages. Herbicides and labeled rates included primisulfuron at 40 g/ha, nicosulfuron at 35 g/ha, fluazifop-P at 210 g/ha, and clethodim at 140 g/ha. Contour graphs to predict johnsongrass control at different growth stages and herbicide rates were developed. Excellent seedling and rhizome johnsongrass control was obtained with reduced rates of herbicides applied at the three- to five-leaf stages. Primisulfuron at 20 g/ha resulted in 90% or greater control of seedling johnsongrass in the three- to four-leaf stage. Rhizome johnsongrass at this growth stage required 30 g/ha of primisulfuron for the same level of control. Nicosulfuron at 17.5 and 26.3 g/ha provided 90% or greater control up to the four-leaf stage of seedling and rhizome johnsongrass, respectively. Fluazifop-P and clethodim were more effective than primisulfuron and nicosulfuron. Fluazifop-P at 105 g/ha resulted in 90% or greater control of seedling and rhizome johnsongrass up to the seven- and five-leaf stages, respectively. Clethodim at 35 g/ha controlled seedling johnsongrass at least 90% up to the eight-leaf stage. Clethodim at 70 g/ha provided 90% or greater control of rhizome johnsongrass if applied at the three- to four-leaf stages.

Dodder is a serious parasitic weed of cranberry bogs. Investigations were undertaken to determine if applications of sand to cranberry bogs might be useful as a control strategy for dodder. Sand is regularly applied to bogs as a cultural practice. In greenhouse studies, applications of sand reduced the seedling emergence, but did not affect survival time of dodder seedlings. At least 2.5 cm of sand was needed to reduce the emergence of dodder seedlings. Sanding may be used as part of a program consisting of combinations of multiple management methods directed toward the control of dodder. In addition, potential exists for using sand as a nonchemical means for obtaining some control, should present herbicidal control methods become unavailable.

Clomazone was applied preplant incorporated, preemergence, delayed preemergence, or postemergence and compared to pendimethalin, quinclorac, and thiobencarb for control of barnyardgrass in rice. Seven days after the clomazone postemergence application, all clomazone treatments except those applied postemergence controlled barnyardgrass ≥ 86%. At 49 d after the postemergence application, control of barnyardgrass with 0.56 and 0.67 kg/ha clomazone postemergence improved to 92 and 93%, respectively. Clomazone and quinclorac applied delayed preemergence generally controlled barnyardgrass less than 70%. Visible injury to rice was ≤ 18% 7 d after emergence and had declined to less than 10% when evaluated 7 d after postemergence treatments. Rice yields were higher with all treatments compared to the nontreated control.

Field experiments were conducted in 1996 and 1997 at five locations in southwestern Ontario to develop dose-response curves for SAN 1269H (SAN 835H plus dicamba) for weed control and crop tolerance in corn. SAN 1269H controlled wild buckwheat (Polygonum convolvulus L.), common ragweed (Ambrosia artemisiifolia L.), common lambsquarters (Chenopodium album L.), pigweeds (Amaranthus retroflexus L. and A. powellii S. Wats.), barnyardgrass [Echinochloa crus-galli (L.) Beauv.], and yellow foxtail [Setaria glauca (L.) Beauv.]. Biologically effective doses of SAN 1269H (BAS 662H) were 440, 430, 180, and 40 g/ha for yellow foxtail, barnyard grass, wild buckwheat, and common ragweed, respectively. The biologically effective dose (that which provides 90% reduction in weed dry matter) for common lambsquarters was 560 g/ha when SAN 1269H was applied preemergence (PRE) and 110 g/ha when applied postemergence (POST). When applied PRE at a rate of 420 g/ha, pigweed was controlled, whereas only 85 g/ha was required when applied POST. Grain yield increased with dose of SAN 1269H and did not differ with time of application. Temporary crop injury was observed when SAN 1269H was applied at the four- to six-leaf growth stage. Optimum corn yields were achieved with doses of 100 to 250 g/ha.

Greenhouse and field experiments were initiated to evaluate rapeseed and mustard species as green manure crops for weed suppression. Under greenhouse conditions incorporating 20 g fresh wt leaf and stem tissue of rapeseed, two white mustards, and brown mustard into 450 g Sharpsburg, silty clay loam soil resulted in significant reductions in weed emergence, biomass, and height. Kochia, shepherd's-purse, and green foxtail emergences were reduced by all green manure crops. Redroot pigweed emergence was reduced by all green manure crops except brown mustard, and velvetleaf emergence was reduced by white mustards only. Kochia and shepherd's-purse fresh weights were reduced by all green manure crops, while redroot pigweed and velvetleaf fresh weights were reduced by brown mustard and white mustard var. Salvo. Green foxtail fresh weight was reduced by all green manure crops except rapeseed. With the exception of shepherd's-purse, no relationship between glucosinolate content of the incorporated green manure and suppression of weed growth was found. Under field conditions, early spring-planted green manure crops reduced early season weed biomass in soybean at one of the two locations. Mustard species as green manure crops reduced total weed biomass in soybean by 40% 4 weeks after emergence (WAE) and 49% 6 WAE. Soybean biomass and yield were sometimes reduced by the incorporation of green manure crops in treatments containing weeds; however, hand-weeded plots with green manure treatments yielded similar to hand-weeded plots without green manure.

A study was conducted in 1996 and 1997 near Rohwer, AR, to evaluate weed control with standard herbicide programs in nontransgenic and glyphosate-resistant soybean (Glycine max) compared with glyphosate-based herbicide programs in glyphosate-resistant soybean. Total postemergence (POST) programs containing 0.84 kg ae/ha glyphosate 14 d after emergence (DAE) followed by 0.42 kg/ha at 42 DAE controlled pitted and entireleaf morningglory (Ipomoea lacunosa and I. hederacea var. integriuscula) 80 to 83%. This control was lower than with any other weed control program in the study. All glyphosate containing herbicide programs controlled barnyardgrass (Echinochloa crus-galli) at least 94%. Soybean injury was 8% or less for all herbicide treatments except for ‘Hartz 5088 Roundup Ready’ treated with pendimethalin plus chlorimuron and metribuzin preplant incorporated (PPI) followed by glyphosate or acifluorfen plus bentazon 42 DAE. All weed control and soybean injury ratings were similar at 24 d after the 42 DAE application (DAA) when compared with the 10 DAA rating. Two weed control programs with ‘Northrup King S-59-60’ soybean using standard herbicide programs yielded 3,015 and 3,070 kg/ha, and net returns were $92.20/ha and $122.82/ha, respectively. Two production systems containing the Hartz 5088 Roundup Ready cultivar had positive net returns of $35.11/ha and $39.49/ha.

A replacement series study was conducted in a greenhouse in 1998 and 1999 to evaluate the interference interactions among two rice cultivars and two red rice ecotypes. Plants were established in proportions of 3:0, 2:1, 1:2, and 0:3 (rice–red rice) plants/pot. Relative yield of Kaybonnet based on the shoot dry weight was lower than that of KatyRR or LA3, whereas PI 312777 was comparable to that of KatyRR and LA3. These results indicate that Kaybonnet was less competitive than PI 312777 when contrasted with KatyRR and LA3 red rice ecotypes. Kaybonnet (commercial rice cultivar) was dominated by both KatyRR (suspected rice × red rice cross) and LA3 (tall red rice ecotype) in tiller production, whereas PI 312777 (weed-suppressive cultivar) was comparable to either KatyRR or LA3. Both KatyRR and LA3 considerably reduced the leaf area of Kaybonnet. In contrast, PI 312777 reduced the growth of KatyRR, and its leaf area was comparable to that of LA3. The data suggest that high tillering capacity, as demonstrated by PI 312777, should be considered when breeding for rice cultivars that are competitive against weeds. This agronomic characteristic of rice may improve the success of reduced herbicide rate application programs.

Applying glyphosate relative to the growth stage of soybean is important for maximizing weed control and profits in glyphosate-resistant soybean under no-till systems. A study was conducted in Ontario for 4 yr to evaluate the effectiveness and gross return on the timing and sequence of applications of glyphosate in glyphosate-resistant no-till soybean. Percent control of various weed species varied among years due to environmental conditions. Timing of glyphosate was critical relative to weed emergence and determined the success of the treatment in terms of optimum soybean yield and gross return. Soybean yield and gross return approximated that the critical period for weed control in glyphosate-resistant no-till soybean was the unifoliolate to the one- to three-trifoliolate stage. Sequential applications of glyphosate provided higher soybean yield and gross return than a single preplant application of glyphosate. Glyphosate applied preplant or at the unifoliolate stage followed by a second application at the one- to three-trifoliolate stage consistently provided maximum average soybean yield and gross return. Gross return of the sequential glyphosate treatments was also more consistent across variable soybean price scenarios. Competition from uncontrolled later emerging weeds resulted in soybean yield loss with the single preplant application of glyphosate. Competition from uncontrolled early-emerging weeds reduced soybean yields when glyphosate was applied only at the one- to three-trifoliolate stage of soybean. Overall, two weed control strategies were identified: (1) two applications of glyphosate, the first at preplant to the unifoliolate stage, followed by a second application at the one- to three-trifoliolate stage of soybean, (b) first application of glyphosate at the unifoliolate stage followed by a second application at the one- to three-trifoliolate stage of soybean if later emerging weeds exceeded threshold densities.

Field studies were conducted to examine the effects of the sulfonylurea-tolerant (ST) soybean and fall tillage on reducing rotational soybean response to soil-applied prosulfuron across a range of soil pH values. Prosulfuron (10 and 20 g ai/ha) was applied in the fall to simulate the maximum amount of carryover theoretically possible from corn weed control systems. ST soybean reduced effects of prosulfuron soil residues on soybean grain yield. Increased soil pH led to greater prosulfuron carryover as indicated by the differential in ST and non-ST soybean and grain yield responses. Tillage (chisel plow) did not decrease non-ST soybean response to prosulfuron soil residues. Soybean injury 30 d after emergence was well correlated with eventual yield losses in non-ST soybean. Greater soybean injury and yield loss was observed on a silt loam soil with 0.8% organic carbon than on silt loams with greater than 2.5% organic carbon.

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.

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.

This study documents the first case of triazine resistance in wild radish and the resistance mechanism involved. The high survival (57 to 97%) of the resistant (R) biotype progeny plants treated at a rate four times higher than the commonly recommended rate of simazine or atrazine clearly established that the R biotype plants were resistant to triazines. All the plants of the susceptible (S) biotype plants were killed when treated at half the commonly recommended rate of atrazine (0.5 kg/ha) or simazine (0.25 kg/ha). The dry weight of the S biotype was reduced by 89 to 96% at the commonly recommended rate of atrazine or simazine, while the dry weight of the R biotype plants was reduced by only 36 to 54% even when treated at a rate four times higher than the commonly recommended rate of atrazine or simazine. The growth-reduction–ratio values indicated that the R biotype progeny plants were 105 and 159 times more resistant to atrazine and simazine, respectively, than the S biotype plants. Leaf chlorophyll fluorescence yield was reduced by 97% in the S biotype 24 h after application of triazine compared with only 9% reduction in the R biotype, indicating that the resistance mechanism involved is target-site based. The R biotype was effectively controlled by herbicides of different modes of action.

Competition from large crabgrass is often responsible for the establishment failure of spring-seeded cool-season turfgrasses. The objective of this study was to determine how soon after seedling emergence dithiopyr could be applied without causing unacceptable damage to Kentucky bluegrass. Dithiopyr at 0.28 and 0.56 kg/ha was applied at various times prior to seeding and after emergence of Kentucky bluegrass in the field and greenhouse. Dithiopyr damaged Kentucky bluegrass seedlings in the greenhouse when applied earlier than 10 d after emergence (DAE) at 0.28 kg/ha and 14 DAE at 0.56 kg/ha. In the field study with heavy large crabgrass infestation, no significant phytotoxicity or long-term thinning occurred with any application of dithiopyr after emergence of Kentucky bluegrass. Dithiopyr applied as early as 3 DAE improved cover of Kentucky bluegrass by reducing competition from large crabgrass. When seeding is done early in spring before large crabgrass germination, dithiopyr could be applied at 0.28 kg/ha 10 DAE or at 0.56 kg/ha 14 DAE without risk of injury. Dithiopyr could be applied as early as 3 DAE on sites with heavy large crabgrass pressure or to late spring seedings when large crabgrass is germinating because the risk of turf thinning is warranted given the benefit of reduced large crabgrass competition for the desired turf.

Field experiments were conducted in 2001 and 2002 at two sites in Ohio to characterize the effect of isoxaflutole herbicide applied the previous year to field corn on processing tomato, bell pepper, cabbage, snapbean, and cucumber. Isoxaflutole was applied preemergence to field corn in 2001 at 0, 53, 70, 105, and 210 g ai/ha. There were no rotational crop cultivar by herbicide rate interactions at either site. Generally, there was a higher level of visible injury on crops at the Fremont site. Isoxaflutole residues at either site did not affect processing tomato yield. Bell pepper yield was reduced 33% when rotated into 210 g ai/ha rate plots only at Fremont. Snapbean marketable yield was reduced by isoxaflutole carryover from 70 and 210 g ai/ha rates resulting in 0.39 and 0.0 t/ha at Fremont. Similarly, isoxaflutole soil residues from 105 and 210 g ai/ha resulted in 14 and 24% visible injury on cucumber but did not reduce marketable yield. Site differences in soil characteristics and precipitation in the application year may have contributed to observed differences in crop response.

The premix atrazine + S-metolachlor is commonly used to control a wide range of weeds in corn, but it is weak on velvetleaf and several other broadleaf species. RPA 201772, used at reduced rates in combination with atrazine + S-metolachlor, may improve the weed control spectrum. In field studies at Urbana and Dekalb in 1998 and 1999, RPA 201772 was combined with a premix of atrazine at 1,820 g ai/ha and S-metolachlor at 1,408 g ai/ha to compare RPA 201772 rate effects on corn injury, weed control, and yield. RPA 201772 was applied at 0, 26, 53, 78, 105, and 132 g ai/ha early preplant (EPP), preplant incorporated (PPI), and preemergence (PRE). Compared with atrazine + S-metolachlor alone, RPA 201772 combinations did not improve giant foxtail control. However, RPA 201772 at 26 or 53 g/ha increased control of redroot pigweed, common lambsquarters, ivyleaf morningglory, common ragweed, giant ragweed, and velvetleaf in some environments. At Urbana in both years, RPA 201772 at 26 g/ha in combination with atrazine + S-metolachlor increased velvetleaf control 15% compared with the premix alone. RPA 201722 at 26 to 78 g/ha decreased velvetleaf densities with all application methods. Greater weed control and higher corn yield occurred in treatments applied PRE or PPI compared with EPP applications, except where conditions were dry after PRE applications. This research demonstrates the benefit of RPA 201772 at low rates for broadleaf weed control with less potential for crop injury compared with high RPA 201772 rates.

Lima bean cultivar ‘Improved Kingston’ was evaluated for sensitivity to herbicide treatments in a field experiment, conducted from 1999 to 2001 in Ontario. Crop was evaluated for visual injury at 7, 14, and 28 d after emergence following preemergence (PRE) applications of metolachlor (1,600 and 3,200 g ai/ha) and imazethapyr (75 and 150 g ai/ha). Crop visual injury to postemergence (POST) applications of imazamox + fomesafen (25 + 200 g ai/ha and 50 + 400 g ai/ha) and quizalopfop-P (72 and 144 g ai/ha) was evaluated at 7, 14, and 28 d after treatment. Plant height and crop yield were also assessed. The imazamox + fomesafen mixture caused significant visual injury and tended to decrease lima bean height and yield. Despite some initial injury observed in the metolachlor, imazethapyr, and quizalofop-P treatments, yield was not significantly decreased. Because of their margin of crop safety, metolachlor applied PRE at 1,600 g/ha, imazethapyr applied PRE at 75 g/ha, and quizalofop-P applied POST at 72 g/ha have excellent potential as weed management tools in Ontario lima bean production.

Field studies were conducted in 2001 and 2002 in the Black Sea Region of northern Turkey to determine the critical period for weed control (CPWC) in corn and the effects of weed interference on corn height. Treatments of increasing duration of weed interference and weed-free period were imposed at weekly intervals from 0 to 12 wk after crop emergence (WAE). The CPWC was determined with the use of 2.5, 5, and 10% acceptable yield loss levels by fitting logistic and Gompertz equations to relative yield data. With 5% yield loss level, the CPWC was 5 wk, starting at 0.2 WAE and ending at 5.2 WAE, which corresponded to the one- to five-leaf stage of corn. The CPWC increased to 8.9 wk, starting at 0 WAE and ending at 8.9 WAE, at the 2.5% yield loss level. At 10% yield loss level, the CPWC decreased to 1.7 wk, starting at 2.1 WAE and ending at 3.8 WAE.

Field studies were conducted at Fayetteville, Arkansas, to determine whether 47 Palmer amaranth accessions from different areas of the southern United States varied in response to postemergence applications of the registered rates of the isopropylamine salt of glyphosate (840 g ae/ha), fomesafen (420 g ai/ha), and pyrithiobac (70 g ai/ha). Glyphosate controlled all Palmer amaranth accessions at least 99% 21 d after treatment (DAT). Palmer amaranth control with fomesafen was equivalent for all accessions and at least 96% 21 DAT. Percent dry weight reductions were at least 92 and 94% for glyphosate and fomesafen, respectively. Palmer amaranth control with pyrithiobac was variable and ranged from 20 to 94% 21 DAT, but differences could not be attributed to accession origin. Herbicides with alternate modes of action from pyrithiobac should be utilized for Palmer amaranth control in regions where pyrithiobac has been used continuously.

White bean producers have a limited number of herbicide options available for annual grass and broadleaf weed control. Tolerance of two white bean cultivars to preemergence (PRE) applications of S-metolachlor, S-metolachlor + imazethapyr, flumetsulam + S-metolachlor, cloransulam-methyl, clomazone, clomazone + imazethapyr, and clomazone + S-metolachlor at the maximum labeled rate in soybean (1×) and twice the labeled rate (2×) were studied at two Ontario locations (Exeter and Ridgetown) in 2001 and 2002. S-Metolachlor, clomazone, and clomazone + S-metolachlor generally had no negative effect on plant height, dry weight, maturity, and yield. S-Metolachlor + imazethapyr and clomazone + imazethapyr reduced plant height, dry weight, and yield as much as 21, 42, and 24%, respectively. Flumesulam + S-metolachlor and cloransulam-methyl reduced plant height, dry weight, and yield as much as 39, 58, and 43%, respectively. White beans are tolerant to PRE applications of S-metolachlor, clomazone, and clomazone + S-metolachlor. White beans are sensitive to PRE applications of S-metolachlor + imazethapyr, flumetsulam + S-metolachlor, clomazone + imazethapyr, and cloransulam-methyl.

Weed management is a major production issue facing otebo bean growers in Ontario. Field trials were conducted at six Ontario locations during a 2-yr period (2003 and 2004) to evaluate the tolerance of otebo bean to the preplant incorporated (PPI) application of EPTC at 4,400 and 8,800 g ai/ha, trifluralin at 1,155 and 2,310 g ai/ha, dimethenamid at 1,250 and 2,500 g ai/ha, S-metolachlor at 1,600 and 3,200 g ai/ha, and imazethapyr at 75 and 150 g ai/ha. EPTC, trifluralin, dimethenamid, and S-metolachlor applied PPI resulted in minimal (less than 5%) visual injury and with exception of the low rate of dimethenamid causing a 16% reduction in shoot dry weight and the high rate causing an 8% plant height reduction had no adverse effect on plant height, shoot dry weight, seed moisture content, and yield. Imazethapyr applied PPI caused up to 7% visual injury and reduced plant height, shoot dry weight, and yield 8, 18, and 12% at 75 g/ha and 19, 38, and 27% at 150 g/ ha, respectively. Seed moisture content was also reduced by 0.4% with both rates. Based on these results, otebo bean is not tolerant of imazethapyr applied PPI at rates as low as 75 g/ha, the proposed use rate. EPTC, trifluralin, dimethenamid, and S-metolachlor applied PPI have a 2× rate crop safety margin for use in otebo bean weed management.