Clodionafop, an acetyl-coenzyme A carboxylase (ACCase) inhibitor, changed the shape of the chlorophyll fluorescence induction curve (Kautsky curve) in barley and oat in greenhouse experiments. Biomass ED50, based on log-logistic dose–response curves, for barley was considerably higher than that for oat in all experiments. Biomass ED50 and relative potency (ED50 [barley]/ED50 [oat]) were consistent among experiments when sprayed at the same phenological stage of plant development. Especially at high doses, clodinafop changed the shape of the Kautsky curve more for oat than for barley. From the numerous parameters that can be derived from the OJIP steps of the Kautsky curve, we found that (1) Fvj, the relative changes at the J step [Fvj = (Fm − Fj)/Fm], (2) area between Kautsky curve and maximum fluorescence (Fm), and (3) Fv/Fm, maximum quantum efficiency of Photosystem II [Fv/Fm = (Fm − F0)/Fm], were closely linked to the biomass dose–response relationships for both species. The linkage between biomass and the fluorescence parameters may be used to shorten the screening period for ACCase inhibitors.

Wild radish is a major weed in winter grain crops in Australia. This weed is poorly controlled by glufosinate. Therefore, factors influencing glufosinate efficacy in this species were examined. Dose–response studies conducted with three populations of wild radish collected from different parts of Australia and one from Europe showed poor control of all populations by glufosinate under Australian winter conditions. Studies conducted in controlled environmental chambers under night/day temperatures of 5/10, 15/20, and 20/25 C and various light intensities demonstrated that wild radish grown under cooler temperatures of 5/10 C were poorly controlled with 1,200 g ai ha−1 glufosinate when the same rate was sufficient to cause 100% mortality under 15/20 and 20/25 C. Light intensity did not significantly influence glufosinate activity at low temperatures. However, under warm temperatures of 20/25 C, glufosinate efficacy was enhanced with low light intensities. Experiments examining absorption and translocation of glufosinate showed that temperature did not have a significant effect on absorption of glufosinate. However, basipetal translocation of glufosinate was greatly increased by higher temperatures. Therefore, the poor control of wild radish by glufosinate at low temperatures is probably because of reduced accumulation of glufosinate in the meristematic regions of the plant.

A number of redroot pigweed and Powell amaranth populations from various locations in Ontario, Canada, have distinct patterns of resistance to the acetolactate synthase–inhibiting herbicides imazethapyr and thifensulfuron. This suggested the presence of diverse ALS gene mutations among these populations. Seven polymerase chain reaction primer pairs were used to amplify the gene to obtain full sequence information and to determine the identity of resistance-conferring mutations. There was a high degree of similarity in the ALS gene of the two species with only five nucleotides and one amino acid differing. A total of four herbicide resistance-conferring mutations were identified in the two species. The Ala122Thr, Ala205Val, and Trp574Leu amino acid substitutions were found in redroot pigweed whereas Ala122Thr, Trp574Leu, and Ser653Thr were detected in Powell amaranth. The pattern of resistance known to be conferred by the mutations concurred with the resistance level observed at the whole plant level. Distinct mutations being found in geographically separated populations suggest that selection for resistance occurred simultaneously in different locations. It reinforces the fact that resistance to ALS inhibitors is easily selected and that growers need to take this into account when formulating weed management strategies.

It is difficult to quantify the mechanism(s) responsible for competition-induced yield loss using traditional experimental techniques. A technique using yield and 13C discrimination (Δ) for wheat, a C3 plant, has been developed to separate total yield loss (TYL) into yield loss due to N (YLNS) and water (YLWS) stresses. The objective of this research was to determine whether the Δ approach could be used in corn, a C4 plant, to separate TYL into YLNS and yield loss due to a combination of water and light stresses (YLWLS). The field study had a factorial design using five corn densities and five N rates and was conducted in western Nebraska in 1999 and 2000. Relationships for YLNS and YLWLS with TYL were derived from only a portion of the yield and Δ data collected in 1999 and validated based on the remaining data collected in 1999 and 2000. In 1999, 20 to 40% of TYL was due to YLWLS, whereas in 2000, a dry year, YLWLS accounted for 60 to 80% of the TYL. Results from using the Δ-based approach were consistent with analysis of variance results. For example, calculated YLWLS values were related to measured YLWLS by the equation: calculated YLWLS = 19 + 0.91 (measured YLWLS) (r2 = 0.95; P < 0.01). The Δ approach, based on a plant's physiological response to the environment, can be used to separate and quantify competition-induced YLNS and YLWLS in corn.

A population of waterhemp was identified in Adams County, Illinois, that survived treatment of several acetolactate synthase (ALS) inhibitors and a postemergence (POST) application of lactofen, a protoporphyrinogen oxidase (PPO)–inhibiting herbicide. Greenhouse studies were conducted to quantify the responses of this waterhemp population, designated ACR, to multiple PPO inhibitors and various other herbicides with different sites of action. Resistance ratios were obtained by comparing herbicide dose–response curves between the ACR population and a herbicide-susceptible waterhemp population. The ACR population was resistant to lactofen (23-fold) and to five other PPO-inhibiting herbicides (ranging from 2.2- to 6.2-fold). Furthermore, the ACR waterhemp population was 17,000-fold and 18,000-fold resistant to imazamox and thifensulfuron, respectively, two ALS-inhibiting herbicides. In response to atrazine, a Photosystem II inhibitor, the ACR population was 38-fold resistant. Plants within the ACR waterhemp population survived treatment of a herbicide mixture containing lactofen at 175 g ai ha−1, imazamox at 44 g ae ha−1, and atrazine at 1,000 g ai ha−1. Thus, individual plants—not just the population as a whole—displayed multiple herbicide resistance. The ACR population was not resistant to glyphosate or paraquat. This is the first reported weed population from the United States with resistances to herbicides inhibiting three unique sites of action. Furthermore, this research identifies a significant reduction in the number of POST herbicide options available for waterhemp control in soybean production.

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.

Hairy nightshade is the most widespread nightshade species in North America. Increased knowledge of hairy nightshade germination biology would facilitate development of an optimum control program. Germination of hairy nightshade seeds as affected by environmental and chemical factors was studied under greenhouse and controlled-environment growth chamber conditions. Hairy nightshade seeds were in an innate dormant state when initially separated from the berries. Moist compared with dry storage was more effective for breaking dormancy at 4 C, but dry storage was more effective at 17 C. Hairy nightshade seeds germinated equally well under both a 14-h photoperiod and continuous darkness. These germinated at constant temperatures ranging from 19 to 39 C, with optimum germination attained between 27 and 33 C. Germination markedly declined as osmotic potential of the germination medium decreased. The optimum pH range for germination of hairy nightshade seeds was between 6 and 8, although some seeds germinated at pH 4 and 9. Maximum hairy nightshade emergence occurred with seeding depths of 2 cm or less. No emergence occurred when seeding depth reached 8 cm.

Several populations of different Amaranthus species have been reported resistant to single or multiple herbicides. Interspecific hybridization among amaranths is hypothesized to contribute to the evolution of herbicide resistance. Although other studies have shown the occurrence of interspecific Amaranthus hybrids, little has been done to establish the likelihood of hybridization under field conditions. The main objective of this study was to determine potential field frequencies of hybridization between tall waterhemp females and smooth pigweed. Field hybridization plots were established during each of two growing seasons. Individuals of the two species were transplanted to field plots and arranged at varying distances from each other. Hybrid progeny were detected using the acetolactate synthase (ALS) gene as a marker. Smooth pigweed parents were homozygous for a herbicide-resistance ALS allele, whereas maternal tall waterhemps were homozygous for a herbicide-sensitive ALS form. Heterozygous interspecific progeny were thus detected by means of herbicide selection. Molecular and cytogenetic tools were used to verify the validity of the data obtained. Averaged among female waterhemp plants and across the two field seasons, hybridization occurred at a frequency of 33%. A single tall waterhemp plant was capable of producing more than 200,000 hybrids, suggesting little if any gametic incompatibility. All flowering hybrids obtained from tall waterhemp females were of dioecious condition and female sex. Observed sexual segregation was consistent with previously postulated chromosomal XY-type system in tall waterhemp sex determination, where males are the heterogametic sex.

The timing of weed seedling emergence relative to the crop is important in planning and optimizing the time of weed control, but very little work has been done to predict seedling emergence of tropical weed species, especially in low-input and small-scale farms. We developed a simple model based on hydrothermal time to predict seedling emergence of tropic ageratum. Hydrothermal time at 2-cm soil depth was calculated from soil moisture and soil temperature simulated from several micrometeorological and soil physical variables. The model was developed using 5 yr of field emergence data from a continuous corn–cassava production system in southwestern Nigeria. Percentage of cumulative seedling emergence from the 5-yr data set was fitted to cumulative soil hydrothermal time using a Weibull function. The predicted cumulative emergence curve significantly matched observed field emergence (r2 = 0.83). Model predictions were evaluated with root mean square error (RMSE) using four field emergence data sets from southeastern Nigeria (RMSE ≤ 10.1) and Los Banos, Philippines (RMSE = 8.9). RMSE values ≤ 10 indicated that predictions represented observations well. With such models, extension personnel working on tropical soils, especially in West Africa, may be able to provide additional advice to farmers on the appropriate time for the management of tropic ageratum.

Previous research with annual weed species indicates that critical timing of weed removal begins primarily after the two-leaf stage of onion, a time when postemergence (POST) herbicides can first be applied. Volunteer potato is difficult to manage and persists in onion fields of western United States. The purpose of this research was to quantify the duration of volunteer potato interference on yield and market grade of onion as well as potato tuber production. Volunteer potato interference caused a 5% yield loss before onions reached the two-leaf stage, at two of three locations. Relative to weed-free plots, onion bulb diameter was reduced as duration of interference increased, resulting in smaller proportions of marketable bulbs. Volunteer potato produced daughter tubers shortly after emergence, which explains, in part, weed persistence despite removal of shoots with contact herbicides, cultivation, and hand-weeding in onion. Significant losses in onion yield and bulb diameter are likely given current volunteer potato management systems.

Contrasting cover-cropping systems were compared to determine whether fundamental differences in cover-cropping strategies affect weed seed predators and resulting seed predation. We conducted typical “feeding” trials in which 25 seeds of each of six weed species, including velvetleaf, wild mustard, yellow foxtail, common lambsquarters, redroot pigweed, and hairy galinsoga, were placed in the field. Exclosures showed that the majority of seed predation could be attributed to invertebrates: 43% out of a total of 56% seed predation for 11 d in 2002 and 40% out of a total of 58% seed predation for 4 d in 2003. The predominant invertebrate seed predator across all entry points of four cropping systems was a ground-dwelling carabid beetle, Harpalus rufipes, which was more abundant in vegetated treatments, particularly red clover, compared with treatments recently tilled and planted to a fall cover crop. In the absence of vertebrates, H. rufipes activity–density was positively correlated with mean seed predation in 2002 (Spearman ρ = 0.489; P < 0.001) but not in 2003 (Spearman ρ = 0.090; P = 0.504), possibly because of a delay between pitfall trapping and predation assay. The activity–density of invertebrate seed predators measured in these systems and the high level of predation imposed on weed seeds at the soil surface indicate that cover-cropping strategies should consider late-season weed management, which maintains seeds on the soil surface and provides desirable habitat for invertebrate predators.

Greenhouse studies were conducted to evaluate the influence of wild radish–amended soil on tomato, bell pepper, and yellow nutsedge growth. In addition, yellow nutsedge interference with tomato and bell pepper was evaluated with and without the wild radish amendment. Leaf margins of tomato and bell pepper plants were necrotic for approximately 2 wk after transplanting into soil amended with 1% (wt/wt) wild radish biomass. Injury to both crops was transient, but bell pepper biomass through 9 wk after transplanting was negatively affected, whereas tomato was not. In a replacement series study, tomato was more competitive than yellow nutsedge in nonamended soil and the competitiveness of tomato further increased at the expense of reduced yellow nutsedge growth in wild radish–amended soil. Bell pepper was less competitive than yellow nutsedge in nonamended soil, but in wild radish–amended soil, bell pepper held a competitive advantage over yellow nutsedge. In addition, yellow nutsedge tuber production was reduced as much as 88% when soil was amended with wild radish, and tuber weight in nonamended soil was 0.32 g tuber−1 compared with 0.05 g tuber−1 in wild radish–amended soil. This research shows that competitiveness of tomato and bell pepper is increased over yellow nutsedge when soil is amended with wild radish, in addition to reducing yellow nutsedge tuber production and size.

Greenhouse and laboratory experiments were conducted to investigate mechanisms of glyphosate resistance in horseweed populations from Mississippi, Arkansas, Delaware, and Tennessee. A nondestructive leaf-dip bioassay was developed to confirm resistance and susceptibility in individual test plants. A single leaf was excised from each plant, and the petiole and bottom one-fourth of leaf was dipped in a 600 mg ae L−1 glyphosate solution for 2 d followed by visually estimating the injury on a scale of 0 to 10. Plants were classified as resistant (R) if the score was 2 to 3 and susceptible (S) if the score was 5 to 6. 14C-glyphosate solution was applied on the adaxial surface of a fully expanded leaf of the second whorl of four-whorl rosette plants. Plants were harvested 48 h after treatment and radioactivity was determined in treated leaf, other leaves, crown, and roots. Absorption of 14C-glyphosate was similar (47 to 54%) between R and S plants from within and among the four states, suggesting absorption is not involved in glyphosate resistance. The amount of radioactivity translocated from the treated leaf was reduced in R plants compared with S plants. The reduction in translocation of 14C-glyphosate ranged from 28% in Mississippi-R biotype to 48% in Delaware-R biotype compared with their respective S biotypes. Epicuticular wax mass ranged from 6 to 80 μg cm−2 among horseweed biotypes, with no differences between R and S biotypes within each state. Treating two leaves with glyphosate solution at the field use rate (0.84 kg ae ha−1) killed S plants but not R plants (38 to 58% control) regardless of state origin. These results suggest that a simple bioassay can be used to screen biotypes for suspected resistance and that reduced translocation of glyphosate plays a major role in glyphosate resistance in R biotypes of horseweed.

Root traits and growth of spreading orach and common lambsquarters were compared in response to soil compaction, drought, and waterlogging under controlled environment conditions. On the basis of the typical habitats occupied, the hypothesis was that spreading orach would be more tolerant of compaction and waterlogging and common lambsquarters more tolerant of drought. When grown in buckets with two soil bulk densities (1.2 and 1.6 g cm−3) for 8 wk, the two species responded similarly to compaction, with the fraction of fine roots reduced by 10%, total root length by 70%, root and shoot dry weight and leaf area by 50 to 60%, and plant height by 30% at the high compared with the low bulk density. When grown for 6 wk in soil columns 1 m long, which were watered daily or allowed to dry, common lambsquarters was deeper rooted than spreading orach at both moisture levels and better able to sustain growth in the drying columns. The watering regime did not alter the rooting depth of either species. Total root length in successive 10-cm increments declined exponentially from the top to the bottom of the watered columns, but root proliferation was reduced in the upper 20 cm of the drying columns. The average root diameter of both species decreased with drought and increased with soil compaction. When grown in waterlogged soil at 10 or 20 C for 4 wk, seedlings of spreading orach survived with little reduction in growth, whereas survival and growth of common lambsquarters were drastically reduced, particularly under cool soil conditions.

Field research was conducted in Tennessee at two locations over 3 yr under no-tillage conditions to examine the interaction of broadleaf signalgrass and corn yield. Broadleaf signalgrass was killed at various time intervals during the growing season by applying postemergent glyphosate. Corn injury was avoided by the use of a glyphosate-tolerant variety. Yield reductions because of broadleaf signalgrass interference occurred with weed densities > 150 plants m−2 and when the corn and weeds emerged simultaneously. Corn had the ability to withstand broadleaf signalgrass presence up to 28 d after planting with no yield loss at all locations.

Two studies investigated off-target exposure of soybean to plant growth regulator (PGR) herbicides and determined if simultaneous exposure to PGR herbicides and labeled soybean herbicides increase PGR injury. The PGR herbicides, 2,4-D, clopyralid, and dicamba, as well as dicamba plus the auxin transport inhibitor diflufenzopyr, were applied to glyphosate-resistant soybean at the V3, V7, and R2 soybean growth stages. Two rates were chosen from previous and preliminary research to approximate threshold rates that would cause a yield reduction so as to distinguish differences in sensitivity between growth stages. All four PGR herbicides caused significant soybean injury, height reduction, and yield loss at one or more application rates and growth stages. Relative to other PGR herbicides, dicamba reduced soybean yield at the lowest rate (a potential rate from residues remaining in improperly cleaned application equipment), followed by clopyralid, with 2,4-D requiring the highest rate to reduce soybean yield (a potential rate from a high level of spray drift). Dicamba and dicamba plus diflufenzopyr were applied at equal fractions of labeled use rates for corn to compare them directly at equivalent levels of off-target movement. Dicamba plus diflufenzopyr caused less injury and yield loss than dicamba applied alone. In a second study, the highest labeled soybean use rates of glyphosate, imazethapyr, imazamox, and fomesafen were applied alone and in combination with the highest rate of dicamba used in the first study (1% of a labeled use rate for corn) at the V3 and V7 stages. Dicamba demonstrated synergistic interactions with imazamox, imazethapyr, and fomesafen (but not with glyphosate) to further reduce yield under some circumstances, especially when applied at the V7 stage. Several treatments that included dicamba reduced soybean seed weight when applied at either the V3 or V7 stage and reduced the number of seeds per pod at the V7 stage.

Any plant not sown from seed is often labeled a weed in improved pastures of New Zealand. Most improved pastures are a mix of perennial ryegrass and white clover but generally are infested with broadleaf weeds. Changes in forage production due to individual weeds were determined using measurements of perennial ryegrass and white clover before and after dairy cattle, beef cattle, or sheep grazing under, near, and far from individual plants of six rosette-forming weed species throughout a growing season. The larger weeds, bull thistle and musk thistle, reduced the amount of forage utilized 42 and 72%, respectively, in beef cattle– and sheep-grazed hill-country pastures. Forage production under and near Canada thistle, hedge mustard, broadleaf plantain, and hairy buttercup in a dairy pasture was greater (136, 140, 178, and 450%, respectively) than in the control areas. Although the dairy pasture was grazed following recommended grazing procedures, our results indicate that this grazing system was not maximizing forage yield potentials of perennial ryegrass and white clover and that these weeds served as an indicator that the pasture was being overgrazed.

Pyrithiobac and imazaquin are persistent herbicides used in the midsouthern region of the United States. Certain rotational crops are extremely sensitive to each herbicide. Field studies were established near Brooksville, MS, and St. Joseph, LA, to examine crop injury from the previous year's application of pyrithiobac and imazaquin. In Mississippi, 690 g ai ha−1 of each herbicide was applied as a preplant-incorporated carryover treatment. The subsequent year, pyrithiobac-treated areas were planted with soybean, corn, and grain sorghum. Imazaquin-treated areas were planted with cotton, corn, and grain sorghum. In plots adjacent to the carryover treatment, pyrithiobac or imazaquin was applied at rates ranging from 0 to 173 g ha−1 as in-field bioassay plots. In Louisiana, pyrithiobac was applied at 70 and 240 g ha−1 as a postdirected broadcast layby treatment in cotton. The subsequent spring, pyrithiobac-treated areas were planted to corn. Assay rates of pyrithiobac ranging from 0 to 140 g ha−1 were established in areas adjacent to the carryover treatment. Grain sorghum was most sensitive to pyrithiobac soil residues, followed by corn and then soybean. Cotton was the most sensitive crop to imazaquin soil residues, followed by corn and then grain sorghum. Residual amounts of pyrithiobac and imazaquin were approximately 15 and 24 g ha−1, respectively. The approximated half-life of pyrithiobac and imazaquin was 61 and 71 d, respectively. Pyrithiobac applied at 280 g ha−1 as a layby treatment the preceding year injured corn and reduced yields in 1 of 2 yr. Pyrithiobac at 70 g ha−1 did not affect corn in either year. The difference in persistence between the 2 yr was attributed to reduced precipitation and lower soil temperatures in the year with observed carryover. Injury and yield reduction observed in the second year of the study correspond to an approximate pyrithiobac half-life of 60 d.

Nonindigenous invasive weed species can have substantial negative impacts on the quantity and quality of outdoor recreational activities such as fishing, hunting, hiking, wildlife viewing, and water-based recreation. Despite the significance of impacts on recreation, very little research has been performed to estimate the corresponding economic losses at spatial scales such as regions, states, and watersheds. This is true primarily because in most jurisdictions the data necessary to estimate recreational impacts are scarce and incomplete. Because of the challenges involved in measuring recreational losses precisely, we illustrate a method that can provide indications of the ranges in which the true economic losses likely lie. To reflect underlying uncertainty in parameters such as the number of acres infested in a jurisdiction and the rate at which wildlife-related recreation decreases as a function of increasing weed infestation, we developed a range of estimates using lower, medium, and higher scenario combinations of parameter and variable values. Our case study jurisdiction is a western state (Nevada) in which nonindigenous weed infestations on public lands have expanded rapidly in recent years. Under conservative assumptions, the negative economic impacts stemming from the adverse influence of nonindigenous weeds on wildlife-related recreation in Nevada likely range from $6 million to $12 million per year. Using the most conservative findings for annual recreation losses, the predicted discounted stream of negative economic impacts over a future time horizon of 5 yr ranges from about $30 million to $40 million in Nevada, depending on actual future expansion rates of weeds.