Laboratory experiments were undertaken to investigate the allelopathic activity of Mexican sunflower in soil as well as to determine the effect of several soil factors on activity. There was a decrease in shoot and root growth of the test plant species when grown in soil previously planted with Mexican sunflower. However, seed germination was not affected. Soil-water separated from the same soil also inhibited shoot and root growth. A water extract from Mexican sunflower leaves applied to soil also reduced shoot and root growth, but the degree of inhibition varied among test plant species. Incorporation of dry Mexican sunflower leaves into soil at the rate of 1 and 2% (w/w) inhibited the growth of rice seedlings. The phytotoxic activity of incorporated leaf residue diminished 4 wk after treatment of soil. The inhibitory effect of a leaf water extract in nonautoclaved soil was less than in autoclaved soil and the phytotoxic effect to rice seedling grown in autoclaved soil was less than in autoclaved sand. It is suggested that the decrease in allelopathic activity of Mexican sunflower extract and its residue in soil is due to the action of soil microorganisms and the soil adsorption of the active components.

Field studies were conducted in 1996, 1997, and 1998 to determine the effectiveness of several grasses as filter strips for reducing sediment and herbicide losses in runoff. Big bluestem, eastern gamagrass, switchgrass, and tall fescue reduced total runoff volume by at least 55, 76, 49, and 46%, respectively. Within the 127-d sampling period, each perennial grass filter strip investigated reduced total sediment loss in surface runoff by at least 66%. All four species reduced total fluometuron loss in runoff at least 59%. Big bluestem and eastern gamagrass reduced norflurazon loss in runoff 63 and 86%, respectively. When a filter strip was present, fluometuron and norflurazon losses did not exceed 5 and 3% of the total applied, respectively, compared to 12 and 5%, respectively, when a filter strip was not present.

Glyphosate-resistant Palmer amaranth is a serious problem in southern cropping systems. Much phenotypic variation is observed in Palmer amaranth populations with respect to plant growth and development and susceptibility to herbicides. This may be related to levels of genetic diversity existing in populations. Knowledge of genetic diversity in populations of Palmer amaranth may be useful in understanding distribution and development of herbicide resistance. Research was conducted to assess genetic diversity among and within eight Palmer amaranth populations collected from North Carolina and Georgia using amplified fragment length polymorphism (AFLP) markers. Pair-wise genetic similarity (GS) values were found to be relatively low, averaging 0.34. The highest and the lowest GS between populations were 0.49 and 0.24, respectively, while the highest and the lowest GS within populations were 0.56 and 0.36, respectively. Cluster and principal coordinate (PCO) analyses grouped individuals mostly by population (localized geographic region) irrespective of response to glyphosate or gender of individuals. Analysis of molecular variance (AMOVA) results when populations were nested within states revealed significant variation among and within populations within states while variation among states was not significant. Variation among and within populations within state accounted for 19 and 77% of the total variation, respectively, while variation among states accounted for only 3% of the total variation. The within population contribution towards total variation was always higher than among states and among populations within states irrespective of response to glyphosate or gender of individuals. These results are significant in terms of efficacy of similar management approaches both in terms of chemical and biological control in different areas infested with Palmer amaranth.

Restoration of historic fire regimes is complicated by concerns about exotic plant invasions, yet little is known of how the two may interact. Seeds of Japanese brome, spotted knapweed, Russian knapweed, and leafy spurge were subjected to fire at six fuel loads (100 to 700 g m−2) and a nonburned control. Fires were simulated with field-cured grass and time–temperature profiles were developed from thermocouples at the soil surface. Emergence was determined by species and fuel load in growth chambers. Fuel load explained 98% of variation in mean heat dosage and emergence decreased with increasing fuel load across species. Emergence was reduced 79 to 88% relative to nonburned treatment with 100 g m−2 of fuel and at least 97% with 200 g m−2 of fuel. Emergence probabilities were less than 0.01 for all species but spotted knapweed with a 300 g m−2 fuel load. Results indicate high potential for fire to disrupt the life cycle of invasive species through direct seed mortality. The relationship between fuel load and seedling emergence provides good predictability of fire effects on surface-deposited seeds. A single fire is unlikely to eradicate many invasive species because they often produce abundant seeds and some will undoubtedly escape fire. However, abrupt reductions in seedling emergence with relatively light fuel loads indicate that fire may be an effective tool for increasing mortality of invasive plant seed across a broad range of habitats.

Kochia is a troublesome weed throughout the western United States. Although glyphosate effectively controls kochia, poor control was observed in several no-till fields in Kansas. The objectives of this research were to evaluate kochia populations response to glyphosate and examine the mechanism that causes differential response to glyphosate. Glyphosate was applied at 0, 54, 109, 218, 435, 870, 1305, 1740, 3480, and 5220 g ae ha−1 on 10 kochia populations. In general, kochia populations differed in their response to glyphosate. At 21 d after treatment, injury from glyphosate applied at 870 g ha−1 range from 4 to 91%. In addition, glyphosate rate required to cause 50% visible injury (GR50) ranged from 470 to 2149 g ha−1. Differences in glyphosate absorption and translocation and kochia mineral content were not sufficient to explain differential kochia response to glyphosate.

The effects of crop rotation and management system on annual variability in weed communities and crop yields were assessed in a 4-yr study in Michigan. Variability of the weed community and corn yields were assessed using the coefficient of variation (CV) and a multivariate dissimilarity index (Bray-Curtis) that accounted for changes in both weed species abundance and composition. The treatments included two rotations: continuous corn and a corn–corn–soybean–wheat rotation, and two management systems: conventional (CONV) and organic-based (ORG). Weed biomass was significantly higher in the ORG system; however, there was no effect of crop rotation on weed biomass or number of weed species in a treatment (species richness). Annual variability in weed community composition and structure was affected by both crop rotation and management system and was highest in the ORG rotation. In contrast to the weed community, variability in corn yield was highest in the least-diverse cropping system (CONV monoculture), despite that system having a more constant weed community. Corn yield in the ORG rotation was not significantly different from that in the CONV monoculture. Results of this study suggest that management aimed at increasing cropping system diversity may have additional effects on weed communities and crop yields beyond those commonly reported, and these may have important implications for the development of more efficient and sustainable weed and crop management practices.

It has been recognized for some time that although short-chain alkyl esters of 2,4-D are very toxic, they do not translocate so well as the parent acid or the heavy esters. Two very plausible reasons for this are apparent: (1) These compounds, being highly lipoid soluble, dissolve readily in plant cuticle, and other lipoid phases, and tend to be held in this phase; (2) penetrating rapidly, these compounds are highly toxic to the treated leaves. This contact toxicity kills the mesophyll and fine vascular bundles of the leaf and stops translocation.

The phytotoxicity from preemergence applications of the herbicides 2-chloro-4,6-bis(ethylamino)-s-triazine (simazine), 3-(3,4-dichlorophenyl)-1,1-dimethylurea (diuron), isopropyl N-(3-chlorophenyl) carbamate (CIPC), 2-chloroallyl diethyldithiocarbamate (CDEC) and 2-chloro-N,N-diallylacetamide (CDAA) was measured at 17 field locations in the coastal plain of North Carolina over three years. Site selections were made with specific regard for soil organic matter, which varied from 0.7 to 48.8 percent. Phytotoxicity responses determined at four to five weeks after treatment were the amounts of herbicide required to cause a 50 percent reduction in (1) soybean dry shoot weight, (2) cotton dry shoot weight and (3) number of weedy annual grasses. Fourteen factors were measured at each location to characterize soil, climatic and biotic factors, and correlated with plant responses obtained. For simazine, diuron and CIPC the factors measured accounted for 60 percent of the plant response variation. The highest simple correlation was usually between soil organic matter and plant response. Phytotoxicity of the aromatic herbicides was inversely correlated with soil organic matter level. CDAA and CDEC usually gave reliable and repeatable performance at constant rates on soils containing between 3 and 40 percent soil organic matter. Below 3 percent soil organic matter, extreme variation in CDAA and CDEC toxicity was encountered.

In the field, soybeans [Glycine max (L.) Merr. ‘Amsoy 71′] required 4 to 6 weeks free of sunflower (Helianthus annuus L.) competition for maximum yield. Competition studies suggested that more than one type of interference was involved when sunflowers grew with soybeans. In greenhouse studies, soybean height, fresh weight, and dry weight were significantly reduced at certain sunflower and soybean densities and levels of competition. Sunflower height was not reduced by any level of competition, but sunflower fresh and dry weights were reduced with high sunflower and soybean populations. Two percent (w/w) or greater amounts of ground, mature sunflower leaves mixed into the soil reduced emergence and growth of soybeans, sorghum [Sorghum bicolor (L.) Moench ‘G-625 GBR′], and sunflower. A mixture of powdered stems and branches of sunflower also reduced growth of the three species, although it was not as phytotoxic as ground leaves. Sunflower roots apparently released exudates into the nutrient solution and soil. Sunflower root exudates inhibited sunflower emergence, and reduced sorghum, soybean, and sunflower height, fresh weight, and dry weight.

Early physiological mechanisms that occur in crop plants in response to neighboring weeds are not well understood. In this experiment, it was hypothesized that, in the absence of direct competition for resources, low red to far red ratio (R:FR) reflected from neighboring weeds will modulate the phenylpropanoid pathway, increase hydrogen peroxide (H2O2), and up-regulate the expression of ethylene biosynthesis and auxin transport genes. Laboratory experiments were conducted under conditions of nonlimiting resources using perennial ryegrass as a model weed species. We discovered that the detection by phytochrome (Phy) of low R:FR signals reflected from both biological and nonbiological sources triggered an up-regulation of ethylene biosynthesis genes and stimulated an auxin transport gene. The low R:FR also modulated the phenylpropanoid pathway resulting in a reduction in anthocyanin content and an enhancement of lignin synthesis. The presence of neighboring weeds also caused an accumulation of H2O2 in the first leaf and crown root tissues of the maize seedling. Stomata were observed to be closed as H2O2 accumulated in leaf tissue. This is the first study to report the modulation of phenylpropanoid pathway and the accumulation of H2O2 attributed to low R:FR. We further suggest that these physiological changes that occur in response to early weed competition result in a physiological cost to the crop plant, which contributes to the rapid loss in yield observed in weed competition studies conducted under field conditions.

The tremendous losses brought about by the presence of weeds in agricultural crops have been much underrated in the past. Recently it has been estimated, probably conservatively, that the total losses to agricultural producers in the United States approaches four billion dollars annually (18). Unless the basic causes of these losses and the places in which they occur can be determined, their significance cannot be fully evaluated.

Glyphosate-resistant (GR) volunteer corn has emerged as a problematic weed in corn:soybean rotational systems, partly because of the rapid increase in adoption of corn hybrids that contain traits for both glyphosate and insect resistance. Volunteer GR corn can decrease soybean yields. The objectives of this study were to quantify the impact of volunteer corn on soybean growth and yield and determine how volunteer corn densities affect western corn rootworm (WCR) emergence. Volunteer corn seed was hand-planted at targeted densities of 0.5, 2, 4, 8, 12, and 16 seeds m−2 at soybean planting and 21 d after planting to evaluate both early- and late-emerging cohorts. WCR emergence was assessed with the use of field emergence traps placed over individual corn plants in the 0.5- and 16-plants-m−2 plots in 2008 and 2009. In 2010, WCR emergence traps were also placed over individual and clumped volunteer corn plants at densities of two and eight plants m−2. Soybean yield reductions ranged from 10 to 41% where early-emerging volunteer corn densities ranged from 0.5 to 16 plants m−2. No soybean yield loss occurred with the late-emerging cohort of volunteer corn. Twice as many adult WCRs emerged from a single volunteer corn plant growing at densities of 8 and 16 plants m−2, compared with plots containing 0.5 and 2 plants m−2. These results demonstrate that controlling volunteer corn will not only prevent soybean yield loss, but also may reduce the risk of WCR larval survival after exposure to Bt (Bacillus thuringiensis Berliner derived) corn.

Activated carbon, applied over the seeded row in a band 2.5 cm wide, protected grasses from herbicides applied preemergence. The carbon rate required to protect the grasses varied with the grass species, the herbicide, and the rate of herbicide application. Most of the herbicides controlled weeds effectively between the rows. The herbicides usually controlled weeds less effectively within the carbon bands. Diuron [3-(3,4-dichlorophenyl)-1,1-dimethylurea] was inactivated most readily by the carbon and showed the greatest safety margin on the crop species. Atrazine [2-chloro-4-(ethylamino)-6-(isopropylamino)-s-triazine], simazine [2-chloro-4,6-bis(ethylamino)-s-triazine], and terbacil (3-tert-butyl-5-chloro-6-methyluracil) required higher carbon rates for crop protection than diuron and showed a much narrower safety margin on crops. Use of this technique made possible the effective control of weeds during establishment and, subsequently, the production of high quality seed in the first seed crop.

Field and greenhouse experiments were conducted to evaluate factors affecting johnsongrass [Sorghum halepense (L.) Pers. # SORHA] control in soybeans [Glycine max (L.) Merr. ‘Essex’] with three foliarly-applied herbicides. In 1980, johnsongrass height at treatment time had little effect on control due to inadequate soil moisture. In 1981, a wet year, best control was achieved on 15- or 40-cm-tall johnsongrass. BAS-9052 {2-[1-(ethoxyimino)butyl]-5-[2-(ethylthio)propyl]-3-hydroxy-2-cyclohexen-1-one} at 0.6 kg/ha gave good to excellent control at all growth stages. Two applications of 0.3 kg/ha of BAS-9052 in both years and PP-009 butyl {2-[4-[5-(trifluoromethyl-2-pyridinyloxy)] phenoxy] propanoate} in 1981 increased johnsongrass control compared to only one treatment. Mixing BAS-9052 with other herbicides significantly reduced johnsongrass control. Greenhouse studies to determine the effect of soil water on herbicide response indicated that drier soils decreased johnsongrass regrowth with PP-009 and increased regrowth with BAS-9052.

Differences in the competitive effects of three major species of foxtail on soybeans were due primarily to differences in the mature plant yields of the species. Giant foxtail (Setaria Faberi) grew more vigorously and caused greater soybean yield reductions than did either yellow foxtail (Setaria lutescens) or green foxtail (Setaria viridis). The competitive effects of velvetleaf (Abutilon theophrasti) per cwt. of mature weeds were approximately double those of yellow and green foxtail, but produced about the same yield reduction.

The compounds 3-(p-chlorophenyl-1,1-dimethylurea (monuron), 5-bromo-3-sec-butyl-6-methyluracil (bromacil), 2-chloro-4-ethylamino-6-isopropylamino-s-triazine (atrazine), 2-chloro-4,6-bis (ethylamino)-s-triazine (simazine), 1,1′-dimethyl-4,4′-dipyridylium salt (paraquat), and 3-amino-1,2,4-triazole (amitrole) were added to a complete nutrient medium on which chlorotic Nicotiana tobacum was grown in the dark. Herbicide concentrations generally were 0, 10-4, 10-5, 10-6, and 10-7 molar. Callus growth was inhibited by molar concentrations greater than 10-7 for the triazines, 10-5 for monuron, 10-4 for bromacil, 10-6 for paraquat and amitrole. At these concentrations, interference with a mechanism other than photosynthesis was responsible for inhibition of growth.

Siamweed is an asteraceous shrub native to the Neotropics that ranks among the world's most widespread and troublesome invasive species. It was introduced in several regions of Africa, Southeast Asia, and the Pacific Islands, where it severely infests natural habitats and plantation crops. Although extensive data document the weed's abundance and distribution throughout the invaded continents, the details of its current range are not fully known, especially within its native region. In this study, we used point-occurrence data and digital maps summarizing relevant environmental parameters to generate predictions for the species' geographic distributional potential—specifically, we modeled the native range of siamweed in the Neotropics using the genetic algorithm for rule-set prediction, an evolutionary computing approach. The native range occurrence data set contained 239 published and herbarium records. Models were trained on a random subset of half the points and tested using the other half. The rule sets of the native-range models were projected onto the invaded continents to predict the weed's potential for invasion, blind to its known occurrences in such regions. Native-range models predicted a wide potential distribution of siamweed throughout tropical America, from southern United States to northern Argentina and southern Brazil. The weed's occurrence has been confirmed on the northern Pacific coast, in southeast Brazil, and in other South American areas, where it was supposed to be absent. Independent model projections to Africa, Asia, and Oceania are supported by known occurrence records. Four regions are predicted to be susceptible to siamweed spread: (1) Central Africa, currently being invaded from Western Africa; (2) Infestations spreading northward from South Africa, which have already reached Swaziland and Mozambique and may extend to East Africa and Madagascar; and (3) northern New Zealand and (4) Australia, which are at risk from uncontrolled infestations on several western Pacific islands.

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.

A small assortment of microbial proteins have the ability to activate defense responses and induce necrosis in plant cells through cell signaling pathways. These proteins are of interest because of their potential use as bioherbicides and inducers of plant resistance in agriculture. A 24-kDa protein (Nep1) was purified from culture filtrates of Fusarium oxysporum, and the effects of this protein on weed leaves were investigated. This protein induced necrosis in detached leaves of Papaver somniferum, Lycopersicon esculentum, Malva neglecta, and Acroptilon repens when taken up through the petiole. The pattern and level of necrosis were dependent on the plant species. Treatment with Nep1 induced the production of ethylene in isolated leaves of various species, and the level of ethylene response was shown to be correlated to the concentration of the protein. Pretreating leaves of P. somniferum, L. esculentum, M. neglecta, and Cardaria draba with 100 µl L−1 ethylene enhanced the protein induction of ethylene biosynthesis in those leaves. Application of Nep1 (200 nM) as a spray to intact plants of Abutilon theophrasti, P. somniferum, Centaurea solstitialis, Centaurea maculosa, and Sonchus oleraceus resulted in extensive necrosis of leaves within 48 h. The results of this research are supplemental to our understanding of the role of specific polypeptides in plant/microbe interactions and demonstrates for the first time that a fungal protein can cause extensive necrosis when applied to weed species as a foliar spray.

A mitochondrial fraction isolated from cotyledons of etiolated Cucumis sativus L. (Burpee's Improved Long Green) showed respiratory activity roughly comparable to that of other reported plant mitochondrial systems. Published procedures on extraction, centrifugation, and determination of cofactor requirements were modified for this system to obtain maximum activity as measured by Warburg manometry. Succinic dehydrogenase activity in the fraction was inhibited by known inhibitors such as sodium azide, DNP, and hydroxymalonic acid; α-ketoglutarate utilization was inhibited by sodium azide.

Dichlobenil (2,6-dichlorobenzonitrile) consistently increased respiration (up to 162% of the controls, using 1.45 × 10-4 M dichlobenil in a succinate substrate mixture); 2,5-diiodo-4-hydroxybenzonitrile (ioxynil) consistently reduced O2 uptake at 0.75 × 10-6 to 0.75 × 10-3 M concentrations. Picloram (4-amino-3,5,6-trichloropicolinic acid) performed in a manner generally similar to the five following substituted benzoic acids: 2,3,6-trichlorobenzoic acid (2,3,6-TBA); 3-amino-2,5-dichlorobenzoic acid (amiben); 3-nitro-2,5-dichlorobenzoic acid (dinoben); 2-methoxy-3,6-dichlorobenzoic acid (dicamba); and 2-methoxy-3,5,6-trichlorobenzoic acid (tricamba). Using succinate as substrate, stepwise inhibition occurred in the range of 10-5 to 10-3 M herbicide; however, the α-ketoglutarate system was much less sensitive to all compounds. For example, the effects of 10-3 M amiben and dicamba on α-ketoglutarate utilization were essentially negligible. Six other herbicides studied showed either no effect or varying degrees of inhibition, depending upon concentration and substrate employed. Dichlobenil uncoupled oxidative phosphorylation at a 1.45 × 10-4 M concentration.