We investigated seasonal variation in germinability of seed reserves in a degraded big sagebrush (Artemisia tridentata Nutt.) community. A bioassay was employed to determine germinability. There were many more germinable seeds and seedling establishment was greater under the shrub canopies than in the interspace. Caryopses of downy brome (Bromus tectorum L.) were the most abundant of any species in the litter where most of the seeds were located. Seasonal germinability of downy brome caryopses depends on the environment of the seedbed when the samples were collected and on the incidence of environmentally-induced dormancy or on loss of dormancy.

This research examined the effect of duration of the warm cycle and number of fluctuating temperature/light cycles on goosegrass seed germination. Less than 10% goosegrass seed germination occurred at constant temperatures of 20, 25, and 35 C, while 99% germinated at daily fluctuating temperature regimes of 20 C for 16 h and 35 C for 8 h with light. Increasing the number of fluctuating temperature cycles increased goosegrass seed germination. Fluctuating temperature cycles with 22 h at 20 C and 2 h at 35 C with light strongly stimulated germination, but not as much as the cycle with 8 h at 35 C. Goosegrass seed germination ceased once the fluctuating temperature regime was discontinued, and constant 20 C with 8 h light daily was reimposed. Germination occurred only on resumption of fluctuating temperature cycles. Seed incubated for several weeks in a moist environment at constant 20 C had 81 to 91% germination with two cycles of 20 C for 16 h and 35 C for 8 h, compared to 14% for fresh seeds.

To determine the competitiveness of common cocklebur (Xanthium pensylvanicum Wallr.) with cotton (Gossypium hirsutum L. 'Stoneville 213′), experiments were conducted on a Lucedale fine sandy loam from 1978 through 1980. Common cocklebur dry weight increased with increasing density up to 16 plants/15 m of row. No further increase in dry matter occurred beyond this density. Regression analysis showed that common cocklebur produced an average of 342 kg/ha of dry weight for each plant per 15-m row. Seed-cotton yields decreased as weed density increased up to 16 common cocklebur plants/15 m of row. Regression equations revealed yield losses ranging from 72 to 115 kg/ha for hand-harvested seed cotton and 57 to 90 kg/ha for machine-harvested seed cotton for each common cocklebur plant/15 m of row. Cotton stem diameter and height were reduced by weed competition in the same manner as seed cotton yields, but reductions were not as pronounced, indicating that these parameters were not good indicators of common cocklebur competition.

Seven insecticides were applied separately and in combination with the herbicide propanil (3,4-dichloropropionanilide) to young rice. None of the insecticides applied alone caused leaf burn. Toxaphene and endrin, in combination with propanil, did not increase leaf burn over that which occurred from propanil alone. Malathion, phosphamidon, Guthion (O,O-dimethyl S-4-OxO-1,2,3-benzotriazin-3(4-H)-ylmethyl) phosphorodithioate and Dylox, (O,O-dimethyl 2,2,2-trichloro-1-hydroxyethyl phosphonate) in combination with propanil, increased leaf burn over that which occurred from propanil alone. In general, increased leaf burn resulted in decreased yields of rough rice.

Nikolai Ivanovich Vavilov was an early 20th century Russian plant scientist who was killed by Joseph Stalin in 1943 for his adherence to basic genetic principles. Vavilov is well known within plant breeding and plant evolutionary biology circles, yet the science of Vavilov is just as important to the field of weed science. Specifically, Vavilov proposed that certain weeds adapted to weed control practices to survive in prehistorical agrarian societies. Most would refer to this adaption as crop mimicry, but the term “Vavilovian mimicry” is more apt. Vavilovian mimicry requires three factors: a model—the crop or desirable plant; a mimic—the weed; and an operator—the discriminating agent, possibly human, animal, or machine. In a modern context, it is proposed that weed adaptation to herbicide applications be included as a form of Vavilovian mimicry, with the acknowledgement that the operator is the herbicide. In this context, Vavilovian mimicry is the adaption of the weed mimic to be perceived by the operator as visually, physically, or biochemically indistinguishable from the crop model. This review will cover the history and legacy of Vavilov in a condensed version in the hope that weed scientists will hold this individual in high regard in our future endeavors and begin to acknowledge Vavilov as one of the first scientists to propose that weeds can mimic the attributes of crops.

Translocation of 14CO2-labeled photoassimilates was compared to movement of 14C-glyphosate [N-(phosphonomethyl)glycine] in Canada thistle [Cirsium arvense (L.) Scop. ♯ CIRAR] under field and controlled-environment conditions. Field-grown Canada thistle plants were treated on mature upper leaves at the midflower stage in mid-June and harvested 8 days later. No differences were found in glyphosate and assimilate distribution, and movement was primarily basipetal. Of the glyphosate and photoassimilates translocated from the treated leaves, 25 and 31%, respectively, were recovered from the roots, while 59 and 58%, respectively, were found in the shoot tissue below the treated leaf. Concentration of labeled glyphosate in the roots as much as 95 cm from the treated shoot was as high as at the base of the treated shoot. Photoperiods that induce flowering (15 h) or maintain vegetative growth (13 h) did not differentially affect the distribution of photoassimilates or glyphosate in 7-week-old Canada thistle plants in the growth chamber. In growth chamber-grown plants equivalent amounts of glyphosate and assimilates were translocated out of treated leaves; however, relatively more glyphosate than photoassimilates accumulated in the roots.

The competitiveness of 10 soybean [Glycine max (L.) Merr.] cultivars to weeds, their tolerance to twice normal rates of commonly used soybean herbicides, and the soil residual properties of these herbicides were studied during 4 years at Lincoln, Nebraska. Three soybean cultivars showing the least percentage yield loss from weeds were ‘Harosoy 63′, ‘Amsoy’, and ‘Corsoy’; whereas cultivars showing the greatest yield loss were ‘Hawkeye 63′, ‘Shelby’, and ‘Lindarin 63′. The two herbicide treatments causing the greatest average soybean yield loss were 3-(3,4-dichlorophenyl)-1-methoxy-1-methylurea (linuron) at 4.5 kg/ha and α,α,α-trifluro-2,6-dinitro-N,N-dipropyl-p-toluidine (trifluralin) at 2.2 kg/ha. Cultivar selection was important in reducing soybean yield loss from herbicides as linuron at 4.5 kg/ha reduced yields 5% for ‘Harosoy 63’ to as much as 31% for ‘Ford’. ‘Neal’ oat (Avena saliva L.) yields showed 4-(methylsulfonyl)-2,6-dinitro-N,N-dipropylaniline (nitralin) to be the most persistent herbicide in soil in this study followed by trifluralin. Plowing the soil eliminated the phytotoxicity of nitralin and trifluralin to subsequently planted oats.

Japanese knotweed is an aggressive alien species in Europe, North America, and Australia, causing a range of environmental problems. Eradication of Japanese knotweed is proving to be a difficult task, because the plant is able to propagate generatively by intra- and interspecific hybridization, and vegetatively from shoot and tiny rhizome pieces. Despite the economic consequences of Japanese knotweed on natural and built environments, its physiology is not yet fully understood; especially important are sink-source relations between old and young parts of the rhizome and growth of lateral and latent rhizome buds. Current methods of chemical control include three types of phloem-mobile herbicides, such as glyphosate, imazapyr, and synthetic auxins. These herbicides have limitations on their use, and all fail to eradicate the plant completely, for the reasons discussed in this review. Our aim is to suggest prospective approaches to enable chemical eradication: use of signals to induce controlled growth and development of quiescent rhizome buds; use of phytohormones, sugars, and light to increase allocation of phloem-mobile herbicides to the rhizome; use of xylem-mobile herbicides to exterminate the old rhizome parts; and use of different phloem-mobile herbicides at different growth stages.

Geostatistical methods were used to describe and map nonrandom distribution and variation (standard deviation) of shoot density and root growth across a well-established patch of Canada thistle, a perennial weed. Semivariogram functions and kriging, an interpolation method, were used to prepare isoarithmic contour maps and associated error maps. Maps consisted of interpolated contours of uniform weed density and other measured or calculated regionalized variables between measured X-Y control points, as well as maps of error (standard deviation) associated with contour estimation. Mapped regions of greatest shoot density across a patch not only had the greatest underlying root biomass and, often, greatest density of adventitious root buds, but also had more deeply growing root biomass.

The critical period for weed control (CPWC) is an integral component of integrated weed management strategies. Several studies have defined the CPWC in soybean under varying agronomic conditions, yet none have described the mechanisms involved in crop yield losses caused by weed competition. We hypothesized that under nonresource-limiting conditions, morphological changes resulting from the expression of shade avoidance could be used to define a period of developmental sensitivity to low red-to-far-red ratio (R : FR) that would overlap with the defined CPWC in soybean. Two experiments (a sequential harvest and a weed addition/removal series) were conducted in 2008 and 2009 under controlled environmental conditions to address this hypothesis. Two light-quality treatments were used: (1) high R : FR ratio (i.e., weed-free), and (2) low R : FR ratio (i.e., weedy). The low R : FR ratio treatment induced shade avoidance responses in soybean, which included increases in height, internode length, and the shoot : root ratio, as well as a reduction in biomass accumulation and leaf number. Using the morphological changes in biomass and leaf number observed in the weed addition/removal series, a period of developmental sensitivity to low R : FR was defined between the first trifoliate (V1) and third trifoliate (V3) stages of soybean development. This period was found to be very similar to the CPWC previously defined by field studies of soybean–weed competition.

Effects of α,α,α-trifluoro-2,6-dinitro-N,N-dipropyl-p-toluidine (trifluralin) rate and placement in soil on growth and anatomy of soybeans (Glycine max (L.) Merr.) were studied in greenhouses. In 7-week-old plants, trifluralin caused pronounced changes in number and organization of palisade cells in leaves. In fourth internodes, xylem elements of treated plants were less organized and walls of pericycle fibers were extremely thick compared to those of untreated plants. There was starch accumulation in nodules and in xylem elements of the upper root, and binucleate and trinucleate cells were present in tips of tap and lateral roots of treated plants. The effects were more pronounced as rate of trifluralin was increased from 0.36 to 0.74 and to 1.1 kg/ha and when trifluralin was incorporated into the 5 to 15-cm depth rather than the top 5 cm of soil. However, occlusions of xylem vessels in lateral and tap roots were most pronounced at 0.74 kg/ha. Plant topgrowth was reduced more when trifluralin was incorporated into the bottom 10 cm of soil, but root growth inhibition was not affected by trifluralin placement. Trifluralin reduced nodulation of soybeans and seemed to inhibit utilization of cotyledonary reserves and redistribution of organic and mineral constituents of unifoliolate leaves.

Experiments were conducted in the field, greenhouse, and controlled environment chambers to determine the extent to which MCPA {[(4-chloro-o-tolyl)oxy] acetic acid} antagonizes wild oat (Avena fatua L.) control with diclofop {2-[4-(2,4-dichlorophenoxy)phenoxy] propanoic acid}. Wild oat control with diclofop at 1 kg/ha was reduced from 96% when used alone to 76, 48, 31, and 14% by tank mixture with IAA (3-indole acetic acid), MCPA, 2,4-D [(2,4-dichlorophenoxy)acetic acid], or dicamba (3,6-dichloro-o-anisic acid), respectively. Wild oat control with diclofop applied alone at 1.1 kg/ha was similar to that of diclofop at 2.2 kg/ha applied as a tank mixture with MCPA at 0.15 or 0.3 kg/ha. MCPA antagonism of wild oat control with diclofop increased as the post-treatment temperature increased from 10 to 30 C. MCPA antagonism of wild oat control with diclofop was the same whether the herbicides were applied to the foliage only or to the foliage and soil. Approximately 20% of the wild oat root inhibition with diclofop applied postemergence, however, was from diclofop uptake from the soil. MCPA at 0.6 kg/ha did not reduce wild oat control when applied as a sequential treatment 2 days before or 1 day after diclofop at 1.1 kg/ha.

The dissociation constant for chlorsulfuron {2-chloro-N-[[(4-methoxy-6-methyl-1,3,5-triazin-2-yl)amino] carbonyl] benzenesulfonamide} in aqueous solution measured by spectrophotometric titration is 3.58 ± 0.05. Chlorsulfuron was more strongly adsorbed on IRA-400-Cl strong anion exchange resin than on IR-4B-OH weak anion exchange resin or Al2O3 anionotropic adsorbent. Hydrogen bonding was probably responsible for the adsorption observed on IR-120-Na(H) cation exchange resin. No chlorsulfuron was adsorbed on Al2O3 cationotropic absorbent, technical montmorillonite, illite, or kaolinite. Adsorption did occur on organic matter derived from a histosol. Chlorsulfuron was strongly adsorbed on activated charcoal but had little affinity for α-cellulose. Adsorption onto hydrophobic polymeric XAD-2 adsorbent at pH 5.2 was not significant for chlorsulfuron concentrations below 30 μM. No significant adsorption occurred on a variety of mineral soils low in organic matter. Adsorption on a Sharpsburg silty clay loam was inversely related to solution pH. Hydrogen bonding and charge transfer bonds were postulated as the major mechanisms responsible for chlorsulfuron adsorption in soil.

Broomrapes are parasitic angiosperms that live on the roots of many important crop plants. We have conducted a series of in vitro experiments to investigate the direct effects of nitrogen fertilizers on broomrape seed germination and early development. These experiments confirm that nitrogen in the ammonium form is more inhibitory than nitrate, but we conclude that it is the elongation of the seedling radicle that is primarily inhibited by ammonium, rather than the seed germination itself. Reductions in radicle length were observed when ammonium solutions were applied during either preconditioning or germination periods. For germinating seeds, exposures to ammonium sulfate of 4 to 8 h (depending on the species) reduced radicle elongation by half, indicating a relatively rapid inhibition. Broomrape species varied in sensitivity to ammonium, with Orobanche aegyptiaca being the most tolerant, followed by O. crenata, O. ramosa, O. cernua, and finally O. minor as compared in the presence of 5 mM ammonium. However, all broomrape species were much more sensitive to ammonium toxicity than were host crop species under similar treatment conditions.

Biochemical and physiological effects of target site resistance to herbicides inhibiting acetolactate synthase (ALS) were evaluated using sulfonylurea-resistant (R) and -susceptible (S) near isonuclear Lactuca sativa ‘Bibb’ lines derived by backcrossing the resistance allele from Lactuca serriola L. into L. sativa. Sequence data suggest that resistance in L. sativa is conferred by a single-point mutation that encodes a proline197 to histidine substitution in Domain A of the ALS protein; this is the same substitution observed in R L. serriola. K mapp (pyruvate) values for ALS isolated from R and S L. sativa were 7.3 and 11.1 mM, respectively, suggesting that the resistance allele did not alter the pyruvate binding domain on the ALS enzyme. Both R and S ALS had greater affinity for 2-oxobutyrate than for pyruvate at the second substrate site. Ratios of acetohydroxybutyrate: acetolactate produced by R ALS across a range of 2-oxobutyrate concentrations were similar to acetohydroxybutyrate: acetolactate ratios produced by S ALS. Specific activity of ALS from R L. sativa was 46% of the specific activity from S L. sativa, suggesting that the resistance allele has detrimental effects on enzyme function, expression, or stability. ALS activity from R plants was less sensitive to feedback inhibition by valine, leucine, and isoleucine than ALS from S plants. Valine, leucine, and isoleucine concentrations were about 1.5 times higher in R seed than in S seed on a per gram of seed basis, and concentrations of valine and leucine were 1.3 and 1.6 times higher, respectively, in R leaves than in S leaves. Findings suggest that the mutation for resistance results in altered regulation of branched-chain amino acid synthesis.

Geostatistical techniques were used to describe and map weed spatial distribution in two sunflower fields in Cabello and Monclova, southern Spain. Data from the study were used to design intermittent spraying strategies. Weed species, overall infestation severity (IS) index, and spatial distribution varied considerably between the two sites. Weed species displayed differences in spatial dependence regardless of IS. The IS mapping of each single weed and of the overall infestation was achieved by kriging, and site-specific application maps were then drawn based on the multi-species weed map and the estimated economic threshold (ET). Herbicide treatment was assumed to be needed for an overall IS score of 2 or 3, and the infested “area exceeding the economic threshold” was determined. The overall weed-infested area varied considerably between locations. About 99 and 38% of the total area was moderately infested (IS ≥ 2) at Monclova and Cabello, respectively. Therefore, if a given herbicide were applied just to the areas exceeding the ET, a significant herbicide saving would be realized in Cabello but not in Monclova. A multi-species spatial analysis provides an opportunity to make site-specific management recommendations from a map of the distribution of IS of the total infestation. Furthermore, only in fields with hard-to-control weed species (e.g., nodding broomrape and corn caraway) would site-specific herbicide application maps developed from total weed infestations need to be complemented with targeted site-specific herbicide treatments to prevent further spread of these species, although their IS might be low.

Foliar applications using volumes of 100 to 500 ml per acre were used to determine the effect of droplet size on herbicide toxicity to five plant species. Several types of plant response were used to indicate effectiveness. Herbicide toxicity increased as the particle diameter decreased from 0.3 mm to 0.1 mm.

Weeds of the genus Orobanche parasitize many dicotyledonous species, causing severe damage to vegetable and field crops worldwide. In Oregon, the number of red clover fields contaminated with small broomrape has increased in recent years. Small broomrape parasitism in red clover is temperature related. In this study, the temperature-dependent relationship was developed into a predictive model based on growing degree-days (GDD) for small broomrape parasitism in red clover. The model was developed in greenhouse studies and validated in the field during three growing seasons. A strong relationship between GDD and parasite size allowed for the creation of a simple predictive model for tubercle number based on GDD. The proposed model is based on a temperature range realistic to western Oregon climatic conditions and predicts lag, log, and maximum phases for four parasitism sizes in relation to GDD. Small broomrape parasitism in red clover began at about 400 GDD, but red clover biomass accumulation was not affected by parasitism before 1,200 GDD. Small broomrape flower stalk emergence began at about 1,100 GDD. Field studies validated that GDD could be a predictive parameter for small broomrape parasitism and could be used to time detection surveys and herbicide applications.

An experiment conducted at five locations in North Carolina during 1998 and 1999 evaluated weed management systems in cotton with CGA-362622 and pyrithiobac. Weed management systems evaluated different combinations with or without fluometuron preemergence (PRE) followed by (fb) CGA-362622 early postemergence (EPOST), postemergence (POST), or EPOST + POST; or pyrithiobac EPOST fb prometryn plus MSMA late postemergence directed (LAYBY) or no LAYBY treatment. The weed species evaluated include common ragweed, entireleaf morningglory, pitted morningglory, prickly sida, sicklepod, tall morningglory, and yellow nutsedge. Fluometuron PRE improved the control of all weed species by at least 17 percentage points and increased cotton lint yield compared with the systems that did not use fluometuron PRE. Prometryn plus MSMA LAYBY improved the control of all weed species and increased lint yield compared with the systems that did not use prometryn plus MSMA LAYBY when PRE or POST herbicides were used. Control with CGA-362622 at all application timings was greater than 70% for all weed species evaluated (common ragweed, entireleaf morningglory, pitted morningglory, sicklepod, tall morningglory, and yellow nutsedge), except prickly sida. Control of all three morningglory species and prickly sida was at least 70% with pyrithiobac, whereas control of common ragweed, sicklepod, and yellow nutsedge was lower. The only cotton that yielded over 800 kg ha−1 was treated with fluometuron PRE fb CGA-362622 EPOST, POST, or EPOST + POST fb prometryn plus MSMA LAYBY. Cotton treated with pyrithiobac EPOST gave yields that were similar to those given by cotton treated with CGA-362622 EPOST in systems with fluometuron PRE and less than those given by cotton treated with CGA-362622 EPOST in systems without fluometuron PRE. Early-season injury with CGA-362622 was greater than 60% at Clayton and Rocky Mount in 1998, whereas 12% or less injury was observed at the other locations. Pyrithiobac resulted in 25 to 45% injury at these two locations. No injury was observed 45 d after treatment.

Field experiments on suppression of three species (scentless chamomile, field poppy, and canola) by winter wheat sown in two different spatial patterns (normal 12.8-cm rows and a uniform, grid-like pattern) and three densities (204, 449, and 721 plants m−2) in two growing seasons were performed. The effects of crop-sowing density and pattern when weeds were controlled by herbicide were also investigated in one season. Weed and crop biomass were measured when weed biomass was at its maximum (late June/early July), and grain was harvested in August. Weed biomass comprised on average 30% of the total (crop + weed) biomass in the first year and only 5% in the second year. Weed biomass decreased and grain yield increased with increasing sowing density. Weed biomass was on average 23% lower and grain yield 14% higher in the uniform pattern than in rows. Weed biomass decreased 27% and 38% in the row pattern and 36% and 50% in the uniform pattern by increasing sowing density from low to medium and from low to high density, respectively. When weeds were controlled with herbicide, increasing sowing density had no influence on grain yield, but grain yield was 7% higher in the uniform pattern. Field poppy was the weed with the largest biomass and the largest impact on yield, whereas canola had the lowest biomass and the least impact on yield.