Patterns of spread of 85 weed species alien to the northwestern U.S. were simulated using principal coordinate analysis on primarily herbaria-derived data. The primary pattern detected was a west-to-east migration of weeds. The point of introduction for most weeds following this pattern appeared to be near Portland, OR, an important shipping port in the past. An east-to-west migration represented a secondary pattern. The major cattle and wheat-cropping areas of east-central Montana were the apparent introduction points for weeds following this pattern. The grain and pulse region of eastern Washington represented the point of introduction of species following a third pattern of migration. These weeds subsequently spread southward to Utah and then laterally to the east and west. Historical weed migration patterns provide insight to probable points of origin and routes of currently spreading weeds as well as those that will spread in the future. Such insight may aid in the containment of future spreading weeds.

The dynamics of buried weed seeds in ridge-tilled corn (Zea mays L.) and soybeans [Glycine max (L.) Merr.] were studied in Minnesota. Soils subjected to continuous corn production harbored at least twice as many buried weed seeds as soils under corn/soybean rotations. Truncation of ridges at the time of crop sowing removed 31 to 37% of the buried weed seeds from ridges of continuous corn, and 80 to 100% of those from ridges of the corn/soybean rotations. From 14 to 36% of weed seeds originally in ridges were lost through germination either on the ridge or in the furrow. Ridging soil at layby stimulated germination of numerous weed seeds. The resulting population of small weeds eventually produced up to 1000 seeds/m2 in continuous corn but only about 100 seeds/m2 in corn/soybean rotations, thoroughly replenishing the soil seed reserve in both cropping systems. After 7 to 8 yr of good to excellent weed control with herbicides, crop yield losses in the absence of herbicides were 10 to 27% in continuous corn, and 0% for corn and 6 to 10% for soybeans in corn/soybean rotations.

In three replacement series experiments, winter wheat (Triticum aestivum L.), jointed goatgrass (Aegilops cylindrica Host. #3 AEGCY), and downy brome (Bromus tectorum L. # BROTE) were paired in all possible combinations to determine competitive relationships during vegetative growth. Under growth chamber conditions of ample fertility and soil moisture and day/night temperatures of 18/10 C, relative yield totals for the three species were similar, indicating that they compete for the same resources. Both winter wheat and jointed goatgrass had greater plant growth and higher relative crowding coefficients than downy brome, which indicated a hierarchy of relative competitiveness of winter wheat > jointed goatgrass >> downy brome. In other growth chamber studies, winter wheat was slightly more competitive than jointed goatgrass regardless of fertility levels. Winter wheat was the superior competitor at 18/10 C and −33 kPa (soil moisture), whereas jointed goatgrass was superior at 27/10 C and −300 kPa, conditions that are frequently encountered in the Pacific Northwest.

Cotton (Gossypium hirsutum L. ‘Coker 315’), spurred anoda [Anoda cristata (L.) Schlecht. # ANVCR], and velvetleaf (Abutilon theophrasti Medik. # ABUTH) were grown in controlled-environment chambers at 29/23 C day/night temperature and 1000 μE·m–2·s–1 photosynthetic photon flux density (PPFD). Groups of plants were subjected to water stress by withholding water for 8 days, beginning at 27 days after planting. Cotton reached a lower leaf water potential than spurred anoda and velvetleaf following 8 days of withholding water. Following the drought, plants were rewatered for a 6-day recovery period. Water stress reduced plant height, leaf number, leaf area, total dry weight, stomatal conductance, transpiration, net assimilation rate, and leaf area duration in all species, in comparison with well-watered controls. Water stress decreased partitioning of plant biomass into leaves and increased partitioning into roots. Following rewatering, stomatal conductance and transpiration rates of previously stressed plants recovered to control levels within 28 h. Net assimilation rates of previously stressed spurred anoda also recovered to control levels. After 6 days of recovery, total dry weights and leaf areas of previously stressed plants of all three species remained below those of control plants. The overall effect of the drought and water stress was to delay the time at which spurred anoda and velvetleaf equaled or surpassed cotton in dry weight and leaf area, suggesting that drought early in the growing season under field conditions might increase the competitiveness of cotton relative to the two weeds.

Season-long interference of shattercane, giant foxtail, common cocklebur, and common lambsquarters in corn was evaluated in the field at weed densities from 0.4 to 13.1 plants or clumps/m of corn row during 1985, 1986, and 1987. Corn seed yields decreased linearly with increases in the density of clumps of 2 to 3 shattercane plants and 5 to 8 giant foxtail plants, reaching 22% yield loss at 6.6 shattercane clumps/m of row and 18% yield loss at 13.1 giant foxtail clumps/m of row. Increases in common cocklebur density caused corn yields to decrease curvilinearly in 1985 with a maximum predicted yield loss of 27% occurring at a density of 4.7 common cocklebur plants/m of row. In 1986 and 1987, yields decreased linearly as common cocklebur density increased to 6.6 plants/m of row, where a 10% yield loss was observed. Common lambsquarters reduced corn yields only in 1985. In this year, yields decreased curvilinearly with increasing weed density, resulting in a maximum yield loss of 12% at 4.9 common lambsquarters plants/m of row. Corn yields averaged 11600 kg/ha when grown without weed interference in these experiments.

The effect of Russian thistle density and duration of interference on spring wheat was investigated in two 3-yr field studies. In the density study, the best fit regression equation predicted a loss in spring wheat yield of 0.5, 0.5, and 0.6% in 1983, 1984, and 1985, respectively, for each percent of the total biomass contributed by Russian thistle. In 1985, when rainfall was 46% below normal, the highest weed density produced greater than 70% of the total plant biomass and reduced yields more than 50%. In contrast, in 1984 when rainfall was 65% above normal, the highest weed density produced less than 20% of the total plant biomass and reduced yields 11%. In the duration study, the predicted yield loss was 2.3 and 0.7% for 1983 and 1984, respectively, for each week of interference. In 1985, yield was not significantly reduced until after 6 weeks of interference. In both studies, spikes/m of row had the highest correlation with yield and were affected the most by weed interference.

Field studies were conducted for 3 yr near Mead, NE, to evaluate the effectiveness of seed safeners CGA-92194 [N-(1,3-dioxalon-2-yl-methoxy)iminobenzeneacetonitrile], NA (1,8-naphthalic anhydride), and R-29148 (2,2-dimethyl-5-methyldichloroacetyloxazolidine) to reduce herbicide injury to big bluestem (Andropogon gerardii Vitman), indiangrass [Sorghastrum nutans (L.) Nash], intermediate wheatgrass [Agropyron intermedium (Host.) Beauv.], sideoats grama [Bouteloua curtipendula (Michx.) Torr.], and switchgrass (Panicum virgatum L.) from preplant-incorporated butylate [5-ethyl bis-(2-methylpropyl)carbamothioate] and metolachlor [2-chloro-N- (2-ethyl-6-methylphenyl) -N- (2-methoxy-l-methylethyl)acetamide], applied at 4.5 and 2.2 kg ai/ha, respectively. Big bluestem stands were satisfactory, regardless of herbicide or safener treatment, although stands were reduced by NA treatment with either herbicide. Indiangrass stands varied by year, with protection from both herbicides by R-29148 in 1984 and by NA in 1985. All safeners reduced injury to intermediate wheatgrass from metolachlor and to a lesser extent from butylate; acceptable stands were obtained with metolachlor treatment when unsafened. Sideoats grama was nearly eliminated with either herbicide, regardless of safener. Switchgrass treated with NA produced stands two- to threefold higher than other safened or unsafened seed in metolachlor plots and equal to unsafened seed in the weeded control plots.

Competition of weeds was characterized by determining the distance down the soybean row that a weed affects soybean biomass and yield. Field studies were conducted for 2 yr to compare competitive effects of common cocklebur, johnsongrass, Palmer amaranth, sicklepod, and tall morningglory on ‘Forrest’ and ‘Centennial’ soybeans. The weeds did not significantly reduce soybean biomass for 6 weeks after emergence. Palmer amaranth, common cocklebur, and tall morningglory had the greatest biomass by 6 weeks after emergence. However, only competition from common cocklebur and Palmer amaranth measurably reduced soybean biomass during the growing season. Biomass of Forrest and Centennial soybeans was reduced when these cultivars were growing within 12.5 and 50 cm of common cocklebur, respectively. Johnsongrass, sicklepod, and tall morningglory grew more slowly than the other weeds and had no measurable competitive effects on soybean biomass. Soybean competition reduced biomass of all weeds 90 to 97%. Soybean cultivar influenced the level and duration of competitiveness depending on the weed species present. Biomass of both soybean cultivars was reduced when they were growing within 50 cm of Palmer amaranth. Soybean seed yield was reduced when soybeans were growing within 25 cm of common cocklebur and Palmer amaranth and also when they were growing within 12.5 cm of tall morningglory. Sicklepod had no effect on soybean seed yield.

Using an accession of velvetleaf having a high degree of purple pigmentation as the male parent made possible the identification of seedlings resulting from controlled or natural crosses. Cross-pollination, even after flowers had opened and anthers had shed pollen, was demonstrated by transmitting pollen manually. Natural outcrossing was demonstrated in the field by growing plants of the purplepigmented accession in close proximity to locally occurring velvetleaf. Insect transmission of pollen was suspected.

Palmer amaranth (Amaranthus palmeri S. Wats. # AMAPA) residue was incorporated into soil to determine its allelopathic effects on the seedling growth of grain sorghum (Sorghum bicolor L. Moench.), cabbage (Brassica oleracea, var. capitata L.), carrot (Daucus carota L.), and onion (Allium cepa L.). Root and shoot growths were equally sensitive to the toxic effects of soil-incorporated Palmer amaranth. Growth of ‘Grand Slam’ cultivar of cabbage was 17 to 30% more sensitive than the growth of 'Sanibel’ cabbage. Growth of onion and carrot seedlings was less inhibited than either cabbage or grain sorghum. Growth of grain sorghum root was severely inhibited by 8000 and 16 000 ppm of Palmer amaranth in soil and was not affected by oven dryings other than lyophilization. Seedling growth was more severely inhibited by thyrsus and leaf tissues than by stem and root tissues of Palmer amaranth.

The morphological and physiological response of the slender oat (Avena barbata Pott ex Link # AVEBA) to the herbicides barban (4-chloro-2-butynyl 3-chlorophenylcarbamate) and difenzoquat (1,2-dimethyl-3,5-diphenyl-1H-pyrazolium) in conjunction with decreased water availability was determined for seven populations, representing three ecotypes, under greenhouse conditions. Generally, within the range of sublethal herbicide doses, with increasing herbicide rates, phytotoxicity ratings increased, but plant dry weight, tiller height, and number of spikelets were decreased. Also, the number of juvenile tillers decreased, while that of fertile tillers increased. Flag leaf area increased and flowering was delayed. The ratio of number of spikelets to plant dry weight and seed weight was influenced the least. Under the highest rates of difenzoquat, the within-plant variance of spikelet number decreased, indicating that there may have been a more equal partitioning of resources amongst tillers for spikelet production. The general influence of water stress was to amplify the effect of the herbicide. For example, the dry treatment reduced dry weight and tiller height, and delayed flowering. Within a particular herbicide treatment, the effect of the water stress was to cause reduced within-plant variance for days to flowering, flag leaf area, and number of spikelets. Three reactions were observed that could have helped buffer decreases in spikelet production: 1) An increased fraction of the dry weight of the plants was partitioned into the spikelets at the expense of other vegetative matter, 2) the increased leaf area of the primary tiller may have helped counterbalance any reduction in photosynthesis caused by herbicide action, and 3) an increased number of juvenile tillers was converted into fertile tillers resulting in an increased number of mature tillers. These data indicate that the slender oat has a remarkable “phenotypic plasticity,” which enables it to maintain reproductive structures under sublethal herbicide doses.

Photosynthesis and growth responses to irradiance level during growth were compared in soybean (Glycine max L. Merr. ‘Century’) and three broadleaf weeds to determine if these responses were associated with differences in shade tolerance among species. In response to reduced irradiance during growth, leaf thickness of all species decreased, while chlorophyll content per unit leaf volume and photosynthetic rate per unit leaf volume, measured at low irradiance, increased. Soybean and common cocklebur (Xanthium strumarium L. #3 XANST) also exhibited a decrease in soluble proteins on a leaf volume basis under reduced irradiance, and common cocklebur further exhibited a decrease in ribulose-1,5-bisphosphate carboxylase (RuBPcase) protein per unit leaf volume. Decreased irradiance during growth did not alter the content of RuBPcase or other soluble proteins per unit leaf volume in jimsonweed (Datura stramonium L. # DATST) or velvetleaf (Abutilon theophrasti Medic. # ABUTH). The superior shade tolerance of common cocklebur compared to the other species was attributed in part to the levels of RuBPcase and other photosynthetic proteins in leaves developed at low irradiance.

The distribution pattern of s-triazine-resistant biotypes of common lambsquarters (Chenopodium album L. #3 CHEAL) and smooth pigweed (Amaranthus hybridus L. # AMACH) in Virginia was determined. Seeds were collected from suspected triazine-resistant biotypes of both species. Triazine resistance was confirmed by measuring chlorophyll fluorescence in the presence of atrazine [6-chloro-N-ethyl-N′-(1-methylethyl)-1,3,5-triazine-2,4-diamine]. Greenhouse bioassay with whole-plant material and a sinking leaf disc assay were also performed as further confirmation of triazine resistance. Triazine-resistant smooth pigweed was confirmed in 19 counties and common lambsquarters in eight counties in Virginia. Triazine-resistant smooth pigweed and common lambsquarters were located mostly in the northern and southwestern highlands of the state where there has been a long history of triazine use in no-till corn (Zea mays L.) production. S-triazine-resistant biotypes were also cross-resistant to other representative s-triazine and as-triazine herbicides but susceptible to the substituted urea herbicide diuron [N′-(3,4-dichlorophenyl)-N,N-dimethylurea].

Field and laboratory studies were conducted to examine the influence of previous pesticide use, number of prior EPTC (S-ethyl dipropyl carbamothioate) and butylate [S-ethyl bis(2-methylpropyl)carbamothioate] applications, and pretreatment of soil with EPTC and butylate metabolites on EPTC and butylate degradation. EPTC degradation was enhanced in Clay Center and Scottsbluff soils following three annual EPTC, butylate, or vernolate (S-propyl dipropylcarbamothioate) applications, but was not affected in soils following three annual atrazine [6-chloro-N-ethyl-N′-(1-methylethyl)-1,3,5-triazine-2,4-diamine], cyanazine {2-[[4-chloro-6-(ethylamino)-1,3,5-triazin-2-yl] amino]-2-methylpropanenitrile}, metolachlor [2-chloro-N-(2-ethyl-6-methylphenyl)-N-(2-methoxy-1-methylethyl)acetamide], alachlor [2-chloro-N-(2,6-diethylphenyl)-N-(methoxymethyl)acetamide], or cycloate (S-ethyl cyclohexylethylcarbamothioate) applications. EPTC degradation rate was not affected in a Scottsbluff soil following three annual carbofuran (2,3-dihydro-2,2-dimethyl-7-benzofuranyl methylcarbamate) applications, but was partially enhanced in a Clay Center soil. Self- and cross-enhanced degradation of EPTC and butylate did not change regardless of the number of prior annual applications of each herbicide. The degradation rates of EPTC and butylate were not affected by soil pretreated with butylate sulfone, but the degradation rates-were fully enhanced by pretreatment with the respective sulfoxides of each herbicide. In cross-enhancement testing, EPTC and butylate degradation was partially enhanced in Clay Center soil pretreated with the sulfoxides of each.

Field studies were conducted to determine the effects of various barnyardgrass populations on growth, yield, and nutrient concentration of transplanted “Jetstar’ tomato. Barnyardgrass densities at 16, 32, and 64 plants/m tomato row were tested in 1982 and 1983. Barnyardgrass shoot fresh weights/unit area increased as density increased. Fresh weight of barnyardgrass shoots ranged from 17 100 kg/ha at 16 plants/m of row to 35 500 kg/ha at 64 plants/m of row. At the vegetative stage, tomato shoot dry weight was unaffected by barnyardgrass. As crop growth progressed, tomato shoot dry weight decreased at all barnyardgrass densities. Season-long interference of barnyardgrass reduced marketable tomato fruit number and fruit weight at all densities compared to weed-free plots. Reductions in marketable fruit weight ranged from 26% to 16 plants/m row to 84% at 64 plants/m row. In 1982, concentrations of N, P, K, Ca, and Mg in tomato shoots were unaffected by season-long interference of barnyardgrass at all densities. However, in 1983, concentrations of N and K decreased and concentration of P increased in tomato leaves as the density of barnyardgrass increased. Concentrations of Ca and Mg in tomato leaves were unaltered by barnyardgrass density.

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.

Greenhouse and growth chamber studies were conducted to determine conditions for infection of the fungal pathogen Colletotrichum truncatum (Schw.) Andrus and Moore on Florida beggarweed (Desmodium tortuosum (Sw.) DC. # DEDTO and to determine the host specificity of this fungus. Optimum conditions for disease development were 14 to 16 h incubation in 100% relative humidity (RH) at 24 to 29 C. Control of Florida beggarweed with 105 to 107C. truncatum spores/ml was greatest in the cotyledon stage and decreased with plant age. Ten of 18 Desmodium species tested were susceptible to the Florida beggarweed isolate of C. truncatum but 13 other plant species and varieties were resistant and 61 were immune.

Patterns of weed emergence in upland and lowland soils were observed for several months after tillage by counting all emerged species. Forty to 50% of weed emergence occurred within 6 weeks of tillage on both sites. Significant emergence occurred within 3 weeks on both sites, but very little emergence occurred within 1 week in lowland soil. The data confirm the strong influence of soil tillage on weed emergence. They also suggest an influence of radiant energy on weed emergence. Peaks of weed emergence 6 or more weeks after tillage often occurred coincident with or soon after peaks of radiant energy.

Effects of propanil [N-(3,4-dichlorophenyl)propanamide] applied to three- to four-leaf rice (Oryza sativa L.) 1 or 7 days before, after, or tank mixed with methyl parathion (O,O-dimethyl-O-4-nitrophenyl phosphorothioate) were determined under different environmental conditions. Field experiments determined the effect on yield of drill-seeded rice, ‘Labelle’ for two planting dates in 1982 and ‘Lemont’ for one planting date in 1986. Treatments were applied at sunrise and at noon. Growth chamber and greenhouse experiments determined the effects of temperature, relative humidity (RH), and soil moisture on response of Labelle rice. In all experiments, propanil, both alone and with methyl parathion, resulted in 20 to 30% leaf burn during the first week after treatment with rapid recovery to less than 10% injury after 3 to 4 weeks. In field experiments, yields were not reduced in the treated rice below that in the untreated controls. In controlled-environment experiments, rice was not injured by propanil plus methyl parathion more than by propanil alone after 2 to 4 weeks. Both treated and untreated rice were injured more by the environmental conditions of high (40 C) temperature, low (40%) RH, and low (near the wilting point) soil moisture than by low (30 C) temperature, high (100%) RH, and flooded soil.

Weed seed and seedling populations, and weed competition were compared in plots of continuous corn and corn/soybean rotation under ridge and conventional tillage. After 7 to 8 yr of standard chemical and mechanical weed control, from 1500 to 3000 weed seeds/m2 (to a 10-cm depth) were found in continuous corn with ridge tillage whereas about two-thirds fewer seeds were found in conventionally tilled corn. Soil from a corn/soybean rotation had from 200 to 700 seeds/m2 in both tillage systems. Annual loss of weed seeds from the soil through germination was from 3 to 12% in ridge tillage and 11 to 43% in conventional tillage. Additions to the seed pool were supplied by small weeds whose germination was stimulated by “layby” cultivation, with up to 10 times more emergence and 140 times more seed production in ridge than in conventional tillage. Withholding herbicides for 1 yr reduced yields of continuous corn by 10 to 27% in ridge tillage, only 2 to 4% in conventional tillage, and negligibly in corn/soybean rotations regardless of tillage. Reducing seed production of small layby weeds in ridge tillage may aid in solving the weed problem in this conservation tillage system.