Fluazifop-butyl {(±) butyl 2-[4-[[5-(trifluoromethyl)-2-pyridinyl] oxy] phenoxy] propanoic acid}, haloxyfop-methyl {methyl-2-[4-[[3-chloro-5-(trifluoromethyl-2-pyridinyl] oxy] phenoxy] propanoic acid}, and sethoxydim {2-[1-(ethoxyimino)-butyl]-5-[2-ethylthio)propyl-3-hydroxy-2-cyclohexen-1-one}exhibited soil herbicidal activity to annual grass species in greenhouse and field experiments. Control of forage sorghum [Sorghum bicolor (L.) Moench. ‘Rox Orange’] with all three herbicides was greater when seeds were planted at or near the soil surface than when planted 2, 4, or 6 cm deep. Imbibition of the herbicides by forage sorghum and soybean [Glycine max (L.) Merr.] seeds did not reduce germination at concentrations of 10-3 M or lower, but forage sorghum seedling survival and vigor were reduced by herbicide concentrations as low as 10-7 M. Fluazifop-butyl, haloxyfop-methyl, and sethoxydim were more phytotoxic when applied to the root zone than the shoot zone. Haloxyfop-methyl exhibited the longest soil residual control in the field followed by fluazifop-butyl and sethoxydim. Haloxyfop-methyl and fluazifop-butyl controlled simulated shattercane [Sorghum bicolor (L.) Moench. ♯3 SORVU] in soybeans with preemergence and early postemergence applications, whereas control from similar applications of sethoxydim was limited and variable.

Field studies were conducted at Rosemount, MN, in 1992 and 1993 to quantify the demographic processes regulating the population dynamics of velvetleaf in soybean as part of a corn-soybean rotation. A consistent 6.8 ± 0.5% of the total velvetleaf seedbank emerged each year. Less than 21% of all velvetleaf seedlings survived each year in mixture with soybean, due in part to Verticillium spp wilt infection. The probability of seedling survival varied across time of emergence. Velvetleaf seed production in the absence of crop competition was 125 and 227 seeds plant−1 in 1992 and 1993, respectively. Velvetleaf plants that emerged early produced greater numbers of seed than later emerging plants. Velvetleaf survival and seed production were reduced up to 82% in the presence of crop competition. Soybean yield varied across soybean densities in both years, but was not reduced across velvetleaf densities.

In 1985, when weed densities were low (169 plants/m2 in untreated control), imazaquin applied at 0.07 kg ai/ha early preplant controlled over 90% of all weeds before no-till planting of soybeans. In 1986 and 1987 when weed densities were higher (589 plants/m2 in untreated control), addition of 1.1 kg ai/ha or more of metolachlor to imazaquin (0.07 kg/ha) before soybean planting controlled 95% or more of the grass weeds and 83% or more of the broadleaf weeds. Imazaquin plus metolachlor applied less than 1 day after soybean planting controlled less than 70% of the emerged weeds in 1986 and 1987; common lambsquarters was most tolerant. Early preplant treatments controlled more weeds throughout the growing season than treatments applied after planting. Splitting herbicide treatments among application times generally did not increase weed control compared to single applications. Early preplant applications resulted in higher soybean densities and taller soybeans 30 days after planting in 1986 and 1987 than treatments applied after planting. Soybean yields increased as weed control increased. Weed control and soybean yields were greater with early preplant treatments than paraquat plus alachlor plus metribuzin applied preemergence in 1986 and 1987. No carryover of imazaquin residue was detected through corn bioassay in the field.

Field research from 1986 to 1988 evaluated control of alfalfa, dandelion, orchardgrass, and annual weeds in corn with sequential herbicide treatments. Glyphosate applied the previous fall controlled 95% or more of the dandelion and orchardgrass before postemergence treatments. Atrazine applied early preplant or with paraquat the day before planting only partially controlled the perennial species. Atrazine applied postemergence usually controlled vegetation escaping initial treatment except orchardgrass following the atrazine treatments. Adding tridiphane increased weed control with atrazine or cyanazine applied postemergence only when fall panicum was present. Herbicides applied postemergence increased corn yield compared to the initial treatments alone, and few differences among postemergence treatments were observed. Sequential treatments which included glyphosate applied in the fall controlled 82% or more of each perennial species before the next growing season. Orchardgrass control was 81% or less the following spring when the initial treatment was atrazine or atrazine plus paraquat.

Formation and distribution of 14C-atrazine degradation products in the top 120 cm of soil were determined over 16 mo under field conditions in an Estherville sandy loam. After 16 mo, 78% of applied 14C was still present in the soil. By 2 mo after treatment (MAT), 14C had moved to the 30- to 40-cm depth; however, movement to depths greater than 40 cm was not observed. Greater than 98% of the 14C remaining in the soil profile after 16 mo was in the top 20 cm. Twenty-seven percent of the 14C applied was atrazine 16 MAT. Atrazine was the predominant 14C-compound in soil below 10 cm. Hydroxyatrazine (HA) was the major degradation product in the top 10 cm of soil. The proportion of 14C as HA in the top 10 cm increased from 15% 2 MAT to 37% 16 MAT. Deethylatrazine (DEA) was the predominant degradation product at the 10- to 30-cm depth and accounted for up to 23% of the 14C present in the 10- to 20-cm depth. Deisopropylatrazine (DIA) accounted for less than 6% of the radioactivity recovered at any soil depth. The proportion of DEA and DIA increased while the proportion of HA decreased as soil depth increased, indicating that DEA and DIA are more mobile in soil than HA. Detection of HA at depths greater than 10 cm appears to be due to in situ degradation of atrazine previously moved to that soil depth. The large amount of 14C remaining in the soil 16 MAT suggests that a large pool of atrazine and its degradation products are present in the soil for an extended period following application and have the potential to contaminate ground water.

Clomazone at 0.7 kg ai/ha or more, applied early preplant, completely controlled weeds before planting of no-till soybeans. Under low weed density (57 plants/m2 in untreated control) in 1985, grass weed control was nearly complete and not affected by clomazone application time. Late-season broadleaf weed control was less with preemergence application of clomazone at 1.1 or 1.4 kg/ha than with an early preplant or early preplant-preemergence split application of the same clomazone rate. Addition of metribuzin at 0.2 kg ai/ha overcame this control deficiency. Under greater weed densities (330 plants/m2 in untreated control) during 1986 and 1987, early preplant-preemergence split applications gave the greatest control of both grass and broadleaf weeds throughout the growing season. Preemergence application of clomazone failed to completely control common lambsquarters emerged at the time of application. Early preplant applications failed to maintain redroot pigweed control throughout the season. Differences in soybean yield were attributed to differences in weed control. No significant carryover of clomazone residue was detected through greenhouse or field bioassays.

The effect of tillage systems on depth of emergence and densities of giant and green foxtail under different environmental and cropping conditions were evaluated from 1985 to 1987 at Arlington, Hancock, and Janesville, WI. Mean emergence depths in no-till were the shallowest, followed by chisel plow and conventional tillage at each location. At least 40% of the giant and green foxtail plants emerged from the upper 1 cm of soil in no-till compared to about 25% in chisel plow and less than 15% in conventional tillage. As many as 25% of the plants emerged from greater than 4 cm in conventional tillage compared to about 10% in chisel plow and less than 5% in no-till. Seedlings emerged from greater depths in a loamy sand than in a silt loam soil regardless of tillage system. At Arlington, green foxtail was the dominant species in conventional tillage, while giant foxtail dominated in chisel plow and no-till. Foxtail densities were greater in chisel plow and no-till than in conventional tillage at all three locations.

Application time did not greatly influence control of velvetleaf (Abutilon theophrasti Medik. # ABUTH) or common lambsquarters (Chenopodium album L. # CHEAL) in no-till corn (Zea mays L. ‘Pioneer 3747’) with fluorochloridone {3-chloro-4-(chloromethyl)-1-[3-(trifluoromethyl) phenyl]-2-pyrrolidinone}. Giant foxtail (Setaria faberi Herrm. # SETFA) control was reduced as much as 25% by 90 days after planting when fluorochloridone was applied early preplant rather than preemergence. Fluorochloridone at 0.8 kg/ha applied preplant or preemergence gave 83% or greater control of common lambsquarters and giant foxtail for the entire growing season. However, velvetleaf control with the same treatments was 61% or less. Fluorochloridone caused minimal corn injury. Greenhouse bioassay indicated that fluorochloridone may carry over and injure soybean [Glycine max (L.) Merr.] the year after application. Imbibition of fluorochloridone by seed of corn and giant foxtail did not reduce germination at concentrations up to 10-3 M. Giant foxtail seedling fresh weight was reduced 80% following imbibition of 10-5 M fluorochloridone. Corn seedling fresh weight was not reduced by imbibition of up to 10-4 M fluorochloridone.

Formation of 14C-atrazine degradation products and their distribution in the top 90 cm of soil was determined over 16 mo in a Webster clay loam in the field. After 16 mo, 64% of the applied 14C could still be accounted for in the 90-cm soil profile. At 1 mo after treatment (MAT), 14C moved to the 70- to 80-cm depth. Rapid movement of radioactivity could be attributed in part to preferential movement through vertical macropores. Atrazine accounted for 32% of the 14C applied 16 MAT and was the predominant 14C-compound in soil below 10 cm through 12 MAT. Hydroxyatrazine (HA) was the major degradation product in the top 10 cm of soil accounting for 9% of the 14C present 1 MAT and increasing to 24% within 6 MAT. Deethylatrazine (BEA) was the predominant degradation product at depths greater than 10 cm, accounting for 26% of the 14C in the 10- to 20-cm depth 16 MAT. Deisopropylatrazine (DIA) accounted for less than 10% of the 14C recovered at any soil depth. Deethyldeisopropylatrazine (DEDIA) and an unidentified product were detected in soil extracts 1 MAT indicating further degradation past primary metabolites. The proportion of DEA and DIA increased while the proportion of HA decreased as soil depth increased indicating that DEA and DIA are more mobile in soil than HA. The large amount of radioactivity remaining in the soil 16 MAT suggests that a large pool of atrazine and its degradation products are present in the soil for a long period of time, having the potential to move deeper in the soil and ultimately contaminate ground water.

The species composition and density of weed seed in the soil vary greatly and are closely linked to the cropping history of the land. Altering tillage practices changes weed seed depth in the soil, which plays a role in weed species shifts and affects efficacy of control practices. Crop rotation and weed control practices also affect the weed seedbank. Information on the influence of cropping practices on the weed seedbank should be a useful tool for integrated weed management. Decision aid models use information on the weed seedbank to estimate weed populations, crop yield loss, and recommend weed control tactics. Understanding the light requirements of weed seed may provide new approaches to weed management. Improving and applying our understanding of weed seedbank dynamics is essential to developing improved weed management systems. The principles of plant ecology must be integrated with the science of weed management to develop strategies that take advantage of basic plant responses in weed management systems for agronomic crops.

Weed control practices affected the weed seed content of the soil, weed seedling populations, and corn and soybean yields in two 4-yr field experiments. When the initial weed seed density was high, giant foxtail seed numbers in soil declined rapidly under weed-free conditions. Changes in seed densities when initial densities were low varied over years. Mechanical and chemical control treatments had variable effects on weed seed numbers over the course of the experiment. Weed seed numbers and weed densities were sometimes greater if herbicides were banded, but crop yields were similar to full-rate herbicides in both experiments in all years. When the initial density was low, giant foxtail seed numbers in soil increased over the course of the experiment regardless of the weed control treatment. Seed densities of broadleaf species decreased under the same conditions. Also, when initial weed densities were low, weed control and crop yields were less sensitive to weed control practices than at high weed densities. Weather factors that influenced treatment efficacy had a major affect on weed populations and crop yield responses to the weed control treatments over years. The interactions of weed control practices, weed populations, and crop yields were complex. Initial weed density and species composition interacted with weed control treatments and weather patterns to generate the results observed.

WEEDSIM is a bioeconomic decision aid for management of annual weeds in corn and soybean. It was field-tested for 4 yr in Minnesota. The decision aid has two categories of management recommendations: soil-applied plus postemergence (PRE+), based on estimated weed seedbank composition and density; and postemergence (POST), based upon observed weed seedling composition and density. Weed densities, weed control, herbicide use, environmental impact of herbicide use, weed management costs, crop yields, and economic returns that resulted from PRE+ and POST recommendations were compared to those associated with herbicide management systems (HERB) that were standard for the region. After 4 yr of applying WEEDSIM recommendations to the same plots, there were no increases in annual weed densities (seedbanks, seedlings, established plants, or seed production) or decreases in weed control or crop (soybean, rotation corn, and continuous corn) yields, compared to HERB. WEEDSIM recommendations resulted in average annual herbicide applications of 1.1 kg ai ha−1 for PRE+ and 1.0 kg ai ha−1 for POST, compared to 3.5 kg ai ha−1 for HERB. Environmental impact indices associated with PRE+, POST, and HERB were 0.75, 0.71, and 0.54, with the lowest value indicating greater environmental risk than the two higher values. Similarly, average weed management costs were $24, $33, and $77 ha−1 for PRE+, POST, and HERB, respectively. Based on crop prices of $94 Mg−1 for corn and $220 Mg-1 for soybean, the average gross margins over weed control costs were higher for PRE+ ($509 ha−1) and POST ($522 ha−1) than for HERB ($455 ha−1). In general, WEEDSIM appeared to make management recommendations that adequately controlled weeds, maintained crop yields, reduced herbicide use, decreased environmental risk, lowered weed management costs, and increased gross margins over weed control costs compared to the use of herbicides standard for the region.

Glyphosate [N-(phosphonomethyl)glycine] was less phytotoxic when applied in 190 L/ha of well water with ionic strengths of 0.005 or greater than in distilled water. At a carrier volume of 24 L/ha, this inhibition of glyphosate was reduced or eliminated. Glyphosate was less phytotoxic when applied in 10-mM solutions of CaCl2, FeSO4, Fe2(SO4)3, MgSO4, NaHCO3, Na2CO3, and ZnSO4 than in distilled water. Adding H2SO4, HCl, HClO4, acetic, and lactic acids at 5, 10, and 50 mM to the spray solution resulted in variable increases in glyphosate phytotoxicity. H2SO4 was the most effective acid tested in increasing glyphosate phytotoxicity when added to well water. Lowering the pH of the spray solution by using potassium biphthalate and phosphoric acid buffers did not increase glyphosate phytotoxicity.

The effect of different levels of tillage and weed management on population, distribution, and germination of weed seed was evaluated in three tillage systems at Arlington and Hancock, WI, in 1989 and 1990. Over 60% of all weed seed in the top 19 cm of soil were found in the top 1 cm in no-tillage at both sites. As depth increased, concentration of weed seed declined logarithmically in no-tillage. In chisel plowing, over 30% of seed were in the top 1 cm and seed concentration decreased linearly with depth. Moldboard plowing had uniform distribution of weed seed in the top 19 cm of soil. Preemergence metolachlor plus atrazine decreased weed seed population by 50% compared with no treatment over all tillage systems. One year of the herbicide treatment plus handweeding to assure weed-free conditions did not reduce seed numbers in chisel plowing or moldboard plowing compared to herbicide alone. Seed numbers with no-tillage and weed-free conditions decreased by 40% relative to herbicide alone. Common lambsquarters germination was 40% greater in moldboard plowing and chisel plowing compared with no-tillage. Germination was highest in seed taken from 9 to 19 cm deep in moldboard plowing and from 0 to 9 cm deep in chisel plowing.

Ridge truncation methods affected weed populations, weed control, and corn yields in a ridge-tillage production system. Ridges were left undisturbed, partially truncated, deeply truncated, or destroyed by full-width tillage. The effect of truncation on weed densities and control (visual estimates of biomass reduction) varied by weed species, herbicide treatment, and time of observation. Without herbicide, deep truncation resulted in the lowest densities of all species except giant foxtail. Band application of herbicide controlled all weed species as well as broadcast application following deep truncation. When herbicides were broadcast, weed control was usually not affected by ridge truncation. Ridge truncation often increased corn yields compared with no truncation due to better weed control. While deep truncation often resulted in lower weed populations following herbicide treatment than shallow truncation or disking, differences were not great enough to affect corn yields. When ridges were truncated, broadcast herbicide did not increase corn yields compared with banded herbicide.

Additional information on mechanical weed management systems is needed so producers can develop systems that meet their production and weed control goals without sacrificing profitability. Field research was conducted at Rosemount, MN, in 1989, 1990, and 1991 to determine the effect of preplant tillage and soybean planting date on weed populations and effectiveness of mechanical weed control operations. Delaying soybean planting from mid-May to early-June reduced weed densities and yield losses from weeds. Weed control with a herbicide treatment was not affected by planting date, but control with rotary hoeing and cultivation was often increased by delaying soybean planting. Early-planted soybean usually yielded better following herbicide treatment than mechanical weed control. However, when preplant tillage and planting were delayed, weed densities were reduced and mechanical weed control operations usually resulted in soybean yield similar to the herbicide treatment. While delaying soybean planting reduced weed densities and improved mechanical weed control, there is potential for reduced soybean yields with delayed planting.

Experiments were conducted in a growth chamber to determine the effects of planting depth on the developmental sequence of velvetleaf seedlings and the effect of cyanazine placement relative to velvetleaf planting depth on cyanazine toxicity. Velvetleaf seedling emergence was delayed when seeds were planted 6 cm deep compared to the 2- and 4-cm planting depths. Lateral roots did not appear until after the cotyledons had emerged and expanded. This was followed by secondary root emergence from the primary root which occurred before adventitious roots appeared from the hypocotyl for the 2- and 4-cm planting depths. The first true leaf did not unfold until after the secondary root system was well developed. The quantity of adventitious roots on the hypocotyl increased with increasing planting depth. Velvetleaf adventitious roots appeared to be involved in cyanazine toxicity when cyanazine was placed above the seed. Increasing planting depth increased the proportion of seedling absorptive tissue above compared to below the seed. This resulted in increased cyanazine exposure for the deeper planted velvetleaf seeds when herbicide was placed above the seed.

Phytotoxicity of fluazifop-butyl {butyl 2-[4-[(5-trifluoromethyl)-2-pyridinyl)oxy] phenoxy] propanoate}, haloxyfop-methyl {methyl 2-[4-[(3-chloro-5-(trifluoromethyl)-2-pyridinyl)oxy)phenoxy] propanoate}, and sethoxydim {2-[1-ethoxyimino)-butyl]-5-[2-(ethylthio)propyl]-3-hydroxy-2-cyclohexen-1-one} increased as carrier volume was decreased from 570 to 24 L/ha. Addition of crop oil concentrate to the spray solution increased phytotoxicity, especially at carrier volumes of 190 L/ha and above. Sorghum plants were treated with the same amount of active ingredient in a single 1-, 2-, 4-, 8-, 16-, or 24-μl droplet. Smaller, more concentrated droplets were more phytotoxic than larger, more dilute droplets, with or without crop oil concentrate. Application site of single herbicide droplets also influenced phytotoxicity. In general, application of concentrated droplets to metabolically active leaf tissue resulted in maximum phytotoxicity.

Behavior of 14C-haloxyfop-methyl {methyl 2-[4-[(3-chloro-5-(trifluoromethyl)-2-pyridinyl] oxy] phenoxy] propanoate} was examined in intact plants of soybean [Glycine max (L.) Merr. ‘Mead’], shattercane [Sorghum bicolor (L.) Moench. ♯ SORVU], and yellow foxtail [Setaria glauca (L.) Beauv. ♯ SETLU] and in cell cultures of soybean and yellow foxtail. Foliar absorption of 14C-haloxyfo-methyl by intact plants was almost complete within 48 h. Significant translocation occurred within 48 h of treatment, with soybean and shattercane plants translocating more 14C than yellow foxtail. Metabolism of 14C-haloxyfop-methyl in intact plants was similar among species. Haloxyfop-methyl was rapidly hydrolyzed to haloxyfop in treated leaves and then translocated to metabolically active areas within the plants. Significant quantities of polar products were also isolated. Soybean cells in suspension culture contained higher levels of 14C than did yellow foxtail cells. Most of the 14C in soybean cells was haloxyfop-methyl, while yellow foxtail contained mostly haloxyfop.

Horseweed emergence and survival were evaluated in no-tillage soybean and corn at Rosemount, MN, and Ames, IA, from the fall of 1992 through the summer of 1995. Most of the horseweed at both locations emerged in the fall. Winter survival of fall-emerged seedlings ranged from 59 to 91%. Timing and extent of horseweed emergence in spring varied by year and location. Spring emergence ranged from 5 to 32% of total emergence, with greater spring emergence at Rosemount than at Ames. Emergence occurred as late as early June at Rosemount and late May at Ames. Results indicated that horseweed may emerge well into the growing season, and spring emergence should be considered in weed management systems for no-tillage crop production.