Giant foxtail (Setaria faberii Herrm.) was seeded the day the crop was planted and 3, 6, 9, and 12 weeks later. Foxtail that began growing with the crop and was left to maturity reduced corn yields 13% and soybean yields 27%. Foxtail seeded 3 weeks or later after the crop was planted did not cause yield reductions in corn or soybeans. Although not competitive, foxtail seeded 3 weeks after corn was planted produced 500 pounds of dry matter per acre, and all seedings in corn produced seed. Soybeans produced more shade than corn, and foxtail from the 3-week or later seedings in soybeans produced little or no dry matter or seed.

Corn (Zea mays L.) and soybeans [Glycine max (L.) Merr.] were grown for 5 yr in rotation without tillage (no-till), with fall and spring tillage (disk), and with conventional tillage (plow). Weed distribution and weed yields were measured in the final year of the experiment. Grass weed yield, primarily due to green foxtail [Setaria viridis (L.) P. Beauv. ♯ SETVI], was significantly higher in the no-till system. Broadleaf weed distribution and yield were similar in all tillage systems.

Increasing crop density is a cultural weed management practice that can compliment the use of cover crops for weed suppression. In this research, we created a range of cover crop biomass and soybean densities to assess their weed-suppressive ability alone and in combination. The experiment was conducted in 2008 and 2009 in Maryland and Pennsylvania using five levels of cereal rye residue, representing 0, 0.5, 1.0, 1.5, and 2.0 times the ambient level, and five soybean densities ranging from 0 to 74 seeds m−2. Weed biomass decreased with increasing rye residue and weeds were completely suppressed at levels above 1,500 g m−2. Weed biomass also decreased with increasing soybean density in 2 of 4 site–years. We evaluated weed suppression by fitting an exponential decay model of weed biomass as a function of rye biomass and a hyperbolic model of weed biomass as a function of soybean density at each of the five tactic levels. We multiplied these individual tactic models and included an interaction term to test for tactic interactions. In two of the four site-years, the combination of these tactics produced a synergistic interaction that resulted in greater weed suppression than would be predicted by the efficacy of each tactic alone. Our results indicate that increasing soybean planting rate can compensate for lower cereal rye biomass levels when these tactics are combined.

The story of agriculture is the story of weed interference. After millennia of weed control we still have weeds. This situation has led many growers to observe that “the weeds always win.” One of the most important reasons weeds are so successful is their biodiversity. Biodiversity is an inevitable consequence of the struggle an individual weed species undergoes in the presence of neighbors, and by occupying a physical space in an agroecosystem. Weeds have evolved in response to cropping system practices by adapting and occupying niches left available in agroecosystems. Forces created by our cropping practices over evolutionary time have led to the weed diversity we observe today. Diversity underlies weed management in several important ways. A plant experiences diversity among its neighbors in at least five different ways. Weeds have adapted to selection in agroecosystems in several ways: (1) genetic variants within a species; (2) somatic polymorphism of plant parts; (3) success in diverse habitat microsites; (4) temporal adaptations within the community; and (5) floristic diversity of a community at higher levels than the species. Herein, weed diversity is discussed in this broader context, in terms of population behaviors that emerge as a consequence of the activities of individual components at lower levels of organization. Diversity is also discussed in terms of its implications for weed management. The potential exists to develop management strategies based on differences in weed and crop diversity. These strategies might be developed by characterization of weedy genetic and phenotypic diversity; enhancement of crop, cropping system, and agroecosystem diversity; and characterization of the spatial distribution of weed populations.

Common ragweed is a native annual that colonizes disturbed habitats including agricultural fields and roadsides. It is especially abundant along roadways receiving regular applications of deicing salt. Anecdotal evidence has suggested that the emergence of common ragweed seedlings often occurs before the emergence of other roadside species and at salinity concentrations as high as 400 mM L−1, a level that can be found in roadside soils in early spring. However, the extent of this tolerance to salinity in common ragweed populations has not been quantified. The objective of this study was to assess the germination behavior of common ragweed seeds collected from three roadside and two agricultural populations across a salinity gradient. Seed germination of these five populations was monitored daily for 21 d across a sodium chloride gradient [0, 100, 200, 300, and 400 mM L−1] under controlled conditions. Seeds from roadside populations showed consistently greater total germination and rate of germination than seeds from agricultural populations. Germination differences were most evident at the 300 and 400 mM L−1 salinity concentrations. Average germination at the 400 mM L−1 sodium chloride concentration was 31% for two roadside populations and only 3% for two agricultural populations. Germination of seeds placed in distilled water after the 21-d salinity exposure treatments (i.e., recovery rates) was also greater for the roadside vs. agricultural populations. Findings indicate that the germination behavior of common ragweed seeds to salinity for roadside populations may be locally adaptive and allows common ragweed to emerge relatively early in spring thus providing a competitive advantage over later emerging roadside plants.

Field experiments were conducted in 1986, 1987, and 1988 to evaluate imazethapyr rate and time of application on postemergence control of 24 weed species. Contour graphs were developed that predicted imazethapyr rates required for various levels of weed control based upon weed leaf number at application. Rates below the labeled rate (70 g ha−1) provided 90% or greater control of common cocklebur, smallflower morningglory, and smooth pigweed if applied to 3 true-leaf or smaller weeds and of barnyardgrass, seedling johnsongrass, and Palmer amaranth if applied while weeds were in the cotyledon or 1 true-leaf stage. A rate of 70 g ha−1 provided 90% control of large crabgrass in the 1 true-leaf stage. Entireleaf morningglory, red rice, pitted morningglory, and velvetleaf are not susceptible enough to imazethapyr for 90% or greater control to be obtained with rates lower than 70 g ha−1 at the 1 true-leaf growth stage. These data demonstrate how control data can be used for developing effective reduced-rate herbicide recommendations based on weed leaf number.

Field studies were conducted over a 2-year period at Urbana, Illinois, to evaluate soybean (Glycine max (L.) Merr., var. Harosoy 63 and Wayne) and weed yields as influenced by row spacing, variety, weed control methods, and 2,3,5-triiodobenzoic acid (hereinafter referred to as TIBA). Soybean yield increased and weed yield decreased as row spacing was decreased. Compared to the yield from 40-inch rows, soybean yield increase was 10, 18, and 20% for 30, 20, and 10-inch rows, respectively. Both cultivation and applications of a,a,a-trifluoro-2,6-dinitro-N,N-dipropyl-p-toluidine (trifluralin) controlled annual grass weeds at all row spacings. Cultivation controlled broadleaf weeds at all row spacings. Some broadleaf weeds, unaffected by trifluralin, made sufficient growth to impair soybean yields in wide rows but not in narrow rows.

Cotton (Gossypium hirsutum L.) was grown at two locations in Alabama during 1966 to 1969 with specific densities of sicklepod (Cassia obtusifolia L.) and tall morningglory (Ipomoea purpurea L. Roth). Competition from sicklepod and tall morningglory caused cotton yield reductions at densities as low as eight weeds per 7.31 m of row. Tall morningglory was usually more competitive than sicklepod in cotton. Both species were more competitive on Lucedale sandy clay loam than on Norfolk sandy loam. Sicklepod at eight weeds per 7.31 m of row reduced yields from 10 to 23% on the Norfolk soil and approximately 40% on the Lucedale soil. At the highest density, 48 sicklepod plants per 7.31 m of row, yields were reduced 45 to 65% on the Norfolk soil and approximately 80% on the Lucedale soil. Tall morningglory at a density of eight weeds per 7.31 m of row reduced cotton yields 10 to 40% on the Norfolk soil and 50 to 75% on the Lucedale soil. Vegetative growth, boll, and seed size were not affected as much as cotton yields by competition. Percentage lint and lint fiber properties were unaffected by competition.

Downy brome (Bromus tectorum L. # BROTE) has developed into a severe weed in several agricultural production systems throughout North America, particularly on rangeland and in winter wheat (Triticum aestivum L.). Several million hectares of winter wheat, pastureland, alfalfa (Medicago sativa L.), grass seed fields, and overgrazed rangeland, as well as other crops, have been invaded by this annual grass since its introduction into this hemisphere. Downy brome is most abundant in the Great Basin and Columbia Basin areas of the western United States, but is found throughout the continental United States and parts of Canada and Mexico. In some cases, the vegetation on overgrazed rangeland consists totally of downy brome, while winter wheat growers in the western United States proclaim it as their worst weed problem. Changes in tillage practices that are currently being implemented for the control of soil erosion coupled with the lack of selective herbicides for the control of downy brome have aided its increase and spread.

Differences in periodicity and depth of weed seedling recruitment due to agronomic management practices, such as reduced tillage, have implications for weed competitive ability and management strategies. Periodicity and depth of seedling recruitment of 10 different weed species was measured in the field in 2004 and 2005. The seedling recruitment of rigid ryegrass, threehorn bedstraw, and wild radish seedlings was higher under minimum tillage than under no-till scenarios. In contrast, the seedling recruitment of Oriental mustard, annual sowthistle, squirreltail fescue, little mallow, and turnipweed was higher under the no-till system. The seedling recruitment of wild oat and African mustard was not influenced by the tillage system. The mean seedling recruitment depth of wild oat, rigid ryegrass, threehorn bedstraw, wild radish, and turnipweed was greater under minimum tillage than under the no-till system. These weeds are able to emerge from deeper in the soil profile. In contrast, the seedling recruitment depth under minimum-tillage and no-till systems was similar for African mustard, Oriental mustard, annual sowthistle, little mallow, and squirreltail fescue. These are all small-seeded species, which failed to emerge from deeper depths under either tillage system. In addition, all of these species except African mustard showed higher total seedling recruitment under the no-till system. Results of this study will facilitate weed-control timing decisions and provide validation data for weed seedling recruitment models.

Laboratory, greenhouse, and field studies were conducted to determine the allelopathic potential of sunflower (Helianthus annuus L.) cultivars to suppress weed germination and growth. Germination of wild mustard [Brassica kaber (DC.) L.C. Wheeler var. pinnatifida (Stokes) L.C. Wheeler] seeds at 25 C in undiluted aqueous extracts of sunflower leaf tissue was inhibited 75%, but was stimulated by up to 150% at 10- and 100-fold dilutions. Stem-tissue extracts at all concentrations stimulated wild mustard seed germination. The germination response of other weed species varied with the sunflower cultivar and concentration of tissue extract. In sand culture, leachates of dried sunflower leaf and stem tissue inhibited broadleaf - weed seedling growth, but had little or no effect on the growth of grass weeds. Sunflower root exudates inhibited seedling growth, but were less effective than leaf and stem tissue leachates. Germination of weed seeds was unaffected by root exudates. Over a 5 -yr period, weed density and percent ground cover increased less in field plots of sunflower than in control plots.

The Broadbalk experiment was started in 1843 to investigate the relative importance of different plant nutrients (N, P, K, Na, Mg) on grain yield of winter wheat. Weeds were controlled initially by hand hoeing and fallowing, but since 1964, herbicides have been applied to the whole experiment with the exception of the 18 plots on Section 8. Approximately 130 weed species have been recorded on Broadbalk and about 30 of these are currently recorded annually on Section 8. Detailed weed surveys, conducted from 1930 to 1979, provide a unique 50-yr record, but the relatively small number of frequency categories used (six) poses a limitation on the interpretation of these data for ecological studies. Weed surveys were restarted in 1991 on Section 8. The current assessment method records the presence of individual weed species in 25 random 0.1-m2 quadrats per plot, which is more appropriate for detecting long-term trends in weed frequencies and population differences between plots. A principal components analysis of the 1991–2002 survey data for 15 species showed clearly the influence of inorganic N fertilizer levels on the frequency of individual species. The frequency of one species (common chickweed) was greatly favored by increasing amounts of nitrogen fertilizer from 0 to 288 kg N ha−1, others were strongly disadvantaged (e.g., black medic and field horsetail), some were slightly disadvantaged (e.g., common vetch and parsley-piert), and some showed little response to differing N rates (e.g., blackgrass and corn poppy). Other weed investigations include studies on the effects of fallowing on the weed seed bank, seed dormancy and persistence, agroecology, and population dynamics of individual weed species. Recently, molecular approaches have been used to study the genetic diversity of weeds found on Section 8, which is one of the few arable sites in the country where herbicides have never been applied. This site also provides an invaluable reserve for seven nationally rare or uncommon species. Broadbalk continues to act as a valuable resource for weed investigations 160 yr after it was established.

Acetochlor [2-chloro-N-(ethoxymethyl)-N-(2-ethyl-6-methylphenyl)acetamide], alachlor [2-chloro-N-(2,6-diethylphenyl)-N-(methoxymethyl)acetamide], and metolachlor [2-chloro-N-(2-ethyl-6-methylphenyl)-N-(2-methoxy-1-methylethyl)acetamide] were applied in 280 L of water/ha to plots covered with 0 to 6720 kg/ha of wheat (Triticum aestivum L.) straw. At straw levels of 1120 kg/ha or greater, 50% or less of the applied herbicides were received by the soil surface before irrigation. Sprinkle irrigation (1.3 cm) washed 15 to 20% of the originally applied herbicide into the soil regardless of straw level. More metolachlor was retained on the straw than acetochlor or alachlor. Analysis of the wheat straw indicated that little water-extractable herbicide remained for all herbicides. Initial herbicidal activity on grain sorghum [Sorghum bicolor (L.) Moench.] was reduced by the presence of wheat straw at the time of application, with acetochlor being least affected and alachlor most affected. Ten days after treatment, less than 10% of the original alachlor and acetochlor remained in the soil. When planted at this time, grain sorghum response was inversely related to the amount of straw mulch that was originally present. Metolachlor residues in the soil on day 10 were 11 to 26% of that on day 0 and there was comparably less reduction in activity on grain sorghum.

The adsorption and desorption of 2-chloro-4-ethylamino-6-isopropyl-amino-s-triazine (atrazine) as affected by pH, temperature, and cencentration of atrazine were studied on Drummer clay loam soil and a humic acid isolated from Leonardite. The adsorption of atrazine on the soil increased as pH decreased, but it was affected only slightly by temperature and cencentration of atrazine. Desorption of atrazine was greater with increasing temperature and pH. The adsorption of atrazine on the soil was reversible and desorption was nearly complete. Adsorption of atrazine on the humic acid increased as pH decreased and temperature increased, and there was very little desorption of atrazine from the humic acid.

The growth regulator, glyphosine [N,N-bis(phosphonomethyl)glycine], and other possible metabolites of glyphosine and glyphosate [N-(phosphonomethyl)glycine] [glycine, sarcosine, and aminomethylphosphonic acid (AMPA)] were tested individually (0.5 mM) or as a mixture (each at 0.5 mM) for their effects on growth, extractable phenylalanine ammonia-lyase (PAL) activity, hydroxyphenolic-compound production, chlorophyll and anthocyanin contents, and on soluble-protein levels in soybean [Glycine max (L.) Merr. ‘Hill’] seedlings. Most chemical treatments caused some inhibition of growth either on fresh weight accumulation or on root elongation in the light and dark over 72 h. Glyphosine was generally the most inhibitory and caused the greatest inhibition on axis dry-weight accumulation. Glyphosine significantly increased extractable PAL activity in axes of light- and dark-grown soybeans to a lesser extent than did glyphosate. AMPA had some inhibitory effects on extractable PAL activity whereas other compounds had little influence on the enzyme. These compounds had little effect on total soluble protein in axes or on soluble protein in PAL preparations from 12 to 72 h in light-or dark-grown seedlings. No in vitro effect of the chemicals on PAL activity was found at concentrations up to 0.5 mM. Hydroxyphenolic compound levels increased within 24 to 72 h (per gram fresh weight basis) in light- or dark-grown soybean axes treated with glyphosine, AMPA, or a metabolite mixture (AMPA, sarcosine, and glycine). Anthocyanin content was decreased by glyphosate and to a lesser extent by glyphosine, but was increased by AMPA and the mixture. Glyphosate significantly increased the chlorophyll a/b ratio and decreased total chlorophyll, but glyphosine decreased the chlorophyll content to a lesser degree.

Adsorption of metribuzin [4-amino-6-tert-butyl-3-(methylthio)-as-triazin-5(4H)-one], metolachlor [2-chloro-N-(2-ethyl-6-methylphenyl)-N-(2-methoxy-1-methylethyl)acetamide] and fluometuron [1,1-dimethyl-3-(α,α,α-trifluoro-m-tolyl)urea] on a Taloka silt loam (Mollic Albaqualf) and a Roxana silt loam (Typic Udifluvent) from the 10- to 20- and 40- to 50-cm soil depths was measured. The order of adsorption of the herbicides was fluometuron = metolachlor > metribuzin. Adsorptivity of the soil from the 10- to 20-cm depth was greater than from the 40- to 50-cm depth. At 7, 15, 23, and 37C, all herbicides degraded more rapidly as temperature increased. Metribuzin was least persistent and fluometuron and metolachlor were similarly persistent. The time required for the initial herbicide concentration to decrease by 50% in the Taloka silt loam from the 10- to 20-cm depth at 23C was 2.6, 9.4, and 10.1 weeks for metribuzin, fluometuron, and metolachlor, respectively.

We investigated the periodicity of germination, longevity of caryopses viability, and reproductive capacity of downy brome (Bromus tectorum L.) populations in relation to chemical and mechanical fallow and fall tillage weed control treatments. The important factors in the dynamics of downy brome populations are: (a) large numbers of viable caryopses are carried from 1 year to the next in the litter and soil; (b) downy brome caryopsis production is density dependent; and (c) the simultaneous germination characteristics of freshly harvested downy brome caryopses can be conditioned environmentally to continuous germination.

Light from sunlight, incandescent, and fluorescent sources was filtered through fresh tobacco (Nicotiana tabaccum L.), corn (Zea mays L.), and soybean (Glycine max (L.) Merrill) leaves. The spectral quality of the leaf-filtered light showed that much more of the incident red energy was absorbed than the far-red. The effect of the leaf-filtered light on the phytochrome-controlled germination of six weed species was generally to inhibit germination of seeds given a stimulatory pre-irradiation of red light from a standard source. Germination of seeds with no pre-irradiation was either not promoted or promoted to various degrees. Unfiltered light, at intensities equivalent to those under the leaf filters, caused no comparable effects. These results indicate an effect of altered spectral quality of the leaf-filtered light on the ratio of inactive/active phytochrome (Pr/Pfr) in the underlying seeds. We suggest that such phenomena could influence the germination of weed seeds in the field.

Few studies on herbicide resistance report data to establish unambiguously the correlation between genotype and phenotype. Here we report on the importance of the EPSPS prolyl106 point mutation to serine (P106S) in conferring resistance to glyphosate in a goosegrass population from Davao, Mindanao Island, the Philippines (Davao). Initial rate-response studies showed clear survivors within the Davao population at glyphosate rates that completely controlled the standard sensitive goosegrass population (STD1). Assessment of potential resistance mechanisms identified the presence of P106S mutant individuals in the Davao population. Polymerase chain reaction (PCR) amplification of specific alleles (PASA) analysis established that the mixed-resistant Davao population was comprised of 39.1% homozygous proline wild-type (PP106), 3.3% heterozygous serine mutant (PS106), and 57.6% homozygous serine mutant (SS106) genotypes. Further rate-response studies on plants with a predetermined genotype estimated the Davao SS106 individuals to be approximately 2-fold more resistant to glyphosate compared to Davao PP106 individuals. Extensive analysis at different goosegrass growth stages and glyphosate rates established strong correlation (P < 0.001) between presence of P106S in EPSPS and the resistant phenotype. Importantly, no differences in the pattern of absorbed or translocated 14C–glyphosate were observed between PP106 and SS106 Davao genotypes or Davao and STD1 individuals, suggesting that glyphosate resistance in the Davao population was attributable to an altered target site mechanism. This study demonstrates that whilst P106S in EPSPS confers a moderate resistance level to glyphosate, the mechanism is sufficient to endow glyphosate failure at the recommended field rates.

Rigid ryegrass is a major weed of southern Australian cropping systems. Increased knowledge about the germination biology of rigid ryegrass would facilitate development of effective weed control programs. The influence of different environmental factors on seed germination of rigid ryegrass was studied. The level of seed dormancy in five different populations, collected from different fields on the same farm, was found to be similar, suggesting consistency in seed dormancy in populations at a single location. Dormancy release was more rapid for seeds after-ripened in the field compared with those after-ripened dry in a greenhouse. Seed decay was found to be an important contributor to the loss of seeds for this species from the seed bank in the field. Seed decay was much greater for seeds after-ripened on the soil surface compared with buried seed. In contrast, germination played an important role in the loss of buried seed as compared with surface seed. Regardless of the burial depth, the dormant seed component present at the end of the growing season was quite small (4 to 16%). Seedling emergence of rigid ryegrass in the field was greater (49%) for seeds buried at 1 cm than for those on the soil surface (16%). No seedlings emerged from seeds buried at 10 cm. Seed germination of rigid ryegrass was tolerant to a wide range of pH and showed some tolerance to salt and osmotic stress. The results indicate rigid ryegrass is unlikely to become a greater weed problem in no-till compared with cultivated seeding systems.