As all herbicides act on pathways or processes crucial to plants, in an inhibitory or stimulatory way, low doses of any herbicide might be used to beneficially modulate plant growth, development, or composition. Glyphosate, the most used herbicide in the world, is widely applied at low rates to ripen sugarcane. Low rates of glyphosate also can stimulate plant growth (this effect is called hormesis). When applied at recommended rates for weed control, glyphosate can inhibit rust diseases in glyphosate-resistant wheat and soybean. Fluridone blocks carotenoid biosynthesis by inhibition of phytoene desaturase and is effective in reducing the production of abscisic acid in drought-stressed plants. Among the acetolactate synthase inhibitors, sulfometuron-methyl is widely used to ripen sugarcane and imidazolinones can be used to suppress turf species growth. The application of protoporphyrinogen oxidase inhibitors can trigger plant defenses against pathogens. Glufosinate, a glutamine synthetase inhibitor, is also known to improve the control of plant diseases. Auxin agonists (i.e., dicamba and 2,4-D) are effective, low-cost plant growth regulators. Currently, auxin agonists are still used in tissue cultures to induce somatic embryogenesis and to control fruit ripening, to reduce drop of fruits, to enlarge fruit size, or to extend the harvest period in citrus orchards. At low doses, triazine herbicides stimulate growth through beneficial effects on nitrogen metabolism and through auxin-like effects. Thus, sublethal doses of several herbicides have applications other than weed control.

Aromatic plants and their volatile compounds affect seed germination and plant growth, and therefore hold potential for agriculture uses as plant growth regulators and bioherbicides. In the present study 17 major monoterpenes were selected, and their mechanisms of plant toxicity were elucidated using transgenic Arabidopsis thaliana at various growth stages. Microtubulin and the plant cell membrane were identified as the focal targets through which phytotoxicity and herbicidal activity acted. Variability in monoterpene mechanisms was observed. Limonene and (+)-citronellal had strong antimicrotubule efficacy, whereas citral, geraniol, (−)-menthone, (+)-carvone, and (−)-citronellal demonstrated moderate antimicrotubule efficacy. Pulegone, (−)-carvone, carvacrol, nerol, geranic acid, (+)/(−)-citronellol, and citronellic acid lacked antimicrotubule capacity. An enantioselective disruption of microtubule assembly was recorded for (+)/(−)-citronellal and (+)/(−)-carvone. The (+) enatiomers were more potent than their (−) counterparts. Citral, limonene, carvacrol, and pulegone were also tested for phytotoxicity and herbicidal activity. Pulegone had no detectable effect on microtubules or membranes. Citral disrupted microtubules but did not cause membrane damage. Carvacrol lacked a detectable effect on microtubules but incited membrane leakage, and limonene disrupted microtubules and membrane leakage. Therefore, only limonene was herbicidal at the tested concentrations. In planta quantification of residues revealed that citral was biotransformed into nerol and geraniol, and limonene was converted into carvacrol, which could explain its dual capacity with respect to microtubules and membrane functionality. The results obtained are an important added value to commercial efforts in selecting appropriate aromatic plants to be sources of bioherbicidal compounds for sustainable weed management with a limited potential for herbicide resistance evolution in weed populations.

Surviving rigid ryegrass plants were collected from a cropping field at Pindar, Western Australia (population WALR 50), after inadequate control by glyphosate applied at the normal field rate. Plants were grown to maturity in pots and seeds were collected. Glyphosate dose–response experiments with known susceptible and resistant control populations confirmed the resistant status of the WALR 50 population. The glyphosate rate resulting in 5% mortality (LD50) and GR50 (the glyphosate rate required to reduce mean growth of individuals to 50% of the untreated control) values for this population were 1,069 and 217 g ae ha−1, respectively, corresponding to R:S ratios of 3.4 and 1.9 for mortality and growth. In addition, a novel root growth–based assay of glyphosate resistance was developed and validated, giving a root growth GR50 R:S ratio of 3.4. A resistance profile was established by assessing population-level survival of WALR 50 after applications at recommended rates of a range of herbicides commonly used for rigid ryegrass control in Australia. High levels of resistance to the acetolactate synthase (ALS)–inhibiting sulfonylurea herbicides chlorsulfuron and sulfometuron, moderate resistance to the acetyl coenzyme A carboxylase (ACCase)–inhibiting herbicide diclofop, and low levels of resistance to the imidazilinone herbicide imazethapyr were found. More detailed dose–response experiments confirmed resistance to chlorsulfuron, sulfometuron, and diclofop. In vitro enzyme-inhibition studies demonstrated that ALS resistance in WALR 50 is due to an insensitive target enzyme and that ACCase resistance is due to a nontarget site–based mechanism. WALR 50 is the first glyphosate-resistant weed population with confirmed resistance to ACCase- and ALS-inhibiting herbicides.

Experiments were conducted to determine potential mechanisms of glyphosate resistance in Lolium rigidum from Australia. 14C-Glyphosate uptake, translocation, and metabolism were compared between resistant (R) and sensitive (S) biotypes. The R seed (48118a) represented the F1 progenies of plants having survived a 1.73-kg ae ha−1 (4.8 L ha−1) application of a Roundup® formulation. The S seed was a sensitive biotype of L. rigidum from Australia. Plants (one to four tillers, 2 to 4 wk old) were presprayed with a high (1.26 kg ae ha−1) or a low (0.28 kg ae ha−1) dose of formulated glyphosate. The first leaf of the first tiller, which was shielded from the spray, was immediately treated with a 14C-glyphosate solution via manual application to the adaxial surface. Harvest was made 6 days after treatment (DAT), and glyphosate residues in the leaf wash, treated leaf, roots, and shoots were quantified based on radioactivity as percentage of applied dose. The overall radioactivity recoveries were very good (90.2 to 97.3% of applied dose). R and S plants showed comparable uptake at the high (79.2 vs. 78.0%) or the low (64.0 vs. 64.7%) doses of glyphosate. About one-half of the absorbed glyphosate in both R and S (32.9 to 38.3% appl.) was translocated into the plant and distributed almost equally into roots (13.6 to 16.0% appl.) and shoots (18.1 to 22.6% appl.). Autoradiography studies demonstrated no difference in tissue localization of glyphosate between the R and S plants. For metabolism studies, tissues from individual plants were homogenized in water, and extracts were analyzed by anion exchange high-pressure liquid chromatography (HPLC) with radioactivity detection. There was little to no metabolism of glyphosate in extracts from various tissues of either R or S plants. Based on these results, we conclude that neither uptake, translocation, nor metabolism play a major role in glyphosate resistance in L. rigidum.

The influence of secondary soil disturbance on the emergence pattern and seed bank depletion of an annual weed community in a long-term, no-tillage corn cropping system was determined in 1992 and 1993. As a component of this research, the seed bank was characterized prior to implementation of soil disturbance treatments. The seed bank was initially composed of common lambsquarters, redroot pigweed, and giant foxtail, with approximately 55, 36, and 8% of the total viable seeds, respectively. The remaining 1% was comprised of five other species in 1992 and eight in 1993. The spatial distribution of viable seeds of each species, except common lambsquarters and redroot pigweed, was described by a negative binomial distribution. Three dispersion indices indicated that seeds of individual and total weed species were aggregated and that the level of aggregation of viable seeds of a species was associated with seed density; at lower seed densities, the level of aggregation was greater. Soil disturbance increased common lambsquarters emergence 6-fold in 1992 relative to nondisturbed soil, but did not influence emergence in 1993. Rainfall was about 50% less in 1993. In contrast, soil disturbance increased giant foxtail and redroot pigweed emergence approximately 6- and 3-fold in 1992 and 1993, respectively. Seedling emergence associated with soil disturbance, relative to nondisturbed soil, increased seed bank depletion of common lambsquarters 16-fold in 1992, and giant foxtail and redroot pigweed and average of 6- and 3-fold in 1992 and 1993, respectively. These results indicated that soil disturbance increased seedling emergence and seed bank depletion of the predominant species in the weed community of a long-term, no-tillage system, but that this response was dependent on rainfall for common lambsquarters.

Peanut (Arachis hypogaea L. ‘Tifspan’ or ‘Florunner’) yields were not reduced when the crop was maintained free of Florida beggarweed [Desmodium tortuosum (Sw.) DC.] or sicklepod (Cassia obtusifolia L.) for 4 weeks after crop emergence and when vigorous crop growth was maintained for the remainder of the season. Sometimes weed-free maintenance for only 2 weeks resulted in near-normal yields, which indicated that the canopy of peanut leaves effectively suppressed the weeds. Conversely, these weeds had to compete with peanuts for more than 10 weeks before crop yield was reduced. Weeds that overtopped the peanuts at harvest emerged during the first 6 weeks after planting.

Laboratory studies were conducted to determine the relative sorption, mobility, and degradation rates of triclopyr and 2,4-D on two surface soils and two subsoils from the rice-producing areas of Arkansas. Triclopyr sorption was slightly greater than 2,4-D sorption. However, mobility of the herbicides on a given soil did not differ. Sorption of both herbicides was greatest and mobility lowest on a subsoil with the lowest pH. Triclopyr degradation rates were lower than 2,4-D degradation rates in a dark incubator. The average half life was 138 d for triclopyr and 21 d for 2,4-D. High soil moisture content (0 versus 100 kPa water tension) increased the rate of 2,4-D degradation. Triclopyr degraded more rapidly at 30 C than at 15 C. The dissipation rates of both herbicides were lowest on the soil on which sorption was greatest.

The persistence of metribuzin [4-amino-6-tert-butyl-3-(methylthio)-as-triazine-5(4H)-one] in several soils from the lower alluvial floodplain of the Mississippi River was studied in the greenhouse and laboratory using bioassay and gas chromatographic methods of residue detection. Bioassay results indicated that metribuzin phytotoxicity was lost in several soil types within a few weeks of application. Longevity of phytotoxicity depended upon soil type. However soil type had little effect on the rate of metribuzin degradation measured chemically. Gas chromatographic techniques indicated that metribuzin degradation followed first-order kinetics with half-life values ranging from 17 to 28 days in six soils under greenhouse conditions. Incubation studies indicated that metribuzin was degraded more rapidly in nonautoclaved field soil and in soil enriched with glucose than in soil that had been air dry for 1 yr or had been autoclaved. Degradation was more rapid at 30 C than at 20 C. Microbiological activity apparently is important in the degradation of metribuzin in soil.

Common lambsquarters, a summer annual weed, has occupied comparatively different ecogeographical regions around the globe. To investigate the extent to which germination and emergence traits have differentiated in two natural populations of common lambsquarters from different environments (Denmark as mesic and Iran as xeric population), experiments were conducted in the laboratory. Germination of both populations was stimulated by light. The greatest germination percentage of xeric and mesic populations occurred at 25/15 and 20/10 C, respectively. The xeric population showed significantly greater germinability at the lowest (15/5 C) and the highest (35/25 C) fluctuating temperature regimes. Germination of the xeric population was only slightly affected, and the mesic population was severely retarded at a salinity level of 20 desi siemens (DS) m−1. Greater than 42% of the xeric seeds germinated at 30 dS m−1 salinity, while the mesic germination almost ceased at this salinity level. Germination of seeds placed in distilled water after the 14-d salinity exposure treatments (recovery rates) was also greater for the xeric vs. mesic populations. Xeric populations showed more than 65% germination up to the osmotic potential of −0.4 MPa, while decreasing osmotic potential from 0 to −0.4 MPa caused an 80% reduction in mesic population germinability (9% germination). The greatest emergence of xeric (77%) and mesic (70%) populations occurred for seeds placed on the soil surface and no seedlings emerged from burial depth 3 cm. These study results suggest that common lambsquarters populations have differentiated for heat, salinity, and drought tolerance at germination stage. These contrasting germination patterns are appeared to be due to either adaptation via natural selection or maternal effects, or some combination of both. Though this study has certain limitations, it, through its findings and their propositions, adds to the existing knowledge about interpopulation differences in germination requirements of common lambsquarters, as a globally distributed species.

Spray application of 24 and 46 g ha−1 MON 37500 was used in efficacy studies, and vacuum infiltration or droplet application of radiolabeled MON 37500 was used in metabolism studies to evaluate temperature and soil moisture on MON 37500 efficacy and metabolism. Day/night temperatures before vs. after application of MON 37500 of 25/23 vs. 25/23, 25/23 vs. 5/3, 5/3 vs. 25/23, and 5/3 vs. 5/3 C were evaluated for the efficacy study, whereas day/night temperatures of 5/3 and 25/23 C were used for the metabolism study. Soil moisture of one-third and full pot capacities was evaluated for both studies. No Triticum aestivum injury was observed at the different temperatures or soil moistures because of rapid metabolism of MON 37500 by T. aestivum. Weed control was greater when the temperature after application was 25/23 C or soil moisture was at full pot capacity than when the temperature was at 5/3 C after application or soil moisture was at one-third pot capacity. Susceptibility to MON 37500 was greatest for Bromus tectorum, moderate for Avena fatua, and least for Aegilops cylindrica. This pattern of susceptibility for the weed species was related to their ability to metabolize MON 37500. Aegilops cylindrica metabolized more MON 37500 in the first 24 h than did A. fatua, whereas B. tectorum metabolized the least MON 37500. Cool air temperatures decreased MON 37500 metabolism in all species, whereas soil moisture had no effect.

Laboratory studies were conducted to evaluate the degradation of butylate (S-ethyl diisobutylthiocarbamate), EPTC (S-ethyl dipropylthiocarbamate), cycloate (S-ethyl N-ethylthiocyclohexanecarbamate), and vernolate (S-propyldipropylthiocarbamate) in soil with prior thiocarbamate herbicide exposure. EPTC degradation was accelerated in soil which had previously been treated with butylate, vernolate, or EPTC in 1979, 1980, and 1981. The evolution rate of 14CO2 from 14C-labeled vernolate was increased in soil which had previously been treated with vernolate or EPTC in 1979, 1980, and 1981. Cycloate degradation was not enhanced in soil with prior exposure to butylate, EPTC, or vernolate. Butylate breakdown was enhanced to the greatest extent in soil with previous exposure to butylate.

Seedlings of fifteen different cultivars of tomato (Lycopersicon esculentum Mill.) varied greatly in tolerance to metribuzin [4-amino-6-tert-butyl-3-(methylthio)-as-triazine-5(4H) one] applied at a concentration of 0.5 mg/L in quartz sand nutrient culture during the three-true-leaf stage. Tolerance increased with seedling age, but acceptable tolerance was observed about 6 days earlier in the tolerant ‘Fireball’ compared to the susceptible ‘Heinz 1706′. Root uptake and acropetal translocation of metribuzin-carbonyl-14 C was similar in 2-week-old seedlings of susceptible Heinz 1706 and tolerant Fireball seedlings. Differential tolerance was related to the rate of detoxification by metabolism within the tomato leaves, which was approximatley two-fold greater in the tolerant Fireball seedlings.

The germination of caryopses on water or GA3 solutions was used to characterize the degree of primary dormancy present in the seed progeny of 10 genetically pure lines of wild oats (Avena fatua L. # AVEFA). These 10 lines represented a range of types from lines exhibiting no dormancy to those with a high degree of dormancy. Repeated propagation of these pure lines under constant environmental conditions identified several genetically inherited characters that were associated with the inherited differences in degree of primary dormancy. Correlation of the seed development period, number of primary caryopses produced, and caryopsis moisture content to the degree of primary dormancy shows: a) Lines characterized by low degrees of primary dormancy were, in general, those that had a short seed development time and produced few heavy seed low in percent water; and b) lines characterized by high degrees of primary dormancy were, in general, those that had a long seed development time and produced many light seed high in percent water. The adaptive advantage of the coupling of physiological and morphological characters with degree of dormancy is discussed and the possibility that some characters are determined by others is indicated.

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.

Resistance to glufosinate has been confirmed in glyphosate-resistant Italian ryegrass populations collected in hazelnut orchards in Oregon. Dose–response, ammonia accumulation, and enzyme activity studies were conducted to test the sensitivity of three glyphosate-resistant and three susceptible Italian ryegrass populations to glufosinate. The glufosinate rates required to reduce the growth by 50% (GR50) were 0.15, 0.18, and 0.21 for the control populations C1, C2, and C3, respectively, whereas for the resistant populations OR1, OR2, and OR3, the GR50 values were 0.49, 0.42, and 0.40 kg ai ha−1, respectively, exhibiting an average resistance index of 2.4. The same trend was observed in ammonia accumulation studies between 48 and 96 h after glufosinate treatment where the susceptible populations accumulated on average two times more ammonia than the resistant populations. The glufosinate concentration required to reduce the glutamine synthetase enzyme activity by 50% (I50) was not different for the resistant and susceptible populations. The I50s ranged from 3.1 to 3.6 µM for the resistant populations and from 3.7 to 4.3 µM for the susceptible populations; therefore, an insensitive target site is not responsible for the glufosinate resistance.

Using climate-controlled glasshouses, the growth of grain sorghum was evaluated with and without the presence of common cocklebur at current and projected future atmospheric concentrations of carbon dioxide [CO2]. Single-leaf photosynthetic rates declined for both species in competition; however, elevated CO2 reduced the percentage decline in common cocklebur and increased it in sorghum by 35 d after sowing (DAS) relative to ambient CO2. In monoculture, elevated CO2 significantly stimulated leaf photosynthetic rate, leaf area, and aboveground dry weight of common cocklebur more than that of sorghum. However, the stimulation of aboveground biomass or leaf area for monocultures of sorghum and common cocklebur at elevated CO2 did not adequately predict the CO2 response of aboveground biomass or leaf area for sorghum and common cocklebur grown in competitive mixtures. Overall, by 41 DAS, plant relative yield (PRY), in terms of aboveground biomass and leaf area, increased significantly for common cocklebur and decreased significantly for sorghum in competitive mixtures at elevated CO2. Data from this study indicate that vegetative growth, competition, and potential yield of economically important C4 crops could be reduced by co-occurring C3 weeds as atmospheric carbon dioxide increases.

Variation in interference relationships have been shown for a number of crop-weed associations and may have an important effect on the implementation of decision support systems for weed management. Multiyear field experiments were conducted at eight locations to determine the stability of corn-foxtail interference relationships across years and locations. Two coefficients (I and A) of a rectangular hyperbola equation were estimated for each data set using nonlinear regression procedures. The I and A coefficients represent percent corn yield loss as foxtail density approaches zero and maximum percent corn yield loss, respectively. The coefficient I was stable across years at two locations and varied across years at four locations. Maximum yield loss (A) varied between years at one location. Both coefficients varied among locations. Although 3 to 4 foxtail plants m−-1 row was a conservative estimate of the single-year economic threshold (Tc ) of foxtail density, variation in I and A resulted in a large variation in Tc . Therefore, the utility of using common coefficient estimates to predict future crop yield loss from foxtail interference between years or among locations within a region is limited.

Field experiments were conducted at Vegreville, Alberta, in 1997, 1998, and 1999 and in Lacombe, Alberta, in 1997 and 1998, to determine if barley row spacing (20 and 30 cm) and seeding rate (75, 125, and 175 kg ha−1) influenced the effects of variable tralkoxydim rates on barley seed yield, net economic returns, and wild oat seed production. In most cases, barley seed yield was unaffected by row spacing or seeding rate. Where no herbicide was applied, the presence of wild oat reduced barley yield at each location each year. When the herbicide was applied at 50, 75, or 100% of the recommended rate, barley yields were not affected by the presence of wild oat. Results were more variable at 25% of the recommended rate, especially at Lacombe, where yield losses occurred both years at this rate. The lowest net economic returns consistently occurred in the absence of herbicide application; however, the influence of herbicide rate on net returns varied among years and locations. Net returns were either higher at the lower herbicide rates or were unaffected by herbicide rate. Seeding rate and herbicide rate affected wild oat seed production at each location each year and also the amount of seeds in the soil seedbank at Vegreville in 1999. Row spacing had little or no effect on wild oat seed production. There was a consistent and highly significant seeding rate by herbicide rate interaction on wild oat seed production. The effects of tralkoxydim on wild oat seed production, especially at relatively low rates, were superior at the higher barley seeding rates. The results suggest that seeding barley at relatively high rates can result in optimum barley yields, undiminished economic returns, and effective wild oat management when tralkoxydim is used at lower than recommended rates.

Glyphosate [N-(phosphonomethyl)glycine] phytotoxicity to greenhouse-grown wheat (Triticum aestivum L. ‘Centurk’) was greatly reduced by Ca2+ in the carrier solution. Addition of ethylenediaminetetraacetic acid (EDTA) restored glyphosate phytotoxicity in hard water and increased phytotoxicity in deionized water. The phytotoxicity of 0.1 kg ae/ha glyphosate plus 1.0% (w/v) EDTA was equal to that observed for 0.2 kg/ha of the herbicide alone in deionized water or 0.4 kg/ha in water containing 200 ppm Ca2+. EDTA was more compatible with glyphosate in combination with X-77 or MON-0011 nonionic surfactant than with cationic surfactant (contained in a commercial formulation). Phytotoxicity could be increased by adding EDTA and nonionic surfactant to the latter formulation. EDTA increased glyphosate phytotoxicity to wheat when applied in water that contained high levels of calcium and bicarbonate. Glyphosate activity was greater at pH 4 than at pH 6 or 10, but EDTA was equally effective at pH 4, 6, or 10 for all levels of calcium tested up to 800 ppm. NaHCO3 probably reduced phytotoxicity by increasing the pH of the glyphosate solution.

Weed seed and seedling populations, and weed competition were compared in plots of continuous corn and corn/soybean rotation under ridge and conventional tillage. After 7 to 8 yr of standard chemical and mechanical weed control, from 1500 to 3000 weed seeds/m2 (to a 10-cm depth) were found in continuous corn with ridge tillage whereas about two-thirds fewer seeds were found in conventionally tilled corn. Soil from a corn/soybean rotation had from 200 to 700 seeds/m2 in both tillage systems. Annual loss of weed seeds from the soil through germination was from 3 to 12% in ridge tillage and 11 to 43% in conventional tillage. Additions to the seed pool were supplied by small weeds whose germination was stimulated by “layby” cultivation, with up to 10 times more emergence and 140 times more seed production in ridge than in conventional tillage. Withholding herbicides for 1 yr reduced yields of continuous corn by 10 to 27% in ridge tillage, only 2 to 4% in conventional tillage, and negligibly in corn/soybean rotations regardless of tillage. Reducing seed production of small layby weeds in ridge tillage may aid in solving the weed problem in this conservation tillage system.