Field studies were conducted in northern Greece, in 1997 and 1998, to investigate the effect of nitrogen fertilization and red rice density on interference between red rice and two rice cultivars (Thaibonnet, Ariette). Interference between rice and red rice began 3 wk after rice emergence, but was not affected by the increasing nitrogen rate from 100 to 150 kg N ha–1. Dry weight of both rice cultivars was proportionally reduced with increasing red rice interference duration and density, but dry weight of Thaibonnet was reduced more than that of Ariette. At harvest, grain yield of Thaibonnet was reduced by 58% because of the occurrence of 40 red rice plants m–2, whereas that of Ariette was reduced by 46%. Red rice interference affected panicle number more than 1,000 grain weight in both rice cultivars. The reduction of all yield components was greater in Thaibonnet than in Ariette. Dry weight and stem or panicle number of red rice plants grown with either of the two rice cultivars increased with increasing red rice density and were greater most of the time when grown with Thaibonnet than with Ariette. Ten weeks after rice emergence, red rice plants were 14 and 35 cm taller than the Ariette and Thaibonnet plants, respectively. Shattering of red rice plants ranged from 63 to 79% and was greater grown with Thaibonnet than with Ariette, but it was not affected by nitrogen fertilization and red rice density.

Giant reed is an extremely aggressive riparian invader in California. Little is known about its response to nitrogen, which is often elevated in watersheds downstream from agricultural fields and wastewater treatment facilities. Two pot-experiments were conducted to quantify physiological responses of giant reed, and a co-occurring riparian species, common threesquare, to added nitrogen and to investigate a possible enhancement effect of nitrogen on the ability of giant reed to spread laterally belowground into a competitive environment. The first experiment measured shoot height, tissue biomass, and leaf area of giant reed and common threesquare, both herbaceous perennials, grown in pots with and without added nitrogen. The second experiment measured lateral rhizome growth, tissue biomass, and tiller production of giant reed in planters subjected to four possible treatments: with or without competition and with or without added nitrogen. Competition planters had previously been colonized by common threesquare and no-competition planters were unoccupied. Nitrogen-treated plants from the first experiment had greater overall shoot height. With added nitrogen, giant reed produced more root and shoot biomass, whereas common threesquare produced more rhizome and shoot biomass. In the second experiment, added nitrogen resulted in significantly greater rhizome length and greater production of tillers by giant reed regardless of competition. In competition plantings without added nitrogen, giant reed tiller production was reduced, whereas the addition of nitrogen nearly restored tiller production to levels attained without competition. Neither nitrogen nor competition significantly affected giant reed biomass production. Results of these experiments indicate the positive response of giant reed and a native riparian species to nitrogen enrichment and suggest that nitrogen can compensate for the effects of competition on giant reed in some cases. As a result, this species might be able to penetrate some environments without negative impacts from competing vegetation.

Species studied at Lincoln, Nebraska for 4 years were downy brome (Bromus tectorum L.), hairy chess (B. commutatus Schrad.), Japanese chess (B. japonicus Thunb.), and cheat (B. secalinus L.). These grasses were usually winter annuals, but also became spring annuals or biennials when sown in late March or early April.

In the greenhouse, an inductive period either of vernalization or short day lengths appeared necessary for panicle production by downy brome and hairy chess. For both species, long day lengths following the inductive period were necessary for flowering.

When a mixture of smooth bromegrass (Bromus inermis Leyss.) and 5.4 per cent of weed bromegrasses was sown in the fall, weed bromes made up 48 per cent of the total basal density and 87 per cent of the total seed yield the following summer. Panicles of weed bromes were absent or nearly so when this mixture was sown soon after April 1.

The best time for mowing hairy chess and downy brome for their control in pasture was approximately one week after the heads had emerged.

Practical control of weed bromegrasses has generally been obtained by prevention of seed production for two years. A practical program has included killing all emerged weed bromegrasses by tillage between April 1 and heading. A spring cereal or row crop, such as corn or sorghum, was then planted. The next season, timely tillage followed by the above crops, or summer fallow, has permitted return of fields to the usual alternate winter wheat and fallow sequence.

Effective control of weed bromegrasses also was accomplished by isopropyl N-phenylcarbamate (IPC), isopropyl N-(3-chlorophenyl) carbamate (CIPC), 3-(p-chlorophenyl)-1,1-dimethylurea (monuron), sodium salt of trichloracetic acid (TCA), 3,6–endoxohexahydrophthalic acid (endothal), 4,6-dinitro-o-sec-amylphenol (DNAP), and mixtures of these herbicides.

Laboratory and greenhouse experiments were conducted to determine the effect of light, temperature, solution pH, solution osmotic potential, and oxygen concentration on pitted morningglory germination and radicle plus hypocotyl elongation, and seeding depth on its subsequent emergence. Daily exposure of seed to natural light resulted in lower germination than in darkness, whereas germination was not influenced by brief exposure to red or far-red light. Germination occurred over a wide range of constant temperatures, from 7.5 to 52.5 C, with optimum germination between 20 and 25 C. Germination occurred at solution pH range of 3 to 10 and was optimal from pH 6 to 8. Radicle plus hypocotyl elongation was influenced by the interaction of temperature and solution pH. A combination of acidic medium (pH 6) and high temperature (30 C) resulted in the greatest radicle plus hypocotyl length of 7.6 cm after a 7-d incubation. Germination and radicle plus hypocotyl elongation, averaged over the 15 and 30 C temperature, decreased with increasing moisture stress, with less than 3% normalized germination at −1.0 MPa. Germination was 29, 40, and 51% at 2, 10, and 20% oxygen, respectively, averaged over 15 and 30 C. Germination of seed lying on the soil surface covered with filter paper or without any cover was similar, averaging 64%. Normalized emergence, relative to germination on the soil surface, decreased with increasing burial depth to 4% emergence at 10 cm, with a mean emergence depth of 4.1 cm, averaged across two soil types.

A model that describes the emergence of ripgut brome was developed using a two-season data set from a no-tilled field in northeastern Spain. The relationship between cumulative emergence and hydrothermal time (HTT) was described by a sigmoid growth function (Chapman). HTT was calculated with a set of water potentials and temperatures, iteratively used, to determine the base water potential and base temperature. Emergence of ripgut brome was well described with a Chapman function. The newly-developed function was validated with four sets of data, two of them belonging to a third season in the same field and the other two coming from independent data from Southern Spain. The model also successfully described the emergence in different field management and tillage systems. This model may be useful for predicting ripgut brome emergence in winter cereal fields of semiarid Mediterranean regions.

Cadillo is an invasive species commonly found in pastures, rangelands, and disturbed areas. It is becoming a significant problem weed in Florida pastures and natural areas. The objectives of this research were to determine effective techniques to break seed dormancy and the effect of light, temperature, pH, water stress, and depth of seed burial on Cadillo germination. Cadillo seeds had significant levels of innate dormancy imposed by a hard seed coat; chemical scarification was the most effective technique for removing dormancy. Seeds germinated from 15 to 40 C, with an optimal temperature of 28 C. Germination was unaffected by pH levels. Water stress below −0.2 MPa reduced seed germination. Cadillo germination was not light-dependent and seeds emerged from depths up to 9 cm, with the greatest occurring emergence near the soil surface. Considering that Cadillo seed can germinate under a wide range of environmental conditions, it is not surprising that it has become a serious invasive weed in Florida.

The nitrogen (N) response of competing plants may be affected by photosynthetically active radiation (PAR) availability and maximum potential growth rate, which determine N requirements. The responses of two crop (corn and soybean) and six weed species (common lambsquarters, common waterhemp, giant foxtail, velvetleaf, wild mustard, and woolly cupgrass) in low and high (150 and 450 μmol m−2 s−1) PAR levels to daily fertilization with either low or high (0.2 or 7.5 mM) NH4NO3 levels were studied. Leaf area of all species responded positively to N by 8 d after emergence (DAE) when grown in high PAR; in low PAR, most species did not respond until 11 DAE. Dry weight and leaf area of all species at 18 DAE were greater with high than with low N. These responses to high N were also greater in high than in low PAR for all species. Dry weights with high N were up to 100% greater in low PAR and up to 700% greater in high PAR than dry weights with low N. These responses suggest that low PAR reduced the benefit of N to the plants. The regression of relative growth rate (RGR) with high N to RGR with low N had a slope that was less than unity (β = 0.79), indicating that species with a higher RGR with high N experienced greater decreases in RGR with low N. Similarly, the sensitivity (change in RGR) of plants grown with high and low N was positively related to RGR with high N. RGR differences among crop and weed species may be related to differences in N requirement that could be exploited for weed management. RGR and seed size were negatively correlated, which may explain previous observations that small-seeded weeds were more sensitive to environmental stress.

Imazethapyr was applied at 0.14 kg ae/ha to soybean, peanut, sicklepod. Florida beggarweed, and redroot pigweed as either a soil, foliar, or soil plus foliar application. Soybean and peanut were the most tolerant species; redroot pigweed was the most sensitive, with sicklepod and Florida beggarweed being intermediate. Foliar or foliar plus soil applications were more effective in reducing sicklepod and Florida beggarweed fresh weights than soil application alone. Foliar absorption of 14C-imazethapyr 72 h after treatment was greater than 90% for soybean, peanut, sicklepod, and redroot pigweed, but only 77% in Florida beggarweed. For the species evaluated, the amount translocated from the treated leaf ranged from 5 to 16% after 72 h. Within this same time period, an average of 90% of the root-absorbed imazethapyr had been translocated to the shoot in all species except peanut. The half-life of imazethapyr was 6.6, 6.5, 14.4, 24.0, and 32.1 days in soybean, peanut, Florida beggarweed, sicklepod, and redroot pigweed, respectively. Tolerance was most closely associated with imazethapyr half-life within these species.

Field studies were conducted from 1991 through 1993 to determine the effects of stale seedbed management practices on weed control in peanut. Main plots were four levels of stale seedbed management: deep till (23 cm) and plant the same day (standard system), deep till 6 wk early and shallow till (7.6 cm) at 2 wk intervals prior to planting, deep till 6 wk early and application of glyphosate (1.1 kg ai ha−1) 1 wk prior to planting, and deep till 6 wk early without additional treatment prior to planting. Sub-plots were three levels of weed management following peanut planting; intensive, basic, and cultivation alone. Stale seedbed management practices stimulated weed emergence when followed by other control measures prior to planting. Populations of Florida beggarweed, Texas panicum, and yellow nutsedge were lower when stale seedbeds were shallow tilled at 2 wk intervals prior to planting, resulting in greater peanut yields. Weeds on nontreated stale seedbeds were difficult to control once peanut was planted and reduced yields. Stale seedbed management practices generally had no effect on the quantity of foreign material contaminants originating from weeds, soil, or peanut plant in harvested peanut. These results indicate that shallow tillage on stale seedbeds can reduce weed populations prior to planting and increase peanut yields.

Crop residues acting as mulches can influence weed seedling emergence and weed biomass. A field study was conducted to evaluate the effect of rice residue amounts (0, 3, and 6 t ha−1) on seedling emergence of eight weed species in zero-till dry-seeded rice. The highest seedling emergence of spiny amaranth, southern crabgrass, crowfootgrass, junglerice, eclipta, goosegrass, and Chinese sprangletop was observed in the absence of residue. Seedling emergence of these weeds declined with increasing residue amounts; however, the greatest and most substantial reductions in emergence occurred with 6 t ha−1 of residue. The presence of residue also resulted in less weed biomass than with the no-residue treatment. The emergence and biomass of threelobe morningglory seedlings, however, were not influenced by residue amounts. The use of residue also increased the time taken to reach 50% of maximum emergence for some species, for example, spiny amaranth and Chinese sprangletop. The results of our study suggest that the use of residue at high rates can help suppress seedling emergence and growth of many weeds. However, there is a need to integrate other weed management strategies with residue retention to achieve season-long weed control.

Preplanting treatment of filter paper or soil containing α,α,α-trifluoro-2,6-dinitro-N,N-dipropyl-p-toluidine (trifluralin) with any of a wide variety of plant lipids prevented the phytotoxic action of the herbicide in laboratory situations. The protective action of externally applied lipids was attributed to preventing sorption of trifluralin by seedlings. However, protection by internal lipids also appeared likely. A significant correlation (1% level) was demonstrated between seedling sensitivity to trifluralin and the lipid content of dry seeds of 11 weed and crop species. Artificially induced increases in the internal lipid content of wheat (Triticum aestivum L. ‘Mediterranean’) seed resulted in greater tolerance of seedlings to trifluralin. Therefore, it is hypothesized that selective phytotoxicity of trifluralin to young seedlings is determined in part by the amount of endogenous lipids available to trap trifluralin and keep it from its site of phytotoxic action. Tolerance above that predicted from seed lipid content was observed in a twelfth species of the study.

Identification of associations between site properties and weed species abundance led to the generation of hypotheses as to why weed populations occur where they do, or do not, in agricultural fields. The objective of this research was to use a multivariate statistical technique, canonical correlation analysis, to identify the associations. Two continuous Zea mays production fields under center-pivot irrigation in the central Platte River Valley of Nebraska were grid-sampled between 1994 and 1997 for nine site properties and six to seven weed species. Weed species were identified and counted just prior to postemergence weed control in two adjacent quadrats (1 by 0.38 m) at each grid sampling point. These quadrats represented untreated weed populations emerging between crop rows and treated populations that survived preemergence herbicide banded within the crop row. Canonical correlation analysis identified one to five significant correlations between linear combinations of site properties and weed species abundance depending on field site, years, and between- vs. on-crop row weed populations. The first pair of linear combinations consistently described an association that separated weed species across a gradient of topography and soil type. The second pair of linear combinations described associations between weed species and soil fertility. In all cases, it was hypothesized that management practices strongly interacted with site properties to create the observed associations with weed populations. Other hypothesized mechanisms for weed patchiness include patchiness in available soil moisture that would influence weed seed germination, emergence, and seedling growth. Additional variation in plant-available preemergence herbicide concentration across the field site would vary weed control efficacy. Another mechanism would be variation in soil fertility that affects the growth, reproduction, and competitive ability of both the crop and the weed.

Purposefully introduced plants that have become weeds cost the United States millions of dollars annually by reducing yields of crops and livestock, clogging navigable and recreational waterways, reducing productivity and value of grazing lands, poisoning humans and livestock, and increasing weed control costs. Research is in progress to develop methods of intercepting foreign plants that might be toxic to livestock if introduced on western ranges and pastures. Methods of intercepting include identifying poisonous plants from seed inventories, chemical analysis, chemotaxonomic investigations of biochemical relationships between related taxa, and feeding suspected plants to laboratory and domestic animals.

Tall fescue vegetative filter strips 0.5 to 4.0 m wide were evaluated for their ability to reduce losses of metolachlor, metribuzin, and runoff (water and sediment) in conventionally tilled soybean. Differences in the parameters studied were significant between filter and no filter strips, regardless of filter strip width. Two days after treatment, metribuzin concentration in runoff from the unfiltered treatment was 231 ng ml−1; filter strips reduced this amount to 119 ng ml−1 or less. Similar trends were observed with metolachlor, with concentrations of 1,009 ng ml−1 from the unfiltered, whereas filter strips of any width reduced this to 523 ng ml−1 or less. Metribuzin loss during the growing season was 41 g ai ha−1, or 9.8% of the amount applied when no filter strip was present. The addition of a filter strip, regardless of width, reduced cumulative metribuzin losses to 11 g ha−1 or less. Similar results were noted with metolachlor. Filter strips, regardless of width, reduced cumulative runoff and sediment loss at least 46 and 83%, respectively.

BAS 9052 {2-[1-(ethoxyimino)-butyl]-5-[2-(ethylthio)propyl]-3-hydroxy-2-cyclohexen-1-one} effectively controlled annual grasses in soybean [Glycine max (L.) Merr.] alone or in combination with bentazon [3-isopropyl-1H-2,1,3-benzothiadiazin-4(3H)-one 2,2-dioxide] in greenhouse and field experiments. The activity of diclofop {2-[4-(2,4-dichlorophenoxy)phenoxy] propanoic acid} on annual grasses was reduced if combined with bentazon in a tank mixture. The wettable powder formulation of bentazon, as well as a day temperature of 30 C, slightly reduced the antagonistic interaction from that of the emulsifiable concentrate formulation of bentazon or a day temperature of 15 C. In greenhouse studies ‘Corsoy’ soybean plants were injured by the diclofop-bentazon combination. In field studies, however, the injury was not sufficient to reduce grain yields.

The evapotranspiration of six dominant emersed and floating weeds was studied. Evapotranspiration of water-chestnut (Trapa natans L. var. bispinosa Roxb.), waterlettuce (Pistia stratiotes L.), and swamp morningglory (Ipomea aquatica Forsk.) did not increase water losses significantly. Evapotranspiration of waterhyacinth (Eichornia crassipes (Mart) Solms.) was 30 to 40% higher, that of narrowleaf cattail (Typha augustifolia L.) was 60 to 70% higher, and that of purple nutsedge (Cyperus rotundus L.) was 130 to 150% higher than evaporation from a free water surface under equivalent conditions. Positive correlations (significant at the 1% level) were found between some meterological factors (daily maximum air temperature and daily average wind velocity), evaporation from free water surface, and evapotranspiration from aquatic plants studied.

Roads and streams are important conduits for the spread of invasive species, and croftonweed is an invasive plant in southwest China. We established six transects along one road and stream in Huili County, Sichuan Province to determine the pattern of invasion and identify major factors potentially influencing it. Results demonstrated that the invasion (cover, abundance, and number of clusters) of croftonweed declined significantly with distance from the road and stream. However, after compensation for environmental factors, only distance from the road was significantly negatively related to number of clusters of this plant. Instead, native plant species cover and richness were significantly negatively associated with croftonweed invasion along the road; elevation and native species cover were negatively correlated with the invasion along the stream. These findings indicated that roads and streams were main conduits for the spread of croftonweed in southwest China, and reduced native species cover could facilitate invasion by croftonweed in habitats along roads and streams.

Buckwheat residues can suppress both emergence and growth of weeds, but the mechanisms of this suppression are not well understood. The main objectives of this research were to evaluate the possible role of (1) low initial nitrogen (N) availability and (2) fungal pathogens in this suppression for three sensitive weed species: Powell amaranth, shepherd's-purse, and corn chamomile. Growth chamber experiments were conducted comparing weed emergence and growth in bare soil or soil with freshly incorporated buckwheat residue at multiple rates of N fertilization with or without fungicide seed treatment. In the absence of N or fungicide addition, emergence of all weed species was reduced 40 to 70%, and dry weight was reduced 85% in buckwheat residue compared with bare soil. For all three weed species, suppression of growth by buckwheat residue was completely overcome with the addition of N. For shepherd's-purse and corn chamomile (2005 only), suppression of emergence was also overcome with the addition of N. In 2006, treatment of corn chamomile seeds with fungicide resulted in a higher emergence in buckwheat residue than in bare soil. In contrast, suppression of Powell amaranth emergence was not overcome with N fertilization or fungicide treatment. The results suggest that buckwheat-mediated changes in N dynamics account entirely for suppression of weed growth but that the mechanisms responsible for suppression of emergence by buckwheat residue vary by species. Fungal and N effects account for suppression of emergence of corn chamomile and shepherd's-purse, but the mechanism of suppression for Powell amaranth remains obscure.

The effect of storage conditions on longevity of crenate broomrape seed was determined and then used to predict and simulate new infestations. Newly harvested broomrape seed mixed with soil and placed in nylon mesh bags was buried in soil 8 and 23 cm deep or kept at room temperature in the laboratory for 6 yr. In the field, seed exhibited a seasonal pattern in germination, showing consistent and poor germination from September to January and for the rest of the year, respectively. The two burial depths tested showed no significant differences in seed germination or survival. Seed stored under laboratory conditions germinated throughout the year. The negative exponential model (% germination = A exp−-B-months of storage) described the time course of germination of crenate broomrape seed. Seed longevity decreased much quicker in the field than in the laboratory. For example, germination capacity decreased 66 and 95% and only 27 and 60% after 33 and 108 mo of soil burial or under laboratory conditions, respectively. According to these findings, the repeatedly reported infestations occurring in plots after many (up to 14) years of not growing broomrape-susceptible crops can not be attributed to the longevity of buried seed.

Curly dock (Rumex crispus L.) seeds failed to germinate in the dark, but germinated readily in light at 15, 20, and 25C. A secondary dormancy was induced in these seeds by 6 days of dark or light incubation at 30C (thermodormancy), by 6 days of dark incubation at 15C, or by a prolonged soaking in −15.7 bar polyethylene glycol-6000 (PEG) solution at 15C in light. The secondary dormancy induced at 15C was partially removed by 2-chloroethylphosphonic acid (ethephon) in presence of light; however, when the dormancy was induced at 30C, an addition of gibberellin A4+7 (GA4+7) to ethephon was necessary for effective removal of the dormancy. The ability of growth regulators to remove the secondary dormancy induced by prolonged dark incubation at 15C decreased progressively in light as the duration of dark incubation increased. The water-potential studies indicate that the thermodormancy can be attributed, at least in part, to the inability of the embryonic cells to lower their water potential to a level sufficient to cause radicle protrusion.