Laboratory assays were conducted to determine the sensitivity of tomato and tobacco cell suspension cultures and tomato and pepper cotyledons to clomazone. A comparison of fresh weight and carotenoid content indicated up to a three-fold difference between the clomazone-tolerant tobacco and clomazone-susceptible tomato cell suspension cultures. In contrast, an approximate 60-fold difference between the tolerant pepper and susceptible tomato cotyledons was observed when total chlorophyll and carotenoid contents were measured. The effect of clomazone and its possible metabolites on in vivo and in vitro extractable IPP isomerase (EC 5.3.3.2) and prenyltransferase (EC 2.5.1.29) activity was investigated. There was no clear inhibitory effect of clomazone or possible clomazone metabolites upon enzyme activity in tomato or tobacco cell suspension cultures or on light or dark grown tomato or pepper cotyledons. No specific enzymatic target site of clomazone was identified in correlation with the reduction in total chlorophyll or carotenoid content.

This paper reports on a microtitre plate version of the 2,6-dichlorophenol-indophenol (DCPIP) reduction assay. DCPIP reduction rates of alfalfa thylakoid membranes were determined by measuring absorbance at 600 nm before and after a 1 min illumination period. The membranes were near light-saturated at intensities above 600 μE m–2 s–1. Saturation of thylakoid membranes with DCPIP occurred above 120 μM. DCPIP reduction rates increased linearly with chlorophyll concentrations from 0.25 to 2 μg. The rate of DCPIP reduction was linear throughout a 2 min illumination period (R2 = 0.99). The assay was sensitive enough to terbacil to differentiate between a 20% change in the I50 concentration. DCPIP reduction rates were sensitive to concentrations of terbacil as low as 100 nM. It only takes 13 minutes to load and read 96 samples using the microtitre plate assay compared to 5.5 h using the conventional procedure.

A 2-yr field study was conducted at Fayetteville, AR, to determine the effect of Palmer amaranth interference on soybean growth and yield. Palmer amaranth density had little effect on soybean height, but soybean canopy width ranged from 77 cm in the weed-free check to 35 cm in plots with 10 plants m–1 of row 12 wk after emergence. Soybean yield reduction was highly correlated to Palmer amaranth biomass at 8 wk after emergence and maturity, soybean biomass at 8 wk after emergence, and Palmer amaranth density. Soybean yield reduction was 17, 27, 32, 48, 64, and 68%, respectively, for Palmer amaranth densities of 033, 0.66, 1, 2, 333, and 10 plants m–1 of row. Soybean yield reduction and Palmer amaranth biomass were linear to approximately 2 Palmer amaranth m–1 of row, suggesting intraspecific interference between adjacent Palmer amaranth is initiated at Palmer amaranth densities between 2 and 3.33 plants m–1 of row.

Field trials were carried out in 1989 and 1990 at St-Augustin, near Québec City, Canada, to determine the critical periods of quackgrass control in potato submitted to three levels of infestation. Potato yield losses due to quackgrass interference increased with quackgrass infestation and length of duration of interference. Quackgrass interference influenced marketable tuber yields more than total tuber yields. Duration of the critical period varied depending on the level of quackgrass infestation and year. Based on an arbitrary 5% level of marketable yield loss, the critical period started at ca. 15 days after emergence (DAE) of potato at low level of infestation, and at ca. 3 DAE at medium level of infestation. At high level of infestation, the critical period began prior to the emergence of potato. The end of the critical period of quackgrass removal was extremely variable across quackgrass infestation level and year and ranged from 23 to 68 DAE of potato at a 5 % yield loss level. It appears that onset of interference varied less than the end of it, indicating that early quackgrass control is necessary to prevent yield loss.

Greenhouse experiments showed that yellow nutsedge shoot number and shoot and root dry weights were reduced by root-knot nematodes and chile peppers. Root-knot nematodes increased and chile peppers decreased the number of yellow nutsedge tubers. Yellow nutsedge tuber germination was reduced by chile peppers but not by root-knot nematodes. Yellow nutsedge established from root-knot nematode-infected tubers produced more tubers than noninfected tubers. Root-knot nematode populations became established on yellow nutsedge root systems when plants were established from tubers previously cultured with root-knot nematodes. Metolachlor stunted chile peppers, eliminated yellow nutsedge, and influenced root-knot nematode populations through reduction of host plant root mass. However, when root-knot nematodes were present, yellow nutsedge tuber germination was not affected by metolachlor. This research indicates that the pests do not exist independently and that their management may be interrelated.

Field research was conducted from 1987 to 1991 to evaluate the influence of four weed management systems on weed population and species dynamics in conventional-tillage and no-tillage grain sorghum production. These weed management systems included zero, low, medium, and high input systems. The weed seedbank increased faster in zero and low input weed management systems than in the high input weed management systems because of differences in weed control. Tillage influenced weed seed densities as well. Common ragweed, common lambsquarters, horseweed, and sicklepod seed densities often were greater in no-tillage than conventional-tillage plots. Common cocklebur and large crabgrass seed densities were usually greater in conventional-tillage than no-tillage plots. Smooth pigweed seed densities were not affected by tillage. Increasing weed management inputs diminished differences between tillage systems. Smooth pigweed dominated the weed populations after 4 yr in both tillage systems even in high input systems.

Kochia pollen dispersion was measured during 24 and 48 h periods from a kochia population in an 8- by 10-m area in the center of a 1.6 ha fallow field. Pollen counts from traps at 50- and 100-cm heights declined rapidly with increasing distance from the pollen source. Pollen deposition was highest along the prevailing wind direction: up to 23 pollen grains cm–2 were recovered 50 m from the pollen source along the southeast (SE) vector. Nonlinear regression analysis of pollen deposition along the SE vector was used to estimate that 99.9% of shed pollen would be deposited within 154.4 m of the source. Viability of pollen from greenhouse- and field-grown plants was measured using staining and germination assays. of four pollen stains tested, only 1,2,3-triphenyl tetrazolium chloride gave consistent results and did not stain heat-killed pollen. Depending on environmental conditions, kochia pollen remained viable from less than 1 d to 12 d. Length of kochia pollen viability was shortest under high temperatures (22 and 28 C) and low relative humidity (7 and 32%). Less than 0.5% germination was observed in 1.1% agar media with various additions; however, up to 17.8% germination was observed after incubation at 28 C in 100% relative humidity.

Field studies were conducted from 1988 through 1990 in Fayetteville, AR, to measure competitiveness of spotted spurge in cotton. Population levels were established by transplanting seedlings from other field areas. There was a positive linear relationship between cotton boll numbers and seed cotton yield. Percent reduction in cotton height, leaf area, dry weight, boll numbers, and seed cotton yield increased as spotted spurge densities increased. Seed cotton yield reduction was 47, 57, and 85% for spotted spurge densities of 5, 10, and 50 spotted spurge plants m–1 of row.

Field studies were conducted to determine the influence of phosphorus (P) fertility and method of application (banded versus broadcast) on the competitive interaction of lettuce and spiny amaranth. Weed density significantly affected lettuce head weight and spiny amaranth shoot biomass after 5 wk of interference. Weed density and duration of interference had little or no effect on P content of lettuce tissue. Duration of interference did affect P concentration of spiny amaranth; however, weed density did not Spiny amaranth competition reduced lettuce yield, but P was not the limiting factor. Duration of interference and method of P application interactively affected lettuce head weight; however, only duration of interference affected spiny amaranth biomass. Seven wk of interference caused a decrease in lettuce head weight of 20, 8, and 24% when P was broadcast, banded, or not applied, respectively. Banding of P reduced the negative impact of spiny amaranth on lettuce. Although method of P application influenced the interaction between lettuce and spiny amaranth, interspecific competition between the two species probably was not due to competition for P but some other factor.

Experiments were conducted to evaluate the influence of temperature and water potential on water uptake, germination, and emergence of large crabgrass in order to predict emergence in the field. Water uptake of seed soaked in polyethylene glycol solutions of 0 to −1400 kPa underwent an initial imbibition phase followed by a lag phase and subsequent increase in water content when radicles emerged from the seed. Maximum germination at 15 C was 12% at 0 kPa and 60% at 25 C at 0 to −200 kPa osmotic potential. In the growth chamber, large crabgrass emergence from soil began 2 to 3 d after planting at 30 or 35 C and within 9 to 10 d at 15 C. Maximum emergence of 77 % occurred at 25 C and at a soil water potential of −30 kPa. Emergence percentage decreased as water potential decreased or as temperature increased or decreased. A logistic equation described emergence of large crabgrass at each combination of temperature and soil water potential at which emergence occurred, and a predictive model was developed and validated by field data. In the field, there was little or no emergence at soil temperatures below 15 C or water potentials below −50 to −60 kPa. The model predicted the time of onset of large crabgrass emergence and the time to reach maximum emergence to within 2 to 4 d of that recorded in field experiments. The model also predicted the correct number of flushes of emergence occurring in the field in three of four experiments.

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.

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.

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.

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.

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.

Field studies conducted at six locations in Georgia and one location in Virginia evaluated imazethapyr and imazethapyr mixtures for weed control, crop tolerance, and peanut yield. Imazethapyr applied early postemergence controlled bristly starbur, coffee senna, common cocklebur, Ipomoea species, jimsonweed, prickly sida, and smallflower morningglory at least 91% and controlled yellow and purple nutsedge 88 and 98%, respectively. Paraquat plus bentazon applied early postemergence did not control the aforementioned weeds as well as imazethapyr or imazethapyr mixtures. Paraquat applied with imazethapyr reduced bristly starbur control 15% compared to imazethapyr alone but did not influence control of the other species. Imazethapyr control of bristly starbur was not improved by the addition of bentazon. Sicklepod control was less than 24% with imazethapyr and was at least 58% with imazethapyr plus paraquat Imazethapyr plus paraquat controlled sicklepod better than paraquat plus bentazon at three of the four locations evaluated. Imazethapyr did not control Florida beggarweed, while imazethapyr plus paraquat controlled at least 53%. Peanut injury was minimal 30 d after application for all treatments.

Field experiments were established to evaluate the effects of various band widths left open directly over the soybean row to improve soybean tolerance to AC 263,222 and chlorimuron applied POST, while maintaining weed control. Drop tubes with even fan tips extended below the spray boom to within 40 and 30 cm from the ground to obtain 5 and 10 cm open bands at the soybean terminals. AC 263,222 applied broadcast at 13, 18, and 36 g ai ha-1 injured soybean 14, 16, and 32%, respectively, 3 DAT. AC 263,222 at 36 g ha-1 applied with a 5-cm and 10-cm open band injured soybean 11 and 3%, respectively. Chlorimuron injury was 12% or less for all treatments. Metribuzin followed by AC 263,222 or chlorimuron was not more injurious to soybean than AC 263,222 or chlorimuron alone. Metribuzin PRE followed by 36 g ha-1 AC 263,222 applied in a 10-cm open band controlled sicklepod and pitted morningglory 86 and 92%, respectively, 6 WAT. Sicklepod control was similar with open-band treatments compared to the same rate applied broadcast; however, 18 g ha-1 AC 263,222 applied in a 10-cm open band provided less pitted morningglory control than the broadcast treatment. Soybean treated with herbicides in a band produced yields similar to or greater than soybean treated with the same herbicides broadcast. Metribuzin followed by 36 g ha-1 AC 263,222 with a 10-cm open band resulted in a soybean yield of 2840 kg ha-1, which was greater than any other herbicide treatment and equivalent to the weed-free check.

BAS 145 138, CGA 154 281, and 1,8 naphthalic anhydride were evaluated as seed treatments to safen 'Silver Xtra Sweet,’ ‘How Sweet It Is,’ ‘Zenith,’ and 'Sweetie 76’ sweet corn against nicosulfuron injury. Nicosulfuron POST injured sweet corn by causing slight leaf chlorosis and leaf base widening in Sweetie 76 and severe plant injury and total yield loss in Silver Xtra Sweet. Naphthalic anhydride or CGA 154 281 provided limited or no safening activity, respectively, and resulted in plant height reduction when applied alone. BAS 145 138 reduced nicosulfuron injury to Silver Xtra Sweet and prevented visible injury from nicosulfuron in How Sweet It Is, Zenith, and Sweetie 76.

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.

Chlorimuron dissipation as determined by bioavailability in nine soils across the Southern United States was characterized by rapid initial loss followed by a slower dissipation rate following field application of the herbicide in soybean. The rate of bioavailable dissipation varied by year and soil The calculated time for 50% dissipation of bioavailable chlorimuron (DT50) ranged from 5 to 18 d in 1988, 5 to 20 d in 1989, and 5 to 11 d in 1990. In 2 of 3 yr, chlorimuron DT50 was positively correlated with temperature 24 h after application. Chlorimuron DT50 was positively correlated with precipitation in 1988 when regional rainfall was below normal and negatively correlated with precipitation 30 d after application in 1989 when regional rainfall was above normal.