A study was conducted under controlled environmental conditions to determine the effect of soil temperature, soil moisture, and depth of seed burial on the emergence of round-leaved mallow. Emergence occurred from 5 to 30 C but was optimal at 15 to 20 C. Soil moisture had a greater effect than soil temperature on percentage emergence. Emergence progressively declined below a soil water content of −0.28 MPa, with less than 20% emergence attained at −1.03 to −1.53 MPa. In contrast, rate of emergence of round-leaved mallow was affected more by soil temperature than by moisture. A decrease in temperature from 30 to 5 C increased the time to reach 50% emergence by 10 to 12 days over the moisture regime of this study. Emergence was greatest at depths of 0.5 to 2 cm. No emergence occurred at 8 cm or below. The potential of using the findings of this study to develop cultural control strategies for round-leaved mallow is discussed.

Corn (Zea mays L.) grain yield was measured in a 5-yr study at two locations in central Ohio in relationship to johnsongrass [Sorghum halepense (L.) Pers. # SORHA] populations infesting the plots. Herbicide treatments were EPTC + R-25788 (S-ethyl dipropyl carbamothioate) + (N,N-diallyl-2,2-dichloroacetamide) at 6.7 kg ai/ha to corn in a monoculture and in a 3-yr rotation of winter wheat (Triticum aestivum L.), corn, and soybeans [Glycine max (L.) Merr.], with each crop being grown each year. There were large increases in johnsongrass populations in the monocultures during the course of the 5-yr study, and corn grain yield fell to zero, even though ear formation was not greatly reduced. Conversely, johnsongrass populations in the rotation were low and maize grain yields and numbers of ears remained high. The differential in grain yields between the two treatments was apparently due chiefly to lack of pollination or kernel development, since ears were formed in approximately equal numbers, except for the last year.

Genetic variation was assessed in a range of populations of wild mustard (Sinapis arvensis L.) using random amplified polymorphic DNA (RAPD) analysis. Sixty markers were used to assess the extent of genetic variation in wild mustard populations sampled throughout 12 different locations in the United Kingdom, including herbicide treated and untreated sites. In addition, selected sites were sampled over two consecutive seasons. Individual plant analysis was required, since a high degree of intra-population genetic variation was observed. The apparent extent of genetic variation in a population at one site increased when the results from two consecutive seasons were assessed. The range of genetic variation was as high in the herbicide-treated populations as in those which were untreated. Genetic diversity was maintained in populations of wild mustard where rotational cropping and herbicide use was practiced. These results confirm the utility of RAPD for the determination of genetic variation in outcrossing species where no prior genotypic knowledge is available.

Measurements of above-ground plant volume were used to quantify corn interference with common cocklebur and velvetleaf. Separate experiments were carried out for each weed species in which neighborhoods with a radius of 50 cm were established around target plants of both species, selected from a range of corn plus cocklebur or velvetleaf densities. Height and canopy area of target plants and neighbor corn and weed populations were measured periodically during the growing season. Target plant (corn, cocklebur, or velvetleaf) size as well as corn and weed population size within each neighborhood were quantified as cylindrical volumes. Regression analysis was used to quantify the relationship between target plant seed production and cylindrical volumes of the target and neighbor species. Both target and neighbor plant volumes were correlated with target plant seed production for all species. The ratio of target plant volume to total neighborhood plant volume (volume ratio) was the independent variable that accounted for the most variation in target plant seed production. These volume-based variables may be used to develop competitive indices in physico-empirical based interference models.

Field studies were conducted at two locations near Manhattan, KS, in 1994 and 1995 to determine the influence of density (0.5, 1, 2, 4, and 12 plants m−1 row) and time of emergence on redroot pigweed growth in monoculture or with sorghum. Redroot pigweed was seeded at sorghum planting and at the three- to four-leaf stage of sorghum in plots with sorghum or alone. When redroot pigweed grew with sorghum, dry matter and seed production were reduced with later times of emergence. In monoculture, there was no reduction in dry matter or seed number between the emergence dates studied. Redroot pigweed dry matter and seed production per plant were reduced as plant density increased for plants grown in monoculture. The same trend was observed for redroot pigweed grown with sorghum that did not emerge early relative to sorghum. Plants grown at low density exhibited more lateral growth than when grown at higher densities because of intraspecific competition.

Uptake of herbicides by excised roots of soybean (Glycine max (L.) Merr.) plants was studied over periods of 30 min, 8 hr, and 48 hr. In the 48-hr experiment, initial rate of herbicide uptake was rapid and was greater at higher temperatures and external herbicide concentrations. Rates of initial uptake varied among herbicides. Over 30-min periods, rapid herbicide uptake by excised roots for 2.5 to 5 min was followed by much slower uptake. Total uptake for 30 min and 1 hr was in the order of 3-(3,4-dichlorophenyl)-1-methoxy-1-methylurea(linuron) > isopropyl-m-chlorocarbanilate (chlorpropham) > S-ethyl dipropylthiocarbamate (EPTC) > 3-amino-2,5-dichlorobenzoic acid (amiben) > 2-chloro-4-(ethylamino)-6-(isopropylamino)-s-triazine (atrazine). However, total uptake after 48 hr was in the order of chloropropham > linuron > EPTC > atrazine > amiben, indicating that rates of uptake for the herbicides changed over 48 hr. Values of Q10 for herbicide uptake were always less than 1.85, indicating that physical processes were important in uptake of herbicides by excised soybean roots. Uptake of herbicides over 8 hr by roots of intact plants was similar at 5, 10, and 20 C to uptake by excised roots, but uptake at 30 C was less for roots of intact plants than for excised roots. Transpiration increased as temperature increased, but herbicide uptake and transpiration were unrelated.

Duration and density experiments were conducted in the field to measure horsenettle (Solanum carolinense L. # SOLCA) interference with Spanish and runner-type peanuts (Arachis hypogaea L. ‘Pronto’ and ‘Florunner′). Spanish peanut yield generally was higher with 6 to 8 weeks of weed-free maintenance. Horsenettle interference for 6 to 8 weeks did not decrease the yield of Spanish peanuts, and interference for 6 weeks did not decrease yields of runner peanuts. Weed-free maintenance for 2 or more weeks allowed increased runner peanut yield when compared to weedy plots. Linear regression predicted a 69 kg/ha Spanish peanut yield increase for each week of weed-free maintenance. Linear regression predicted a Spanish peanut yield reduction of 40 kg/ha for each week of weed interference in 1983, the only year in which the slopes of the regressions were statistically significant Curvilinear equations with the runner-type cultivar predicted an 81 kg/ha yield increase or 96 kg/ha decrease for each week of weed-free maintenance or weed interference, respectively. In 1 of 2 yr, Spanish peanut yield was reduced by horsenettle at a density of 32 plants/10 m of row.

In at least three metabolic processes in wheat (Triticum aestivum L. var. ‘Arthur’) shoots inhibitory activity can be related to the chemical structure of substituted pyridazinones. Inhibitory activities include: inhibition of the Hill reaction and photosynthetic CO2 fixation; inhibition of carotenoid biosynthesis accompanied by photodestruction of chlorophyll; and interference with the formation of chloroplast membrane polar lipids. The parent compound, pyrazon [5-amino-4-chloro-2-phenyl-3(2H)-pyridazinone], inhibits the Hill reaction and photosynthetic CO2 fixation. Trifluoromethyl substitution of the phenyl ring of pyrazon, mono-methyl substitution of the amine, or substitutions at both positions result in inhibition of carotenoid biosynthesis. However, both substitutions are required for maximum effect. Substitutions onto the molecular structure of pyrazon are also related to alterations in the formation of membrane polar lipids. Dimethyl substitution of the amine of pyrazon is related to a decrease in linolenic acid accompanied by an increase in linoleic acid without a shift in the relative proportion of saturated to unsaturated fatty acids of the membrane lipids. The trifluoromethyl substitution of the phenyl ring and mono-methyl substitution of the amine are related to a shift toward a higher proportion of saturated fatty acids of chloroplast membrane lipids. Results obtained with diuron [3-(3,4-dichlorophenyl)-1,1-dimethylurea] and with dark-grown wheat tissue indicated that activity of the pyridazinones on the formation of membrane lipids was probably not related to inhibition of the Hill reaction.

Growth chamber and laboratory experiments evaluated the effects of seed-soil microsite characteristics on seed germination. When corn (Zea mays L. ‘Pioneer 3541’), soybean [Glycine max (L.) Merr. ‘Corsoy 79’], velvetleaf (Abutilon theophrasti Medic. ♯ ABUTH), and giant foxtail (Setaria faberi Herrm. ♯ SETFA) were seeded among soil aggregates 2.50 to 1.25 cm in diameter in the growth chamber, there was decreased seedling emergence with decreasing frequency of irrigation. These same species seeded inside artificial soil aggregates showed increased seedling emergence with decreasing frequency of irrigation. The germination of corn and soybean seed inside 0.4-g, fully moist soil aggregates incubated in the laboratory under high relative humidity conditions was significantly decreased relative to seed incubated in the absence of soil. Velvetleaf and giant foxtail germination was significantly reduced by 0.1-g soil aggregates. Rice (Oryza sativa L.) had 68% emergence from 1.0-g, fully moist soil aggregates, whereas larger seeded corn and soybean had only 20 and 10% emergence, respectively. Germination inhibition of giant foxtail seed by fully moist soil was partially reversed by incubating seed-containing aggregates in an atmosphere of 75% oxygen.

Weed interference with crop growth is often attributed to water, nutrient, or light competition; however, specific physiological responses to these stresses are not well described. This study's objective was to compare growth, yield, and gene expression responses of corn to nitrogen (N), low light (40% shade), and weed stresses. Corn vegetative parameters from V2 to V12 stages, yield parameters, and gene expression using transcriptome (2008) and quantitative polymerase chain reaction (qPCR) (2008/09) analyses at V8 were compared among the stresses and with nonstressed corn. N stress did not affect vegetative parameters, although grain yield was reduced by 40% compared with nonstressed plants. Shade, present until V2, reduced biomass and leaf area > 50% at V2, and recovering plants remained smaller than nonstressed plants at V12. However, grain yields of shade-stressed and nonstressed plants were similar, unless shade remained until V8. Weed stress reduced corn growth and yield in 2008 when weeds remained until V6. In 2009, weed stress until V2 reduced corn vegetative growth, but yield reductions occurred only if weed stress remained until V6 or later. Principle component analysis of differentially expressed genes indicated that shade and weed stress had more similar gene expression patterns to each other than they did to nonstressed or N-stressed tissues. However, corn grown in N-stressed conditions shared 252 differentially expressed genes with weed-stressed plants. Ontologies associated with light/photosynthesis, energy conversion, and signaling were down-regulated in response to all three stresses. Shade and weed stress clustered most tightly together, based on gene expression, but shared only three ontologies, O-METHYLTRANSFERASE activity (lignification processes), POLY(U)-BINDING activity (posttranscriptional gene regulation), and stomatal movement. Based on morphologic and genomic observations, weed stress to corn was not explained by individual effects of N or light stress. Therefore, we hypothesize that these stresses share limited signaling mechanisms.

Medusahead, Elymus caput-medusae L., is currently the worst range weed in Idaho. It has gained this distinction because of its rapid migration, its vigorous competitive nature, and its extremely low forage value. In the absence of effective control measures, the medusahead problem is becoming more acute each year. The threat that the weed poses to Idaho's livestock industry is sufficient to merit grave concern.

Weeds are selected to produce overwhelming propagule pressure, and while vertebrate and invertebrate seed predators destroy a large percentage of seeds, their ecosystem services may not be sufficient to overcome germination site limitations. Cover crops are suggested to facilitate seed predation, but it is difficult to disentangle reductions in weed recruitment attributable to granivores from those due to plant competition. Using common lambsquarters as a focal weed species, we used experimental seed subsidies and differential seed predator exclusion to evaluate the utility of vertebrate and invertebrate seed predators in fallow, killed cover crop, and living mulch systems. Over two growing seasons, we found that seed predators were responsible for a 38% reduction in seedling emergence and 81% reduction in weed biomass in fallow plots following simulated seed rain, suggesting that granivory indeed overcomes safe-site limitation and suppresses weeds. However, the common lambsquarters densities in ambient seedbanks across fallow and cover crop treatments were high, and seed predators did not impact their abundance. Across the study, we found either neutral or negative effects of vertebrate seed predators on seed predation, suggesting that invertebrate seed predators contribute most to common lamnsquarters regulation in our system. These results imply that weed seed biocontrol by invertebrates can reduce propagule pressure initially following senescence, but other tools must be leveraged for long-term seedbank management.

Fifty-three fungal strains belonging to 15 mainly Fusarium species were isolated from branched broomrape plants. Their virulence was assessed using a plastic bag system, and they were grown both in liquid and on solid media, extracted, and the extracts were chemically analyzed and biologically assayed to find new metabolites that inhibit germination of branched broomrape to estimate the production of fusaric and dehydrofusaric acids by Fusarium strains and evaluate their possible involvement as virulence factors and their practical use as biomarkers to make the selection of potential mycoherbicides easier, and to ascertain whether toxins affected mammals. Nine strains proved to be highly virulent and 18 strains produced fusaric and dehydrofusaric acid at concentrations from 4 to 165, and from 9 to 204 mg L−1 respectively. Fifteen extracts from solid cultures caused high mortality when assayed on brine shrimps. Five extracts from liquid cultures caused total inhibition of seed germination. Some strains may be considered as sources of new biocontrol agents but virulence was not always positively correlated to the production of phytotoxic compounds.

Silverleaf sunflower (Helianthus argophyllus Torr. & Gray) is a weed in the sandy soils of south and southeast Texas. The young plant parts are densely white-tomentose. This pubescence greatly increased laboratory and field-measured light reflectance over the 0.5- to 2.5-μm waveband compared with the sparsely-hairy leaves of common sunflower (Helianthus annum L.). This increased reflectance caused silverleaf sunflower's image on Eastman Kodak Aerochrome infrared color type 2443 film to be “pinkish” compared with darker magenta images for other plant species. Therefore, aerial photography may be useful to distinguish silverleaf sunflower plants from other plant species to locate its endemic areas, monitor its spread, and delineate areas needing weed control.

Green foxtail [Setaria viridis (L.) Beauv. # SETVI] and proso millet [Panicum miliaceum (L.) #PANMI] growing under high water stress for 3 days before + 3 days after foliar applications of the methyl ester of haloxyfop {2-[4-[[3-chloro-5-(trifluoromethyl)-2-pyridinyl] oxy] phenoxy] propanoic acid} developed less herbicide injury than plants growing under low water stress. There was a 32% reduction in the retention of haloxyfop sprays by proso millet when growing under high water stress than when growing under low water stress. Haloxyfop spray retention by green foxtail was not affected by the degree of water stress. The translocation of 14C out of 14C-haloxyfop-treated leaves was significantly greater in plants of both species when growing under low water stress than when growing under high water stress. The conversion of 98% of the absorbed 14C-haloxyfop-methyl to 14C-haloxyfop and unidentified polar metabolites occurred in both species within 12 h of application. The green foxtail growing under low water stress contained greater concentrations of 14C-haloxyfop than did plants growing under high water stress. Levels of 14C-haloxyfop were the same in proso millet growing under high and low water stress. The reduced injury of green foxtail and proso millet growing under high water stress from postemergence sprays of haloxyfop can be attributed to a reduction in herbicide translocation in both species, plus a reduction in spray retention by proso millet or changes in the metabolism of the herbicide in green foxtail, which results in a reduction in the concentration of haloxyfop.

Experiments were conducted in the North Carolina State University Phytotron greenhouse and field locations in Clayton, Rocky Mount, and Lewiston-Woodville, NC, in 2002 to determine the effect of glyphosate on pollen viability and seed set in glyphosate-resistant (GR) corn. Varieties representing both currently commercial GR corn events, GA21 and NK603, were used in phytotron and field studies. All glyphosate treatments were applied at 1.12 kg ai ha−1 at various growth stages. Regardless of hybrid, pollen viability was reduced in phytotron and field studies with glyphosate treatments applied at the V6 stage or later. Scanning electron microscopy of pollen from affected treatments showed distinct morphological alterations correlating with reduced pollen viability as determined by Alexander stain. Transmission electron microscopy showed pollen anatomy alterations including large vacuoles and lower starch accumulation with these same glyphosate treatments. Although pollen viability and pollen production were reduced in glyphosate treatments after V6, no effect on kernel set or yield was found among any of the reciprocal crosses in the phytotron or field studies. There were also no yield differences among any of the hand self-pollinated (nontreated male × nontreated female, etc.) crosses. Using enzyme-linked immunosorbent assay to examine CP4-5-enolpyruvlshikimate-3-phosphate synthase expression in DKC 64-10RR (NK603) at anthesis, we found the highest expression in pollen with progressively less in brace roots, ear leaf, anthers, roots, ovaries, silks, stem, flag leaf, and husk.

The level of genetic variation for tolerance to herbicides was quantified in populations of slender wild oat (Avena barbata Brott. # AVEBA), wild oat (Avena fatua L. # AVEFA), and godetia (Clarkia williamsonii Lewis & Lewis) that had not been previously exposed to herbicides. Seedlings of wild oat and godetia were treated with barban (4-chloro-2-butynl-m-chlorocarbanilate) and bromoxynil (3,5-dibromo-4-hydroxybenzonitrile), respectively. The plants were rated for phytotoxic effects following treatment. A one-way analysis of variance on arcsin-transformed phytotoxicity ratings showed significant amounts of inter- and intrapopulation variability for herbicide reaction. Furthermore, the amount of genetic variance for herbicide reaction is higher than expected on the basis of mutation alone, suggesting selection favoring genes conferring herbicide tolerance occurs in natural populations.

Threehorn bedstraw is an important dicotyledonous weed in southern Australia that is particularly difficult to control in pulse crops. Knowledge of the germination ecology of this weed would facilitate development of effective weed-control programs. Experiments were conducted to study the germination of two populations, Roseworthy Campus (RC) and Yorke Peninsula (YP), of threehorn bedstraw from South Australia. In the absence of chilling, seeds germinated only in the darkness. Germination was considerably higher under an alternating day/night temperature range of 13/7 C compared with 20/12 or 25/15 C day/night temperature. Germination was inhibited by light; however, when seeds were subsequently transferred to complete darkness they germinated readily. Potassium nitrate (0.005 M KNO3) and gibberellic acid (0.001 M GA3) stimulated germination in the darkness in both populations. This concentration of KNO3 increased germination of the RC and YP populations from 26 and 37% to 56 and 68%, respectively; however, higher concentrations of KNO3 inhibited germination. GA3 added in combination with KNO3 further increased germination to 81 and 94%, respectively. Germination was also promoted by cold-stratification treatment (5 C). Complete germination (100%) was achieved within 4 wk of cold stratification, when seeds were incubated in sand. In the field, seedling recruitment of both populations was higher under minimum tillage (25 to 27%) than no-till (15 to 18%) conditions, reflecting greater exposure of seeds to light under no-till systems.

Information about weed biology and weed population dynamics is critical for the development of efficient weed management programs. A field experiment was conducted in Fayetteville, AR, during 2014 and 2015 to examine the effects of Palmer amaranth (Amaranthus palmeri S. Watson) establishment time in relation to soybean [Glycine max (L.) Merr.] emergence and the effects of A. palmeri distance from the soybean row on the weed’s height, biomass, seed production, and flowering time and on soybean yield. The establishment time factor, in weeks after crop emergence (WAE), was composed of six treatment levels (0, 1, 2, 4, 6, and 8 WAE), whereas the distance from the crop consisted of three treatment levels (0, 24, and 48 cm). Differences in A. palmeri biomass and seed production averaged across distance from the crop were found at 0 and 1 WAE in both years. Establishment time had a significant effect on A. palmeri seed production through greater biomass production and height increases at earlier dates. Amaranthus palmeri that was established with the crop (0 WAE) overtopped soybean at about 7 and 10 WAE in 2014 and 2015, respectively. Distance from the crop affected A. palmeri height, biomass, and seed production. The greater the distance from the crop, the higher A. palmeri height, biomass, and seed production at 0 and 1 WAE compared with other dates (i.e., 2, 4, 6, and 8 WAE). Amaranthus palmeri establishment time had a significant impact on soybean yield, but distance from the crop did not. The earlier A. palmeri interfered with soybean (0 and 1 WAE), the greater the crop yield reduction; after that period no significant yield reductions were recorded compared with the rest of the weed establishment times. Knowledge of A. palmeri response, especially at early stages of its life cycle, is important for designing efficient weed management strategies and cropping systems that can enhance crop competitiveness. Control of A. palmeri within the first week after crop emergence or reduced distance between crop and weed are important factors for an effective implementation of weed management measures against A. palmeri and reduced soybean yield losses due to weed interference.

Products were collected from rhizospheres of a hydroponically grown sorghum-sudangrass hybrid [Sorghum bicolor (L.) Moench. × Sorghum sudanense (P.) Stapf. ‘DeKalb SX17+’] (SSH) and tested for effects on seedling root growth of selected species. Phytotoxic activity was recovered in CH2Cl2 extracts of aqueous solutions of the rhizosphere products. Significant inhibition of annual ryegrass (Lolium multiflorum L.) and alfalfa (Medicago sativa L. was produced by ca. 200 ppmw (H2O weight basis) of the most phytotoxic fraction in filter-paper-disk bioassays. In soils, 70 ppmw (soil weight basis) inhibited alfalfa but not ryegrass. Among several bioassay species, alfalfa was the most sensitive and curly dock (Rumex crispus L. ♯ RUMCR) the least. Treatment of SSH plants with herbicides increased the phytotoxicity but not the amounts of materials collected. Sethoxydim {2-[1-(ethoxyimino)-butyl]-5-[2-ethylthio)propyl]-3-hydroxy-2-cyclohexen-1-one} was detected in rhizosphere products from plants treated with the herbicide. Amounts of products collected from rhizospheres of common rye (Secale cereale L. ‘Wheeler’), johnsongrass [Sorghum halepense (L.) Pers. ♯ SORHA], and cucumber (Cucumis sativus L. ‘PI 169391’) were similar to those from SSH, but their phytotoxicities differed. Johnsongrass products were most phytotoxic, those from common rye and SSH were similar in phytotoxicity, and those from cucumber were least phytotoxic.