Field studies were conducted in 2004 and 2005 to determine the effect of soybean row width and population on weeds, canopy closure, crop yield, and economic return in glyphosate-resistant soybean. Soybean leaf area index (LAI) was greater in 19- and 38-cm, compared with 76-cm rows from 8 to 12 wk after planting in the low, moderate, and high soybean populations. Canopy closure was delayed by 2 wk in the moderate population in 76-cm rows compared with the high population in 19-cm rows. Fewer weeds emerged in 19-cm, compared with 76-cm rows following glyphosate application, and increasing the soybean population within a row width did not influence late-season weed emergence. Weed biomass in the weedy control was greater in the very low soybean population compared with the high soybean population within each row width; however, weed biomass in the weedy control was similar in the high and moderate soybean populations. Soybean yield in the weed-free and 10-cm glyphosate treatment did not differ, and yield was greater in 19-cm rows planted at moderate or high, compared with low populations. There was no difference in weed-free soybean yield at low, moderate, and high populations within 38- and 76-cm rows. Gross margins were usually greater in 19- and 38-cm, compared with 76-cm rows. The gross margin for soybean planted in 19-cm rows was usually greater at moderate or high soybean populations compared with lower populations. In 76-cm rows, the gross margin was greatest at the low and moderate soybean populations. When rainfall or other factors limited soybean yield, increasing the soybean population from approximately 300,000 plants/ha to 445,000 plants/ha in 19-, 38-, and 76-cm rows did not result in quicker canopy closure, reduced weed emergence, or greater soybean yield and gross margins.

Field experiments were conducted at two locations in 2003 and 2004 to determine dark tobacco tolerance and weed control from postemergence over-the-top (POT) and postemergence-directed (PD) applications of trifloxysulfuron and halosulfuron. Trifloxysulfuron was applied at 3.6 or 5.3 g ai/ha and halosulfuron was applied at 36 or 53 g ai/ha. Trifloxysulfuron POT injured ‘Narrowleaf Madole’ dark tobacco 16 to 33% and halosulfuron POT caused 32 to 33% injury at 1 wk after treatment (WAT). Tobacco plant height at 1 WAT with POT applications was also reduced by up to 31% at one of two locations. Tobacco injury from POT applications had dissipated to 6 to 12% by 4 WAT. Despite extensive early-season injury, most POT herbicide applications did not significantly reduce tobacco yield, mean grade index, and gross revenue compared to tobacco that only received pretransplant applications of sulfentrazone plus clomazone. Dark tobacco was much more tolerant to PD applications of either herbicide, with no more than 4% injury observed at 1 WAT. Neither herbicide controlled horsenettle more than 57% at either application rate or method. Although trifloxysulfuron and halosulfuron could contribute to tobacco weed control programs by providing postemergence control of several common weed species, PD applications would be the only acceptable method of application due to excessive injury observed from POT applications of these herbicides.

Sulfometuron was applied at 0.1, 0.15, and 0.2 kg ai/ha over the top of oak seedlings. Three separate field studies were completed. In 1997, 0.1 and 0.2 kg ai/ha was applied PRE over six oak species. In 1998 and 1999, 0.1 and 0.15 kg ai/ha were applied PRE and 0.1 kg ai/ha was applied POST over two species of oaks. Results demonstrate that first-year survival of oak seedlings is greater in areas which receive competition control. Survival was 21 to 44% greater in treated areas as compared to nontreated areas. Observations indicate that survival differences are increased in droughty years. Competition control appears essential to obtaining desirable levels of survival when oak seedlings are planted in areas with established herbaceous competition.

Quinoclamine has been evaluated for POST control of liverwort in nursery crop production. Nearly all previous research has assumed that high spray volumes (e.g., > 935 L/ha) were required for quinoclamine to be effective. To test this assumption, quinoclamine was applied to liverwort in a factorial treatment arrangement of three rates (1.4, 1.9, and 3.8 kg ai/ha), three spray volumes (374, 1,112, and 1,871 L/ha), and two spray pressures (276 and 414 kPa). Control was influenced primarily by quinoclamine rate, although there was a trend for greater control with higher spray pressure. Absorption and translocation studies using 14C-quinoclamine established that 14C absorption into liverwort thalli approached 70% of the amount applied within 9 h after application. Although liverwort lacks vascular tissue, 14C is readily translocated away from the site of entry and tended to accumulate at thallus margins. Neither absorption nor translocation was influenced by spray volume. The perceived requirement for high spray volume may be a misdirected assumption on the basis of the high proportion of inert ingredients within the current formulation. Conversely, this assumption has no basis within liverwort biology or quinoclamine behavior.

Transgenic, herbicide-resistant cultivars and equipment to spindle-pick 38-cm rows has renewed interest in narrow-row cotton production. Field experiments were conducted at four locations in North Carolina during 2004 and 2005 to evaluate weed management systems in glufosinate-resistant cotton planted in 38- and 97-cm rows. Weeds included broadleaf signalgrass, goosegrass, fall panicum, large crabgrass, Palmer amaranth, smooth pigweed, pitted morningglory, and tall morningglory. Greater than 90% control of annual grasses and Amaranthus spp. in 2004 and Ipomoea spp. in both years was obtained in narrow-row cotton with glufosinate applied early POST (EPOST) and mid-POST (MPOST) to two- and six-leaf cotton, respectively. With good early-season control by glufosinate and rapid canopy closure, there was little benefit from pendimethalin, fluometuron, or pyrithiobac applied PRE, S-metolachlor or pyrithiobac mixed with glufosinate applied MPOST, or trifloxysulfuron applied late POST (LPOST) to 11-leaf cotton in 2004. In 2005, with larger weeds at initial application, glufosinate EPOST and MPOST did not adequately control annual grasses and Amaranthus spp. Pendimethalin PRE increased control to greater than 90% and increased yields 59 to 75%. Pendimethalin PRE followed by S-metolachlor or pyrithiobac mixed with glufosinate at MPOST was no more effective than pendimethalin alone. Without PRE herbicides, trifloxysulfuron applied LPOST increased Amaranthus but not annual grass control. Cotton row spacing had no effect on cotton yield and little effect on weed control.

In 2004 and 2005, field research was conducted in Idaho to compare tank mixtures of flumioxazin at the 53 g ai/ha potato use rate with comparable tank mixtures of rimsulfuron at 26 g ai/ha for broad-spectrum weed control. Flumioxazin in two-way tank mixtures with metribuzin, EPTC, pendimethalin, S-metolachlor, or ethalfluralin provided greater than 90% hairy nightshade control, which was comparable with control by similar rimsulfuron two-way tank mixtures. Flumioxazin plus metribuzin or rimsulfuron were the only two-way mixtures with flumioxazin consistently providing 90% or greater redroot pigweed, common lambsquarters, and green foxtail control. Control of these weeds by any of the rimsulfuron two-way mixtures was almost always greater than 90%. Three-way tank mixtures containing flumioxazin or rimsulfuron controlled hairy nightshade, redroot pigweed, and common lambsquarters similarly, and control ranged from 89 to 100%. When metribuzin was not included with flumioxazin in three-way mixtures, control was 80 to 97% and not always comparable with the 89 to 100% control by similar rimsulfuron mixtures. Green foxtail control by flumioxazin or rimsulfuron three-way mixtures usually was similar and greater than 90%.

Field trials were established in Fayetteville, AR, to determine the effects of glyphosate on biomass and seed production of spurred anoda, entireleaf morningglory, hemp sesbania, and Florida pusley. Inconsistent control of these species by glyphosate allows them to enrich the soil seedbank. Glyphosate rates of 0.42, 0.84, and 1.68 kg ae/ha were applied at 3, 6, and 9 wk after emergence (WAE) and were compared with a nonsprayed control. Spurred anoda seed production was reduced 99.9% by glyphosate at 1.68 kg/ha averaged over application timings and 99.7% by glyphosate applied at 6 WAE averaged over rates. Entireleaf morningglory seed production was reduced 100% by glyphosate at 1.68 kg/ha at 3 WAE and > 93% by the same rate applied at 6 or 9 WAE. Hemp sesbania seed production was reduced 100% by glyphosate at 1.68 kg/ha at 3 WAE and > 96% by glyphosate at 0.42, 0.84, and 1.68 kg/ha applied at 6 or 9 WAE. Florida pusley seed production was reduced 99.9% by glyphosate at 1.68 kg/ha averaged over application timings and 99.4% by application at 3 or 6 WAE averaged over rates. Biomass accumulation for all species was reduced most by early applications, particularly at 1.68 kg/ha.

Field studies were conducted in Fayetteville, AR, to determine the response of 38 pitted morningglory accessions to fomesafen, chlorimuron, and glyphosate when applied postemergence over-the-top (POT) at 9 g ai/ha, 420 g ai/ha, and 840 g ae/ha, respectively, to four-leaf, 15-cm-tall pitted morningglory. Visual control following chlorimuron application ranged from 82 to 95% 3 wk after treatment (WAT). Visual control of accessions with fomesafen ranged 34 to 84% 3 WAT. Variability in visual control following fomesafen application was documented among accessions collected from similar geographic locations in west-central and southwest Arkansas, central and southeast Louisiana, and west-central Mississippi. Glyphosate controlled pitted morningglory accessions 81 to 89% 3 WAT. Data indicate that weed management programs should be field specific where fomesafen use is intended.

Annual grasses constitute a major weed problem in winter annual crops in Northern Europe and especially in cropping systems where ploughing is omitted. At the optimum growth stage for control with POST herbicides, grasses have a predominantly vertical leaf orientation. This represents a very difficult spray target using the standard technique where nozzles are mounted more or less vertically downward. In this study, efficacy of the foliar-acting herbicide, haloxyfop, on perennial ryegrass at the two- to three-leaf stage was investigated in field experiments using some alternative configurations of nozzle mounting on the sprayer. Angling the spray either forward or backward relative to the direction of travel increased herbicide efficacy using standard commercially available flat-fan and pre-orifice nozzles. Efficacy increased generally with increasing angling relative to vertically downward and the forward-angled spray improved efficacy most. The largest improvement in efficacy was obtained using a 60° forward-angled spray in combination with a reduced boom height. Using this configuration, herbicide dose could be reduced by approximately 30% without loss of efficacy in comparison with the standard vertical mounting of nozzles. There was no advantage of using combinations of forward- and backward-angled nozzles.

Dallisgrass response to various MSMA and foramsulfuron herbicide combinations was evaluated from 2004 through 2006 in North Carolina. Dallisgrass control declined with herbicide rates; therefore, only the highest rates (foramsulfuron, 0.15 kg ai/ha; MSMA, 2.5 kg ai/ha) are discussed. Foramsulfuron followed by (fb) foramsulfuron 1 wk after initial treatment (WAIT) provided 60% control of dallisgrass 1 mo after initial treatment (MAIT), but control declined to 40% 3 MAIT. MSMA (2.5 kg ai/ha) applied three times provided 89% dallisgrass control 1 MAIT, but control declined to 71% 3 MAIT. Among 22 MSMA and foramsulfuron sequential application programs, MSMA fb foramsulfuron 2 WAIT fb MSMA 3 WAIT provided the highest control (94%) of dallisgrass 1 MAIT, and control levels remained high (93%) 3 MAIT. This herbicide program provided 85% control 1 yr after initial treatment (YAIT), whereas no other treatment provided greater than 37% control 1 YAIT. Timing of foramsulfuron application following the initial MSMA application affected the control of dallisgrass. Waiting 2 WAIT, rather than 1 WAIT, before applying foramsulfuron increased control of dallisgrass by 20% 1 MAIT and 19% 3 MAIT. However, the increase in control 1 YAIT was only 5% regardless of herbicide rate. The addition of a second application of MSMA following the application of foramsulfuron 2 WAIT of MSMA further increased control by 10% 1 MAIT, 37% 3 MAIT, and 48% 1 YAIT. Results from this study suggest dallisgrass may be controlled with applications of MSMA fb foramsulfuron 2 WAIT fb MSMA 3 WAIT applied during early to midsummer. The use of MSMA was required to achieve dallisgrass control, however, the relatively high level of control achieved 1 YAIT with a program including sequential applications of MSMA and foramsulfuron may help reduce the total amount of arsenical herbicides used to control dallisgrass infestations over time.

Field studies were conducted in 2002 and 2005 to evaluate autumn vegetable tolerance to residual herbicides applied the previous spring under low-density polyethylene (LDPE) mulch. Spring applications of 1.12 kg/ha S-metolachlor, 0.027 kg/ha halosulfuron, 0.28 kg/ha sulfentrazone, and 1.12 kg/ha S-metolachlor plus 0.027 kg/ha halosulfuron were made under LDPE mulch in March of each year and included a nontreated control. After removal of the spring crop, vegetables were planted the following August. Seeded and transplanted squash, seeded cucumber, transplanted eggplant, and transplanted cabbage were evaluated. Injury to eggplant, cucumber, and transplanted and seeded squash ranged from 8 to 16% for halosulfuron, sulfentrazone, and S-metolachlor plus halosulfuron in 2002, but no injury was observed in 2005. Cabbage injury was less than 5% for any herbicide treatment either year. There were no differences for cabbage biomass for three harvests for any herbicide treatment relative to the nontreated control. Vine length for cucumber and transplanted squash was significantly reduced by sulfentrazone relative to the nontreated control. Eggplant yield for the first harvest was significantly reduced by sulfentrazone as compared with the nontreated control in 2002 but not in 2005. To avoid injury to rotational crops, growers should read all herbicide labels when considering spring herbicide applications under LDPE mulch when autumn vegetable plantings are part of their production scheme to ensure successful crop production.

Field research was conducted in 2002 and 2003 to determine the effect of twin- and single-row spacing and POST glyphosate application timing on light interception, weed control, and grain yield of glyphosate-resistant corn and soybean. Row spacing did not affect light interception measured 10 to 11 wk after planting. Corn grain yield in 2002 was 1.0 Mg/ha higher in single rows compared with twin rows when averaged over glyphosate timing, but was unaffected by row spacing in 2003. Soybean grain yield was similar in 19- and 38-cm single rows, and single-row grain yield was 0.2 to 0.4 Mg/ha higher than the twin-row spacing. Corn grain yields were similar to the weed-free control when glyphosate was applied to weeds 10 to 15 cm tall in 2002 and 10 cm tall in 2003. Soybean yield was maximized by application of glyphosate to weeds 15 to 30 cm tall in 2002 and 60 cm tall in 2003.

Four field studies were conducted in 2004 to evaluate corn tolerance, weed control, grain yield, and net returns in glufosinate-resistant (GUR), glyphosate-resistant (GYR), imidazolinone-tolerant (IT), and nontransgenic (NT) corn with various herbicide systems. No significant differences between hybrid systems were observed for weed control. Limited corn injury (< 5%) was observed for all herbicide treatments. A single early POST (EPOST) system without S-metolachlor and sequential POST over the top (POT) herbicide systems, averaged over corn hybrids and PRE and late POST-directed (LAYBY) herbicide options, provide 93 and 99% control of goosegrass, respectively, and at least 83 and 97% control of Texas panicum, respectively. A single EPOST system without S-metolachlor, averaged over corn hybrids and LAYBY treatment options, provided at least 88% control of large crabgrass. When averaged over corn hybrid and PRE herbicide options, a sequential POT herbicide system alone provided at least 98, 99, 98, and 100 control of large crabgrass, morningglory species, Palmer amaranth, and common lambsquarters, respectively. The addition of ametryn at LAYBY to a single EPOST system without S-metolachlor was beneficial for improving control of morningglory species, common lambsquarters, and Palmer amaranth, depending on location. However, the observed increases (7 percentage points or less) are likely of limited biological significance. Grain yield was variable between hybrids and locations because of environmental differences. Consequently, net returns for each hybrid system within a location were also variable. Any POT system with or without ametryn at LAYBY, averaged over corn hybrid and PRE herbicide options, provided at least 101, 97, 92, and 92% yield protection at Clayton, Kinston, Lewiston, and Rocky Mount, NC, respectively. Net returns were maximized with treatments that provided excellent weed control with minimal inputs.

Viable horseradish roots of various sizes remain in the soil after harvest and can develop into volunteer plants in subsequent crops. Experiments were conducted to evaluate POST herbicides on volunteer horseradish control and to determine if efficacy is dependent upon horseradish root segment size, herbicide rate, horseradish cultivar, or horseradish shoot size at application. In the greenhouse, horseradish root segment size did not affect herbicide efficacy. Chlorimuron, cloransulam, imazamox, (2,4-dichlorophenoxy)acetic acid (2,4-D) amine, halosulfuron, and imazethapyr plus imazapyr provided greater than 95% foliar control of volunteer horseradish. Chlorimuron, halosulfuron, and 2,4-D amine were also among the herbicides that provided the greatest reduction in horseradish root biomass (69% or greater). Glyphosate provided little foliar control (76%) and root biomass reduction (57%) after one application. The efficacy of 2,4-D amine on horseradish foliage and root biomass increased with increasing herbicide rate; however, the response of horseradish to halosulfuron was similar for all rates evaluated. Root biomass reduction of the horseradish cultivar ‘1573’ was less responsive to 2,4-D amine and halosulfuron applications compared with ‘1038’ and ‘1722’. However, foliar injury from 2,4-D amine and halosulfuron was less for the horseradish cultivar ‘1038’ compared with ‘1573’ and ‘1722’. In field studies, 2,4-D amine applied to 15- and 30-cm-tall horseradish and halosulfuron applied to 15-cm-tall horseradish resulted in the greatest foliar and root biomass reduction. This study indicated that in-season control of volunteer horseradish in rotational crops may be achieved through proper herbicide selection.

Field experiments were conducted to evaluate the influence of three nitrogen fertilizer application timings on corn and giant ragweed dry weight, corn and giant ragweed nitrogen accumulation, and corn grain yield. By the V8 growth stage, corn plants in treatments which included nitrogen at planting accumulated up to 27% more dry weight and 26% more leaf area than corn in sidedress-only treatments that had not yet received any fertilizer nitrogen. Nitrogen fertilizer application timing did not influence early-season giant ragweed dry weight, but did influence N accumulation. Giant ragweed at 0.5 plants/m2 accumulated up to 16 kg N/ha in 40-cm-tall plants by the V8 corn growth stage and up to 104 kg N/ha when allowed to remain in corn until the end of the growing season. At this density of giant ragweed, corn yields were not reduced if removed by the V8 corn growth stage. However, season-long interference reduced corn yields by up to 19% regardless of nitrogen application timing. Early-season giant ragweed growth was less responsive to nitrogen application timing than corn, suggesting that nitrogen management programs should not be altered significantly to influence giant ragweed competitiveness in corn. However, giant ragweed should be removed in a timely manner to minimize nitrogen accumulation in nitrogen-limiting corn production situations.

Much research has evaluated herbicide safety on established bermudagrass turf, but information is lacking on seedling response to herbicides. Three field experiments were conducted in Blacksburg, VA, to assess turfgrass and smooth crabgrass response to selected sulfonylurea herbicides during seedling bermudagrass establishment. Herbicides injured bermudagrass 9 wk after treatment in the following order from most to least injurious: flazasulfuron = trifloxysulfuron > rimsulfuron > metsulfuron = sulfosulfuron > foramsulfuron. Herbicides applied 3 wk after seeding (WAS) were generally more injurious than when applied 1 WAS. Flazasulfuron and trifloxysulfuron-sodium controlled smooth crabgrass greater than 90% at 6 WAS, and metsulfuron and rimsulfuron suppressed smooth crabgrass 76 and 84%, respectively. Foramsulfuron, metsulfuron, and sulfosulfuron appear safe to apply 1 and 3 WAS, causing very little chlorosis and no reduction in turf cover. Rimsulfuron is injurious when applied at those timings; however, long-term cover is not reduced. Flazasulfuron and trifloxysulfuron-sodium are not safe to use 1 and 3 WAS unless weed pressure is extreme, and a delay in bermudagrass cover is acceptable.

Path rush can become problematic on golf course fairways, especially on roughs where continuous golf cart or maintenance equipment traffic occurs. Research trials conducted in 2003 and 2004 compared the efficacy of 2,4-D and mixtures containing 2,4-D in single or sequential applications to herbicides without 2,4-D for path rush control in bermudagrass roughs. Sequential applications of 1.1 kg ae/ha 2,4-D or 0.8 + 0.2 + 0.1 kg ae/ha 2,4-D + MCPP + dicamba provided 95% path rush control with no injury to bermudagrass, 15 wk after initial treatment (WAIT). Single applications of 1.1 + 0.3 + 0.1 kg/ha 2,4-D + MCPP + dicamba or 1.1 kg/ha 2,4-D provided at least 81% path rush control, 15 WAIT. Treatments without 2,4-D did not provide acceptable levels of control (34% or less, 15 WAIT). 2,4-D applied alone or in preformulated mixtures provided excellent path rush control compared to treatments without 2,4-D. Sequential applications of these treatments provided greater path rush control than single applications. However, only the sequential application of 0.8 + 0.2 + 0.1 kg/ha 2,4-D + MCPP + dicamba provided greater path rush control than the single application. Results from these studies suggest that 2,4-D is a key herbicide for path rush control.

Conservation tillage systems, used successfully by cotton producers on the Texas southern High Plains, have facilitated the development of new weed problems including horseweed (Conyza canadensis) and Russian thistle (Salsola iberica). Studies were conducted in 2004 and 2005 near Lubbock, TX to evaluate winter weed control with (2,4-dichlorophenoxy)acetic acid (2,4-D), dicamba, and diflufenzopyr plus dicamba. All of these herbicides have current registration restrictions limiting their use in cotton. Cotton response studies were initiated in 2003 and repeated in 2004 and 2005 to evaluate cotton injury and yield from dicamba (0.14 and 0.28 kg ai/ha), diflufenzopyr plus dicamba (0.10 and 0.20 kg/ha), and 2,4-D (0.56 and 1.12 kg/ha) applied 4, 2, and 1 wk before planting (WBP); and to determine the minimum interval between application of these herbicides and cotton planting without affecting yield. 2,4-D controlled both horseweed and Russian thistle, and could be applied as close as 2 WBP without injuring cotton. However, dicamba as well as diflufenzopyr plus dicamba were less effective on horseweed and Russian thistle as size increased and both injured cotton regardless of interval between application and planting.

A 3-yr field experiment was initiated in 2001 to evaluate weed suppression and sweetpotato productivity in three organic sweetpotato production systems. Organic systems were (1) compost and no cover crop with tillage (Org-NC), (2) compost and a cover crop mixture of hairy vetch and rye incorporated before transplanting (Org-CI), and (3) compost and the same cover crop mixture with reduced tillage (Org-RT). A conventional system with tillage and chemical controls (Conv) was included for comparison. Suppression of monocot and dicot weed density and biomass was similar between Org-NC and Org-CI each year, and were frequently similar to Conv. Org-RT was as effective as Org-NC in controlling dicot weed density and biomass each year, but did not suppress monocot weeds. At sweetpotato harvest, an increase in cover crop surface residue biomass in Org-RT was associated with a decrease in cumulative total weed density (R 2 = 0.43, P = 0.0001); however, the amount of that residue was insufficient to suppress late-emerging monocot weeds. Total sweetpotato yield in Org-RT was at least 45% lower than other systems in 2002 and was most likely due to an increase in monocot weed density and biomass concurrent with a decrease in sweetpotato vine biomass. Total sweetpotato yield was similar among all systems in 2001 and 2004; the exception was lowest yields obtained in the Org-RT system in 2002. Organically managed sweetpotato with or without an incorporated cover crop produced sweetpotato yields equal to conventionally managed systems despite difficulties controlling weeds that emerged later in the season.

Clomazone is an effective herbicide widely used for PRE grass control in rice. However, use of clomazone on sandy textured soils of the western Texas rice belt can cause serious rice injury. Two field experiments at three locations were conducted in 2002 and 2003 to determine the optimum rate range that maximizes barnyardgrass and broadleaf signalgrass control and minimizes rice injury across a wide variety of soil textures and planting dates. At Beaumont (silty clay loam), Eagle Lake (fine sandy loam), and Ganado (fine sandy loam), TX, PRE application of 0.34 kg ai/ha clomazone applied to rice planted in March, April, or May optimized barnyardgrass and broadleaf signalgrass control and rice yield while minimizing rice injury. Data suggest that, although injury might occur, clomazone is safe to use in rice on sandy textured soils.