Showing papers in "Weed Science in 1998"

Evolved resistance to glyphosate in rigid ryegrass (Lolium rigidum) in Australia

Abstract: Following 15 yr of successful use, glyphosate failed to control a population of the widespread grass weed rigid ryegrass in Australia. This population proved to be resistant to glyphosate in pot dose-response experiments conducted outdoors, exhibiting 7- to 11-fold resistance when compared to a susceptible population. Some cross-resistance to diclofop-methyl (about 2.5-fold) was also observed. Similar levels of control of the resistant and susceptible populations were obtained following application of amitrole, chlorsulfuron, fluazifop-P-butyl, paraquat, sethoxydim, sirnazine, or tralkoxydim. The presence of glyphosate resistance in a major weed species indicates a need for changes in glyphosate use patterns.

A biotype of common waterhemp (Amaranthus rudis) resistant to triazine and ALS herbicides

Abstract: A common waterhemp biotype that was not controlled by triazine or acetolactate synthase (ALS)-inhibiting herbicides was isolated from a field in Bond County, IL, in the fall of 1996. Greenhouse and laboratory experiments determined resistance to atrazine and three ALS-inhibiting herbicides in this biotype. Based on whole-plant response, the Bond County common waterhemp biotype required over 1,000 times more imazethapyr relative to a susceptible biotype to reduce growth 50%. Cross-resistance to thifensulfuron, a sulfonylurea, and flumetsulam, a triazolopyrimidine sulfonanilide, was also detected. Based on in vivo enzyme assays, ALS in the Bond County common waterhemp biotype was 20-, > 8-, and 68-fold less sensitive than ALS in the susceptible biotype to imazethapyr, thifensulfuron, and flumetsulam, respectively. Whole-plant efficacy trials also indicated that the Bond County common waterhemp biotype required more than 20 kg ha−1 of atrazine to inhibit growth 50%. Chlorophyll fluorescence assays revealed that 100 nM atrazine inhibited photosynthesis in the susceptible biotype, whereas 10 M did not affect photosynthesis in the resistant biotype. Regions of the genes encoding ALS and D1 proteins were sequenced to determine the molecular basis for the resistances. Triazine resistance was conferred by a glycine for serine substitution at residue 264 of the D1 protein, while ALS resistance was conferred by a leucine for tryptophan substitution at residue 569 of ALS.

Weed flora and the relative importance of site, crop, crop rotation, and nitrogen

Abstract: Weed species composition and density were recorded in three identical field experiments established 26 to 30 yr ago in southern Sweden. Each experiment compared three 6-yr crop rotations and four rates of nitrogen application. The rotations differed by having (1) a 2-yr rotational grassland, (2) a 2-yr mixed rotational grassland (legume/grass), or (3) spring wheat followed by fallow. Other crops in the rotations were winter turnip rape, winter wheat, spring oats, and spring barley. Using multivariate analyses, the relative importance of site, crop, crop rotation, and nitrogen application rate on the weed flora was determined. The greatest difference was found between sites, and the second most important factor was crop species. Nitrogen application rate weakly influenced the weed flora, while differences between crop rotations were hardly detectable.

Imazethapyr resistance in common sunflower (Helianthus annuus)

Abstract: Resistance to imazethapyr was identified in a population of common sunflower that had been treated with imazethapyr for seven consecutive years. The imazethapyr-resistant biotype of common sunflower was approximately 170 times more resistant to imazethapyr than the susceptible biotype based on the rate required for 25% control. Resistance was due to altered acetolactate synthase (ALS) that is less sensitive to imazethapyr. The imazethapyr concentration required to inhibit in vitro ALS activity by 25% was 210–fold higher in the resistant biotype than in the susceptible biotype. Differences in absorption, translocation, and metabolism of imazethapyr in common sunflower biotypes were not sufficient to explain the resistance to imazethapyr.

Quinclorac belongs to a new class of highly selective auxin herbicides

Abstract: Substituted quinolinecarboxylic acids, including quinclorac (BAS 514H), are a new class of highly selective auxin herbicides, which are chemically similar to naturally occurring compounds isolated from plants and soils. Quinclorac is used in rice to control important dicot and monocot weeds, particularly barnyardgrass. The herbicide has also been developed for application in turfgrass areas, spring wheat, and chemical fallow. Quinclorac is readily absorbed by germinating seeds, roots, and leaves and is translocated in the plant both acropetally and basipetally. By mimicking an auxin overdose, quinclorac affects the phytohormonal system in sensitive plants. The compound stimulates the induction of 1-aminocyclopropane-1-carboxylic acid (ACC) synthase activity and thus promotes ethylene biosynthesis. In susceptible dicots, increased levels of ethylene trigger an accumulation of abscisic acid (ABA), which, as part of the intrinsic auxin activity of quinclorac, plays a major role in growth inhibition and the induction of epinasty and senescence. In sensitive grasses, such as barnyardgrass species, large crabgrass, broadleaf signalgrass, and green foxtail, quinclorac leads particularly to an accumulation of tissue cyanide, formed as a co-product during increased ACC and ethylene synthesis. This causes phytotoxicity characterized by the inhibition of root and particularly shoot growth with tissue chlorosis and subsequent necrosis. These effects were not observed in tolerant rice and a resistant biotype of barnyardgrass. No significant differences in uptake, translocation, or metabolism of quinclorac between resistant and sensitive grasses were found. Hence, a target-site-based mechanism of selectivity is suggested. The induction process of the ACC synthase activity plays the primary role in the selective herbicide action of quinclorac. This is a common effect of auxin herbicides and auxins, which lead to the accumulation of cyanide and/or ABA depending on the plant species and tissues, the compound concentration in the tissue, and their biological activity.

Potential for gene transfer between wheat (Triticum aestivum) and jointed goatgrass (Aegilops cylindrica)

Abstract: C. A. Mallory-Smith Department of Crop and Soil Science, Oregon State University, Corvallis, OR 97331 Jointed goatgrass is a major weed in the wheat-producing areas of the western U.S. It shares the D genome with wheat, and interspecific hybrids between the two species occur in the field. The objective of this research was to determine if wheat x jointed goatgrass hybrids could serve to transfer genes from wheat to jointed goatgrass. A backcrossing program was initiated in the greenhouse between wheat x jointed goatgrass hybrids and either jointed goatgrass or wheat to determine the potential for seed set and the restoration of self-fertility. Seed was set by backcrossing with either species as the recurrent parent. Female fertility increased from 2% in the hybrid to 37% in the BC2 plants with jointed goatgrass as the recurrent parent. Partial self-fertility was restored in the second backcross (BC2) generation using jointed goatgrass as the recurrent parent. This indicates that genes could be transferred between wheat and jointed goatgrass after only two backcrosses. The number of bivalents observed in the plants during meiosis appeared to be key to increasing female fertility and self-fertility. Based on the results of this study, it is possible for genes to move from wheat to jointed goatgrass. Any release of a herbicide-resistant wheat should be accompanied by a management plan that would minimize the potential for gene movement between these species.

Biochemical mechanism and molecular basis for ALS-inhibiting herbicide resistance in sugarbeet (Beta vulgaris) somatic cell selections

Abstract: Three sugarbeet selections differing in cross-resistance to three classes of acetolactate synthase (ALS)-inhibiting herbicides have been developed using somatic cell selection. Sugarbeet selections resistant to imidazolinone herbicides, Sir-13 and 93R30B, do not metabolize [14C]-imazethapyr any faster or differently than sensitive, wild-type sugarbeets or a sulfonylurea-resistant/imidazolinone-sensitive selection, Sur. ALS specific activity from the three herbicide-resistant selections ranged from 73 to 93% of the wild-type enzyme extracts in the absence of herbicide, indicating enzyme overexpression was not a factor in resistance. Acetolactate synthase from Sir-13 plants showed a 40-fold resistance to imazethapyr but no resistance to chlorsulfuron or flumetsulam. Polymerase chain reaction amplification and sequencing of two regions of the ALS gene spanning all known sites for ALS-based herbicide resistance in plants indicated a single nucleotide change in the Sir-13 gene (G337 to A337) resulting in a deduced substitution of threonine for alanine at position 113 in the sugarbeet amino acid sequence. Sur ALS was not significantly resistant to imazethapyr, but was 1,000- and 50-fold resistant to chlorsulfuron and flumetsulam, respectively. Sur gene sequencing indicated a single nucleotide change (C562 to T562) resulting in a serine for proline substitution at position 188 of the ALS primary structure. The 93R30B nucleotide sequence indicated two mutations resulting in two deduced amino acid substitutions: threonine for alanine at position 113 plus serine for proline at position 188. The 93R30B double mutant incorporated the changes observed in each of the single mutants above and correlated with higher resistance levels to imazethapyr (> 1,000-fold), chlorsulfuron (4,300-fold), and flumetsulam (200-fold) at the ALS level than observed in either of the single mutants. 93R30B represents the first double mutant derived by a two-step selection process that incorporates two class-specific ALS-inhibitor resistance mutations to form a single broad cross-resistance trait. The interaction of the two altered amino acids is synergistic with respect to enzyme resistance vs. the resistance afforded by each of the individual mutations.

Assessment of weed and crop fitness in cover crop residues for integrated weed management

Abstract: Cover crop residues are not widely used for weed control because, as a stand-alone tactic, they do not effectively suppress all weeds and their duration of weed control is too short. Field experiments were conducted in 1995 and 1996, under both irrigated and rainfed conditions, to quantify Amaranthus spp., Setaria spp., and soybean emergence and growth in residues of fall-planted, spring-killed barley, rye, triticale, wheat, and hairy vetch. For both weed species, seedling emergence was reduced 3 wk after soybean planting by rye and wheat residues (≥ 2, 170 kg ha−1) in 1996. In 1996, Amaranthus spp. canopy volume was reduced 38 to 71% by residues 3 wk after planting. Likewise, Setaria spp. canopy biomass was reduced 37 to 97% in residues 5 wk after planting over both years. The response comparison index was used to identify frequency by which weed growth was placed at a disadvantage relative to soybean growth. Amaranthus spp. and Setaria spp. growth suppressions 3 to 5 wk after planting indicate potential times for intervention with other integrated weed management tactics such as reduced postemergence herbicide rates and interrow cultivation.

Interaction of glyphosate with chlorimuron, fomesafen, imazethapyr, and sulfentrazone

Abstract: Lawrence R. Oliver Corresponding author. Department of Agronomy, University of Arkansas, Fayetteville, AR 72701; oliver@comp.uark.edu Field experiments were conducted on eight weed species to determine if chlorimuron, fomesafen, imazethapyr, or sulfentrazone at two rates (labeled and one-half the labeled rate) were complementary tank mixtures with glyphosate at 210 and 420 g ai ha-1. Laboratory experiments were conducted on barnyardgrass, pitted morningglory, Palmer amaranth, and velvetleaf using radiolabeled glyphosate, chlorimuron, and imazethapyr to determine the absorption and translocation pattern of these herbicides applied alone and in combination. In the field, glyphosate plus chlorimuron tank mixtures were generally additive. Adding chlorimuron did not decrease absorption or translocation of 14C-glyphosate by barnyardgrass, pitted morningglory, or velvetleaf. Adding glyphosate increased absorption of 14C-chlorimuron by Palmer amaranth and velvetleaf. All four fomesafen plus glyphosate rate combinations were antagonistic to goosegrass, sicklepod, Palmer amaranth, and velvetleaf, and three of the four were antagonistic to barnyardgrass and entireleaf morningglory. Fomesafen decreased absorption and translocation of 14C-glyphosate in barnyardgrass, pitted morningglory, and velvetleaf. Ninety percent of glyphosate plus imazethapyr combinations were additive or synergistic, with all rate combinations synergistic for pitted morningglory. Adding glyphosate to imazethapyr increased absorption of 14C-imazethapyr by Palmer amaranth and velvetleaf. Glyphosate plus sulfentrazone tank mixtures were antagonistic at all rate combinations for barnyardgrass and Palmer amaranth and at three of the four combinations for goosegrass and entireleaf morningglory, indicating that these herbicides are not complementary in tank mixtures.

Influence of temperature, photoperiod, and irradiance on the phenological development of common ragweed (Ambrosia artemisiifolia)

Abstract: Implementation of an integrated weed management system requires prediction of the effect of weed competition on crop yield. Predicting outcomes of weed competition is complicated by genetic and environmental variation across years, locations, and management. Mechanistic models have the potential to account for this variability. Weed phenological development is an essential component of such models. Growth cabinet studies were conducted to characterize common ragweed's phenological response to temperature, photoperiod, and irradiance. Ragweed development occurred over a temperature range of 8.0 to 31.7 C, and this response to temperature was best characterized using a nonlinear function. A maximum leaf appearance rate of 1.02 leaves d-I occurred at 31.7 C. Ragweed has a short juvenile phase, during which it was not sensitive to photoperiod. Following this juvenile phase, sensitivity to photoperiod was constant and continued until pistillate flowers were observed. Photoperiods of 14 h or less were optimal and resulted in maximum rates of development. Irradiance level affected ragweed phenological development only when combined with the additional stress of low temperatures. Data generated in this study can be used for the development of mechanistic weed competition models.

Interference between pigweed (Amaranthus spp.), barnyardgrass (Echinochloa crus-galli), and soybean (Glycine max).

Abstract: Field experiments were conducted to determine the influence of time of emergence and density of single and multispecies populations of pigweed and barnyardgrass on soybean yield and competitive abilities of pigweed and barnyardgrass. Pigweed and barnyardgrass were established at selected densities within 12.5 cm on either side of the soybean row. Pigweed and barnyardgrass seeds were sown concurrently with soybean and at the cotyledon stage of soybean growth. Time and density of pigweed and barnyardgrass seedling emergence relative to soybean influenced the magnitude of soybean yield loss. Maximum soybean yield loss ranged from 32 to 99%, depending upon time of emergence relative to soybean. Pigweed was more competitive than barnyardgrass across all locations, years, and time of weed emergence. When pigweed was assigned a competitive index of 1 on a scale from 0 to 1, the competitive ability of barnyardgrass ranged from 0.075 to 0.40 of pigweed, depending upon location and time of emergence. This is the first multiple weed species study to include time of weed emergence relative to the crop. Competitive index values for multiple weed species must be calculated from field experiments in which weeds are grown with the crop under differing environmental conditions.

Influence of corn (Zea mays) population and row spacing on corn and velvetleaf (Abutilon theophrasti) yield

Abstract: Research was conducted to determine the optimum population and row spacing for corn production and for suppressing velvetleaf growth and seed production. Corn was grown in a factorial arrangement of three populations targeted at 64,000 (1 ×), 96,000 (1.5 ×), or 128,000 (2 ×) plants ha−1 and two row spacings of 38 or 76 cm. Influences on corn were determined in weed-free plots, and influences on velvetleaf were determined for target plants established at 1.5–m intervals along the center of corn interrows. Four velvetleaf plantings were made at weekly intervals beginning at corn planting. Corn row spacing had little influence on corn or velvetleaf. Corn yield exhibited a parabolic response to population with a maximum of approximately 90,000 plants ha−1 in one year, no response to population in another year, and a linear decline with increasing population in a dry year. Velvetleaf seed production was reduced 69 to 94% by the 1.5 × population and 99% by the 2 × population compared to the standard 1 × population when velvetleaf emerged with corn. Velvetleaf seed production was eliminated when velvetleaf emerged at or later than corn leaf stages 3, 5, and 6 for corn populations of 2 ×, 1.5 ×, and 1 ×, respectively. Reduced velvetleaf seed production was correlated with lower positioning of plants in the corn canopy and reduced light availability. Results suggest that higher corn populations could aid integrated weed management strategies by reducing seed production and limiting the build-up of weed populations.

Influence of surface and subsurface soil properties on atrazine sorption and degradation

Abstract: Studies to predict pesticide fate often lack measurements of model input parameters. Using independent data sets and understanding how soil properties affect herbicide retention and degradation may result in more accurate prediction of herbicide fate. We conducted laboratory studies to determine the influence of soil properties on atrazine adsorption and degradation. These data will be used in a separate study involving a pesticide fate model. Atrazine adsorption and desorption isotherms were constructed for six soil depths of a Hastings silty clay loam (fine, montmorillonitic, mesic Udic Argiustoll) using batch equilibration. The Freundlich adsorption constants (log Kf ) ranged from 0.38 (60 to 90 cm) to 2.91 (0 to 30 cm). Adsorption was higher in the low pH, high organic matter-containing surface soil compared to the lower soil depths. Multiple regression of the adsorption constants against selected soil properties indicated that organic matter content was the best single predictor of atrazine adsorption (R 2 = 0.98) followed by soil pH (R 2 = 0.82). Combining organic matter and cation exchange capacity in the model produced the lowest Cp statistic (2.33) and highest R 2 value (0.99). We observed hysteresis in atrazine adsorption–desorption isotherms by higher adsorption slopes (1/n)ads compared to desorption slopes (1/n)des. Soils that adsorbed more atrazine also desorbed less atrazine. Desorption correlated negatively with organic matter content and positively with soil pH. Atrazine degradation after 84 d of incubation generally decreased with increasing depth. The first-order degradation rate was highest 0 to 30 cm deep (0.0187 day−1) and lowest 270 to 300 cm deep (0.0031 day−1). Atrazine degradation was faster in soil treated annually for 12 yr than in soil with no previous atrazine history (p = 0.01).

Effect of proximity factors on competition between winter wheat (triticum aestivum) and italian ryegrass (lolium multiflorum)

Abstract: Density and spatial arrangement (rectangularity) effects on the competitive relationships, yield performance, and dynamics in canopy dominance of winter wheat and Italian ryegrass were evaluated using two addition series experiments. In experiment 1, combinations of six densities of each species formed the treatment matrix of addition series. In experiment 2, each species was tested at four densities and three rectangularities (RE) of winter wheat. In monocultures, crop density (plants per square meter) explained 82 to 85% of the total variation in the per-plant biomass of winter wheat in experiment 1. In mixtures of crop and weed, initial wheat density (N1) and initial ryegrass density (N2) and interaction of N1 and N2 explained 74 to 80% of the total variation in the per-plant biomass of winter wheat and 68 to 79% of Italian ryegrass in experiment 1. Intraspecific competition was apparent between 15 and 90 days after emergence (DAE) in winter wheat and between 90 and 170 DAE in Italian ryegrass. In mixtures, RE influenced plant size of Italian ryegrass up to 50 DAE only. Maximum winter wheat intraspecific competition occurred at 170 DAE, but maximum interspecific competition occurred during reproductive stages in mixtures. High RE increased seed yield, seed size, and harvest index of winter wheat and reduced biomass of Italian ryegrass. Grain yield of winter wheat was reduced up to 92% by competition from ryegrass. Even nine ryegrass plants in 100 winter wheat plants m−2 reduced winter wheat grain yield by 33%. However, the extent of loss in winter wheat grain yield was less in RE 16 (wider spacing) than in RE 1 (square planting) or 4 (close row spacing). Winter wheat was the stronger competitor during vegetative stages, but Italian ryegrass became the stronger competitor during the reproductive stages of development. Winter wheat leaves dominated at the top canopy during the vegetative stage, but ryegrass dominated at the top canopy during the reproductive stages. In the top canopy of mixtures at 200 DAE, the leaf area indices (LAI) of ryegrass was 6.6 times greater than winter wheat at RE 1 compared to only 1.6 times at RE 16. Greater LAI of Italian ryegrass in the top canopy reduced photosynthetically active radiation available to winter wheat by 68% at booting stage.

Resistance to acetolactate synthase inhibitors and quinclorac in a biotype of false cleavers (Galium spurium)

Abstract: A false cleavers population that survived treatment with triasulfuron/bromoxynil in 1996 was identified in central Alberta, Canada, in a field that had been treated with acetolactate synthase (ALS) inhibitors in 3 of the previous 6 yr. In greenhouse studies, this biotype was highly resistant to the ALS inhibitors triasulfuron, thifensulfuron/tribenuron, and sulfometuron and moderately resistant to imazethapyr; GR50, values were > 16, > 5, > 1.0, and 9.9, respectively. In addition, cross-resistance was identified to the auxin-type herbicide quinclorac (GR50 value > 6.7) but not to fluroxypyr (GR50 value 1) or MCPA/mecoprop/dicamba. Quinclorac had not been used previously in this field. Analysis of ALS extracted from the resistant biotype and a susceptible biotype from a nearby location indicated that resistance to ALS inhibitors was due to an altered target site with reduced sensitivity to a broad range of ALS inhibitors. The ALS I50 values for triasulfuron, metsulfuron, chlorsulfuron, thifensulfuron, and imazethapyr were 36, 34, 92, 96, and 14 times higher, respectively, for the resistant compared to the susceptible biotype. The mechanism of resistance to quinclorac is unknown. This is the first report of high-level herbicide resistance in this weed species.

Sulfentrazone sorption, desorption, and mineralization in soils from two tillage systems.

Abstract: Martin A. Locke Southern Weed Science Research Unit, U.S. Department of Agriculture, Agricultural Research Service, Stoneville, MS 38776 Sulfentrazone sorption kinetics, desorption, and mineralization were evaluated in surface 7.5 cm of soils collected from long-term conventional-till (CT) and no-till (NT) plots. The soils used were Miami silt loam and Drummer silty clay loam from Illinois and Dundee silt loam from Mississippi. Sulfentrazone sorption kinetics in Dundee silt loam CT and NT soils were adequately described by a simple two-site equilibrium/kinetic model. Rapid initial sorption (within 1 h) was followed by a slower sorption and equilibrium, largely achieved by 72 h of shaking, with a negligible increase in sorption thereafter. The sorption Kf ranged from 1.02 to 3.44 among the six CT and NT soils. The Kf values were greater for NT compared to their respective CT soils. Overall, Kf1values were higher in Drummer silty clay loam followed by Dundee silt loam and Miami silt loam soil. The N values were less than unity in all soils indicating nonlinear sorption. Sulfentrazone desorption was hysteretic with a very low rate of desorption. The total amount desorbed in four desorptions ranged from 58 to 72% of that sorbed. Less than 2.1% of applied 14Csulfentrazone was mineralized to 14C02 in Dundee silt loam CT and NT soils during a 77-d incubation. Relatively low mineralization of sulfentrazone suggests poor adaptability of native microbial populations that have not been exposed to this herbicide. Higher sorption and lower desorption of sulfentrazone in NT soils compared to CT soils suggest that NT systems (which tend to increase plant residues) may prolong sulfentrazone residence time in soil.

Phytochrome-mediated Amaranthus germination I: effect of seed burial and germination temperature

Abstract: This research was conducted to evaluate the light requirement for redroot and smooth pigweed germination in soil, and how this requirement is affected by germination temperature and seasonal periodicity in seed dormancy. Seed enclosed in nylon mesh bags was buried in the field in December 1993 and 1994 and was recovered throughout the spring and summer of the following year, respectively. Germination was highest with red light or at 30 C. The requirement for red light was more pronounced at 20 vs. 30 C. The saturating fluence of red light was as low as 3 μmol m−2 in buried seed and 1,000 μmol m−2 in unburied control seed, depending on germination temperature. The effect of light and germination temperature pigweed germination also changed throughout the growing season. Our results indicate that light may be a requirement for germination in only the most dormant weed seed in the soil seedbank.

Tolerance and velvetleaf (Abutilon theophrasti) suppressive ability of two old and two modern corn (Zea mays) hybrids

Abstract: Improved crop tolerance and weed suppressive ability are tactics that may reduce the negative effect of weeds on crop yield. Irrigated field experiments were conducted to compare leaf area index (LAI), intercepted photosynthetic photon flux (PPF), and relative tolerance and velvetleaf suppressive ability among two old (circa 1940) and two modern corn hybrids. Each hybrid was grown in monoculture and in mixture with velvetleaf at 1, 4, 16, and 40 plants m−1 row. Plants were periodically harvested in monoculture plots to obtain estimates of corn LAI, and PPF interception was measured. Variation in hybrid tolerance to velvetleaf competition for light was evaluated by comparing among hybrids the coefficients of a regression of corn yield loss on velvetleaf density. Velvetleaf seed capsule production in the presence of each hybrid was compared to evaluate variation in velvetleaf suppressive ability among hybrids. Maximum corn yield loss was 32% lower for the two old hybrids, and velvetleaf capsule production was reduced by 62% at low velvetleaf densities in 1995 compared to the modern hybrids. In 1996, yield loss of the modern hybrid 3394 was 74% lower than that of the other three hybrids at low velvetleaf densities, whereas maximum yield loss of the old hybrid 336 was 44% lower at high densities. Velvetleaf capsule production did not vary among hybrids at any velvetleaf density in 1996. Hybrids with greater tolerance and velvetleaf suppressive ability also had greater LAI and PPF interception, suggesting optimized corn LAI and PPF interception may be useful in an integrated weed management program.

Allelopathic activity of mexican sunflower (tithonia diversifolia) in soil

Abstract: Laboratory experiments were undertaken to investigate the allelopathic activity of Mexican sunflower in soil as well as to determine the effect of several soil factors on activity. There was a decrease in shoot and root growth of the test plant species when grown in soil previously planted with Mexican sunflower. However, seed germination was not affected. Soil-water separated from the same soil also inhibited shoot and root growth. A water extract from Mexican sunflower leaves applied to soil also reduced shoot and root growth, but the degree of inhibition varied among test plant species. Incorporation of dry Mexican sunflower leaves into soil at the rate of 1 and 2% (w/w) inhibited the growth of rice seedlings. The phytotoxic activity of incorporated leaf residue diminished 4 wk after treatment of soil. The inhibitory effect of a leaf water extract in nonautoclaved soil was less than in autoclaved soil and the phytotoxic effect to rice seedling grown in autoclaved soil was less than in autoclaved sand. It is suggested that the decrease in allelopathic activity of Mexican sunflower extract and its residue in soil is due to the action of soil microorganisms and the soil adsorption of the active components.

Effect of imazamox, fomesafen, and acifluorfen soil residue on rotational crops

Abstract: Field studies were conducted at Goiania, GO, Brazil, on an Oxisol (clayey, kaolinitic, isothermic, Typic Haplustox) and at Jussara, GO, Brazil, on an Oxisol (loamy sand, kaolinitic, isothermic, Typic Haplustox) during 1995 and 1996 to determine the carryover effect of fomesafen, imazamox, and acifluorfen, applied to edible bean, on rotational crops (maize, sorghum, rice, and millet) and to estimate the level of soil residues under Brazilian Savanna conditions. Averaged across locale, year, and rate, fomesafen dissipation time (DT50) (37.5 d) was longer than acifluorfen (27.5 d) and imazamox (25.9 d). For both locations, soil herbicide persistence (average of herbicides) was longer in 1995 than in 1996. This was due to higher soil moisture content in 1996. The sensitivity of rotational crops to fomesafen and imazamox residues was, in decreasing order: sorghum, corn, millet, and rice, and for acifluorfen: sorghum, corn, rice, and millet. The period between herbicide application and rotational crop planting (PAP) varied in agreement with the sensitivity of rotational crops to herbicide residues in soil and the persistence of the herbicide. Considering both location and year, the PAP for fomesafen (250 g ai ha(-1)) ranged from 69 to 132 d for corn, 114 to 179 d for sorghum, 29 to 95 d for rice, and 52 to 111 d for millet; the PAP for imazamox (40 g ai ha(-1)) ranged from 68 to 111 d for corn, 78 to 139 d for sorghum, 25 to 75 d for rice, and 40 to 102 d for millet; and the PAP for acifluorfen (170 g ai ha(-1)) ranged from 56 to 89 d for corn, 96 to 139 d for sorghum, 61 to 95 d for rice, and 43 to 82 d for millet

Integrating crop management and crop rotation effects into models of weed population dynamics: a review

Abstract: Current weed demography models were reviewed to evaluate how the effects of cultural practices on weed dynamics were integrated into the models and to suggest possible ways to improve the simulation of cropping system effects. Several models were chosen to illustrate the interactions between cropping systems and weed dynamics. The first one described the structure of the weed life cycle. The second model integrated the effects of a wide set of cultural practices; the comparison of this example with other models suggested how the integration of cropping system effects could be improved. The last two models introduced the interactions of cultural practices with intraplot weed variability, either spatial variability of weed densities or genetic and phenotypic variability within weed populations. This review indicates some ways to make weed population models more comprehensive, robust, and accurate in order to improve their contribution to the evaluation and management of cropping systems.

Effect of environmental conditions on the dormancy and germination of volunteer oilseed rape seed (Brassica napus).

Abstract: Peter J. W. Lutman Department of Crop and Disease Management, IACR-Rothamsted, Harpenden, Herts, AL5 2JQ, U.K. The conditions causing the onset of secondary dormancy in seed of winter oilseed rape cultivars Falcon and Libravo were investigated in a series of laboratory experiments to provide practical guidance on how to minimize the persistence of volunteer oilseed rape seed. The germination of recently harvested oilseed rape seed in moist conditions at 12 C was close to 100% in both darkness and under white light. Imbibition in water stress with an osmotic potential (4,) of -1,500 kPa at 12 C in far-red light (FR) or dark for more than 5 d induced secondary dormancy in both cultivars. Treatment with FR was more effective than darkness at inducing secondary dormancy. Libravo produced more dormant seed than Falcon. Imbibition in FR and absence of water stress (+ = 0 kPa) decreased germinability especially at the low temperature (6 C). These conditions resulted in 19.4 and 27.2% dormant seed for Libravo and Falcon, respectively. The results suggest that phytochrome is involved in the induction of secondary dormancy in oilseed rape. In practice, much oilseed rape seed is buried by postharvest cultivation in late summer (low hydric conditions) and exposed to water stress and darkness for some time. To avoid soil seed persistence and subsequent volunteer oilseed rape, seed must be retained at the soil surface when there is adequate moisture available. Therefore, incorporation of seed by tillage should be avoided or delayed as long as possible.

Evaluation of soybean injury from sulfentrazone and inheritance of tolerance

Abstract: Experiments were conducted in the field and greenhouse to evaluate the effects of cultivar and sulfentrazone rate on soybean injury and yield from soil-applied sulfentrazone and to determine soybean inheritance of sulfentrazone tolerance. Excessive rainfall and cool growing conditions in 1996 enhanced sulfentrazone injury and caused yield reduction in the susceptible cultivars ‘KS4895’ and ‘Hutcheson’ at 0.56 kg ai ha−1 and KS4895 at 0.42 kg ha−1. Yields of tolerant cultivars ‘Deltapine 3478’ and ‘Manokin’ were not reduced. Sulfentrazone at 0.42 kg ha−1 reduced soybean stand 17 and 35% for tolerant and susceptible cultivars, respectively. Field and greenhouse inheritance studies were conducted on plant families developed from two crosses: Manokin (tolerant) by ‘Asgrow A4715’ (susceptible) and ‘Northrup King S59–60’ (tolerant) by KS4895 (susceptible). The sulfentrazone tolerance of Manokin and Northrup King S59–60 appeared to be controlled by a single gene, with tolerance being dominant to susceptibility. The allelic relationship of tolerance from these two sources needs to be determined.

Functional and quantitative analysis of seed thermal responses in prostrate knotweed (polygonum aviculare) and common purslane (portulaca oleracea)

Abstract: A screening method was used to characterize seed thermal responses of prostrate knotweed and common purslane, two important weeds invading wheat in the humid Pampa. Through this method, it was possible to detect thermal conditions that induce or break dormancy in both species. In addition, we were able to quantify changes in dormancy level in seed populations as a function of time of burial after dispersal, through changes in width of the thermal range within which germination can occur. Plotting the overlap of this thermal range and observed soil temperature throughout the year allowed the prediction of the seedling emergence period. This prediction was in agreement with observed seedling emergence in the field for both species, during 2 consecutive yr. From the analysis carried out under laboratory conditions, it was also possible to estimate required thermal time for germination of the nondormant fraction of the population and the base temperature above which thermal time is accumulated. The results obtained from this study are the basis for the formulation of seed germination models that predict not only the occurrence of seedling emergence in the field, but also the dynamics of germination within those periods.

Light effects on rhizome morphogenesis in nutsedges (Cyperus spp.): implications for control by soil solarization.

Abstract: Salvadore J. Locascio Horticultural Sciences Department, University of Florida, Gainesville, FL 32611 Soil solarization is a process of soil disinfestation that involves solar heating of moist soil covered with transparent polyethylene film. This nonchemical approach to controlling soil-borne pests is being investigated as an alternative to methyl bromide fumigation. Summer solarization controlled annual weed species and suppressed purple nutsedge. Although some nutsedge tubers sprouted despite the solarization treatment, the resulting shoots were almost always trapped under the clear solarization film. Conversely, in rows that were mulched with black film, nutsedge rhizomes punctured the film so that leaf expansion occurred above the film. In controlled pot experiments conducted in darkness, yellow nutsedge rhizomes readily penetrated 19and 30-jxm clear films as effectively as opaque films. Thicker clear films and bubble film reduced nutsedge penetration. In the greenhouse and laboratory, nutsedge penetration of transparent polyethylene film was inversely related to irradiance levels when the film was in direct contact with the soil. However, when there was a 5to 10-mm space between the soil and the film, the lowest irradiance level (30 ,umol m-2 s-1) was as effective as 320 ,umol m-2 s-1 in reducing penetration by purple nutsedge. The film penetration by nutsedge rhizomes appears to be linked to a lightdependent morphological change from rhizome growth to leaf development, which occurs before film penetration with clear mulch and after film penetration with opaque mulch. The alternate sprouting and foliar scorching of nutsedge shoots trapped under clear films could potentially be exploited to deplete nutsedge tubers that occur at soil depths that do not develop lethal temperatures under soil solarization.

The effect of light during and after soil cultivation with different tillage implements on weed seedling emergence

Abstract: The purpose of this study was to investigate the role of sunlight perceived by weed seeds during and after soil cultivation with moldboard and chisel plows on induction of seed germination. Daytime cultivation with a moldboard plow increased weed seedling emergence by as much as 200% above the levels recorded following nighttime cultivation; however, the magnitude of this promotion varied depending on season and field history, and occasionally it was nil. In contrast, when a chisel plow was used in primary tillage, no differences in seedling emergence were observed between daytime and nighttime cultivation. The absolute germination level in plots cultivated with a chisel plow was at least 200% greater than in plots cultivated with a moldboard plow. This observation suggests that microenvironmental factors other than light played an important role in promoting seed germination in chisel-plowed plots. Covering moldboard-plowed plots with opaque plastic immediately after cultivation did not reduce weed seedling emergence compared to plots that were either covered with transparent film or left uncovered. Our results suggest that only the light stimulus perceived by the seeds during soil cultivation is effective in triggering germination.

Control of parasitic witchweeds (Striga spp.) on corn (Zea mays) resistant to acetolactate synthase inhibitors

Abstract: Parasitic witchweeds inflict most of their damage while still underground and attached to crop roots. Most selective translocated herbicides are detoxified by crops such as corn and thus cannot reach the attached parasites. Corn with target site resistance to acetolactate synthase (ALS)-inhibiting herbicides was tested to ascertain whether these herbicides could control witchweeds, assuming that witchweeds do not obtain amino acids from the crop. Postemergence directed sprays of 27 g ae ha−1 imazapyr 54 d after planting (DAP) delayed Striga asiatica emergence on corn in South Carolina from 3 wk (control) to 7 wk and to 11 wk when mixed with 45 g ae ha−1 AC 263 222. Treatments with up to 71 g ae ha−1 imazamox, and up to 71 g ae ha−1 AC 263 222 only delayed Striga emergence by 1 wk, and 71 g ae ha−1 imazethapyr was ineffective. ALS-inhibiting herbicides were far more effective when applied in 1-ml drenches above the seed at planting. Chlorsulfuron (10 g ai ha−1) and sulfometuron (50 g ai ha−1) were somewhat phytotoxic to Pioneer 3245IR. Rimsulfuron (30 g ai ha−1), metsulfuron (10 g ai ha−1), halosulfuron (120 g ai ha−1), and imazethapyr (140 g ae ha−1) were marginally active in Kenya, with some mature Striga hermonthica seed-bearing capsules appearing at harvest (12 wk). Imazapyr at 15 g ae ha−1 gave 70 to 95% suppression of capsule formation, whereas no capsules appeared at 30 g ae ha−1. The use of imazapyr in Kenya increased the harvest index by 17% when corn plants in Striga-infested soils were kept insect and disease free by using insecticides and fungicides. Thus, complete control can be achieved at affordable cost by farmers in subsistence conditions.

Metolachlor and metribuzin losses in runoff as affected by width of vegetative filter strips

Abstract: Michele Boyette Glen P. Murphy Department of Plant and Soil Sciences, Mississippi State University, Mississippi State, MS 39762 Tall fescue vegetative filter strips 0.5 to 4.0 m wide were evaluated for their ability to reduce losses of metolachlor, metribuzin, and runoff (water and sediment) in conventionally tilled soybean. Differences in the parameters studied were significant between filter and no filter strips, regardless of filter strip width. Two days after treatment, metribuzin concentration in runoff from the unfiltered treatment was 231 ng ml-'; filter strips reduced this amount to 119 ng ml-' or less. Similar trends were observed with metolachlor, with concentrations of 1,009 ng ml-' from the unfiltered, whereas filter strips of any width reduced this to 523 ng ml-' or less. Metribuzin loss during the growing season was 41 g ai ha-', or 9.8% of the amount applied when no filter strip was present. The addition of a filter strip, regardless of width, reduced cumulative metribuzin losses to 11 g ha-I or less. Similar results were noted with metolachlor. Filter strips, regardless of width, reduced cumulative runoff and sediment loss at least 46 and 83%, respectively.

Critical weed-free period for large crabgrass (Digitaria sanguinalis) in transplanted watermelon (Citrullus lanatus)

Abstract: Removal and plant-back studies were conducted in North Carolina in 1991 and 1992 to determine the critical period of large crabgrass competition in transplanted triploid (seedless) watermelon. For every week that large crabgrass remained in watermelon, medium (3.6 to 7.3 kg) melon yield decreased 3,996 kg and 716 fruit ha−1. For every week that large crabgrass emergence was delayed, yield increased by 814 kg and 142 fruit ha−1. Likewise, for every week that large crabgrass remained in watermelon, marketable (3.6 kg and over) yield decreased 5,582 kg and 911 fruit ha−1. For every week that large crabgrass emergence was delayed, yield was increased 881 kg and 151 fruit ha−1. Large crabgrass emerging after 6 wk had no effect on marketable fruit or number of watermelon. To achieve the greatest quality or quantity of medium or marketable fruit, a large crabgrass-free period between 0 and 6 wk after transplanting was necessary.