Kudzu (Pueraria lobata Ohwi # PUELO) was grown from seeds in controlled-environment chambers at 350, 675, or 1000 μl·1−1 CO2. Biomass and leaf area production, morphological characteristics, and growth analysis components were determined at 14, 24, 45, and 60 days after emergence. At 60 days, plants grown at 1000 μl·1−1 CO2 had 51% more biomass, 58% longer stems, and 50% more branches than plants grown at 350 μl·1−1 CO2. Plants grown at 675 μl·1−1 CO2 were intermediate. Growth analysis components indicated that CO2 enrichment increased growth by compounding effects due to increased net assimilation rates and increased leaf area duration. Relative growth rates were not significantly affected. The large CO2-induced increase in stem height versus stem diameter is in marked contrast to previously reported responses of woody erect growth forms. Possible ecological implications for competitive abilities are discussed.

Field research was conducted in 1973, 1974, 1977, and 1978 to compare water-culture practices and rates of molinate (S-ethyl hexahydro-1H-azepine-1-carbothioate) for control of “strawhull” red rice (Oryza sativa L.) in water-seeded rice (O. sativa L.). Water-culture treatments (continuously drained, alternately drained and flooded, or continuously flooded during the first 28 days after seeding) alone, or preplant, soil-incorporated treatments of molinate (3.4, 4.5, and 6.7 kg/ha) alone did not control red rice. Treatments of preplant-incorporated molinate at 4.5 and 6.7 kg/ha in a continuously flooded culture or 6.7 kg/ha in an alternately drained-flooded culture controlled 87 to 93% of the red rice and permitted production of satisfactory yields of high quality rice grain.

Extensive field and greenhouse studies were done to characterize varietal response of three recently commercialized sulfonylurea corn herbicides: nicosulfuron, primisulfuron, and thifensulfuron. Most of the 94 varieties tested were highly tolerant to these herbicides. The 37 inbreds represented all major inbred families now used in hybrid seed production as well as several sensitive experimentals. Twenty-one defined hybrids from these inbreds as well as 36 commercially coded hybrids were also tested. Sensitive inbreds produced tolerant hybrids when crossed with tolerant inbreds. Sensitive hybrids occurred when both parents were sensitive. Genetic analysis of sensitive by tolerant crosses showed that sensitivity is controlled by a single recessive gene. Nicosulfuron had the widest corn safety margin and fewest sensitive varieties. Dose response analysis showed varieties can vary more than 40 000-fold in sensitivity. Only corn varieties with the AHAS-modified XA-17 gene showed any change in enzyme sensitivity. This gene overcame sensitivity to sulfonylureas, even when the organophosphate insecticide terbufos was present. Thus, breeders have three options to eliminate sulfonylurea sensitivity: backcross sensitive inbreds with tolerant, always use at least one tolerant hybrid parent, or use the XA-17 gene.

Field experiments over 3 yr at Lethbridge, Alberta, determined the effect of various downy brome densities and times of its emergence on winter wheat biomass and seed yield. Downy brome reduced wheat biomass up to 59% and seed yield up to 68%. Time of downy brome emergence relative to wheat affected the magnitude of these yield reductions more than the density of downy brome. At comparable densities, downy brome caused 2- to 5-fold greater reductions in yield when it emerged within 3 wk after winter wheat than when it emerged 6 wk after wheat or in early spring. Late-emerging downy brome caused significant wheat yield or biomass losses only at densities of 200 to 400 plants m-2. Late-emerging downy brome plants were strongly shaded (70 to 90%) by winter wheat throughout much of the growing season.

Sulfentrazone sorption kinetics, desorption, and mineralization were evaluated in surface 7.5 cm of soils collected from long-term conventional-till (CT) and no-till (NT) plots. The soils used were Miami silt loam and Drummer silty clay loam from Illinois and Dundee silt loam from Mississippi. Sulfentrazone sorption kinetics in Dundee silt loam CT and NT soils were adequately described by a simple two-site equilibrium/kinetic model. Rapid initial sorption (within 1 h) was followed by a slower sorption and equilibrium, largely achieved by 72 h of shaking, with a negligible increase in sorption thereafter. The sorption Kf ranged from 1.02 to 3.44 among the six CT and NT soils. The Kf values were greater for NT compared to their respective CT soils. Overall, Kf values were higher in Drummer silty clay loam followed by Dundee silt loam and Miami silt loam soil. The N values were less than unity in all soils indicating nonlinear sorption. Sulfentrazone desorption was hysteretic with a very low rate of desorption. The total amount desorbed in four desorptions ranged from 58 to 72% of that sorbed. Less than 2.1% of applied 14C-sulfentrazone was mineralized to 14CO2 in Dundee silt loam CT and NT soils during a 77–d incubation. Relatively low mineralization of sulfentrazone suggests poor adaptability of native microbial populations that have not been exposed to this herbicide. Higher sorption and lower desorption of sulfentrazone in NT soils compared to CT soils suggest that NT systems (which tend to increase plant residues) may prolong sulfentrazone residence time in soil.

Approaches to crop production that successfully reduce weed seed production can benefit farming systems by reducing management inputs and costs. A 5-yr rotation study was conducted in order to determine the effects that interactions between crop rotation, tillage, and amount of herbicide and fertilizer (management inputs) have on annual grass and broad-leaved weed seed production and fecundity. There were 10 crop rotation and tillage system combinations and three levels of management inputs (high, medium, and low). Green and yellow foxtail were the major weed species, and together they yielded between 76 and 93% of collected weed seeds. From 1990 to 1994, average grass weed seed productions were 7.3 by 103, 3.7 by 103 6.1 by 103 and 5.7 by 103 seeds m−-2, whereas average broad-leaved weed seed productions were 0.4 by 103, 0.4 by 103, 1.4 by 103, and 0.4 by 103 seeds m−-2 in crop rotations using conventional tillage (moldboard plow), conservation tillage, no tillage, and ridge tillage, respectively. Crop rotations using conventional or ridge tillage consistently produced more grass and broad-leaved weed seeds, especially in low-input plots. There was little difference in weed seed production among input levels for crop rotations using conservation tillage. Comparing rotations that began and ended with a corn crop revealed that by increasing crop diversity within a rotation while simultaneously reducing the amount of tillage, significantly fewer grass and broad-leaved weed seeds were produced. Among the rotations, grass and broad-leaved weed fecundity were highly variable, but fecundity declined from 1990 to 1994 within each rotation, with a concomitant increase in grass and broad-leaved weed density over the same period. Crop rotation in combination with reduced tillage is an effective way of limiting grass and broad-leaved weed seed production, regardless of the level of management input applied.

A soil column system for determination of relative movement of herbicides in soils is described. A data presentation system which expresses the results for each herbicide in a given soil as a single number is proposed. Comparisons of mobilities of new herbicides with those known for other older, better understood herbicides should make it possible to predict the behavior of new herbicides in the field. The aromatic acid herbicides were most mobile, and the insoluble toluidines least mobile. Results for 28 herbicides are presented.

The effects of drop-size, dosage, and spray volume on the toxicity of aqueous sprays of paraquat (1,1′-dimethyl-4,4′-bipyridinium ion) were measured using sunflower (Helianthus annuus L. ‘Peredovik’). Small drops (100 μ) were more phytotoxic than large ones (350 μ), and concentrated solutions (5.5 L/ha) were more phytotoxic than dilute solutions (2.2 L/ha) at the same dosage rate.

The inheritance of resistance to dicamba in wild mustard was determined by making reciprocal crosses between a resistant (R) population derived from a field treated repeatedly with auxin-type herbicides, and a known susceptible (S) population. The resulting F1 hybrids were selfed to produce F2 populations and backcrossed to the S parent. At the three- to four-leaf stage, parental, F1, F2, and backcross populations were screened for resistance to dicamba at three dosages (50, 200, and 400 g ai ha−1). F1 progeny survived all dosages and exhibited levels of injury similar to the R parental population. F2 populations segregated in a 3:1 ratio of R to S phenotypes. Progeny of backcrosses segregated in a 1:1 (R:S) ratio. Responses of the F1, F2, and backcross populations to treatment with dicamba indicate that resistance is determined by a single, completely dominant nuclear allele.

Sixty isolates of the plant pathogenic fungus Sclerotinia sclerotiorum (Lib.) de Bary and six isolates of Sclerotinia minor Jagger were evaluated in a controlled environment for virulence on leaves excised from 8-week-old dandelion plants. Significant negative correlations were obtained between the relative virulence of the isolates and the dry weights of dandelion plants inoculated in a controlled environment; and positive correlations were detected between the relative virulence of isolates and reduction in number of dandelion plants in turfgrass swards infested with inoculum of the isolates. In August 1988, an 80.7% reduction in the number of dandelion plants was detected in a turfgrass sward treated in 1987 with four applications of heat-killed seed of perennial ryegrass (100 g m−2 application–1) infested with isolate R30 of S. sclerotiorum, followed by six applications at the same rate in 1988. Populations of dandelions in untreated swards increased by 22.2% during the same period. Heat-killed seed of perennial ryegrass (100 g m−2) infested with isolate R30 applied simultaneously with dandelion seed (25 g m−2) onto a sward of Kentucky bluegrass reduced the establishment of dandelion seedlings by 85.5%. Necrosis or discoloration did not develop on Kentucky bluegrass, creeping bentgrass, annual bluegrass, or quackgrass treated with inoculum of Sclerotinia. Sclerotia of Sclerotinia spp. were found, on occasion, in crowns but not on the foliage of diseased dandelions.

Four greenhouse experiments were conducted to study the effects of plant residues and extracts of yellow nutsedge (Cyperus esculentus L.) plant residues on the growth of corn (Zea mays L.) and soybeans [Glycine max (L.) Merr.]. At equal concentrations, tuber residues reduced the dry weight of corn and soybeans more than foliage residues. As the concentration increased, growth decreased, affecting soybeans more than corn. Soybean growth was significantly reduced by the addition of tuber extracts. At a constant residue concentration, increasing the percentage of sand in the soil mixture reduced the growth of corn and soybeans. Growth inhibition was greatest when tuber residues were in contact with the corn or soybean seed. We conclude that extracts and residues of yellow nutsedge have an allelopathic effect on corn and soybeans under greenhouse conditions.

The effect of various soil parameters on metribuzin [4-amino-6-(1,1-dimethylethyl)-3-(methylthio)-1,2,4-triazin-5(4H)-one] efficacy was studied on seven soils and metribuzin adsorption was investigated in nine soils. Soil organic-matter and clay contents were correlated with metribuzin activity. Specific surface area, as measured by ethylene glycol monoethyl ether (EGME) was highly correlated with metribuzin activity. Soil organic-matter content and EGME surface area measurements were also highly correlated with metribuzin adsorption in soils. Since metribuzin is highly water soluble (water solubility greater than 1000 ppm), it was probably adsorbed at the hydrophilic sites on the soil surfaces that were measured by EGME. Metribuzin was much more mobile than atrazine [6-chloro-N-ethyl-N′-(1-methylethyl)-1,3,5-triazine-2,4-diamine] in a leaching study.

Common waterhemp is one of the most commonly encountered and troublesome weeds in the midwestern United States. It is well known that water stress adversely affects crop growth and yield; however, the effects of water stress on weed growth and seed production are poorly understood. The objective of this study was to determine the effects of degree and duration of water stress on growth, development, and fecundity of two common waterhemp biotypes in greenhouse experiments conducted at the University of Nebraska–Lincoln. No difference was observed in growth, development, and seed production between two biotypes in response to degree and duration of water stress; therefore, data were combined. The degree of water stress study included five treatments, where the amount of water applied to each pot at 2-d interval was equivalent to 100, 75, 50, 25, and 12.5% of pot (soil) water content. The highest plant height (163 cm), number of leaves (231 plant−1), and growth index (4.4 × 105 cm3) were recorded at 100% of pot water content (no water stress). Similarly, aboveground biomass, total leaf area, and seed production reached their maximum at 100% of pot water content treatment, whereas they were reduced as degree of water stress increased. The study of water stress duration included five treatments, where amount of water applied to each pot at 2-, 4-, 6-, 8-, and 10-d intervals was equivalent to 100% of pot water content. The highest plant height (150 cm), number of leaves (210 plant−1), and growth index (3.8 × 105 cm3) were observed at 2-d interval of water stress, whereas seed production was similar at 2-d (36,549 seeds plant−1) and 4-d (34,176 seeds plant−1) intervals. This study shows that common waterhemp has capacity to survive and reproduce even under a higher degree and duration of water stress.

Weed control decision rules were derived for the application of postemergence herbicides to control pigweed species in soybean. Field experiments were conducted at two locations in 1992 and 1993 to evaluate soybean-pigweed interference. A damage function was determined that related yield loss to time of pigweed emergence, density, and soybean weed-free yield. A control function described pigweed species response to variable doses of imazethapyr and thifensulfuron. The integration of these two functions formed the basis of an economic model used to derive two weed control decision rules, the biologist's “threshold weed density” and the economist's “optimal dose.” Time of weed emergence had a more significant role than weed density in the economic model. Later-emerging pigweed caused less yield loss and therefore, decision rules lead to overuse of herbicides if emergence time is not considered. The selected herbicide dose influenced the outcome of the control function. Depending on the desired level of weed control, a herbicide could be chosen to either eradicate the escaped weed species (label or biologically-effective doses) or reduce the growth of the weed species and thereby offset interference (optimal dose). The development of a biologically-effective dose by weed species matrix was recommended. Decision rules should not be utilized as an exclusive weed management strategy but rather as a component of an integrated weed management program.

Siam weed and coat buttons are among the most troublesome Asteraceae weed species of rainfed rice. The influence of various environmental factors on seed germination and seedling emergence of these weeds was determined. Germination response of both species was greater at the warmer fluctuating temperatures (30/20 and 35/25 C) than at the colder temperatures (25/15 C). Light stimulated germination in both species; however, some seeds still germinated in the dark. Both species were moderately tolerant of salt and water stress, but Siam weed tolerated more stresses than coat buttons. At the soil surface, Siam weed and coat buttons emergence was 75 and 76%, respectively, but this declined rapidly with increasing soil depths. Neither species emerged from depths exceeding 3 cm. Seedling emergence and seedling dry matter of both species were greatly reduced with the addition of crop residue to the soil surface at rates equivalent to 4 to 6 t ha−1. The information gained in this study will be used to facilitate development of effective weed control programs.

Greenhouse and laboratory experiments were conducted to evaluate foliar absorption, translocation, and efficacy of glufosinate on four weed species. The rate of glufosinate required to reduce shoot dry weight by 50% (GR50) varied between weed species. GR50 values for giant foxtail, barnyardgrass, velvetleaf, and common lambsquarters were 69, 186, 199, and 235 g ai ha−1, respectively. Absorption of 14C-glufosinate increased with time and reached a plateau 24 hours after treatment (HAT). Absorption of 14C-glufosinate was 67, 53, 42, and 16% for giant foxtail, barnyardgrass, velvetleaf, and common lambsquarters, respectively. Translocation of absorbed 14C-glufosinate from the treated leaf was greatest for giant foxtail and barnyardgrass (15 and 14% 24 HAT of absorbed 14C-glufosinate, respectively). This compared to 5 and < 1% for translocation of absorbed 14C-glufosinate from the treated leaves of velvetleaf and common lambsquarters. The majority of 14C-glufosinate translocated by giant foxtail and barnyardgrass was found below the treated leaf and in the roots, indicating phloem mobility of the herbicide. Differential absorption and translocation of 14C-glufosinate may be contributing factors to the differential sensitivity observed between weed species.

A root bioassay was used to compare the adsorption of herbicides by activated carbon with that of muck soil, bentonite clay, a cation exchange resin, and an anion exchange resin. The effectiveness of different adsorbents was determined by comparing the concentrations of herbicide required to give 50% root inhibition of the test plant. Of eight herbicides tested, six were more strongly adsorbed by activated carbon than by any of the other adsorbents. The relative amount of adsorption by activated carbon as measured by the reduction in biological activity was as follows: isopropyl N-(3-chlorophenyl)-carbamate (CIPC) > α,α,α,trifluro-2,6-dinitro-N, N-dipropyl-p-toluidine (trifluralin) > 2,4-dichlorophenoxyacetic acid (2,4-D) > N,N-dimethyl-2,2-diphenylacetamide (diphenamid) > dimethyl 2,3,5,6-tetrachloroterephthalate (DCPA) > 4,6-dinitro-o-sec-butylphenol (DNBP) > 3-amino-2,5-dichlorobenzoic acid (amiben). The biological activity of 1,1'-dimethyl-4,4'-bipyridinium salt (paraquat), a cationic herbicide, was not reduced by activated carbon, but was reduced by bentonite clay and the cation exchange resin. DNBP was more strongly adsorbed by the anion exchange resin than by activated carbon. Desorption from activated carbon varied greatly for the herbicides tested. The most readily desorbed herbicide was 2,4-D while CIPC and DNBP showed little or no desorption.

The effects of sethoxydim {2-[1-(ethoxyimino) butyl]-5-[2-(ethylthio)propyl]-3-hydroxy-2-cyclohexene-1-one} rate, order, and timing of sequential herbicide applications, and the addition of oil concentrate on sethoxydimbentazon [3-isopropyl-1H-2,1,3-benzothiadiazin-4(3H-one 2,2-dioxide] antagonism were studied on broadleaf signalgrass [Brachiaria platyphylla (Griseb.) Nash. ♯3 BRAPP], fall panicum (Panicum dichotomiflorum Michx. ♯ PANDI), and large crabgrass [Digitaria sanguinalis (L.) Scop. ♯ DIGSA] in field experiments during 1980 and 1981. Tank-mixing bentazon with sethoxydim reduced control of these species compared to applications of sethoxydim alone. Sequential applications of the herbicides prevented the antagonism. Variations in time between and order of sequential applications had no consistent effect on efficacy. Increases in sethoxydim rate reduced the severity of the antagonism in some cases, while the addition of oil concentrate had no effect.

The cross-tolerance of imidazolinone-tolerant (IMI-tolerant) rice to various acetolactate synthase (ALS)-inhibiting herbicides at one and two times labeled rates was studied. The IMI-tolerant rice is cross-tolerant to imazaquin, imazapyr, nicosulfuron, pyrithiobac, thifensulfuron plus tribenuron, and triasulfuron; is partially tolerant to imazamethabenz and metsulfuron; and is susceptible to chlorimuron, flumetsulam, imazamox, imazapic, primisulfuron, and rimsulfuron. In the greenhouse, IMI-tolerant rice injury with 70 and 140 g ai ha−1 imazethapyr was 17 and 34%, respectively, 28 DAT. Both rates of imazapyr, imazaquin, rimsulfuron, nicosulfuron, thifensulfuron plus tribenuron, and pyrithiobac, and 25 g ai ha−1 triasulfuron, injured rice the same as imazethapyr. Red rice control with 70 and 140 g ha−1 imazethapyr was 97 and 98%, respectively, 28 DAT. At label and two times the label rate, all imidazolinones, nicosulfuron, and primisulfuron controlled red rice equivalent to imazethapyr. Red rice control with 28 g ai ha−1 rimsulfuron was similar to control with 70 and 140 g ha−1 imazethapyr 28 DAT. In the field, barnyardgrass control with two times the labeled rate of imazamox, imazapic, imazapyr, imazaquin, imazamethabenz, rimsulfuron, and nicosulfuron was equal or greater than control with imazethapyr 30 DAT; however, at two times the labeled rate of imazamox, imazapic, and rimsulfuron, injury was greater than imazethapyr. Of all the herbicides tested, only nicosulfuron, imazaquin, and imazapyr offer a combination of low rice injury and high red rice control compared with imazethapyr.

Sulfentrazone was foliar applied at 34 and 56 g ai ha−1 alone or in combination with surfactants to soybean cultivars Hutcheson and Centennial and to sicklepod, coffee senna, smallflower morningglory, velvetleaf, and yellow nutsedge. The most sensitive weeds, including coffee senna, smallflower morningglory, and velvetleaf, were severely injured by the lowest rate when sulfentrazone was applied with surfactants. Sulfentrazone provided the highest control of yellow nutsedge with X-77. Soybeans were not severely injured by sulfentrazone applied alone, but 55% foliar injury occurred when the herbicide was applied with X-77. However, the seedlings were not killed. Sicklepod was the most tolerant of the weeds tested. In the absence of surfactants, the order of radiolabeled sulfentrazone absorption by the foliage was Centennial (5.8%) = Hutcheson (8.5%) = coffee senna (10.4%) < yellow nutsedge (17.0%) < velvetleaf (22.3%) = smallflower morningglory (24%). Sicklepod leaves did not retain droplets containing sulfentrazone when no surfactant was used. Species with the highest foliar absorption also showed the greatest phytotoxic response to the herbicide. Addition of surfactants to the spray mixture enhanced the foliar absorption and overall phytotoxicity of sulfentrazone in the weeds. An inverse relationship was detected between the foliar absorption of sulfentrazone without surfactants and the amount of cuticular wax present on the leaves. No such correlation was observed when surfactants were used. Thus, surfactants overcame the barrier to absorption imposed by the cuticular wax and, under these conditions, selectivity apparently became dependent upon species-specific cellular tolerance to sulfentrazone.