An elutriation system was developed to estimate weed seed densities in soil. The system provided reliable estimates of seed densities for weed species with a broad range of seed weights (0.06 to 9.80 mg) and had no effect on seed viability of three weed species tested. Elutriation of soil samples (up to 60 g) taken from a cultivated field took approximately 15 min. Separating, classifying, and counting seeds is time consuming (20 to 30 min per sample) but provides an accurate estimate of seed densities for weeds with a diameter of 0.5 mm or more.

The host range of an isolate of the fungal pathogen Alternaria macrospora Zimmerman from infected spurred anoda [Anoda cristata (L.) Schlecht.] leaves was studied in the greenhouse and growth chamber. The fungus was inoculated to representative plants in the Malvaceae, Solanaceae, Leguminosae, and Gramineae families. Evidence of infection was limited to plants in the Malvaceae, with spurred anoda being the most susceptible species tested. Only negligible damage was incited on cotton (Gossypium hirsutum L. ‘Stoneville 213′) and long staple cotton (G. barbadense L. ‘Pima S–5′), hollyhock [Althaea rosea (L.) Cav.], okra (Hibiscus esculentus L. ‘Clemson spineless’), prickly sida (Sida spinosa L.), velvetleaf (Abutilon theophrasti Medic.), and Venice mallow (Hibiscus trionum L.). Disease symptoms induced in cotton by the spurred anoda isolate of A. macrospora were much less severe than those reported for other isolates of the fungus. Therefore, the spurred anoda isolate may be a specialized form of the fungus.

Field experiments were conducted to determine if seeds would be produced on Triticum aestivum by Aegilops cylindrica hybrids in the field and, if it were, to determine the viability of the seeds produced. One, five, or 10 hybrids were planted into varying proportions of A. cylindrica and T. aestivum in a replacement series design. Percent seed set ranged from 0 to 5.5% in 1996 and from 0 to 9.2% in 1997. Seeds were set in all treatments. The average seed set was 2.3% in 1996 and 3.8% in 1997. No differences in seed set were found among treatments. The seeds produced were separated according to seed condition, either full or shriveled, and tested for germination. The germination of the seeds produced on the hybrids was not significantly different between years. The average germination for full seeds was 94% in both years and 79 and 84% for shriveled seeds in 1995 and 1996, respectively. Greenhouse studies were conducted to evaluate the rate of self-fertility of the BC1 generation and to identify morphological characteristics that could be used to identify the probable pollen donor parent and to predict the occurrence of seed set. In 1997 4.1% and in 1998 2.1% of BC1 plants set seeds. The average seed set was 0.3% in 1997 and 0.06% in 1998. It was not possible, using any morphological characteristic measured, to determine the identity of the parent serving as the pollen donor in the previous generation or to predict the occurrence of seed set in the BC1 generation. This is the first reported study to show that hybrids between T. aestivum and A. cylindrica have the ability, although limited, to backcross under field conditions and set seeds. Furthermore, the seeds produced are viable and will germinate and produce plants. With the millions of hectares of T. aestivum infested with A. cylindrica, even the limited ability to backcross is of concern for the movement of a herbicide-resistance gene.

The International Institute of Tropical Agriculture (IITA) is one of the agricultural institutes financed through the Consultative Group on International Agricultural Research (CGIAR) sponsored by FAO, UNDP (United Nations Development Program), and the World Bank. One of IITA's mandates is to conduct research on farming systems in the humid and subhumid tropical zones to identify viable alternatives to shifting cultivation capable of maintaining productivity of food crops under continuous cultivation (14). The research efforts of the institute are organized into four programs. The three commodity-oriented research programs—the Cereals Improvement, the Grain Legume Improvement, and the Root and Tuber Improvement programs – provide improved planting materials that the scientists in the fourth program – the Farming Systems Program – use to develop land use systems capable of sustaining soil productivity as land and economic pressures accelerate the transition from shifting to continuous cultivation (13).

Two red rice accessions from Arkansas have been found to be resistant to the labeled rate of imazethapyr, which is used to control red rice in ClearfieldTM rice. Full-length amplification of the acetolactate synthase (ALS) gene in imazethapyr-resistant red rice revealed a coding sequence of 1,935 base pairs, which is the same as that of the cultivated rice. Coding sequences were generated from four red rice accessions collected from different geographical regions in Arkansas, consisting of accessions that were either resistant or susceptible to imazethapyr. Nucleotide sequence alignments identified six base polymorphisms, three of which resulted in amino acid substitutions in the ALS gene. One amino acid substitution, Gly654Glu, involves a residue required for imazethapyr binding to the ALS. The other substitution, Val669Met, implies conformational changes in the ALS structure that enhances binding of thiamine diphosphate, an ALS cofactor. These novel amino acid substitutions first reported for ALS-resistant red rice accessions support the hypothesis that ALS-resistant red rice can evolve with sustained herbicide selection pressure. Thus, it behooves growers to integrate the Cleafield rice technology with other tools to achieve a successful, long-term weed management program.

Experiments were conducted to determine the efficacy of glyphosate on four common lambsquarters populations collected from Kansas, Nebraska, North Dakota, and Ohio. Glyphosate dose-response studies for common lambsquarters treated at 2.5-, 7.5-, and 15-cm heights showed that glyphosate at 1.1 kg ae ha−1 caused more than 80% injury to 2.5-cm plants but less than 55% injury to 7.5- and 15-cm plants. All populations were susceptible to glyphosate at the 2.5-cm height. The glyphosate rate required to cause 50% injury (GR50) was 430, 500, 500, and 560 g ha−1 for the Kansas, North Dakota, Ohio, and Nebraska populations, respectively. Differential response of common lambsquarters populations was evident with 15-cm plants where the GR50 was glyphosate at 1,010, 1,230, 1,650, and 2,770 g ha−1 for the Kansas, North Dakota, Nebraska, and Ohio populations, respectively. Reduced injury on 15-cm common lambsquarters plants by glyphosate may be partly attributed to reduced glyphosate accumulation per unit of plant tissues and enhanced calcium content in more-developed plants. All four common lambsquarters populations at the early seedling stage were susceptible to glyphosate, but tolerance increased as the plant developed and the extent of tolerance differed among populations.

The effectiveness of dicamba (3,6-dichloro-o-anisic acid), picloram (4-amino-3,5,6-trichloropicolinic acid), and triclopyr [(3,5-6-trichloro-2-pyridinyl)oxy]acetic acid and mixtures of these herbicides with 2,4-D [(2,4-dichlorophenoxy)acetic acid] for the control of horsenettle (Solanum carolinense L.) in Kentucky bluegrass (Poa pratensis L.) pastures was evaluated at three different field sites in Southwest Virginia over a period of 3 yr. Picloram applied at 0.6 and 1.1 kg/ha significantly reduced horsenettle roots, as indicated by a 98% reduction in shoots 1 yr after the first annual application. No shoots emerged within 18 months after three annual picloram applications. There was no significant difference between picloram applied alone and in a mixture with 1.1 kg/ha of 2,4-D. Three annual applications of triclopyr at 3.4 kg/ha reduced horsenettle shoots from 82% to 92%. The 3.4-kg/ha rate of triclopyr was slightly more effective in controlling horsenettle roots than was triclopyr at 1.7 kg/ha alone and in mixture with 1.1 kg/ha of 2,4-D. Dicamba at 0.6 and 1.1 kg/ha reduced the horsenettle shoots by 74% and 81%, respectively. The reduction was similar from mixtures of dicamba (0.1 and 0.3 kg/ha) with 2,4-D (0.4 and 0.8 kg/ha). There were no effects on the bluegrass forage that could be attributed to the herbicides.

Experiments were conducted to determine the response of purple nutsedge tuber sprouting to diurnally alternating temperature. These experiments compared the response to alternating and constant temperatures and determined the effect of the amplitude of alternation and time of exposure to the maximum temperature. Tuber sprouting was more rapid and complete with alternating temperatures than with constant temperatures. Increasing temperature fluctuation from 0 to 6 C for 12 h daily linearly increased total tuber sprouting. As little as 30 min exposure to high temperature per day provided nearly the same level of sprouting as a 12 h alternating temperature cycle. This phenomenon should be considered when conducting studies to describe tuber temperature responses or when predicting tuber sprouting and emergence.

Experiments were conducted in 1985 to 1987 to evaluate the effects of conventional and no-tillage systems on the weed control provided by clomazone applied preemergence in soybeans. The persistence of clomazone in soil of the two tillage systems was also determined. Increasing the clomazone rate from 0.8 to 1.4 kg/ha did not increase weed control. Clomazone controlled 80% or more of jimsonweed, velvetleaf, and giant foxtail. Common cocklebur control ranged from about 50 to 70% in no-till and from 80 to 90% in conventional tillage. Generally, soybean pods/plant and yields were lower from clomazone treatments than from handweeded treatments due to inadequate common cocklebur control. Over 40% of the clomazone applied did not reach the soil surface; it was either intercepted by wheat straw, volatilized, or both. Clomazone persisted longer in conventional tillage than in no-tillage in. However, in 1986, clomazone was equally persistent in the two tillage systems. The half-life of clomazone was 34 and 6 days in 1985 in conventional and no-tillage, respectively, and in 1986, 18 and 16 days in conventional and no-tillage, respectively. Significant clomazone concentrations were not found below 10 cm in the soil profile. Corn planted without tillage (no-till) approximately 1 yr after clomazone application was not injured and yields were not reduced due to prior clomazone use.

The effects of acid scarification, prechilling, summer irrigation, and root extract stimulant from faba bean plants grown during different seasons on crenate broomrape seed dormancy and germination were determined. Immersion of 6- to 12-mo old seed in 0.5 to 5 N H2SO4 solution from 1 to 20 min increased germination by 20%. Similarly, seed pretreatment at temperatures between −4 and 6 C increased germination, regardless of the pretreatment duration. Results suggest that the hard seed coat is an important dormancy mechanism in newly formed crenate broomrape seeds. Summer irrigation did not substantially change the germination capacity of crenate broomrape seeds. The germination stimulant activity of the root extract from faba bean plants grown during different seasons varied considerably, decreasing in this order: autumn > winter > spring > summer. This suggests an ecological adaptation of crenate broomrape to its main host crop.

Alligatorweed is well known for its potassium (K+)-accumulating capabilities and its strong resistance to undesired growth conditions. The results of this study revealed properties of K+ accumulation and its contribution to drought stress in alligatorweed. In addition, we attempted to characterize the molecular mechanisms of K+ accumulation in this plant. Alligatorweed plants showed a consistent increase in biomass in response to external K+ concentrations, ranging from micromolar levels up to 50 mmol L−1; K+ was also accumulated accordingly in the plants. The stem was the most K+-accumulating organ, accumulating up to 13% of the K+. Moreover, this K+ superaccumulation caused improved resistance to drought stress. The apparent K+ uptake by the roots showed a typical high-affinity property, and the Michaelis constant increased at higher rates of plant K+ in the starting materials. Furthermore, three putative, K+-uptake transporter complementary DNAs (cDNAs) were isolated from alligatorweed (ApKUP1, ApKUP2, and ApKUP3, respectively) using degenerated primers and rapid amplification of cDNA end techniques. The expression of ApKUP1 and ApKUP3 was predominately localized to the leaves, whereas ApKUP2 was expressed throughout the entire plant. The expression of ApKUP1 and ApKUP3 was stimulated in the stems and roots when K+ was depleted from the external medium. Moreover, ApKUP3 expression was enhanced in the stem in response to abscisic acid treatment and drought stress. In conclusion, our findings provide further insight into the mechanisms of K+ accumulation linked to K+ uptake in alligatorweed.

Glufosinate inhibits glutamine synthetase (GS), a key enzyme for amino acid metabolism and photorespiration. Protoporphyrinogen oxidase (PPO) inhibitors block chlorophyll biosynthesis and cause protoporphyrin accumulation, a highly photodynamic intermediate. Both herbicides ultimately lead to plant death by a massive accumulation of reactive oxygen species (ROS) through different mechanisms. We investigated a potential synergistic effect by the mixture of the two herbicide mechanisms of action (MoAs). The tank mix between a low rate of glufosinate (280 g ai ha−1) with an ultra-low dose of saflufenacil (1 g ha−1) provided enhanced herbicidal activity compared with the products applied individually on Palmer amaranth (Amaranthus palmeri S. Watson). The synergism between the two herbicides was also confirmed by isobole analysis and field trials. The herbicide combination provided high levels of efficacy when applied at low temperature and low humidity. Mechanistically, glufosinate caused a transient accumulation of glutamate, the building block for chlorophyll biosynthesis. Consequently, inhibition of both GS and PPO resulted in greater accumulation of protoporphyrin and ROS, forming the physiological basis for the synergism between glufosinate and PPO inhibitors. While the synergy between the two herbicide MoAs provided excellent efficacy on weeds, it caused low injury to PPO-resistant waterhemp [Amaranthus tuberculatus (Moq.) Sauer] and high injury to both glufosinate-resistant and glufosinate-susceptible soybean [Glycine max (L.) Merr.]. Glufosinate enhances the activity of PPO inhibitors through glutamate and protoporphyrin accumulation, leading to increased levels of ROS and lipid peroxidation. The synergism between the two herbicide MoAs can help to overcome environmental effects limiting the efficacy of glufosinate. Future research is needed to optimize the uses for this herbicidal composition across different cropping systems.

The employment of selective herbicidal sprays to kill or inhibit one or more undesirable plant species growing in close association with desirable species has become a common practice in producing certain crop and pasture species. Many crop and weed plants which are equally inhibited if exposed to certain herbicides during the germination period may show marked differential response when the tops of the same plants are sprayed with the materials. For example, the common small grains and corn are readily inhibited by the herbicide 2,4–dichlorophenoxyacetic acid (2,4–D) if exposed to solutions of the herbicide at the germination stage. Notwithstanding, 2,4–D is widely used in spraying these crop plants at certain stages to control weeds without causing significant damage to the crops. The lack of serious damage to these species from applications of 2,4–D to their tops may be attributed to several factors. Firstly, the spray may be repelled and shed harmlessly to the soil by the crop species and retained by the weedy ones. Secondly, the spray may be retained about equally well by the foliage of all species, but absorption and translocation of the herbicide to susceptible tissues, such as the stem and root apical meristems, may be less efficient in the non-susceptible species than in the susceptible ones. Possible differential accumulation of herbicides at the site of toxic action by susceptible and tolerant plants also may be important. Thirdly, the differential response of plant species to certain herbicidal sprays may be attributable to an inherent physiological capacity of the non-susceptible plant to tolerate the herbicide even in intimate association with the cellular components. Fourthly, some herbicidal materials appear to be readily taken up by the normal absorbing organ, the roots, of plants, but are not absorbed efficiently by the aerial plant parts because of certain physio-chemical relationships and internal morphological barriers. Fifthly, the position of the stem growing point may be a factor influencing the differential response of plants to herbicidal sprays. The apical bud of the stem in most broadleaf plants is usually wet by foliage sprays, whereas the stem bud of grasses is protected by several enveloping leaves. Sixthly, some species may inactivate the herbicide more rapidly than others and hence are not highly responsive.

The present extensive use of herbicides in soil application has resulted in a great deal of interest in the breakdown of these chemicals in soil (1, 4, 6, 8). An increasing demand exists for information that will permit fairly accurate predictions of the residual life of these chemicals (4). The breakdown of chemicals in soil has been repeatedly shown to be the result of microbiological activity in part but other factors are also known to contribute to the disappearance (1, 4, 5, 7, 8, 9).

A Lolium multiflorum Lam. biotype resistant to diclofop-methyl was found in a Triticum aestivum field in France (Normandy) that had been treated for several years with diclofop-methyl. Based on plant survival evaluated 21 d after herbicide application, the biotype exhibited a high level of resistance to diclofop-methyl and moderate resistance to CGA-184927-propargil and PP-604. The resistant biotype exhibited a small increase in tolerance to haloxyfop-methyl, quizalofop-ethyl, sethoxydim, and BAS-517-H, but was controlled by recommended field rates for these herbicides. The mechanism of resistance was investigated for diclofop-methyl. There was little or no difference in diclofop-methyl absorption by leaves of resistant and susceptible biotypes measured 48 h after treatment. For both biotypes, less than 1% of absorbed radiolabel was translocated during 48 h following foliar application of 14C-diclofop-methyl. Metabolism of diclofop-methyl was not significantly altered in the resistant biotype. In both biotypes, diclofop-methyl was rapidly metabolized to diclofop acid followed by a slow rate of formation of a polar conjugate. Two multifunctional acetyl coenzyme A carboxylase isoforms (ACCase I and ACCase II) were isolated from leaf tissue of resistant and susceptible biotypes. Both isoforms exhibited a subunit molecular mass of 203 kDa. For both resistant and susceptible biotypes, ACCase I constituted approximately 80% of total ACCase activity. Graminicide concentrations required to inhibit ACCase activity by 50% (I50 values) were determined for both ACCase isoforms from resistant and susceptible biotypes. The ACCase II isoform was highly resistant to graminicides in both biotypes. In contrast, the I50 value for diclofop inhibition of ACCase I was 19-fold greater for the enzyme isolated from the resistant biotype compared with the susceptible biotype. It is concluded that diclofop resistance in the L. multiflorum biotype from Normandy is caused by the presence of a resistant form of the ACCase I isoform.

Soilborne bacteria with novel metabolic abilities have been linked with enhanced atrazine degradation and complaints of reduced residual weed control in soils with an s-triazine use history. However, no field study has verified that enhanced degradation reduces atrazine's residual weed control. The objectives of this study were to (1) compare atrazine persistence and prickly sida density in s-triazine-adapted and nonadapted field sites at two planting dates; (2) utilize original and published data to construct a diagnostic test for identifying s-triazine-adapted soils; and (3) develop and validate an s-triazine persistence model based on data generated from the diagnostic test, i.e., mineralization of ring-labeled 14C-s-triazine. Atrazine half-life values in s-triazine-adapted soil were at least 1.4-fold lower than nonadapted soil and 5-fold lower than historic estimates (60 d). At both planting dates atrazine reduced prickly sida density in the nonadapted soils (P ≤ 0.0091). Conversely, in the s-triazine-adapted soil, prickly sida density was not different between no atrazine PRE and atrazine PRE at the March 15 planting date (P = 0.1397). A lack of significance in this contrast signifies that enhanced degradation can reduce atrazine's residual control of sensitive weed species. Analyses of published data indicate that cumulative mineralization in excess of 50% of C0 after 30 d of incubation is diagnostic for enhanced s-triazine degradation. An s-triazine persistence model was developed and validated; model predictions for atrazine persistence under field conditions were within the 95% confidence intervals of observed values. Results indicate that enhanced atrazine degradation can decrease the herbicide's persistence and residual activity; however, coupling the diagnostic test with the persistence model could enable weed scientists to identify s-triazine-adapted soils, predict herbicide persistence under field conditions, and implement alternative weed control strategies in affected areas if warranted.

The effectiveness and profitability of weed control with herbicides and cultivation were determined in peanuts (Arachis hypogaea L. ‘Florunner′) infested with Texas panicum (Panicum texanum Buckl. # PANTE), sicklepod (Cassia obtusifolia L. # CASOB), and pitted morningglory (Ipomoea lacunosa L. # IPOLA). Highest peanut yield (4260 kg/ha) and net returns were achieved in 2 of 3 yr with benefin [N-butyl-N-ethyl-2,6-dinitro-4-(trifluoromethyl)benzenamine] applied preplant incorporated followed by alachlor [2-chloro-N-(2,6-diethylphenyl)-N-(methoxymethyl) acetamide] and dinoseb [2-(1-methylpropyl)-4,6-dinitrophenol] plus naptalam {2-[(1-naphthalenylamino)carbonyl] benzoic acid} applied at ground cracking, with two timely cultivations. Systems utilizing herbicides alone did not provide highest net returns, peanut yield, or weed control.

Laboratory and field studies were conducted to examine the influence of R-33865 (O, O-diethyl-O-phenol phosphorothioate) on the degradation rate of EPTC (S-ethyl dipropylthiocarbamate) in three soils that had a history of repeated EPTC application and exhibited accelerated degradation of EPTC. In field studies, addition of R-33865 to EPTC extended EPTC persistence in soil and provided increased shattercane [Sorghum bicolor (L.) Moench] control. In two soils that exhibited rapid EPTC degradation, addition of R-33865 to EPTC extended the half-life of EPTC from 9 to 18 days and from 6 to 15 days, respectively. Evolution of 14CO2 from 14C-carbonyl EPTC-treated soils with a prior history of EPTC treatment was significantly reduced when R-33865 was added; however, the initial lag period of EPTC degradation was not restored. Addition of R-33865 to EPTC on soils with no prior EPTC treatment did not extend persistence of EPTC in soil.

Herbicidal residues in water and soil were determined after treating eight ponds with six herbicides. Relatively high concentrations of 2,6-dichlorobenzonitrile (dichlobenil), 2,3,6-trichlorophenylacetic acid (fenac), and 2,4-dichlorophenoxyacetic acid (2,4-D) were present in the upper 1-in of the hydrosoil following treatments with granular formulations of these herbicides. Fenac and dichlobenil residues persisted in both water and soil for periods exceeding 160 days. Low concentrations of 2,4-D were found in water for 24 days and in the soil for 55 days following treatment. The mono-N,N-dimethylcocoamine salt of 7-oxabicyclo(2.2.1)heptane-2,3-dicarboxylic acid (amine salt of endothall), 1,1′-dimethyl-4,4′-bipyridinium salt (paraquat), and 6,7-dihydrodipyrido(1,2-a:2’,1′-c)-pyrazidiinium salt (diquat) were relatively less persistent in water and were no longer found after 24, 8, and 4 days, respectively. However, paraquat and diquat were held by the hydrosoils and persisted in high concentrations for more than 85 and 160 days, respectively. In ponds with dense weed infestations, a large fraction of the herbicide may be held by the plants for some time.

An investigation of the adsorption of chlorsulfuron by four selected soil constituents, i.e. humic acid, two iron oxides, and montmorillonite, was carried out under concentration and pH conditions similar to those in most natural soils. CaCl2 (0.01 M) was used as background electrolyte to suppress nonspecific adsorption. Negligible amounts of chlorsulfuron were adsorbed by montmorillonite, whereas humic acid and the iron oxides were found to be important adsorbents. For these adsorbents, chlorsulfuron adsorption decreased when pH increased from 4 to 8, with little adsorption occurring at pH 8. Adsorption by iron oxides was a function of their surface area. Chlorsulfuron adsorption was found to be closely related to the surface charge of the adsorbents, but in weakly acidic solution, also to the acid-base properties of chlorsulfuron itself.