Growth chamber and greenhouse studies were conducted to evaluate effects of long-term low-light intensity on wild oat control with imazamethabenz and fenoxaprop. Seventy percent shading imposed during the entire experimental period resulted in enhanced activities for both herbicides applied at early and later growth stages. Such shading also reduced wild oat regrowth following application of imazamethabenz and fenoxaprop. When applied to plants exposed to 70 to 90% prespraying shading, both herbicides had phytotoxicity similar to, or better, than plants grown under continuous shading. Postspraying shading has less effect on herbicidal activity than prespraying shading or prolonged shading, especially with imazamethabenz. Full-light treatment more adversely affected fenoxaprop activity than imazamethabenz activity.

Seeds of fringed sage were collected in four different years in the Northern Mixed Prairie and placed in sealed vials in the field for up to 5 yr. The seeds were exhumed monthly and incubated at 10 or 20 C in light or darkness. Germination was greater at 10 than 20 C and greater in light than darkness. The overriding influences of light and temperature generally decreased as seeds aged. No well-defined dormancy/nondormancy or conditional dormancy/nondormancy cycles were revealed. Longevity of seeds varied among collections, ranging from 3½ to more than 5 yr. Patterns in germination of different collections were similar when they experienced the same annual changes in temperatures, indicating that germination of previously buried seeds of fringed sage was regulated by temperature. This regulating effect varied among years of storage; germination either increased or decreased with increasing temperatures, or was not affected. Results suggest that fringed sage seeds can germinate and emerge anytime during the growing season if temperature and moisture conditions are suitable. Continuous germination may enable fringed sage to occupy safe sites as they develop over time.

Redroot pigweed is a major weed in corn throughout Ontario. Field experiments were conducted at two locations in 1991 and 1992 to determine the influence of selected densities and emergence times of redroot pigweed on corn growth and grain yield. Redroot pigweed densities of 0.5, 1, 2, 4 and 8 plants per m of row were established within 12.5 cm on either side of the corn row. In both years, redroot pigweed seeds were planted concurrently and with corn at the 3- to 5-leaf stage of corn growth. A density of 0.5 redroot pigweed per m of row from the first (earlier) emergence date of pigweed (in most cases, up to the 4-leaf stage of corn) or four redroot pigweed per m of row from the second (later) emergence date of pigweed (in most cases, between the 4- and 7-leaf stage of corn) reduced corn yield by 5%. Redroot pigweed emerging after the 7-leaf stage of corn growth did not reduce yield. Redroot pigweed seed production was dependent upon its density and time of emergence. The time of redroot pigweed emergence, relative to corn, may be more important than its density in assessing the need for postemergence control.

Field experiments conducted from 1987 through 1991 at the Delta Branch Experiment Station, Stoneville, MS, determined the effect of postemergence topical applications of fluometuron and MSMA on cotton yield and fruiting. Herbicide treatments applied to cotton in the cotyledon to 1-leaf growth stage were 1.12 kg ai ha-1 fluometuron plus 0.25% by vol nonionic surfactant, 2.2 kg ai ha-1 MSMA, and a combination of 1.12 kg ha-1 fluometuron and 2.24 kg ha-1 MSMA. The commercial formulation of MSMA contained nonionic surfactant. Visual injury of cotton 14 d after treatment ranged from 14 to 28% for fluometuron, 9 to 26% for MSMA, and 22 to 34% for the combination. Seed cotton yield at the first harvest was reduced by all herbicide treatments except in 1987 and for DES 119 cotton in 1990. In 1990, yield of DES 119 cotton treated with MSMA and the combination was less than the untreated control for the second harvest interval. Cotton yield from herbicide treatments at later harvests was equal to or greater than the untreated control. Only in 1988 did all treatments reduce total seed cotton yield below the untreated control. Fiber quality was not adversely affected by the herbicides. Compared to the control, plant mapping indicated that the number of sympodial branches per plant was reduced by MSMA and by the combination for DPL 50 cotton in 1990. Averaged over 4 experiment years, herbicide treatments did not reduce total bolls per plant; percentage of bolls in first, second, or outer positions; highest sympodium with two bolls; or number of sympodium with bolls in the first or second position. However, MSMA and the combination increased node number of the first sympodia by one and 1.5 positions, respectively, when compared to the control, indicating delayed maturity.

Greenhouse studies were conducted to determine the effect of phosphorus (P) fertility on intra- and interspecific competition between lettuce and spiny amaranth for 4 wk after emergence. Total lettuce shoot biomass per pot and weight per plant increased 39 and 44% in response to increased P fertility, respectively. P fertility had no impact on growth of spiny amaranth. Total shoot biomass of spiny amaranth increased with increasing density from four to eight plants, however, lettuce did not. Total shoot biomass of both species increased as density increased from 4 to 16 plants. Spiny amaranth, but not lettuce, weight per plant decreased in response to intraspecific competition. Reciprocal yield analysis showed that spiny amaranth produced 2.4 times more biomass than lettuce when competing intraspecifically and four times more biomass under interspecific competition. Lettuce weight per plant was not affected. Relative yield analysis indicated that spiny amaranth was more competitive than lettuce regardless of P fertility. However, increased P fertility increased competitiveness of lettuce. Relative crowding coefficients indicated that spiny amaranth at the low density with low P fertility was 33 times more competitive than lettuce. Addition of P caused lettuce and spiny amaranth to be equally competitive at the lowest density; however, at the highest density, spiny amaranth was 4 tunes more competitive than lettuce regardless of additional P.

Potato response to simulated drift of imazamethabenz, imazethapyr, or imazapyr was evaluated in field studies conducted near Aberdeen, ID, in 1989 and 1990. Herbicides were applied at rates corresponding to 0.02, 0.1, and 0.5 times the typical use rate (x rate) of each herbicide. Simulated drift treatments were applied at potato emergence, tuber initiation, or tuber bulking. Foliar injury symptoms varied with herbicide, rate, and potato growth stage at the time of application, but were generally most severe with imazapyr and least severe with imazamethabenz. Yield losses also varied with herbicide, rate, and potato growth stage at the time of application, but generally were greater when drift occurred at tuber initiation or tuber bulking than at potato emergence. Losses in U.S. Number 1 (highest quality) yield were greater than losses in total yield, indicating that tuber quality was affected more by the herbicides than was tuber biomass accumulation. U.S. Number 1 yield was not reduced by simulated drift of imazamethabenz at 0.02x but was reduced 25% by the 0.1x rate applied at tuber bulking. At the 0.5x rate, U.S. Number 1 yield was reduced 36, 85, or 92% when drift occurred at potato emergence, tuber initiation, or tuber bulking, respectively. For imazethapyr, U.S. Number 1 yield losses ranged from 0 to 68% for the 0.02x rate; 19 to 98% for the 0.lx rate; and 64 to 100% with the 0.5x rate, depending on potato growth stage at the time of application. Losses were least when drift occurred at potato emergence. Simulated drift of imazapyr at the 0.02x rate applied as potatoes were emerging reduced U.S. Number 1 yield 79%. All other imazapyr treatments caused ≥99% loss in U.S. Number 1 yield.

Tank-mix combinations of PRE and POST herbicides were applied to control emerged large crabgrass and goosegrass in common bermudagrass turf. Large crabgrass control was better when MSMA at 2.2 kg ai ha-1 was tank mixed with either pendimethalin at 1.7 kg ai ha-1 (≤ 85%) or dithiopyr at 0.3 kg ai ha-1 (≤ 94%), when compared with MSMA at 2.2 kg ai ha-1 (≤ 50%) alone. When MSMA plus metribuzin at 2.2 + 0.14 kg ai ha-1 was tank mixed with pendimethalin in 1992, large crabgrass control was better (≥ 96%) than when MSMA plus metribuzin was applied alone (≤ 73%). Quinclorac applied alone at 0.8 kg ai ha-1 controlled ≥ 94% large crabgrass, but control with diclofop was 32%. There was no increase in control when either quinclorac or diclofop was tank mixed with any PRE herbicide. Methyl ester of diclofop applied alone at 1.1 kg ai ha-1 provided 97% control of goosegrass at one location, but control was ≤ 62% at two other locations. Goosegrass control was better when diclofop at 1.1 kg ha-1 was tank mixed with dithiopyr at 0.6 kg ha-1 (≥ 77%) than when either diclofop (≤ 62%) or dithiopyr (≤ 50%) was applied alone. Goosegrass control was consistently better across years from combinations of dithiopyr plus diclofop at 0.6 + 1.1 kg ha-1 (82%) than from combinations of MSMA plus metribuzin with dithiopyr (34%) or pendimethalin (53%). Common bermudagrass injury at Griffin-Spalding Golf Course in 1991 was higher at 1 wk after combination treatments of MSMA plus metribuzin with dithiopyr (46 to 54%) or pendimethalin (50%) than with MSMA plus metribuzin applied alone (26%). Injury from the combinations was higher from single MSMA plus metribuzin treatment for 4 wk. Similar results occurred at the Georgia Station in 1992 from combinations of dithiopyr with MSMA plus metribuzin.

Greenhouse studies were conducted to determine the response of purple and yellow nutsedge to selective placement of 5 cm of soil treated with AC 263,222 above and/or below nutsedge tubers. AC 263,222 applied early POST (EPOST) or POST at 71 g ai ha-1 as a foliar, soil, or foliar + soil application also was evaluated. AC 263,222 applied below nutsedge tubers did not reduce purple or yellow nutsedge shoot number, shoot dry weight, shoot regrowth dry weight, or root-tuber dry weight. However, when it was applied above purple nutsedge tubers, shoot dry weight was reduced; and when it was applied above yellow nutsedge tubers, shoot dry weight and root dry weight were reduced. AC 263,222 applied 5 cm above + 5 cm below purple nutsedge tubers reduced shoot number, shoot dry weight, shoot regrowth dry weight, and root dry weight to 9, 4, 10, and 16% of the control, respectively, in purple nutsedge and to 23, 16, 9, and 15% of the control, respectively, in yellow nutsedge. AC 263,222 applied EPOST or POST reduced shoot dry weight, shoot regrowth dry weight, and root dry weight of purple nutsedge to less than or equal to 11, 7, and 27% of the control, respectively, and to less than or equal to 10, 16, and 29% of the control, respectively, for yellow nutsedge with no differences between application methods. The foliar and soil activity of AC 263,222 for purple and yellow nutsedge control provides an advantage over currently registered peanut herbicides.

The combined influence of plant competition and defoliation on development of Canada thistle was investigated in a 2-yr field study. Plant competition was induced by seeding tall fescue and crown vetch. Artificial defoliation was used to simulate various levels of leaf removal by insects. Both defoliation and induced competition reduced biomass of Canada thistle but their impact varied with environmental conditions. Defoliation had a greater detrimental influence than induced competition on thistle biomass in a dry year when growth of the plant competitors was suppressed. In a wet year, induced competition was more important in suppressing Canada thistle than defoliation, and moderate levels of defoliation (25%), applied once when the thistles were 12 to 15 cm in diam, stimulated root weight within the top 20 cm of soil. Reduction in thistle biomass increased with level of defoliation and was greatest when defoliation was applied repeatedly at 14-d intervals in the presence of induced competition. Crown vetch showed very little growth in one season and tall fescue was the primary source of competition for the thistles. The results confirm the hypothesis that combined stresses can substantially reduce biomass development of Canada thistle plants.

Field experiments were conducted to determine the effect of DPX-PE350 on the efficacy of fluazifop-P, sethoxydim, and quizalofop. DPX-PE350 severely antagonized the activity of fluazifop-P on large crabgrass. Antagonism of sethoxydim and quizalofop was dependent on rate of graminicide and large crabgrass stage of growth at application. DPX-PE350 did not reduce control of johnsongrass with quizalofop. Sethoxydim alone controlled 36 to 71% more johnsongrass than sethoxydim mixed with DPX-PE350 at 35 to 105 g ai ha-1. Antagonism of fluazifop-P activity was dependent upon the rate of DPX-PE350 and fluazifop-P applied. Applying DPX-PE350 3 d after fluazifop-P or sethoxydim reduced antagonism. Grass species, growth stage, rate, and sequential application timing all appear to play a role in the severity of antagonism.

Greenhouse experiments were conducted to determine the effects of drought stress length and relief on velvetleaf growth and bentazon efficacy with crop oil concentrate (COC) and 28% urea ammonium nitrate (UAN). Drought stress caused leaf water and osmotic potentials to decline linearly over time. Leaf water potential ΨL) declined 0.02 and 0.08 MPa day−1 for velvetleaf subjected to −0.03 and −0.4 MPa soil water potential (ΨP), respectively. Osmotic potential (ΨO) declined 0.02 and 0.21 MPa day−1 for velvetleaf subjected to −0.03 and −0.4 MPa ΨP respectively. Bentazon did not reduce drought-stressed compared to non-drought-stressed velvetleaf dry weight 21 d after treatment with either COC or no adjuvant but did decrease dry weight with UAN. Reduced velvetleaf dry wights in drought-stressed velvetleaf compared to non-drought-stressed velvetleaf could not be accounted for by increased bentazon penetration in the drought-stressed plants. Rewatering velvetleaf subjected to 7 d of −0.4 MPA ΨP caused them to be more sensitive to bentazon than velvetleaf subjected to −0.03 MPa ΨP except when UAN was the adjuvant.

Effects of root-shoot interference, soil water regimes, and soil nitrogen were evaluated to determine aggressivity for pea relative to mayweed chamomile in the greenhouse using replacement series experiments. Interference between pea and mayweed chamomile occurred mainly below ground, and soil water was more important than soil nitrogen in controlling the outcome of interference. Pea was a stronger competitor than mayweed chamomile under all conditions investigated. Leaf area, root weight, and shoot weight of mayweed chamomile were reduced 55 to 87% by full interference and 27 to 60% by root only interference from pea. Full interference from mayweed chamomile for 53 days after planting released the remaining pea from intraspecific interference and it grew 80% larger and produced 100% more pod weight compared to a pea in monoculture. Low soil water potential (–175 kPa) compared to high water potential (–33 kPa) reduced the height, leaf area, and shoot weight of pea in 1:1 mixtures, but had no effect on mayweed chamomile. Decreasing soil water increased the aggressivity of mayweed chamomile relative to pea. Nitrogen added at 20 mg wk-1 compared to no added nitrogen had no effect on peas, but more than doubled the size of mayweed chamomile in monoculture and in mixtures with pea. Added nitrogen did not increase the aggressivity of mayweed chamomile relative to pea.

Greenhouse experiments were conducted to evaluate the effect of carbon chain length and percent ethoxylation of primary linear ethoxylated alcohols on postemergence control of johnsongrass and quackgrass with primisulfuron. Control of johnsongrass and quackgrass was greatest with primisulfuron plus the experimental ethoxylated alcohols 1412-60 and 1412-70 which had chain lengths of 14 and 12 carbon atoms (60:40 ratio) and 60 or 70% ethoxylation. Control of these two weed species decreased when primisulfuron was applied with ethoxylated linear alcohols of similar chain lengths but decreased percent ethoxylation or with alcohols of similar percent ethoxylation but decreased carbon chain length. Quackgrass and johnsongrass control with primisulfuron plus 1412-60 or 1412-70 was equal to or greater than the control obtained with primisulfuron plus the commercial adjuvants of the nonionic, organosilicone, sunflower methyl ester, or crop oil concentrate types. The experimental adjuvant 1412-70 provided equal or greater uptake and translocation of 14C-primisulfuron in johnsongrass and quackgrass compared to four commercial adjuvants.

Research was conducted to determine the effect of starch encapsulation on soil dissipation and weed control with alachlor, metolachlor, and atrazine on an Estherville sandy loam in the field. Starch encapsulation increased persistence of alachlor in the surface 15 cm of soil compared to the emulsifiable concentrate formulation during the first 60 d after treatment More alachlor was detected 30 to 75 cm deep with emulsifiable concentrate than the starch-encapsulated formulation 30 d after treatment. Little alachlor was detected below 15 cm thereafter. Starch encapsulation also increased persistence of metolachlor in the surface 15 cm, but reduced concentrations at 15 to 30 cm compared to the emulsifiable concentrate 30 d after treatment. By 120 and 340 d after treatment, metolachlor concentrations at 15 to 30 cm were greater with starch-encapsulated than emulsifiable concentrate formulation. Starch encapsulation greatly increased atrazine persistence in the surface 15 cm compared to the dry flowable formulation. Although encapsulation increased atrazine concentration in the surface 15 cm, it reduced the concentration below 15 cm compared to the dry flowable formulation 30 and 60 d after treatment However, by 120 and 340 d after treatment, starch encapsulation often resulted in greater atrazine concentrations below 15 cm than the dry flowable formulation. Control of green foxtail and redroot pigweed with starch-encapsulated herbicides was similar or superior to the commercial formulations.

A 2-yr field study was conducted to determine effects of posttreatment irrigation timing on pendimethalin efficacy and dissipation in turfgrass. Factors investigated included herbicide rate, formulation, and the interval between pendimethalin application and the initial posttreatment irrigation. Plots received an initial posttreatment irrigation of 1.25 cm 0, 7, 14, 21, and 28 d after treatment. Pendimethalin efficacy on smooth crabgrass was evaluated, and turfgrass foliage and the upper 2.5-cm layer of soil were periodically assayed for pendimethalin residues. Pendimethalin 1.71% granular provided better weed control than pendimethalin 60% wettable powder at all rates, irrigation events, and years. Efficacy of granular pendimethalin was not affected by a delay in posttreatment irrigation, whereas efficacy of pendimethalin in the wettable powder formulation was reduced when irrigation was applied later than the day of treatment. Chromatographic analyses indicated that an average of 54% of the applied pendimethalin (wettable powder formulation) was retained on turfgrass foliage immediately after treatment, compared to 9% for the granular formulation. Soil residue analyses confirmed that a greater proportion of applied pendimethalin reached the soil surface immediately after treatment in the granular formulation than in the wettable power formulation.

Experiments were conducted in the greenhouse and field to investigate differential tolerance among tall, pitted, red, entireleaf, and ivyleaf morningglory species to DPX-PE350. Tall morningglory in the greenhouse was more tolerant of DPX-PE350 at 7 and 14 g ai ha-1 applied POST than were other species of morningglory. Increased tolerance of tall morningglory also was observed under field conditions with DPX-PE350 applied PRE and POST. Adjuvants did not increase biomass reduction of tall morningglory by DPX-PE350 applied POST.

Laboratory studies were conducted to determine the influence of degradation and sorption processes on the dissipation of alachlor in one Colorado soil (Kim clay loam) and three Minnesota soils (Port Byron silt loam, Webster silty clay loam, and Estherville sandy loam) as a function of soil depth. Persistence and movement of alachlor in an irrigated corn production system also were determined on the Kim soil. Laboratory degradation data fit first-order kinetics, and rate constants ranged from 0.0094 to 0.0251 d-1 and varied with soil type and depth. For instance, in 60- to 75-cm-depth Kim soil, alachlor degraded at a slower rate (k = 0.011 d-1) than in surface soil samples (k = 0.022 d-1). Alachlor sorption to the four soils was moderate (Kf = 0.7 to 7.4; Kf,oc = 71 to 470) and concentration dependent (1/n < 1.0). Significant hysteretic desorption of alachlor from soils also was observed (1/n desorption < 1/n sorption). The combined effect of degradation and sorption processes has been used to classify a chemical's potential to leach to groundwater. Based on Kf,oc and dissipation half-life, alachlor would be classified as a “leacher” in Kim, Port Byron, and Estherville soils and classified as transitional between “leacher” and “nonleacher” in the Webster soil. The dissipation first-order rate constant (k) of alachlor in Kim soil in the field was 0.036 α 0.012 d-1. Dissipation was apparently not due to leaching since bromide applied at the same time remained in the top 15 cm during the first 28 d. It appears that laboratory-derived leaching indices may overestimate actual leaching and should be used with caution for predictive or regulatory purposes.

The influence of moisture stress intensity and duration, as well as tank mixtures with pendimethalin, which mitigated the antagonistic effects of drought, on fenoxaprop efficacy were evaluated. In growth chamber tests, specific soil matric potential (Ψm) levels of −0.01 to −1 MPa were imposed and maintained using a polyethylene glycol semipermeable membrane system. Moisture stress was alleviated by irrigation 48 or 96 h after herbicide treatment. Fenoxaprop efficacy was influenced by moisture stress intensity, posttreatment stress duration, and application rate. Under conditions of mild stress (Ψ ≥ −0.2 MPa), both increased application rate and irrigation 48 h after treatment alleviated the effects of drought on fenoxaprop efficacy. Under moderate stress (Ψm = −4.4 MPa), the antagonistic effects of drought were alleviated by a combination of increased rate and irrigation 48 h after treatment However, under more severe stress (Ψm ≤ −0.8 MPa), irrigation coupled with increased rate did not increase crabgrass control above 50%. Where irrigation within 48 h of treatment is not feasible, tank mixes of fenoxaprop with pendimethalin were shown to enhance smooth crabgrass control. Field lysimeter tests indicated that under moisture stress, Ψm of −0.7 MPa, smooth crabgrass shoot dry weight was reduced 65 to 94% with tank-mix combinations of ≥ 0.27 kg ai ha−1 fenoxaprop plus ≥ 2.2 kg ai ha−1 pendimethalin. These rate combinations were shown to be synergistic and able to alleviate the antagonistic effect of moisture stress on fenoxaprop efficacy.

A 3-yr field study was conducted to compare an in-row cultivator versus a standard row-crop cultivator to decisions made with WEEDCAM, a weed/corn management computer decision aid, for controlling annual weeds within the row in irrigated corn. In the absence of herbicides, weeds were always controlled better with the in-row cultivator than with the standard row-crop cultivator. However, grain yield and gross margin were affected only in 1991 when weeds emerged simultaneously with corn, and rain delayed the first cultivation 10 d. The in-row cultivator plots not only averaged 34% more grain ha-1 than the standard row-crop cultivator plots, but gross margin was $143 ha-1 more. Weed densities each year were about 95% less in plots managed in accordance with the computer model WEEDCAM simulations than in the non-herbicide treated post-planting tillage plots. Grain yields and gross margins were not affected by weed seedbank density, pre-cultivation tillage, or type of cultivator when weed management decisions were based on WEEDCAM simulation ranking. In the absence of herbicides, weeds can be controlled successfully in corn with an in-row cultivator, but success will depend on such factors as weed seedbank density, cultivation timeliness, and relative time of weed and corn emergence.

Surface microrelief and agrichemical placement influence agrichemical movement through soil. This study determined if anhydrous ammonia shank placement influenced atrazine movement in a ridge tillage system. Atrazine and Br− were broadcast over the plot or banded on the ridge while N fertilizer was Injected into the ridge or valley. Rainfall (17 cm) was applied and leachate was collected incrementally from grid lysimeters placed 75 cm below the soil surface. Amounts of atrazine and Br− in leachate and postrainfall soil atrazine distribution were determined. The injection slot created by fertilizer application influenced chemical movement. When atrazine and the fertilizer injection slot overlapped, atrazine movement through the soil was enhanced. These data suggest that herbicide applications and fertilizer injection slots that are not covered should not overlap due to increased potential for movement.