The nonionic surfactants Tween 20 [oxysorbic (20 POE) polyoxyethylene sorbitan monolaurate] and Tween 80 [oxysorbic (20 POE) polyoxyethylene sorbitan monooleate] at concentrations of 0.05, 0.1, 0.2, and 0.4% v/v stimulated germination of barnyardgrass [Echinochloa crus-galli (L.) Beauv. # ECHCG] seeds in petri dishes. Stimulation occurred under both 30/20 and 25/15 C diurnal temperature cycles whether or not light was supplied during the 8-h high-temperature portion of the cycle. Barnyardgrass responded most to treatment under environmental conditions in which untreated seeds germinated the least. A 0.1% surfactant concentration often stimulated germination of weed seeds as effectively as did higher concentrations. Germination of redroot pigweed [Amaranthus retroflexus L. # AMARE] seeds was inhibited by Tween 80 in the light under both the low- and high-temperature regimes and by Tween 20 at low temperature in the light. Germination of tumble pigweed (Amaranthus albus L. # AMAAL) seeds was inhibited by both surfactants in the high-temperature regime whether or not light was supplied. Common purslane (Portulaca oleracea L. # POROL) seeds were insensitive to treatment.

Artemisinin (qinghaosu), a sesquiterpenoid lactone peroxide constituent of annual wormwood (Artemisia annua L. # ARTAN) that is used as an antimalarial drug, was tested for phytotoxic properties. It inhibited germination of lettuce (Lactuca sativa L.) and annual wormwood, and growth of roots and shoots of lettuce, redroot pigweed (Amaranthus retroflexus L. # AMARE), pitted morningglory (Ipomoea lacunosa L. # IPOLA), annual wormwood, and common purslane (Portulaca oleracea L. # POROL) was inhibited at 33 μM. No effects of 33 μM artemisinin were detected on growth of velvetleaf (Abutilon theophrasti Medik. # ABUTH) and grain sorghum [Sorghum bicolor (L.) Moench.]. Chlorophyll content was not affected in lettuce, and chlorosis was not observed in any species tested. The probable biosynthetic precursors of artemisinin, arteannuin B and qinghao acid, had no effect on growth or chlorophyll content of lettuce; however, they inhibited lettuce seed germination. Artemisinin and cinmethylin {exo-1-methyl-4-(1-methylethyl)-2-[(2-methylphenyl)methoxy]-7-oxabicyclo [2.2.1] heptane} were equally effective in reducing growth of lettuce; however, cinmethylin had no effect on germination. Respiration of lettuce roots or cotyledons was not inhibited by artemisinin. Artemisinin only marginally increased the mitotic index of lettuce root tips at 33 μM. At the ultrastructural level, however, chromosomes were less condensed during mitosis in artemisinin-treated than control meristematic cells. The growth-inhibiting ability of artemisinin could not be reduced by feeding the plants with hydrolyzed protein or treatment with putrescine. Artemisinin is a selective phytotoxin that reduces growth by a mechanism other than mitotic disruption or inhibition of protein synthesis.

This paper describes the communities of weeds in cereal and oilseed crops in Saskatchewan, using data collected in a 4-yr survey that sampled more than 400 fields. The survey data for the 40 most common weeds were analyzed in an attempt to distinguish natural groups of weed species and to compare the weed communities associated with the different crops and with different soils. The crops were barley (Hordeum vulgare L.), flax (Linum usitatissimum L.), oats (Avena sativa L.), rape (Brassica campestris L., Brassica napus L.), and wheat (Triticum aestivum L.). Phytosociological association and cluster analysis indicated that the associations of the weed species were more or less independent of the crop, although some differences existed and were determined more by soil or the associated climate. The 40 most common species were divided into three groups related to the soil and climatic subregions of the province.

In a 2-yr field study conducted on a Weswood silt loam soil (Fluventic Ustochrepts), interspecific competition between soybeans [Glycine max (L.) Merr. ‘Hutton′] and velvetleaf (Abutilon theophrasti Medik. # ABUTH) resulted in greater than 40 and 50% reductions in soybean and velvetleaf seed yield, respectively. Leaf area index, number of mainstem nodes, total number of leaves, and plant dry weight of monocultured and intercropped velvetleaf differed significantly as early as 4 weeks after emergence. Interspecific competition had litttle or no effect on soybean morphology before 8 weeks after emergence. Soil water extraction occurred to 1-m depths in a monoculture of velvetleaf (five plants/m2) in 1984 and 1985. Monocultured soybeans (32.5 plants/m2) extracted water from a 1.5-m or greater depth of the soil profile during the same years. Soil water extraction in the intercropped plots resembled that of the monocultured velvetleaf treatment until soybeans attained R6, when soil water was extracted to a 1.5-m depth. The potential for interspecific competition for water existed early in the season before late-season soybean root development. Relative water content and leaf water potential (Ψw1) did not differ (0.05) between monocultured and intercropped soybeans in 1984 or 1985. In 1985, Ψw1 differed between monocultured and intercropped velvetleaf during anthesis. Leaf water potential values in the youngest, fully expanded leaves were approximately 0.3 and 0.4 MPa lower during midmorning and midday hours, respectively, in intercropped and monocultured velvetleaf. Transpiration and stomatal conductance did not differ between monocultured and intercropped soybeans or velvetleaf at any time during 1984. Photosynthetic and transpiration rates, stomatal conductance, and Ψw1 were lower in intercropped than in monocultured velvetleaf during anthesis in 1985, suggesting interspecific competition for soil water. Soybean water relations were not affected in either year. The data suggest that soybean yield reductions in soybean-velvetleaf interspecific competition are attributable to resource limitations other than water in south-central Texas.

Field experiments were conducted during 1981 and 1982 in Ontario, Canada, on the effects of quackgrass [Agropyron repens (L.) Beauv. # AGRRE] interference in soybean [Glycine max (L.) Merr.] and the usefulness of infrared thermometry in predicting critical periods of weed interference. Soybean seed yield, dry weight, number of leaves, height, and number of pods were substantially reduced due to quackgrass interference. High levels of P and K fertility did not overcome the quackgrass interference. Part of the competitive effects of quackgrass was alleviated by irrigation. Infrared thermometry successfully detected the first occurrence of quackgrass-induced stress during the early soybean flowering stage, when the quackgrass was in the four-leaf gtowth stage. This coincided with the onset of the first significant soybean yield loss. No additional soybean yield loss occurred after quackgrass reached the five-leaf growth stage. There was an inverse relation between accumulated stress degree days and soybean yield reductions due to quackgrass interference. The use of the stress degree day concept may be a valuable tool in predicting soybean yield losses due to quackgrass interference.

Factors that govern the germination of cutleaf groundcherry (Physalis angulata L. # PHYAN), smooth groundcherry [Physalis virginiana var. subglabrata (Mackenz. and Bush) Waterfall # PHYSU], and eastern black nightshade (Solanum ptycanthum Dun. # SOLPT) were evaluated. Maximum germination of all three species occurred at a constant 30 C or alternating temperature regimes that included 30 C. Seed of all species germinated in light and dark, but germination was reduced in the dark. Germination and seedling vigor of all species decreased as substrate pH decreased. Cutleaf groundcherry seed were slightly less sensitive than seed of other species to simulated moisture stress. All species produced 90% or more viable seed by 8 weeks after anthesis.

Studies were conducted to investigate the reproductive biology and herbicide response of horsenettle (Solanum carolinense L. # SOLCA) and robust horsenettle (Solanum dimidiatum Raf. # SOLDM) that are common in peanut (Arachis hypogaea L.) production. Of these two species, horsenettle had a higher rate of seed germination, greater ability for seedling establishment in a competitive situation (pasture), greater ability to regenerate from smaller root fragments, and greater ability to regenerate from progressively greater depths in soil. Horsenettle was relatively more sensitive to glyphosate [N-(phosphonomethyl)glycine] (1.1 and 2.2 ai kg/ha), propylene glycol butyl ether esters of 2,4-D [(2,4-dichlorophenoxy)acetic acid] (0.8 and 1.7 ae kg/ha), and the ethanolamine salt of 2,4-D (1.7 ae kg/ha) than robust horsenettle. Conversely, robust horsenettle was more sensitive to dicamba [3,6-dichloro-2-methoxybenzoic acid)] (0.3 and 0.6 ae kg/ha). In a field study, dicamba applied annually in July or August for 2 yr at a rate of at least 1.1 kg/ha controlled 92% or more.

Mechanical wounding broke dormancy and promoted germination of primary dormant pure-line wild oat (Avena fatua L. # AVEFA) caryopses that were in the first physiological stage of dormancy. Caryopses incubated in petri dishes responded to wounding by germinating more rapidly than wounded caryopses incubated in soil. There was no difference in germination between wounded and lanolin-covered wounded caryopses when incubated in soil. The less dormant AN265 caryopses responded to the wounding treatment more rapidly than the more dormant caryopses of M73 in both petri dish and soil incubation systems. Chlorocholine chloride (CCC), an inhibitor of gibberellic acid (GA) biosynthesis, reduced the germination rate of wounded AN265 and M73 caryopses above and at 1 mM, respectively. Application of 25 mM CCC to wounded dormant AN265 caryopses decreased the level of hexose in the embryo to 47% of the control level. There was no significant effect of CCC on the endosperm hexose, embryo and endosperm imbibed fresh weight, and sucrose and starch content. The results suggest GA or GA biosynthesis is the limiting factor to germination.

Genetic diversity within and among populations of yellow nutsedge (Cyperus esculentus L. # CYPES) was analyzed to evaluate and quantify the genetic consequences of the reported predominance of asexually-produced tubers as colonizing agents. Ten populations were examined using starch gel electrophoresis for allozyme analysis. Four populations of purple nutsedge (Cyperus rotundus L. # CYPRO) were surveyed for comparison. Twelve loci were identified in yellow nutsedge among the eight enzyme systems examined; ten of these loci were found in purple nutsedge. Yellow nutsedge showed relatively low genetic diversity. Most of the genetic diversity occurred as differences among individuals within populations (Hs), compared to differences among populations (Dst) for the four variable loci identified in this species. Thus, most genetic distances between its populations were small. Generally, only a few genotypes occurred within each population. Purple nutsedge was found to possess even lower within- and among-population gene and genotypic diversity. This study supports the view that tubers account for most of the establishment of new populations of both species.

Data obtained from two experiments were used to identify and describe factors affecting crop loss due to weeds in forage stands. Timothy (Phleum pratense L. ‘Champ′) was established in presence or absence of barley (Hordeum vulgare L. ‘Bruce′), red clover (Trifolium pratense L. ‘Florex′), broadleaf weeds, and grass weeds. Botanical composition, soil water, soil nutrients, light interception, and plant cover were determined 4 and 6 weeks after seeding, and crop yield was determined at periodic forage harvest dates. Principal components analyses permitted the extraction of 12 factors and regression analyses demonstrated that these factors can be used to describe crop yield. Each factor can be estimated either by obtaining the rotated factor scores or by selecting a representative variable that can be substituted for the rotated factor scores in the regression models. The approach described in this paper should be tested with other crops and other weed complexes. It is suggested to use weed density in combination with weed and crop cover data to predict crop loss.

Density of seed of three species of locoweed, salt milkvetch [Astragalus lentiginosus var. salinas (Howell) Barneby], Lambert locoweed (Oxytropis lambertii Pursh. # OXRLA), and white locoweed (O. sericea Nutt ex. T.&G. # OXRMA) in the soil was determined at four locations in New Mexico, Wyoming, and Utah. Seed density ranged from 50 to 2820/m2. Greater than 77% of the seed were viable as determined by tetrazolium tests. Longevity of white locoweed seed was determined by sewing seed into nylon mesh packets and returning them to the site of origin in northwestern Utah. Greater than 50% of seed buried 1 cm deep in the soil were still present after 6 yr, but only 33% of seed placed on the surface remained. Germinability of the buried seed was less than 5%, but an average of 14% of seed placed on the soil surface germinated each year. Viability of retrieved seed ranged from 89 to 99%.

Crownbeard [Verbesina encelioides (Cav.) Benth. & Hook. f. ex. A. Gray # VEEEN (Compositae)] exhibited rapid seedling, vegetative, and reproductive growth. Seed germination under ideal conditions was high and occurred in varying soils except in gravel. It was significantly suppressed under drought and waterlogged conditions and was maximum at 21% moisture regimes. Larger and heavier seeds which exhibited higher germination were produced by the plants growing in open and sand dune habitats. Seedling emergence was highest from surface-sown seeds and least in those sown at 2.5-cm soil depth, beyond which no emergence occurred. High phenotypic plasticity, ecological variability, phenological diversity, seed germination in varied soils and under diverse moisture regimes, quick seedling growth and subsequent establishment, coupled with versatile breeding system, contribute to the successful growth, propagation, and spread of this species in nature in India.

Acid-scarified hogpotato (Hoffmanseggia densiflora Benth. ex. Gray # HOFDE) seed incubated in distilled water germinated at least 94% when incubated at constant 15, 20, 30 C, and at alternate 20/30 C (20 C for 16 h and 30 C for 8 h) temperatures. Highest germination in buffered solutions occurred at pH 5.0 and 6.0 with reduced germination at lower and higher pH levels. Sodium chloride concentrations of 50 mM and greater reduced the germination rate. Percent germination after 9 days was reduced at NaCl concentrations of 100 mM and greater. Radicle lengths measured after 3 days were significantly reduced with increasing NaCl concentration. Twenty-day-old hogpotato seedlings having three true leaves were able to resprout after topgrowth removal. Regrowth occurred on 15% of the seedlings approximately 15 days after top removal.

The interference and seed production potential of pitted morningglory (Ipomoea lacunosa L. # IPOLA) with conventional-row (1 m) and narrow-row (20 cm) ‘Hill’ soybeans [Glycine max (L.) Merr.] was studied for 2 yr. Pitted morningglory densities were 3.3, 10, 20, and 40 plants/m2 while soybean densities were 23 and 50 plants/m2 in conventional and narrow rows, respectively. Pitted morningglory interfered with soybean growth earlier in conventional-row soybeans than in narrow-row soybeans due to its rapid increase in leaf area index (LAI) and biomass from 4 to 8 weeks after emergence. Soybeans were competitive with pitted morningglory until the soybean reproductive stage began at 7 weeks after emergence. Pitted morningglory reduced soybean yields 17% more in a dry year than in a wet year. Yield of narrow-row soybeans was equal or greater than yield of conventional-row soybeans at all pitted morningglory densities. Conventional-row soybean yields were reduced an average of 42 and 81% at pitted morningglory densities of 3.3 and 40/m2, respectively, but yield of narrow-row soybeans was reduced only 6 and 62% at equivalent densities. Pitted morningglory grown without soybean interference produced an average of 52.3 million seeds/ha. Total seed production of pitted morningglory growing in soybeans increased as pitted morningglory density increased and was greater in conventional rows than in narrow rows. Narrow rows reduced total seed production an average of 90 and 68% at pitted morningglory densities of 3.3 and 40/m2, respectively.

Research was conducted from 1982 through 1984 on a Shipps clay soil (Udic chromustert) in east-central Texas to determine the density-dependent effects and critical periods of johnsongrass [Sorghum halepense (L.) Pers. # SORHA] competition with cotton (Gossypium hirsutum L.). Two experiments were conducted each year, one to assess cotton yield response to increasing johnsongrass density and the second to establish the johnsongrass-free requirement of cotton and the johnsongrass competition period. A critical minimum density of 2 plants/9.8 m of row was observed. Full-season johnsongrass competition densities exceeding 2 plants/9.8 m of row resulted in seed cotton yield reductions each year. Yield decline was most rapid as density increased from 2 to 8 plants/9.8 m of row. Yield loss averaged 1, 4, 14, 40, 65, and 70% for johnsongrass densities of 1, 2, 4, 8, 16, and 32 plants/9.8 m of row, respectively. Percent yield loss was proportional to yield potential at johnsongrass densities of 4 and 8 plants/9.8 m of row. Three to four weeks of rhizome johnsongrass competition or 6 weeks of seedling johnsongrass competition resulted in significant yield reductions. Maintaining cotton johnsongrass free for 4 weeks did not prevent significant yield reductions.

Effects of a natural mixed stand of giant green foxtail [Setaria viridis var. major (Gaudin) Pospichel # SETVM] and fall panicum [Panicum dichotomiflorum Michx. # PANDI] on soybean [Glycine max (L.) Merr. ‘Union’ and ‘Essex′] biomass and seed yield were investigated during 1981, 1982, and 1983. Plots maintained weed free for 2 weeks after soybean emergence produced yields equal to those maintained weed free for the season despite any grass reinfestation. Grasses growing naturally with soybeans for 8 weeks after soybean emergence did not reduce crop yields. Weeds not removed after 8 weeks reduced soybean yields by decreasing the number of pods per plant. Under drought conditions, yield reduction due to grass competition did not occur until 12 to 16 weeks after soybean emergence. Hand-established densities of 1 grass plant/7.5 cm of crop row reduced soybean yield by 0 to 11%. Natural grass infestations reduced soybean yield by 21 to 41%.

Field experiments were conducted at Lincoln, NE, in 1983 and 1984 to determine the effectiveness of subsurface jet injection of herbicides on a simulated shattercane [Sorghum bicolor (L.) Moench # SORVU] infestation in corn (Zea mays L.). Alachlor [2-chloro-N-(2,6-diethylphenyl)-N-(methoxymethyl)acetamide] and EPTC (S-ethyl dipropyl carbamothioate) plus dichlormid (2,2-dichloro-N,N-di-2-propenylacetamide), incorporated with a disc or applied from the subsurface and surface with a jet injector attached to a sweep plow, generally produced higher corn yields and shattercane control than herbicides applied with the sweep to the subsurface only. EPTC plus dichlormid controlled shattercane better than alachlor. Cultivation improved shattercane control by at least 10% in conjunction with most jet injection and disc-incorporated alachlor treatments. Cultivation increased corn yields by about 10 to 20% when averaged over all herbicides and application patterns. A subsurface plus surface herbicide injection system supplemented with cultivation would be a possible method of incorporating herbicides in reduced tillage to selectively control shattercane in corn.

Greenhouse and field experiments were conducted to investigate degradation rates and efficacy of EPTC (S-ethyl dipropyl carbamothioate), butylate [S-ethyl bis(2-methylpropyl)carbamothioate], vernolate (S-propyl dipropylcarbamothioate), and cycloate (S-ethyl cyclohexylethylcarbamothioate) in soils with and without prior history of thiocarbamate use. Herbicide persistence was measured by an oat (Avena sativa L.) bioassay, and giant foxtail (Setaria faberi Herrm. # SETFA) control was visually estimated in field studies. Both greenhouse and field studies indicated that enhanced thiocarbamate degradation occurred when the same herbicide (EPTC, butylate, or vernolate) was applied over successive years. Addition of dietholate (O,O-diethyl O-phenyl phosphorothioate) or fonofos (O-ethyl-S-phenylethylphosphonodithioate) increased thiocarbamate persistence and giant foxtail control in soils with prior thiocarbamate exposure. Use of dietholate or fonofos two or three consecutive years with EPTC or butylate again resulted in enhanced degradation. EPTC persistence and giant foxtail control were not affected by prior treatments or tank mixes with carbofuran (2,3-dihydro-2,2-dimethyl-7-benzofuranyl methylcarbamate). Rotation away from successive applications of the same thiocarbamate herbicide (EPTC, butylate, or vernolate) improved herbicide efficacy.

Sorghum-almum (Sorghum almum Parod. # SORAL) is a weakly perennial, rhizomatous grass that in northern climates reproduces mainly by seeds. Environmental factors influencing seed germination and the longevity of buried seeds were studied in growth chamber and field experiments. Alternating temperatures of 20 C for 16 h and 30 C for 8 h were optimum for germination. Light was not critical to germination. Germination varied only slightly across a pH range from 5.0 to 8.8 but decreased sharply below pH 5. Osmotic potentials greater than −0.9 MPa reduced germination substantially, and no germination occurred at −1.5 MPa. Limited germination occurred at NaCl concentrations of 340 mM, and no germination occurred at 510 mM. Seeds that were either mechanically scarified or afterripened for 6 months had a higher percent germination and germination rate than untreated seeds. Sorghum-almum seedling emergence was 11% from seeding depths of 20 cm, but emergence was 67% or higher from seeding depths of 15 cm or less. In field studies, viability of seeds placed on the soil surface or buried 3 or 8 cm was 2% or less at 12 months and 0% at 18 months after burial. Viability of seeds buried 15 and 23 cm was 0.3 and 26%, respectively, at 24 months after burial. Thus, tillage that buried sorghum-almum seeds 20 cm or more could reduce emergence but would also prolong seed persistence. Shallow tillage should favor depletion of sorghum-almum seed reserves.