A 4-yr field experiment was conducted to determine the merits of combining cultural and chemical controls to manage foxtail barley in reduced-tillage systems. Factors studied were crop row spacing, seeding rate, and application rate and timing of glyphosate within a spring wheat-flax cropping sequence. Glyphosate applied preseeding at 400 or 800 g/ha killed foxtail barley seedlings but only suppressed established perennial plants. Glyphosate applied postharvest at 800 g/ha killed 60 to 70% of established plants. Combinations of preseeding and postharvest glyphosate gave the greatest reductions in foxtail barley biomass and seed production and resulted in the greatest increases in crop yield. Including flax in the rotation allowed use of grass herbicides such as quizalofop or sethoxydim that effectively controlled foxtail barley seedlings and provided some suppression of perennial plants. An increase in wheat seeding rate from 75 to 115 kg/ha reduced foxtail barley growth and increased wheat yield in 3 of 4 yr. Increasing the flax seeding rate from 40 to 80 kg/ha or reducing wheat and flax row spacing from 30 to 20 cm provided little benefit in managing foxtail barley or increasing crop yield. A multiyear approach combining agronomic practices and timely use of herbicides should allow growers to effectively manage foxtail barley in annual cropping systems using conservation tillage.

Full canopy closure and light interception are critical to obtaining full yield potential of ultra-short–season soybean in the midsouthern United States. We hypothesized that herbicide applications that resulted in soybean leaf injury would reduce season-long light interception and yield of ultra-short–season soybean grown in this environment. Experiments were conducted in 2001, 2002, and 2003 at Fayetteville, AR, to determine the effect of the diphenylether herbicides acifluorfen and lactofen on light interception and yield of maturity group (MG) 0 and II soybean. Factors evaluated included soybean MG, herbicide rate, treatment timing, and soybean seeding density. When applied at soybean growth stage (GS) V3, 0.2 kg ai/ha lactofen reduced green leaf area immediately after application and final canopy closure relative to soybean treated with 0.6 or 0.2 kg/ha acifluorfen and untreated soybean. Herbicide application did not affect yield of well-watered soybean when applied at GS V3 in 2001 or at early reproductive development in 2003. In 2002, an irrigation problem resulted in a period of water-deficit stress during seed fill of MG II soybean. Under these conditions, treatment with acifluorfen at GS V3 reduced soybean yield, and treatment with lactofen during early reproductive development reduced soybean yield, relative to untreated soybean. This research indicates that diphenylether herbicides can be safely applied to well-watered ultra-short–season soybean, but yield reduction can occur when applied to soybean that is not well watered.

Glyphosate-resistant (Roundup Ready®) soybean is widely grown to provide broad-spectrum weed control following sequential applications of glyphosate. Studies were conducted during 1998 and 1999 at Keiser, AR, and in 1999 at Pine Tree, AR, to examine the effect of glyphosate-resistant soybean populations on the number of glyphosate applications needed to maintain 90% or greater control of all weed species and on the gross profit margin from weed management. Soybean was seeded in 19-cm rows at 12 rates ranging from 185,000 to 1,482,000 seeds/ha. Each seeding rate received a single application of 0.56 and 1.12 kg ai/ha glyphosate once weeds were 5 to 7 cm tall, followed by repeat applications when control of any species fell below 90%. Soybean yield and gross profit margin at Keiser were similar both years, and a quadratic curve best described the relationship between seed yield and seeding rate, with maximum predicted and observed yield at 988,000 seeds/ha. Soybean seeding rate affected light interception by soybean and the number of glyphosate applications needed to maintain 90% control of all species. By the later stages of vegetative development, soybean at Pine Tree achieved ≥95% light interception, maximizing yield at all seeding rates. Late-season light interception at Keiser ranged from 88 to 99%, causing soybean yield to vary by seeding rate. Three applications of 0.56 kg/ha glyphosate were needed at seeding rates of 185,000 and 247,000 seeds/ha, whereas higher seeding rates sometimes required only a single application for season-long weed control. Application and herbicide costs were offset by reduced seeding costs; thus, predicted gross profit margin from weed management was optimized at the lowest seeding rate of 185,000 seeds/ha.

A 3-yr study was conducted to establish if the presence of corn had an effect on the emergence patterns and total weed seedling density under growing conditions in southwestern Québec. Weed seedling emergence was monitored in permanent quadrats throughout the growing season in the presence and absence of growing corn. Common lambsquarters and barnyardgrass were prevalent in most site-years. The presence of corn did not affect the patterns of common lambsquarters and barnyardgrass emergence nor their total weed seedling density except in 1994. Corn canopy was probably not sufficiently developed to affect light levels or soil temperature needed for weed germination and, consequently, seedling emergence. In 1994, in the absence of corn, some soil crusting was observed on a fine-textured soil, and the total number of seedlings was reduced. The results of these weed emergence studies in corn can be extended to other crops growing with wide row spacing and relatively slow canopy closure similar to those of grain corn.

A field study was conducted to determine the combined effects of row spacing, plant density, and herbicides on weed management and dry bean (Phaseolus vulgaris) yield. In weed-free dry bean, a reduction in row spacing from 69 to 23 cm increased yield by 19% and an increase in density from 20 to 50 plants/m2 increased yield by 17%. In the presence of weeds, narrow rows and high plant densities increased dry bean yield, but without herbicides, yields remained low. However, when combined with herbicides, narrow-row and high-density production practices resulted in better weed control and higher dry bean yield than that attained in a wide-row and low-density production system. Herbicide combinations, often at reduced rates, controlled weeds as well or better than the full rate of any individual herbicide. Ethalfluralin applied preplant incorporated followed by reduced rates of imazethapyr or bentazon postemergence (POST) consistently controlled weeds. Imazamox exhibited the potential to provide a total POST weed control option in dry bean production. Information gained in this study will be used to develop improved weed management programs appropriate for either wide- or narrow-row dry bean production systems.

A field experiment was conducted in 1998 and 1999 at Keiser, AR, to evaluate glyphosate timing and soybean population in reducing hemp sesbania and pitted morningglory interference with dryland and irrigated glyphosate-resistant soybean under a narrow row, no-till production system. Soybean densities following emergence were 247,000, 475,000, and 729,000 plants/ha. Glyphosate was applied at 0.56 kg ai/ha at the V2; V4; V2 and V4; and V2, V4, and R2 stages of soybean. In dry portions of the growing season, glyphosate increased moisture availability for dryland soybean because of control of hemp sesbania and pitted morningglory. As soybean population increased from 247,000 to 729,000 plants/ha, pitted morningglory and hemp sesbania control increased from 60 to 91%, respectively, for the V2 glyphosate application. Control of both species at 14 wk after emergence was at least 90% following the V4 alone treatment and sequential applications, with no differences in control among soybean populations. Untreated, irrigated hemp sesbania produced up to 32 million seeds/ha in 1999. Irrigation did not influence pitted morningglory seed production either year, and untreated pitted morningglory produced a maximum of 1 million seeds/ha in 1998 at 247,000 soybean plants/ha. Three sequential applications of glyphosate reduced pitted morningglory seed production to 9,000 seeds/ha and eliminated hemp sesbania seed production. Soybean yielded 1,297 kg/ha greater under irrigated than dryland conditions, whereas increasing soybean density from 247,000 to 729,000 plants/ha resulted in 416 kg/ha improvement in seed yield. At the densities of pitted morningglory and hemp sesbania present in this study, seed yield of drill-seeded soybean can be maximized following a V4 alone treatment or sequential glyphosate applications.

Greenhouse and field experiments were conducted to investigate the effect of glyphosate rate and degree of glyphosate spray coverage on pitted morningglory control. Pitted morningglory in the two-, four-, and six-leaf growth stages were treated with the isopropylamine salt of glyphosate at 0.28, 0.56, 0.84, 1.12, 1.40, and 1.68 kg ai/ha. Two- and four-leaf plants were controlled 98% with 1.68 kg/ha glyphosate, whereas six-leaf plants were controlled 68%. Control of two-, four-, and six-leaf plants with the commonly used field rate of 1.12 kg/ha was 68, 60, and 50%, respectively. In a separate greenhouse study, four-leaf pitted morningglory plants with 0, 33, 66, or 100% of their total leaf area exposed to herbicide spray were treated with 0.84, 1.68, or 3.36 kg/ha glyphosate. Increasing glyphosate rate from 0.84 to 3.36 kg/ha increased control from 36 to 88%. In contrast, increasing percent leaf exposure to glyphosate from 0 to 100% increased control from 57 to 75%. Increasing glyphosate rate from 0.84 to 1.68 kg/ha always improved control. However, increasing glyphosate rate from 1.68 to 3.36 kg/ha was beneficial only when no leaves were exposed to the spray solution. In the field, glyphosate spray coverage decreased from 85 to 40% as plant density increased from 1 to 32 plants/m2. However, control decreased only 11% (90 to 79%) between the highest and lowest levels of glyphosate spray coverage. These results demonstrated that inadequate control of pitted morningglory with glyphosate was more related to tolerance than glyphosate spray coverage. Glyphosate rates higher than 1.68 kg/ha may be beneficial when spray coverage is severely limited or when plants are beyond the four-leaf growth stage.

Laboratory studies were conducted to determine if electrolyte leakage from either leaf tissue, germinating seeds, or excised roots correlated with previously established soil-applied field response of soybean cultivars and target weeds to sulfentrazone. Sulfentrazone-induced electrolyte leakage from leaf tissue of coffee senna (sensitive), sicklepod (tolerant), and soybean cultivars ‘Asgrow 6785’ and ‘Carver’ (sensitive) and ‘Stonewall’ and ‘DPL 3606’ (tolerant) was monitored over time. Electrolyte leakage from leaf tissues, caused by 25 ppm (65 μM) sulfentrazone, agreed directly with the known response of these weeds, but response of the four soybean cultivars was equivalent. Furthermore, sulfentrazone-induced electrolyte leakage from leaf tissue of Asgrow 6785 and Stonewall was not affected by sulfentrazone concentration as high as 100 ppm (258 μM) nor by light intensity (4 and 120 μmol/m2/s photosynthetically active radiation). For germinating seeds, sulfentrazone-induced electrolyte leakage was also independent of soybean cultivar. In contrast, electrolyte leakage from excised roots of germinal soybean seedlings did concur directly with the previously established cultivar sensitivity to soil-applied sulfentrazone. Results indicate that electrolyte leakage from excised roots of soybean germinal seedlings can be used to assess cultivar sensitivity to soil-applied sulfentrazone.