Although many pests constrain rice production, weeds are considered to be the major barrier to achieving optimal yields. A predictive model based on naturally occurring mixed-species infestations in the field would enable growers to target the specific weed group that is the greatest contributor to yield loss, but as of now no such models are available. In 2013 and 2014, two empirical hyperbolic models were tested using the relative cover at canopy closure of groups of weed species as independent variables: grasses, sedges, broadleaves, grasses and sedges combined, grasses and broadleaves combined, and all weed species combined. Models were calibrated using data from experiments conducted at the California Rice Experiment Station, in Biggs, CA, and validated across four sites over 2 years, for a total of 7 site-year combinations. Of the three major weed groups, grasses, sedges, and broadleaves, the only groups positively related to yield loss in the multispecies infestation were grasses. At the model calibration site, grasses and sedges combined best predicted yield loss (corrected Akaike information criterion [AICc]=−21.5) in 2013, and grasses alone best predicted yield loss (AICc=−19.0) in 2014. Across the validation sites, the model using grasses and sedges combined was the best predictor in 5 out of 7 site-years. Accuracy of the predicted values at the model validation sites ranged from 6% mean average error to 17% mean average error. No single model and set of parameters accurately predicted losses across all years and locations, but relative cover of grasses and sedges combined at canopy closure was the best estimate over the most sites and years.

This study reports evolved resistance to fenoxaprop-P in a population of sprangletop from a rice field in Thailand (BLC1). After eight applications of fenoxaprop-P, the herbicide appeared no longer effective. To confirm herbicide resistance in the BLC1 population, three experiments were conducted. First, glasshouse experiments revealed that the BLC1 population survived 600 g ai ha−1 of fenoxaprop-P without visual injury. Second, the BLC1 population was treated with fenoxaprop-P and other acetyl coenzyme A carboxylase (ACCase)–inhibiting herbicides (quizalofop-P, cyhalofop-butyl, and profoxydim) under field conditions; BLC1 exhibited resistance to all of these herbicides. Third, seeds of susceptible SLC1 and resistant BLC1 were germinated on 0.6% (v/v) agar across a range of herbicide concentrations. The resistant BLC1 population exhibited 61-, 44-, 9- and 8-fold resistance to fenoxaprop-P, cyhalofop, quizalofop-P, and profoxydim, respectively, compared with a susceptible SLC1 population. At the enzyme level, ACCase from the resistant BLC1 exhibited 30, 24, 11, 4, and 5 times resistance to fenoxaprop, cyhalofop-butyl, haloxyfop, clethodim, and cycloxydim, respectively. The spectrum of resistance at the whole plant level correlated well with resistance at the ACCase level. Hence, the mechanism of resistance to ACCase-inhibiting herbicides in this biotype of sprangletop is a herbicide-resistant ACCase. The specific mutation(s) of the ACCase gene that endows resistance in this population remains to be investigated.

Invasive species and acid rain cause global environmental problems. Creeping daisy, an invasive exotic allelopathic weed, has caused great damage in southern China, where acid rain is prevalent. The impact of the acidity of simulated acid rain (SAR) on soil nutrients, the decomposition of creeping daisy litter, and on the allelopathic potential of the surrounding soils was investigated. Litter was treated with SAR at different acidity (pH 2.5, 4.0, 5.6) or with water (pH 7.0) as a control. After 70 d, the remaining amount of creeping daisy litter, nutrient contents, and allelopathic potentials in the surrounding soil were determined. The litter decomposition was commensurate to the increase in the acidity of the SAR. Total C and N contents, NO3 −-N and available P increased, levels of NH4 +-N, the ratio of C/N and soil pH values decreased, water contents increased and then decreased, whereas available K did not significantly change in the soil surrounding the litters in response to the increase in the acidity of the SAR. Bioassays showed that SAR promoted the allelopathic activity in the soil surrounding the litter, as measured by seedling growth of turnip and radish. In conclusion, our results indicated that SAR influenced soil nutrient status, accelerated creeping daisy litter decomposition, and enhanced the allelopathic potential of its litter in the surrounding soil, suggesting that acid rain may enhance the invasiveness of creeping daisy plants.

Narrow bands of activated charcoal sprayed on the soil surface directly over the rows seeded with alfalfa (Medicago sativa L., var. Cayuga) in the field protected the legume from damage by a broadcast application of 2-isopropylamino-4-(3-methoxypropylamino)-6-methylthio-s-triazine (hereafter referred to as G-36393). Charcoal suspended in water and applied at 25 to 50 lb/A, gave adequate protection from 1.5 lb/A G-36393. Higher rates of charcoal, 100 lb/A, were necessary to protect the legume from a 3.0 lb/A application. Charcoal did not protect alfalfa seedlings from 1.5 lb/A 2-chloro-4-ethylamino-6-isopropylamino-s-triazine (atrazine).

The relationship between species richness and sample area has been characterized in many natural communities but has rarely been examined in crop–weed communities. We determined the species–area relationship in short-term (≤4 yr) and long-term (>15 yr) moldboard-plowed (MP), chisel-plowed (CP), and no-tillage (NT) fields cropped to corn and in short-term MP, CP, and NT fields cropped to soybean. A total of 10 corn fields and 10 soybean fields were sampled for species richness in 14 nested sample areas that ranged from 0.0625 to 512 m2. The influence of sample area on frequency of species occurrence was also determined. Species richness was greater in long-term NT fields than in tilled or short-term NT fields. The species–area relationship in tilled and short-term NT fields was best described by an exponential function. In contrast, a power function was the best fit for the species–area relationship in long-term NT fields. The functional minimum area required to represent 75% of the total weed species in tilled and short-term NT fields was 32 m2. A functional minimum area could not be determined in long-term NT fields because species richness continued to increase over the range of sample areas. Regression functions predicted that sample areas of 1 m2 would contain less than 50% of the observed maximum species richness in these fields. Sample areas of 36 m2 in tilled and short-term NT fields and 185 m2 in long-term NT fields were predicted to measure 75% of observed maximum species richness in these fields. Pigweed species and common lambsquarters occurred at high frequencies and were detected in most sample areas. This information could be used to better define sample area requirements and improve sampling procedures for species richness of weed communities.

A 50-year study at Fairbanks, AK, was started in 1984 to determine soil seed longevity of 17 weed species. Seed were buried in mesh bags 2 and 15 cm deep and were exhumed 0.7, 1.7, 2.7, 3.7, 4.7, 6.7, and 9.7 yr later. Viability was determined by germination and tetrazolium tests. All common hempnettle and quackgrass seed were dead after 2.7 and 3.7 yr, respectively. Less than 1% of wild oats and foxtail barley seed were viable after 3.7 yr, but > 6.7 yr were required for loss of all viability. By 9.7 yr, < 1% seed viability remained for: bluejoint reedgrass, corn spurry, pineappleweed, prostrate knotweed, and wild buckwheat From 2 to 5% of seed from common chickweed, common lambsquarters, flixweed, Pennsylvania smartweed, rough cinquefoil, marsh yellow-cress and shepherd's-purse were viable, while 62% of American dragonhead seed was still alive. Seed longevity in agricultural fields is not greater under subarctic conditions than under warmer conditions.

Intraspecific interference of populations of sunflower (Helianthus annuus L.) and velvetleaf (Abutilon theophrasti Medic), as well as interspecific populations of these two broadleaf weeds, in sugarbeets (Beta vulgaris L. ‘Mono Hy D2′) was determined in 2-yr field experiments. Sunflower was more competitive in sugarbeets than was velvetleaf. At densities of 6, 12, 18, and 24 sunflower plants/30 m of row, root yields were reduced 40, 52, 67, and 73%, respectively. At the same densities of velvetleaf, root yields were reduced only 14, 17, 25, and 30%, respectively. Interspecific interference of these two broadleaf weeds at the same densities reduced root yields 19, 36, 43, and 56%, respectively. The minimum number of weeds required/30 m of row to reduce sugarbeet root yields was predicted to be 1 for sunflower, 9 to 12 for velvetleaf, and 2 to 7 for an equal population of sunflower and velvetleaf.

Palmer amaranth (Amaranthus palmeri S. Wats. # AMAPA) residue was incorporated into soil to determine its allelopathic effects on the seedling growth of grain sorghum (Sorghum bicolor L. Moench.), cabbage (Brassica oleracea, var. capitata L.), carrot (Daucus carota L.), and onion (Allium cepa L.). Root and shoot growths were equally sensitive to the toxic effects of soil-incorporated Palmer amaranth. Growth of ‘Grand Slam’ cultivar of cabbage was 17 to 30% more sensitive than the growth of 'Sanibel’ cabbage. Growth of onion and carrot seedlings was less inhibited than either cabbage or grain sorghum. Growth of grain sorghum root was severely inhibited by 8000 and 16 000 ppm of Palmer amaranth in soil and was not affected by oven dryings other than lyophilization. Seedling growth was more severely inhibited by thyrsus and leaf tissues than by stem and root tissues of Palmer amaranth.

The technique of microradioautography was used to study the absorption and translocation of a,a,a-trifluoro-2,6-dinitro-N,N-dipropyl-p-toluidine (trifluralin) by cotton (Gossypium hirsutum L., var. Coker 201) and soybean (Glycine max (L.) Merr., var. Lee). Radioactivity from 14C-trifluralin was retained primarily on the surfaces of the roots of cotton and soybean. This was apparently due to a tenacious adsorption or binding of the radioactivity to the epidermis and cuticle. Entrance of radioactivity into the roots of these species was greatly facilitated by breaks in the epidermis, such as might occur from seedling diseases or mechanical damage. Within the roots, radioactivity was noted in the walls of the xylem vessels and cortical cells. Little movement out of the soybean roots was observed, but limited movement of radioactivity into the leaves of cotton, apparently via the metaxylem, was noted. Radioactivity accumulated in the protoxylem of the cotton stem where many elements appeared to be plugged. Results of this study indicate that care should be used in interpreting data from studies where loss of radioactivity from nutrient media is used as a criterion of herbicide absorption, and where gross radio autography is used as an indicator of herbicide translocation.

Experiments were conducted over 4 yr to investigate the effects of four postemergence herbicides on soybean [Glycine max (L.) Merr. ‘Williams 79’] yield under weed-free conditions. Bentazon [3-(1-methylethyl)-(1H)-2,1,3-benzothiadiazin-4(3H)-one 2,2-dioxide], acifluorfen {5-[2-chloro-4-(trifluoromethyl)phenoxy]-2-nitrobenzoic acid}, and naptalam/dinoseb {2-[(1-naphthalenylamino)carbonyl] benzoic acid}/[2-(1-methylpropyl)-4,6-dinitrophenol] alone and in combination with 2,4-DB [4-(2,4-dichlorophenoxy) butyric acid] applied to soybeans at the V3 and V5 soybean growth stages caused early crop injury, but soybeans outgrew most of the injury by 21 days and there was no effect on yield. Bentazon plus 2,4-DB with or without adjuvant did not affect the yield of five soybean cultivars tested.

One approach to site-specific weed control is to map weeds within a field and then divide the field area into smaller grid units. The decision to apply a herbicide to individual grid units, or decision units, is made by using yield loss models to establish an economic threshold level. However, decision units often contain weed populations with aggregated distributions. Many yield loss models have not considered this because experiments dealing with weed–crop competition typically assume uniform weed distributions. Therefore, these models may overestimate yield losses. Field experiments conducted in 1999 and 2000 compared the effects of common ragweed having a uniform distribution vs. an aggregated distribution on soybean seed yield, moisture content, and dockage. Field experiment data were used to calculate and compare economic thresholds for both distributions. Economic thresholds that considered drying costs and dockage also were compared. There was no significant difference in I parameters (yield loss as density approaches zero) between the two ragweed distributions in either year. Seed moisture content and dockage increased with increasing common ragweed densities, but increases were not significant at the break-even yield loss level. Economic threshold values were similar for both distributions with differences between aggregated and uniform of 0.14 and 0.01 plants m−2 in 1999 and 2000, respectively. The economic threshold values were reduced by 0.01 to 0.06 plants m−2 when drying costs and dockage were considered.

Venice mallow is an emerging weed problem in many regions of the world in various crops. Studies on its hard seed coat were conducted by scanning electron, light, and fluorescence microscopy. The effects of environmental factors on germination and seedling emergence were examined in laboratory experiments. Seeds possessed physical dormancy (90%) that required immersion for 0.5 h in concentrated sulfuric acid to break without damaging the seed. After scarification, 29% of seeds exhibited primary embryo dormancy. Exposure of seeds to even short periods of 15 d of prechilling induced secondary dormancy (10% germination). The surface of hard seeds had a velvety appearance from numerous papillate structures and deposits (hydrophilic material). The location of the water barrier was very superficial in the outer seed coat. Although, in hard seeds, the hilar area appeared to have vertical ruptures and the hilum fissure appeared to be open, there was no water entry. It was observed that microruptures could be the region of structural weakness of hard seeds in relation to water permeability during prolonged burial (8 mo).

The highest (60%) germination was recorded at a day/night temperature of 30/20 C with a 12-h photoperiod. No germination was measured at either 10 or 45 C constant temperature. Germination was recorded with a broad range of pH (3-11) and seeds were rather tolerant to low water potential (20% germination at –1.2 MPa). Seedling emergence was higher for seeds buried at 2 cm than for those placed on the surface (54 vs. 38%, respectively). These results showed that Venice mallow is a rather unique species that possesses a complex mechanism of dormancy (physical, primary, and secondary). Information gained in this study will be used in developing management strategies for this species.

Field studies were conducted to examine both density and duration of glyphosate-resistant (GR) horseweed interference in cotton. Two studies, one examining the effect of horseweed density and a second the duration of horseweed interference, were conducted on a site with a natural population of horseweed that were treated with glyphosate at 0.84 kg ae ha−1 prior to planting and at the 2nd and 4th cotton node growth stages. GR horseweed density effect on cotton height, maturity, and lint yield was determined at horseweed densities of 0, 5, 10, 15, 20, and 25 plants m−2. Duration of horseweed interference was evaluated when 20 horseweed m−2 were allowed to interfere with cotton from emergence to 2nd node, 6th node, 10th node, 12th node, and 1st bloom stage of cotton. The maximum cotton lint yield loss (46%) occurred when horseweed was allowed to compete with cotton from emergence to maturity at the two highest densities (20 and 25 horseweed m−2). When the data were fit to the Cousens model the estimated a (maximum yield loss) and i (yield loss per unit density as density approaches zero) were 53 ± 7.3 and 2.8 ± 0.6 SE, respectively. In both years of the study, horseweed interference from emergence to the 2nd cotton node did not reduce cotton lint yields. In 2006, cotton lint yield loss was 28% compared to 39% in 2005 when horseweed interfered with cotton from emergence until the 6th cotton node. Cotton lint yield loss was 37 and 44% when horseweed competed to the 8th cotton node in 2005 and 2006, respectively. Maximum horseweed seed production was 134,000 to 148,000 seeds m−2.

Common ragweed is a troublesome allergenic invader and noxious weed of several crops. Despite extensive research to understand the factors affecting its invasion, the role of environmental stresses and soil types on survival and growth is poorly understood. The objective of this study was to determine the effects of drought, salinity, and soil types on survival, growth, and nutrient uptake of ragweed in greenhouse experiments to predict its invasiveness in Turkey. Three separate experiments, with five drought intensities (100, 75, 50, 25, and 12.5% of field capacity [FC]), four levels of salinity (0, 3, 6, and 12 dS m−1), and five soil types varying in sand, silt, and clay content were performed. Severe drought and salinity levels reduced seedling survival, while soil type had no effect. Increasing drought and salinity negatively affected growth and nutrient uptake; the poorest growth was observed under severe drought intensity. Ragweed exhibited intensive tolerance to drought, even severe levels, while it tolerated salinity up to 6 dS m−1 for seedling survival. Growth was negatively affected above 3 dS m−1. The highest and lowest nutrient accumulations were recorded under moderate and extreme drought intensities, respectively. Similarly, the highest Na accumulation was observed under extreme saline conditions, whereas the highest P uptake and K/Na ratio were achieved under nonsaline conditions (0 dS m−1). Variation of soil texture had no effect on growth and nutrient uptake. The highest Ca, Mg, and Na accumulations were recorded on clay soil, while higher P accrued on sandy-loam soil. Increased tolerance of ragweed to severe drought and moderate salinity and its nonselective nature for soil type indicate that semiarid and partially arid regions in Turkey have plenty of vacant niches for ragweed invasion.

Catchweed bedstraw (Galium aparine L.) is a problematic dicot weed that occurs in major winter wheat (Triticum aestivum L.) fields in China. Tribenuron-methyl has been widely used to control broadleaf weeds since 1988 in China. However, overuse has led to the resistance evolution of G. aparine to tribenuron-methyl. In this study, 20 G. aparine populations collected from Shandong and Henan provinces were used to determine tribenuron-methyl resistance and target-site resistance mechanisms. In dose–response experiments, 12 G. aparine populations showed different resistance levels (2.92 to 842.41-fold) to tribenuron-methyl compared with the susceptible population. Five different acetolactate synthase (ALS) mutations (Pro-197-Leu, Pro-197-Ser, Pro-197-His, Asp-376-Glu, and Trp-574-Leu) were detected in different resistant populations. Individuals heterozygous for Pro-197-Ser and Trp-574-Leu mutations were also observed in a resistant population (HN6). In addition, pHB4 (Pro-197-Ser), pHB7 (Pro-197-His), pHB8 (Pro-197-Leu), pHB5 (Asp-376-Glu), and pHB3 (Trp-574-Leu) subpopulations individually homozygous for specific ALS mutations were generated to evaluate the cross-resistance to ALS-inhibiting herbicides. The pHB4, pHB7, pHB8, pHB5, and pHB3 subpopulations all were resistant to sulfonylurea, pyrazosulfuron-ethyl, triazolopyrimidine, flumetsulam, sulfonylamino-carbonyl-triazolinone, flucarbazone-sodium, pyrimidinyl thiobenzoate, pyribenzoxim, and the imidazolinone imazethapyr. These results indicated the diversity of the resistance-conferring ALS mutations in G. aparine, and all these mutations resulted in broad cross-resistance to five kinds of ALS-inhibiting herbicides.

Knowledge of the effects of burial depth and burial duration on seed viability and, consequently, seedbank persistence of Palmer amaranth (Amaranthus palmeri S. Watson) and waterhemp [Amaranthus tuberculatus (Moq.) J. D. Sauer] ecotypes can be used for the development of efficient weed management programs. This is of particular interest, given the great fecundity of both species and, consequently, their high seedbank replenishment potential. Seeds of both species collected from five different locations across the United States were investigated in seven states (sites) with different soil and climatic conditions. Seeds were placed at two depths (0 and 15 cm) for 3 yr. Each year, seeds were retrieved, and seed damage (shrunken, malformed, or broken) plus losses (deteriorated and futile germination) and viability were evaluated. Greater seed damage plus loss averaged across seed origin, burial depth, and year was recorded for lots tested at Illinois (51.3% and 51.8%) followed by Tennessee (40.5% and 45.1%) and Missouri (39.2% and 42%) for A. palmeri and A. tuberculatus, respectively. The site differences for seed persistence were probably due to higher volumetric water content at these sites. Rates of seed demise were directly proportional to burial depth (α=0.001), whereas the percentage of viable seeds recovered after 36 mo on the soil surface ranged from 4.1% to 4.3% compared with 5% to 5.3% at the 15-cm depth for A. palmeri and A. tuberculatus, respectively. Seed viability loss was greater in the seeds placed on the soil surface compared with the buried seeds. The greatest influences on seed viability were burial conditions and time and site-specific soil conditions, more so than geographical location. Thus, management of these weed species should focus on reducing seed shattering, enhancing seed removal from the soil surface, or adjusting tillage systems.

Thirty-eight percent of monoecious hydrilla [Hydrilla verticillata (L.f.) Royle # HYLLI] grown from tubers produced new tubers after 28 days exposure to a 10-h photoperiod. One hundred percent of the plants grown at a 10-h photoperiod produced tubers by 56 days while only thirty-eight percent of those grown at a 12-h photoperiod did so. Plants grown at 14- or 16-h photoperiods did not produce tubers. Tubers appeared to be produced at the expense of new root and shoot tissue. Dioecious hydrilla (female) grown under similar conditions did not produce tubers by 56 days at any photoperiod examined. Relative growth rates (total dry weight) for both types did not differ with photoperiod and ranged between 81 ± 63 and 284 ± 52 mg·g-1·wk-1 (regression coefficient ± standard error; n = 30). In general, total chlorophyll (a+b) was greater for dioecious than for monoecious plants. The ratios of chlorophyll “a” to chlorophyll “b” and carotenoids to chlorophyll a changed with increasing photoperiod and plant age in a similar manner for both monoecious and dioecious plants.

Hairy nightshade is a common weed in potato rotations in the western United States. As a close relative of potato, hairy nightshade can host numerous potato nematodes, diseases, and insect pests. Hairy nightshade hosts three common parasitic nematodes of potato, Columbia and northern root-knot nematodes, and stubby root nematode. Tubers of a potato breeding line with roots that are resistant to Columbia root-knot nematode—race 1, were damaged when grown in the presence of hairy nightshade. The weed provided an alternate host for the nematode, which then allowed the nematode to infect susceptible tubers. Stubby root nematodes transmit tobacco rattle virus (TRV), the causal agent for corky ringspot disease (CRS) of potato. CRS disease was maintained in soil when hairy nightshade was present in rotation crops of alfalfa or Scotch spearmint that otherwise eliminated the disease. Hairy nightshade also is a host of potato leaf roll virus (PLRV), which is transmitted by green peach aphids (GPA). GPA preferentially land and readily reproduce on hairy nightshade. Aphid transmission of PLRV from hairy nightshade to potato was four times greater than the virus transmission rate from potato to potato. Integrated management of these potato nematodes, diseases, and insect pests also should include strategies to control hairy nightshade in potato and rotation crops.

Soybeans [Glycine max (L.) Merr.] and oats (Avena sativa L.) were grown in a rotation using reduced or no-tillage crop production systems at Lincoln, Nebraska, over a 4-yr period. Oat stubble was treated after harvest with 3.4 kg/ha of metribuzin [4-amino-6-tert-butyl-3-(methylthio)-as-triazin-5(4H)-one] to reduce the growth of late summer weeds. The following spring soybeans were planted directly into the undisturbed stubble or into a seedbed prepared by tandem discing. Three seedbed preparations, two soybean cultivars, and six preemergence weed control treatments were compared. Glyphosate [N-(phosphonomethyl)glycine] applied at 0.8 kg/ha or tandem discing were equally effective in producing a weed-free seedbed. Herbicides applied preemergence on soybeans were still necessary for the reduced tillage or no-tillage production systems if weeds were to be adequately controlled in soybeans without cultivation. Differences in seed-yield occurred between cultivars only when late summer rains benefited the later maturing ‘Williams' soybeans over earlier maturing ‘Wells'. With adequate weed control, soybeans and oats can be grown in a no-tillage, crop rotation, production system in eastern Nebraska to produce high yields with a minimum of labor and soil exposure.

Dispersal studies of musk thistle (Carduus nutans L = C. thoermeri Weinmann) seeds by wind were performed to determine the percentage of seeds removed from the vicinity of the plant where the seeds originated. For the different wind velocities tested (up to 5.6 m/s) less than 1% of the seeds were blown further than 100 m. Most seeds were deposited within 50 m of the point of release. A Gaussian model of seed dispersal based on experimentally determined parameters and published data adequately described these results.