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.

Broadleaf signalgrass is sensitive to nicosulfuron and resistant to primisulfuron, but corn is resistant to both. Research was conducted to determine the effect of varying light level and air temperature on absorption, translocation, and metabolism of nicosulfuron and primisulfuron in broadleaf signalgrass and corn. Corn absorbed between 60 and 85% of the applied nicosulfuron and primisulfuron within 72 h after treatment (HAT), depending on environmental treatment. Absorption, translocation, and metabolism all tended to be more rapid at higher temperature and light intensity. Nicosulfuron and primisulfuron translocation out of the treated leaf was < 4.5% of herbicide absorbed through 72 HAT. Corn rapidly metabolized both herbicides in both environments. However, primisulfuron was metabolized more rapidly (high = 99%, low = 92%) than nicosulfuron (high = 95%, low = 78%). Broadleaf signalgrass absorbed 20% more nicosulfuron than primisulfuron through 72 HAT. Nicosulfuron translocation out of the treated leaf in broadleaf signalgrass was ≤ 15% absorbed through 72 HAT, while primisulfuron translocation was ≤ 4% during the same time period. Primisulfuron metabolism was more rapid than nicosulfuron in broadleaf signalgrass. During the first 4 HAT, broadleaf signalgrass metabolized > 20 times more primisulfuron than nicosulfuron. By 72 HAT, broadleaf signalgrass under conditions of high light and temperature had metabolized nearly 90% of the primisulfuron absorbed but ≤ 7% of the nicosulfuron absorbed was metabolized during the same time. These results suggest that differential activity of nicosulfuron and primisulfuron on broadleaf signalgrass may be based on differential rates of metabolism to nonphytotoxic compounds; uptake and translocation differences agree with the differential broadleaf signalgrass activity. Additionally, environment has the potential to affect rates of sulfonylurea absorption, translocation, and metabolism.

Resistance to propanil by a barnyardgrass (BYG) biotype has been reported, and its occurrence is becoming widespread in U.S. rice-producing regions. Interactions between propanil and the herbicides anilofos, molinate, pendimethalin, piperophos, quinclorac, and thiobencarb and the insecticide carbaryl were evaluated in laboratory studies for improved control of propanil-resistant barnyardgrass (R-BYG). Chlorophyll fluorescence measurements of BYG leaf segments were used to ascertain electron transport inhibition and to assess the synergy/antagonism of propanil (100 μM) with the various additives (50 μM). Synergistic effects on photosynthetic inhibition were found with anilofos, carbaryl, pendimethalin, and piperophos in combination with propanil. Such synergistic actions of chemicals with propanil may lead to chemical combinations useful for R-BYG control.

A kochia biotype from McDonough County, Illinois, was suspected to be resistant to both triazine and acetolactate synthase (ALS)-inhibiting herbicides. We performed greenhouse and laboratory experiments to confirm, quantify, and determine the molecular basis of multiple herbicide resistance in this biotype. Whole-plant phytotoxicity assays confirmed that the biotype was resistant to triazine (atrazine), imidazolinone (imazethapyr), and sulfonylurea (thifensulfuron and chlorsulfuron) herbicides. Relative to a susceptible kochia biotype, resistance to these herbicides ranged from 500- to > 28,000-fold. The kochia biotype from McDonough County also displayed high levels of resistance (2,000- to 9,000-fold) to ALS-inhibiting herbicides in in vivo ALS enzyme assays, indicating that resistance to these herbicides was site-of-action mediated. Results from chlorophyll fluorescence assays indicated that triazine resistance was also site-of-action mediated. Foliar applications of atrazine had little or no effect on photosynthesis in the resistant biotype, even when atrazine concentrations were 108-fold higher than needed to inhibit photosynthesis in the susceptible biotype. A region of the gene encoding the D1 protein of photosystem II and all of the open reading frame of the gene encoding ALS were sequenced and compared between the resistant and susceptible biotypes. Resistance to triazine and ALS-inhibiting herbicides in the kochia biotype from McDonough County was conferred by, respectively, a glycine for serine substitution at residue 264 of the D1 protein and a leucine for tryptophan substitution at residue 570 of ALS.

The mode of action of BAS 517 in a susceptible plant species, corn, was investigated using an excised root system and 14C-tracer techniques. The root system of a tolerant species, soybean, was used for comparison. When UL-14C- glucose was used as a precursor, 14C incorporation into lipids was reduced in BAS 517-treated corn roots, although 14C incorporation from UL-14C-glucose into lipids was relatively low. Inhibition of 14C incorporation into water-soluble compounds was not definite because of a high degree of variability. Using 14C-acetate as a precursor, 49, 43, and 34% of the recovered radioactivity was found in the lipid fractions of root tips treated with 0, 1.0, and 10 μM BAS 517, respectively. In nontreated soybean root tips, 47% of the recovered radioactivity was found in the lipid fraction compared to 49% in root tips treated with 10 μM BAS 517. Further analysis of lipids showed that BAS 517 inhibited the incorporation of 14C from 14C-acetate into phosphatidylethanolamine, a phospholipid, whereas the labeling of sterols in treated corn roots was not adversely affected. Acetyl CoA carboxylase extracted from root systems of corn and soybean showed different sensitivity to BAS 517, suggesting its role as the herbicide target site and as a basis for the selectivity.

A growth chamber study was conducted to evaluate the effects of temperature and soil moisture on the absorption and translocation of the new sulfonylurea herbicide, MON 37500, in wheat, jointed goatgrass, wild oat, and downy brome. Treatment combinations of temperatures—25/23, 15/13, and 5/3 C—and soil moistures of one-third, two-thirds, and full field capacities were assigned to the germinated plants. Radiolabeled MON 37500 was applied to the second leaf of plants at the three-leaf stage. The plants were then harvested at 6, 24, and 96 h after application. The plants absorbed MON 37500 in the greatest quantity during the first 6 h; however, absorption did continue to increase over time. Absorption was highest in wheat and lowest in wild oat. Temperature influenced absorption, while soil moisture status appeared to have no effect. Translocation of radiolabeled material out of the treated leaf increased over time; however, 90 to 97% of absorbed radioactivity remained in the treated leaf 96 h after treatment. Temperature affected translocation differently in each species, but increasing soil moisture increased translocation.

Field horsetail was grown from rhizome segments for 28 and 52 d under 35, 60, 120, 170, and 320 μmol m–2 s–1 photosynthetic photon flux (PPF) and four continuous nitrogen (N) supply rates corresponding to a total of 1 to 6 kg N ha–1 28 d after planting (DAP) and 6 to 41 kg N ha–1 52 DAP. There was no response to the supply of N in the first growth period, probably due to the mobilization of nitrogen available in the planted rhizomes, and only dry weight (DW) of new belowground production, mostly roots, responded to increased light. Rhizomes were produced during the latter part of the 52-d period. Requirements for light and N during the 52-d period were low, because total new tissue production was only reduced at the lowest levels of light (35 μmol m–2 s–1 PPF) and the two lowest N supply rates (6 and 18 kg N ha–1). The ratio of shoot DW to belowground organ DW increased only as the N supply increased. Shoot height response to an increased N supply was also small. Field horsetail can therefore be considered tolerant of low levels of N but will be overtopped by fast-growing species when competing for increased supplies of N. Both an increase in N supply and a reduction in light increased the reduction of DW of the planted rhizomes, whereas tuber production was favored by low N supply and high light intensities. Field horsetail can tolerate low levels of N but is not likely to be competitive in a dense crop stand.

Field horsetail shoots were severed from the rhizomes and grown hydroponically with and without shade (40 and 190 μmol m–2 s–1 photosynthetic photon flux) under six different nutrient regimes, by using nutrient solutions with different amounts of nitrogen (N) and potassium (K) (6/42 mmol L–1 N; 7/15/24 mmol L–1 K). Nutrients were supplied daily with the same volumes for the six different nutrient solutions at exponentially increasing rates corresponding to a maximum relative growth rate (RGR) of 12.5% from days 0 to 31 and 6% from days 32 to 48. RGR was calculated for the highest N and K rate. Photosynthetic rate and stomatal conductance to water vapor were determined on day 48, then the plants were harvested. The fresh weight (FW) and dry weight (DW) of different plant components were determined. Shade completely inhibited growth of tubers and fertile shoots and drastically reduced RGR, average aerial shoot height, and DW of shoots and rhizomes. N supplied at the high rate to unshaded plants increased the growth of all parts or the plant except fertile shoots and tubers. No effects of K were detected at the low N rate, whereas at the high N rate, K significantly increased total DW and belowsurface DW. RGR never exceeded 10% d–1. In the unshaded treatments, the K concentration was higher in aerial shoots, 150 to 240 μmol g–1 FW, than in belowsurface organs, 30 to 90 μmol g–1 FW. Shoot growth was significantly reduced at shoot K concentrations below 150 to 210 μmol g–1 FW, and root growth was significantly reduced at root K concentrations below 30 to 50 μmol g–1 FW. K significantly increased maximum net photosynthetic rate, 10 μmol CO2 m–2 s–1, at light saturation, 1,300 μmol m–2 s–1, in unshaded plants, whereas it reduced the light compensation point and respiratory losses of CO2 in darkness. It is concluded that field horsetail is a typical sunny habitat species with a rather high K demand.

Field experiments were conducted in Thessaloniki, Greece, from 1991 to 1994 to study the competition of littleseed canarygrass and short-spiked canarygrass biotypes in wheat and to evaluate their response to diclofop, fenoxaprop, tralkoxydim, and CGA-184927. Competitive ability of all canarygrass biotypes was similar, though species differed in panicle numbers. All littleseed canarygrass biotypes showed faster growth rate than the short-spiked canarygrass biotypes. Wheat grain yield decreased as canarygrass density increased. For example, 152 and 304 canarygrass plants m–2 reduced wheat yield 32 and 42%, respectively. None of the canarygrass biotypes at any density had any effect on grain yield of wheat when cold and dry weather prevailed during the early stages of growth. CGA-184927 and tralkoxydim were more effective herbicides against all short-spiked canarygrass biotypes than fenoxaprop and diclofop. CGA-184927 and fenoxaprop provided > 98% control of all littleseed canarygrass biotypes; control with tralkoxydim was < 80%; and diclofop gave 82 to 96% control of this weed species. Herbicide control of canarygrass increased wheat yield relative to the weedy control, and only diclofop was phytotoxic to wheat.

Data on the spread of invasive weeds into arid western lands are used to evaluate the environmental and economic importance of controlling invasive weed infestations early. Variable rate and constant rate infestation expansion paths are estimated. The implications of variable vs. constant infestation growth rates for projecting both biophysical and economic effects are illustrated. The projections derived from both constant and variable growth rate expansion paths support the contention that it is expedient to control new infestations early.

In a semiarid Mediterranean site in central Spain, field experiments were conducted on a Calcic Haploxeralf (noncalcic brown soil), which had been managed with three crop rotations and two tillage systems (no-tillage and conventional tillage) since 1987. The crop rotations consisted of barley→vetch, barley→sunflower, and a barley monoculture. The study took place in two growing seasons (1992–1994) to assess the effects of management practices on the weed seedbank. During this period, spring weed control was not carried out in winter crops. In the no-tillage system, there was a significant increase in the number of seeds of different weed species: anacyclus, common purslane, corn poppy, knotted hedge-parsley, mouse-ear cress, spring whitlowgrass, tumble pigweed, venus-comb, and Veronica triphyllos. Conversely, the presence of prostrate knotweed and wild radish was highest in plots under conventional tillage. These results suggest large differences in the weed seedbank as a consequence of different soil conditions among tillage systems, but also the necessity of spring weed control when a no-tillage system is used. With regard to crop rotations, the number of seeds of knotted hedge-parsley, mouse-ear cress, and spring whitlowgrass was greater in the plots under the barley→vetch rotation. Common lambsquarters dominated in the plots under the barley→sunflower rotation, whereas venus-comb was the most frequent weed in the barley monoculture. Larger and more diverse weed populations developed in the barley→vetch rotation rather than in the barley→sunflower rotation or the barley monoculture.

Our objective was to identify traits in winter wheat important to competitiveness against jointed goatgrass, measured as increased wheat yields and reduced jointed goatgrass seed production. Jointed goatgrass is an important winter annual grass weed that cannot be controlled selectively in winter wheat. Seven cultivars of soft white winter wheat were grown with and without competition from jointed goatgrass over two growing seasons. Measurements of numerous traits of winter wheat and jointed goatgrass were recorded throughout each growing season. The data were analyzed using path analysis with latent variables to determine which traits most enhanced competitiveness. In a drier year, increased rate of height development was important in maintaining wheat yields when wheat was growing in competition with jointed goatgrass. Increased rate of height development also was an important trait in reducing jointed goatgrass seed production. In a wet year compared to a dry year, the number of wheat heads per plant, the rate of water use, and weight gain were positively correlated to maintaining winter wheat yields. Jointed goatgrass seed production in the wet year was reduced overall compared to the dry year, but from the cultivars tested, there were no traits identified that were critical in enhancing this loss of seed production. This study suggests that cultivars with greater height development rates will be more competitive when growing in fields infested with jointed goatgrass.

Interactions between initial weed seedling density and postemergence herbicide and mechanical weed control were studied in two field experiments conducted between 1994 and 1996. Increasing seedbank densities of velvetleaf (0 to 500 seed m–2) in soybean or common sunflower (250 to 2,500 seed 1.3 m–2) in corn or soybean were established at Lincoln and Mead, NE, respectively. Emerged seedlings were treated with increasing intensities of weed control from none to bentazon alone or with interrow cultivation. A positive linear relationship between initial seedling density and density of surviving seedlings was consistently observed. As initial seedling density increased, more survivors were present after treatment. As intensity of weed control increased, the number of seedling survivors decreased. Resulting reproductive fitness decreased with increasing management intensity but remained positive when regressed against surviving seedling densities. Weed management outcomes were dependent on initial seedling density, such that the absolute number of survivors increased, while proportion of survivors appeared constant within the density ranges studied. These research findings emphasize the need to account for weed infestation level when assessing efficacy of weed management systems and provide evidence that patchy weed distributions may persist in part because of the need for considerably higher management intensities in high density patch centers.

Field experiments were conducted in Louisiana over 2 yr to evaluate the influence of full-season interference from johnsongrass, common cocklebur, or hemp sesbania at densities of 2.5, 0.5, and 2.0 plants m–1 of row, respectively, and simulated insect defoliation of soybean on weed and soybean growth. Defoliation at R2 (full bloom) and R5 (beginning seed development) soybean growth stages was accomplished by removal of zero, one, or two leaflets per soybean trifoliate, which approximated 0, 33, and 66% defoliation, respectively. Height and dry weight of all weeds were not affected by soybean defoliation level or defoliation stage. Soybean height 3 wk after defoliation at R5 was not influenced by weed interference, soybean defoliation level, or defoliation stage in either year. Averaged across soybean defoliation levels and stages in 1994, johnsongrass, common cocklebur, and hemp sesbania reduced soybean yields 30, 15, and 14%, respectively. In 1995, johnsongrass reduced soybean yield 35%. As soybean defoliation level increased, a linear decrease in soybean yield was observed. Averaged across weeds and soybean defoliation stages, 33 and 66% defoliation reduced soybean yield 6 and 20% in 1994 and 12 and 33% in 1995, respectively. Defoliation at R5 resulted in 10% lower yield than defoliation at R2 in one of two years. Yield reduction due to combinations of weeds and soybean defoliation was additive.

A better understanding of the influence of various crop and weed management practices on spatiotemporal dynamics of weeds could improve the design of integrated weed management systems. We examined the influence of 18- and 76-cm soybean row spacings on emergence pattern and spatial aggregation of giant foxtail, common lambsquarters, and velvetleaf seedling cohorts. In addition, we characterized the soil seedbank and determined the quantitative and spatial relationship between the seedbank and seedling populations. Viable seeds of about 10 weed species and twice as many species of seedlings were identified in the weed community. Giant foxtail and common lambsquarters were the predominant species in the seedling and seedbank population, respectively, each accounting for 60 to 70% of the total weed species density. Emergence of giant foxtail, common lambsquarters, and velvetleaf depleted 12 to 33%, < 2% and 12 to 49% of the seedbank in the upper 10 cm of the soil profile. Peak time and periodicity of weed emergence was not influenced by soybean row spacing, and peak time of emergence of giant foxtail, common lambsquarters, and velvetleaf occurred 3 to 4, 3 to 6, and 3 to 9 weeks after soybean planting (WAP), respectively. Magnitude of giant foxtail emergence 5, 6, and 9 WAP was 98, 96, and 76% greater in 76- than in 18-cm row soybeans only when the population of 76-cm row soybeans was 57% lower than the 18-cm soybeans in 1997. Giant foxtail and common lambsquarters seeds in the seedbank were aggregated in 1996 and 1997 according to the Taylor power law (TPL) and the negative binomial distribution (NBD). The TPL and the NBD were similar in describing the spatial aggregation of giant foxtail and common lambsquarters but not some velvetleaf seedling cohorts. The spatial aggregation of seedlings varied among cohorts for different weed species and was likely due to species-specific biological characteristics that influence seed dispersal, germination, and seedling emergence. Within a 1.5-ha area, aggregation declined with decreasing density. Within a 24-m2 area, the level of aggregation of all weed species decreased as seedling densities increased. These results indicated that soybean row spacing influenced neither weed emergence pattern nor weed spatial aggregation; thus, several management decisions can be similar in 18- and 76-cm row soybeans.

Two field experiments were carried out from 1993 to 1995 to evaluate the critical period of weed control and to develop suitable weed management practices for jungle rice, horse purslane, and cockscomb in soybean. Horse purslane was more competitive during early growth stages (up to 45 days after sowing [DAS]) and cockscomb was more competitive during later growth stages, whereas jungle rice was competitive throughout the growing season. The critical period of weed control was found to be 30 to 45 DAS. Weed-free maintenance up to 45 DAS resulted in a 74% increase in grain yield of soybean over the unweeded control. Keeping soybean weed free for 45 d or allowing weeds to remain in the crop for less than 30 d resulted in no significant yield loss. Sequential application of a reduced rate of soil-applied trifluralin 1.0 kg ha–1 (0.67 ×) with postemergence fluazifop 0.75 kg ha–1 (0.75 ×) or a reduced rate of soil-applied trifluralin or pendimethalin at 1.0 kg ha–1 (0.67 ×) followed by hand hoeing 35 DAS provided better control of a broad spectrum of weeds than a single application of a postemergence herbicide applied at reduced or recommended rates. Integration of reduced rates of soil-applied herbicides with post-emergence herbicides or hand hoeing 35 DAS produced soybean yields similar to the hand-weeded treatment. Compared to the weed-free or integrated weed control, a single application of soil-applied or postemergence herbicide did not control a broad spectrum of weeds and reduced soybean yield.

Generalizations concerning the effects of management practices on weed community dynamics often lack robustness, most likely because of the concomitant effects of agronomic and environmental factors. However, such generalizations, when valid, provide useful grounds for predictions and are thus desirable. This study attempted to evaluate the relative importance of crop rotation, tillage, and weed management as factors affecting weed communities and tested the hypothesis of an association between management practices and weeds from certain life cycle groups. Principal component analysis (PCA) of weed density data from a 4-yr field study conducted on a Kamouraska clay and a Saint-André gravelly sandy loam at La Pocatière QC, Canada, identified groups of weed species, while an analysis of variance (ANOVA) of PCA scores associated these groups with management factors. A multivariate analysis of variance (MANOVA) of regression coefficients describing time courses of density for each species confirmed treatment effects. Species segregated roughly according to life cycles. Interactions among weed management intensity, tillage, and crop rotation mostly explained species dominance in the various cropping systems. A first group of species, mostly annual dicots, largely dominated in minimum weed management treatments; their relative importance in each rotation varied with their level of susceptibility to postemergence herbicides. A second group included annuals and perennials, whose commonality seemed to be their tolerance to herbicides; these species also had a particular affinity for chisel and no-till treatments. A third group was formed by perennial species, each with a different response to tillage. The tenuous correspondence between commonly used classification schemes and management factors suggests that other aspects of weed biology (e.g., seed size, dispersal, production, germination requirements, and seedbank longevity) should be considered when trying to explain and predict the presence and dominance of certain weed species with regard to management practices.

Greenhouse experiments were conducted to determine the effect of mixing bromoxynil with clethodim, sethoxydim, fluazifop-P, fluazifop-P plus fenoxaprop-P, or quizalofop-P on annual grass control. Antagonism with mixtures of bromoxynil and graminicides was dependent upon species and the graminicide in the mixture. Goosegrass control was unaffected by mixing bromoxynil with any graminicide. Mixing bromoxynil with clethodim, sethoxydim, fluazifop-P, or fluazifop-P plus fenoxaprop-P had no effect on control of Texas panicum, but the rate of quizalofop-P required for 80% control was increased 240%. Mixing bromoxynil with clethodim or sethoxydim had no effect on control of large crabgrass or yellow foxtail, but the rate of fluazifop-P, fluazifop-P plus fenoxaprop-P, or quizalofop-P required for 80% control was increased 180 to 290%. Antagonism of large crabgrass and yellow foxtail control with mixtures of quizalofop-P and bromoxynil increased as the rate of bromoxynil increased. Antagonism toward large crabgrass and yellow foxtail was alleviated by applying bromoxynil 6 d before the graminicides or 3 or 6 d after the graminicides.