The Arable Area Payments Scheme of the European Union has been in operation since 1992, paying farmers to set aside a portion of their arable land from production for 1 to 5 yr. Management of the land is prescribed, both in terms of timing and type. The scheme is potentially beneficial to the environment, as set-aside land can be used to improve bird, mammal, and insect habitats. However, environmental and agronomic objectives frequently conflict, particularly in the area of weed management. To predict weed growth and succession, a tool was required to model vegetation at set-aside sites using the key environmental factors, location, and soil that could account for differences in the weed species likely to be invasive. To this end, a Geographical Information System has been developed that links modeled species distributions to a model of successional change. The system has a simple menu structure allowing speedy manipulation of the data and models to predict the effects of management on the weed communities at known geographical locations.

Producers rely on cultural practices to manage downy brome, jointed goatgrass, and feral rye in winter wheat because there are no effective herbicides for in-crop control. This study characterized seedling emergence, growth, and development of these winter annual grasses, with the goal of suggesting or improving cultural control strategies. Feral rye seedlings emerged within 4 wk, whereas downy brome and jointed goatgrass seedlings emerged over a 10-wk period. Emergence patterns of these grasses suggest that delay of winter wheat planting may be effective in reducing feral rye densities, but this strategy most likely will be ineffective with downy brome or jointed goatgrass. Downy brome began anthesis 1 to 2 wk earlier than the other two grasses and winter wheat. Both downy brome and jointed goatgrass were shorter than winter wheat during the growing season, whereas feral rye was at least as tall as wheat. Producers mow infested wheat to prevent weed seed production, but this practice may not be effective with jointed goatgrass and downy brome because of their short stature and downy brome's earlier development. Conversely, mowing has potential in preventing feral rye seed production. The grasses produced between 340 and 770 seeds/ plant.

Jointed goatgrass and downy brome continue to plague winter wheat producers in the western United States. Because there are no effective herbicides for in-crop control of these weeds, producers are seeking cultural practices that stimulate seed germination and deplete the soil seed bank. We determined the effect of limited tillage and crop canopy on seedling emergence of these grasses. One tillage operation with a sweep plow increased jointed goatgrass seedling emergence 74% in the first year but did not affect emergence in later years. Downy brome emergence was not affected by tillage. Jointed goatgrass seedlings emerged over 5 yr, whereas downy brome did not emerge after 3 yr. Seedling emergence of both species was two times greater in corn and barley than in proso millet. Producers will accrue more benefit for seedbank management with cultural strategies such as alternative rotations and competitive wheat canopies than with limited tillage using a sweep plow.

Field experiments were conducted to compare performance of glyphosate with three different boom arrangements in a winter wheat-fallow rotation near North Platte, NE, in 1994 and 1995. One boom was optically controlled, and the other boom was for broadcast herbicide applications. Spraying with both booms at the same time was called “dual boom.” The sprayers were tested during May, June, and July on two weed density levels established by applying glyphosate at 0.42 kg ae/ha with and without atrazine at 0.84 kg ai/ha in October following wheat harvest. The dual-boom and the broadcast herbicide applications were more efficient in controlling weeds than the optically controlled system. The dual boom reduced weed density 4.5-fold compared with the optically controlled sprayer used alone. Horseweed < 8 cm tall was more difficult to control with the optically controlled sprayer than redroot pigweed and kochia because of its cylindrical-shaped growth patterns. Barnyardgrass and green foxtail seedlings with an erect growth pattern were also difficult for the sensors to detect. Poorer control with the optically controlled sprayer was associated with failure to identify small weeds, chlorotic plants, inconsistency among sensors, and too wide a field of view (FOV), as sensors were spaced farther apart than presently recommended. The number of sensors on a boom needs to be increased to improve the performance of the optically controlled sprayer.

Few postemergence (POST) herbicides are available for broad-spectrum weed control in pea. Field experiments were conducted to determine the suitability of imazamox to provide selective control of grass and broadleaf weeds in pea. Imazamox controlled several grass and broadleaf species, but the rate required to attain 90% control ranged from 7 to 36 g ai/ha depending on the weed species. Imazamox efficacy differed with the adjuvant included in the spray mixture. Merge adjuvant often increased imazamox activity on weeds more than Sun-It II methylated seed oil or a combination of Agral 90 adjuvant plus 28–0–0 liquid fertilizer. Pea exhibited excellent tolerance to imazamox up to the highest rate applied, 40 g/ha. Pea yield comparable to that of the hand-weeded control was attained with 20 to 30 g/ha of imazamox. Imazamox offers growers a POST option for broad-spectrum weed control in pea.

Atrazine detection in drinking water has raised questions about how to reduce levels of this herbicide. Field experiments were conducted near Ottawa, KS, to determine atrazine levels in runoff from grain sorghum grown under three tillage systems in three growing seasons. Atrazine at 1.12 kg ai/ha was applied several weeks before planting to areas where no-till (NT), ridge-till (RT), and chisel-disk (CD) systems were used, followed by an additional 0.56 kg ai/ha applied at planting. Surface-water runoff was collected from enclosed metal frames within each tillage system, and total water runoff and atrazine concentration were determined and used to compute total atrazine loss. Atrazine loss was less from CD than from the other tillage systems in years when atrazine was soil incorporated. Atrazine loss was highly variable among years and tillage systems. This suggests that other factors besides till systems influenced atrazine runoff.

Greenhouse and field experiments were conducted to evaluate early postemergence (POST) tank mixtures of SAN 582 with fluazifop-P, imazethapyr, or sethoxydim. In the greenhouse, SAN 582 synergistically improved barnyardgrass, broadleaf signalgrass, and johnsongrass control from imazethapyr and sethoxydim. Half-rates of imazethapyr and sethoxydim tank-mixed with SAN 582 controlled grass weeds as well as full rates of either herbicide applied alone. Grass weed control with imazethapyr increased up to 40% with the addition of SAN 582. In field experiments, SAN 582 increased grass control with imazethapyr to a lesser degree than observed in the greenhouse. In a multispecies study, grass weed control increased up to 15% when SAN 582 was tank-mixed with a reduced rate of imazethapyr, although the full rate of imazethapyr applied POST with or without SAN 582 controlled grass weeds 80% or less. The combination of SAN 582 with sethoxydim was synergistic for barnyardgrass and johnsongrass control in this experiment. When applied POST in soybean, SAN 582 plus fluazifop-P or sethoxydim controlled barnyardgrass throughout the season better than a single POST application of a graminicide.

Field experiments were carried out from 1987 to 1990 to develop an integrated weed control management system for the control of blackgrass in winter wheat in Greece. The weed control treatments included both herbicides and cultural measures. The performance of preemergence (PRE) herbicides against blackgrass was better than postemergence (POST) herbicides; selectivity was dependent on application rate. Late timing of wheat sowing resulted in significant reduction of the blackgrass population competing with wheat. The critical period for blackgrass infestation occurred 4.0 to 5.5 wk after wheat emergence. Blackgrass density of 170 to 1,170 plants/m2 caused wheat yield reduction equal to 10 and 30%, respectively. A quadratic relationship describing the percent reduction of wheat yield due to competition with different blackgrass densities was developed. The economic threshold level for blackgrass varied between 100 and 125 plants/m2. The results emphasize the importance of using a combination of chemical and cultural controls, including critical periods for herbicide application in maintaining an integrated approach for blackgrass control in wheat.

A study was conducted from 1994 through 1996 to evaluate barnyardgrass control, rice yield, and estimated economic returns with delayed preemergence (DPRE) applications of pendimethalin, quinclorac, thiobencarb, pendimethalin plus quinclorac, and pendimethalin plus thiobencarb applied alone or followed by postemergence (POST) applications of propanil or propanil plus molinate. Late-season barnyardgrass control with pendimethalin plus quinclorac was greater than with pendimethalin, quinclorac, thiobencarb, or pendimethalin plus thiobencarb. Control with pendimethalin, quinclorac, and pendimethalin plus thiobencarb exceeded that with thiobencarb. Propanil plus molinate controlled barnyardgrass more effectively than propanil. Quinclorac and pendimethalin plus quinclorac were the only DPRE herbicide treatments where yield and estimated economic returns were not improved by POST herbicides.

Broomrape is a severe problem in the continuous cropping of bidi tobacco in India. Fifteen herbicides were evaluated for postemergence control, at different dates and rates, in field experiments in Nipani, Karnataka State. The effects of the herbicides on Orobanche control and growth and yield of tobacco were compared to the effects of handweeding and an untreated check. Following this screening experiment, four herbicides were selected for further testing. Glyphosate at 500 g ai/ha applied at 60 days after transplanting (DAT) and imazaquin at 10 g ai/ha applied at 30 DAT reduced the number of aboveground broomrape spikes by 75 to 80% and increased the tobacco yields by 80 to > 100% compared to the untreated plots. Imazethapyr and EPTC were found to be less effective for broomrape control, but they still led to substantially higher tobacco yield. None of the herbicide treatments resulted in yields higher than those in the hand-weeded plots. These plots probably did not yet reflect the potential yield of tobacco under broomrape-free conditions, due to nonemerged spikes. Toxicity to tobacco of all herbicides was higher for the higher rates tested. It was lowest with imazaquin.

Purple nutsedge collections from 13 states within the continental United States and from 21 other locations around the world were studied to compare their reproduction and morphological characteristics. Differences occurred in the number of shoots produced from single tubers, the number of leaves per shoot, and the length and width of leaves. Differences were also found with respect to flowering, length of culms supporting the inflorescences, and number, length, and width of involucral bracts at the apex of the culms. Variations occurred in the flower parts, including the number and length of rachises and the length of rachillae and spikelets. There were differences in the growth patterns of the leaves, with some collections having leaves mostly erect and others having leaves lie more closely to the ground. The color of the leaves varied between light and dark green, and the color of the inflorescence varied between light and dark purplish-brown. Greater variation was found among plants from different locations around the world than from within the continental United States.

In greenhouse experiments, translucent polyethylene film mulches reduced or eliminated purple nutsedge shoot emergence compared to a conventional, opaque, white/black polyethylene film mulch. The translucent mulches reduced purple nutsedge shoot biomass, tuber and rhizome number, and tuber biomass 85–99%. In a nonilluminated growth chamber, purple nutsedge shoots emerging from tubers readily penetrated translucent mulch as well as opaque mulch. Shoots emerging in pots illuminated by a mixture of fluorescent and incandescent lighting failed to penetrate translucent mulch but penetrated the opaque mulch. In field experiments, translucent mulches reduced emergence and growth of purple nutsedge 70–88% compared to opaque mulch. Numbers of viable tubers in the soil were reduced 65–76%. In greenhouse and field experiments, the translucent mulches elevated soil and air space temperatures 4–13 C compared to opaque mulch. However, in the growth chamber experiment, air space temperatures were not affected by mulch type, and soil temperatures were only 0.4–0.6 C higher under the translucent mulches. Results suggest that the suppression of nutsedge emergence and growth by translucent mulch cannot be attributed to solarization effects.

Glyphosate was evaluated at 0.8, 1.3, and 1.7 kg ae/ha applied at the two-leaf, four-leaf, or two- to three-tiller growth stage for red rice control. In addition, red rice seedheads were counted concurrently with soybean harvest at each of three locations to assess treatment effect on seedhead reduction. Field studies were conducted at Starkville, MS, in 1994 and 1995 and Shaw, MS, in 1995. A significant rate response was not observed for red rice control 2 and 4 wk after treatment (WAT) or for seedhead reductions. Glyphosate controlled red rice 88, 91, and 88% 2 WAT when applied to two-leaf, four-leaf, or two- to three-tiller red rice, respectively. Due to subsequent seedling emergence, control from glyphosate applications to two- or four-leaf red rice 4 WAT was 51 and 84%, respectively. Red rice treated at the two- to three-tiller stage was controlled 91% 4 WAT. When compared to the nontreated control, seedheads were reduced 97% by two- to three-tiller applications, compared to 87 and 56% reductions from four- and two-leaf applications, respectively.

Weed management strategies are needed for no-till soybean grown on clay soils. The effect of several weed management strategies on weed biomass, soybean yield, and gross return were evaluated in 1993, 1994, and 1995 on clay soils at two locations in southern Ontario. Soybean seeds were planted in narrow (19 cm) and wide (76 cm) rows with or without a rye cover crop. Herbicide treatments included glyphosate alone, glyphosate followed by imazethapyr + metribuzin applied PRE, and glyphosate followed by acifluorfen + bentazon applied POST. Two additional treatments with interrow cultivation were included in the wide-row soybean plots with glyphosate and glyphosate + PRE treatments. A nontreated check plot without rye was also included. Presence of a cover crop did not affect weed biomass or soybean yield. The glyphosate + broadcast PRE treatment provided the most consistent weed control both in narrow- and wide-row soybean. The weed control in this treatment ranged from 92 to 100%. The other treatments provided variable weed control across years and locations. The narrow- row plots with glyphosate + broadcast PRE treatment provided the most consistent soybean yields that were generally higher than the other treatments and ranged from 2,560 to 3,420 kg/ha. Soybean yields varied across locations and years in other treatments. Similar weed control and soybean yields were obtained with banded PRE herbicide + interrow cultivation and PRE treatments; however, herbicide use was 60% lower in banded PRE herbicide + interrow cultivation treatment. Narrow-row soybean averaged 27% higher gross returns than wide-row soybean for all broadcast herbicide treatments. Narrow-row soybean with PRE herbicide provided the highest gross returns. No-till soybean in narrow rows with preplant glyphosate and broadcast PRE treatment was the most risk-efficient weed management system on clay soils.

Field studies on a sandy loam soil determined the influence of application method and rate and subsequent tillage on corn response to imazaquin and imazethapyr residues remaining in the soil. Imazaquin was applied preplant incorporated (PPI) and preemergence (PRE) at 70, 140, and 280 g ai/ha and postemergence (POST) at 70 and 140 g/ha. Imazethapyr was applied PPI and PRE at 70, 105, and 140 g ai/ha and POST at 70 and 105 g/ha. Subsequent tillage included fall moldboard plowing followed by spring field cultivation, fall chisel plowing followed by spring field cultivation, and no tillage. Herbicide dissipation was determined by analyzing soil cores taken in the spring at the time of corn planting and by measuring corn height and grain yield. Imidazolinone residues were detected in only 1 of 2 yr. Imazaquin was detected more frequently than imazethapyr. Imazaquin concentrations in the upper 10 cm of soil 11 mo after a PPI application of 280 g/ha were 5, 6, and 7 ng/g of soil in moldboard plow, chisel, and no-tillage systems, respectively. Imazaquin (5 ng/g) was also detected in the 10–18-cm soil depth in the moldboard plow system. Corn height and grain yield were not reduced from imazaquin or imazethapyr, regardless of application method or rate in any tillage system.

Shattercane seeds were collected in 1992 from 16 fields in southeastern and south-central Nebraska that had been treated for 3 consecutive yr with primisulfuron or nicosulfuron. Approximately 11,200 plants, from a total of 16,000 seeds (1,000 per field) planted, were evaluated for primisulfuron resistance in greenhouse screening. A single resistant plant was found. No other primisulfuron resistance was found in evaluating an additional 1,718 plants from the remaining seeds collected in 1992 from the field where the original resistant plant was found. Shattercane seeds were collected in 1993 and 1994 from that field. All 17,038 plants from these collections were killed by primisulfuron. A field assessment of 118 descendents of the original resistant plant (S1 progeny) produced three responses to primisulfuron at an approximate 1:2:1 ratio: (1) death; (2) some interveinal chlorosis; or (3) no visible injury or symptoms. Discovery of this resistant shattercane biotype indicates the potential for selection of primisulfuron-resistant shattercane populations under field conditions.

Field validation studies were conducted in seven Mississippi environments at three application timings to confirm postemergence (POST) recommendations generated by the computer herbicide decision aids HERB and SWC. HERB and SWC agreed on herbicide treatments in only 14% of the location–application timing combinations. Weed scientists involved in the study agreed on treatment recommendations approximately 33% of the time. The HERB model agreed with a faculty member on only one herbicide treatment, while the SWC model was slightly more agreeable in this regard. Subsequent weed flushes, varied production practices, and delayed weed emergence accounted for a majority of the underestimated predictions given by HERB. Only 55% of the predicted values presented for estimated weed control ratings were similar to actual weed control ratings. Over 75% of the predictions that differed from actual weed control values were underpredictions. Recommendations from both computer models were effective in reducing yield loss below that of the untreated check, and recommendations from the HERB model generally improved yield more than those from the SWC model in most instances. HERB and SWC predictions of yield losses with no weed control were not significantly different from the actual yield loss from the untreated check in nine of the 12 instances at Starkville, seven of the 12 instances at Brooksville, six of the 12 instances at Newton, and three of the six instances at Hollandale. The HERB model estimated yield loss similar to that of the actual yield loss 83% of the time, while predictions from the SWC model were accurate 76% of the time. HERB overestimated yield loss in six of 21 application timing–experiment combinations and underestimated yield loss only once. Yield loss was overpredicted as high as 78%. SWC overpredicted yield loss in five of 21 instances and also underestimated in five instances. SWC did not overpredict yield loss to the same magnitude as HERB in many instances.

Experiments were conducted near Duke and Forgan, OK, on land enrolled in the Conservation Reserve program (CRP) that had been seeded to Old World bluestem (OWB) to evaluate tillage systems for returning CRP grassland to winter wheat production. Glyphosate controlled OWB 72% or less in no-till (NT) wheat. Disk tillage (DT) and moldboard plow tillage (MPT) for wheat seedbed preparation controlled OWB 87 and 99%, respectively, at Forgan and 96 and 100%, respectively, at Duke. At Forgan, OWB control in NT was higher when glyphosate was applied in July than when applied in May. Soil water content to a depth of 120 cm at planting was as high in DT and MPT without herbicide as in NT with 1,680 g ae/ha glyphosate. Within NT and MPT, glyphosate did not consistently increase soil water content compared to the respective nontreated checks. In DT, soil water content to a depth of 120 cm was greater following glyphosate at 1,680 g/ ha than without glyphosate. Wheat density was greater in DT and MPT than in NT. Compared to the NT no herbicide treatment, tillage tripled wheat yields. Wheat yields were often greater where glyphosate was applied before tillage. No-till wheat production immediately after CPR in Oklahoma without prior destruction of accumulated OWB residue does not appear feasible.

The contribution of nitrogen (N) fertilizer placement to management of annual weeds in spring wheat grown under two tillage systems was examined at sites located in three ecoregions of the northern Great Plains from 1994 to 1996. The effect of three different fertilizer placements on narrowleaf and broadleaf weed emergence, growth, and N uptake were determined in a no-till and a conventional tillage system. Fertilizer was broadcast, subsurface-banded in the spring prior to seeding, or side-banded at the time of seeding. Tillage system generally did not influence weed and crop response to fertilizer placement. Broadcast-applied fertilizer was more effective than banded fertilizer in promoting wild oat and broadleaf weed emergence and growth over the growing season. Weed densities, biomass, and N uptake measured early in the growing season averaged 20 to 40% less, and grain yield of wheat at maturity averaged 12% higher where fertilizer was side-banded compared to broadcast. In contrast, the addition of fertilizer, regardless of placement, was detrimental to green foxtail because of enhanced crop competitiveness. Banding fertilizer at recommended rates can be an effective cultural practice for managing weeds in no-till and conventional tillage wheat-cropping systems in semiarid to subhumid regions of the northern Great Plains, but it is not reliable when used as the sole method of weed management.

Field studies were conducted in 1996 and 1997 at three locations throughout southern Ontario with the objective of developing dose-response curves of RPA 201772 for weed control and crop tolerance in corn. The biologically effective doses required to control redroot pigweed, velvetleaf, and wild mustard were 100, 90, and 80 g/ha, respectively. Yellow foxtail was controlled with 100 to 120 g/ha, while rates for common lambsquarters varied from 60 to 130 g/ha, depending on the year and location. Wild buckwheat control was poor (> 30%) at all of the doses tested. RPA 201772 did not reduce corn grain yield; however, temporary crop injury was evident on coarse sandy soils.