Soil water availability is the most important factor limiting crop yield worldwide. Understanding crop and weed transpiration in response to water supply may provide valuable insight into the mechanisms of crop yield loss in water-limited environments. A greenhouse experiment was conducted to quantify corn and velvetleaf transpiration in response to drying soil. Five plants of each species were well watered by adding back the equivalent water loss each day to reach field capacity, and five plants were subjected to drought stress (dry-down) by not replacing lost water. Normalized daily transpiration of dry-down plants was regressed on soil water content expressed as the fraction of transpirable soil water (FTSW). The critical soil water content below which plants begin to close their stomates occurred at FTSWcr = 0.36 ± 0.015 for corn and 0.41 ± 0.018 for velvetleaf. Total water transpired did not differ among species. Velvetleaf also responded to drought by senescing its oldest leaves, whereas corn mainly maintained its leaf area but with rolled leaves during peak drought stress. During a short-term drought, corn is expected to perform better than velvetleaf because it maintains full transpiration to a lower FTSW and does not senesce its leaves. Under severe long-term drought, the species that closes its stomates at greater FTSWcr will conserve water and increase its chances of survival. Moreover, senescing all but the youngest leaves may ensure at least some seed production. Research is needed to evaluate the effects of soil water supply on corn–velvetleaf interference in the field.

Methods of extraction, cleanup, and determination of 2-chloro-4-ethylamino-6-isopropylamino-s-triazine (atrazine) residues from soil were studied. Two-hr soxhlet extraction with methanol was more efficient than shaking extraction with methanol or other solvents. The most successful cleanup method was the use of grade IV basic alumina columns. Polyethylene coated alumina columns were useful if the soil extracts contained high amounts of pigments. A pyridine-alkali-ethylcyanoacetate method of determination was more sensitive and had better color stability than the pyridine-alkali method. Another advantage of the method with ethylcyanoacetate was that a pink chromophore was determined rather than a yellow chromophore which is subject to high interference from yellow pigments from soil extracts.

Emergence of 23 medusahead (Taeniatherum asperum (Sim.) Nevski) selections from four seeding depths in two soils was investigated and compared with that of other grasses. Increased depth markedly reduced total emergence of medusahead and variability among selections. Selections of intermediate wheatgrass (Agropyron intermedium (Host) Beauv) and pubescent wheatgrass (Agropyron trichophorum (Link) Richt.) and downy brome (Bromus tectorum L.), among other grasses, greatly exceeded medusahead in emergence from all depths. Standard crested wheatgrass (Agropyron desertorum (Fisch. ex Link) Schult.) had less emergence than medusahead from all depths. The 3 and 4-inch planting depths exceeded the coleoptile length of medusahead. When medusahead emerged from these depths, the first true leaf was recurved and chlorotic.

Bioeconomic models are predicated upon the relationship between weed fecundity and crop yield loss in consecutive growing seasons, yet this phenomenon has received few empirical tests. Residual effects of wild proso millet (WPM) fecundity in sweet corn upon WPM seedling recruitment, weed management efficacy, and crop yield within a subsequent snap bean crop were investigated with field experiments in Urbana, IL, in 2005 and 2006. WPM fecundity in sweet corn showed strong positive associations with WPM seedbank density, seedling recruitment, and demographic transitions within snap bean. A negative exponential relationship between WPM initial seedling density and seedling survival of a single rotary hoe pass indicated that the rotary hoe was ineffective at low weed population densities, but its efficacy increased with increasing weed population density to a maximum of 75% seedling mortality. Efficacy of postemergent chemical control of WPM was unaffected by WPM population density. Path analysis models demonstrated dependence between WPM fecundity in sweet corn, WPM seedling recruitment in snap bean, and reductions in snap bean yield in subsequent growing season, mediated by negative impacts of WPM seedling establishment on snap bean stand. These results underscore the importance of expanding integrated weed management programs to include management of annual weed populations both at the end of their life cycle, by reducing fecundity and seed survival, and at the very beginning of their life cycle, by reducing seedling recruitment and establishment.

Germination of prostrate spurge (Euphorbia supina Raf.) seeds collected in August, September, October, and November 1977 at Riverside, California was 53, 30, 18, and 16%, respectively. Stratification of seeds collected in August, September, and October at 5 C for 3 weeks or more increased germination to 70 to 80%. Prostrate spurge seeds germinated at constant temperatures of 20 to 40 C, with optimum temperatures of 25 to 30 C. Maximum germination occurred under alternating temperatures with a high temperature of 30 to 35 C and a low temperature of 15 to 25 C. Germination in the dark was much lower than in the light. Seedling establishment of seeds collected in August sown at 0.0- to 0.5-cm depths was 30 to 35%, but establishment from 2 cm declined to near 0%. Light intensity did not affect the number of seedlings established, but the seedlings established were larger under higher light intensities.

Laboratory studies enumerated butylate-, EPTC-, vernolate-, pebulate-, and cycloate-utilizing bacteria and actinomycetes in a Dothan loamy sand following repeated use of butylate, EPTC, or vernolate. Herbicide utilizers were isolated on a defined medium composed of minimal salts plus 200 mg/L herbicide as the primary carbon source plus 25 mg/L TTC as an indicator of organisms capable of metabolizing the herbicide substrates. Media inoculated with a butylate-history soil developed more TTC-red actinomycetes on the butylate-indicator agar than did media inoculated with a non-herbicide-history soil. Similarly, media inoculated with a vernolate-history soil developed more TTC-red bacteria on the vernolate-indicator agar. Cross-adaptation from vernolate to butylate and cycloate was evident, with inoculant from the vernolate-history soil exhibiting an increase in bacterial counts when butylate or cycloate was present as the primary carbon source. Likewise, previous butylate exposure corresponded with an increase in the number of vernolate- and pebulate-utilizing actinomycetes. Previous exposure to EPTC resulted in an increased population of butylate- and pebulate-utilizing actinomycetes.

Field research was conducted in 1994 and 1995 to monitor the dissipation of flumetsulam in five Mississippi soils for a period of 16 wk. These soils allowed evaluation of flumetsulam persistence across three soil series with a relatively constant soil pH of 6.0, as well as across three soils of varied pH from 6.0 to 7.6 within a constant soil series. Bioavailability was determined with a rice root length bioassay. Predicted half-lives ranged from 20 to 26 d in 1994 and from 23 to 46 d in 1995. Persistence decreased with increasing cumulative rainfall and decreasing organic matter content (within a range of 1.2 to 3.5%). Soil pH had no measurable effect on flumetsulam persistence.

Transpirations of cotton and velvetleaf plants competing for water and light were estimated with heat balance stem flow gauges in a controlled environment. Cotton and velvetleaf plants were grown together in pots and monitored during fluctuations in soil water, irradiance, air temperature, and humidity. Shading from other plants imposed light competition on velvetleaf or cotton. Velvetleaf used significantly more water than cotton given ample water and light. Transpiration by both species was reduced in dry soil, but velvetleaf transpiration was reduced primarily by leaf abscision, while cotton leaves did not abscise. Both cotton and velvetleaf plants transpired significantly less when shaded. Leaf water potentials indicated that shaded plants experienced reduced evaporative demand suggesting that shading reduced plant stress in dry soil. Competition for water was strongly influenced by competition for light.

The herbicides 2,6-dichlorobenzonitrile (dichlobenil), 7-oxabicyclo-(2.2.1)heptane-2,3-dicarboxylic acid (endothall), and 3,5-dibromo-4-hydroxybenzonitrile (bromoxynil) were examined for their influence on the activity and synthesis of proteolytic enzyme(s) in the cotyledons of squash (Cucurbita maxima Duchesne) seedlings. None of these herbicides affected the activity per se of the isolated proteolytic enzyme(s). The presence of the herbicides in the culture solution during germination caused different effects on the development of the proteolytic activity in the cotyledons for each of the three herbicides. Dichlobenil reduced the development of the activity in both the excised cotyledons and the cotyledons of intact embryos. This inhibition was overcome in part by the addition of benzyladenine to the culture solution. The inhibition of hormone synthesis or export by the embryonic axis caused by dichlobenil could explain the observed results. Endothall inhibited the development of the proteolytic activity in the cotyledons of the intact embryo treatment but had little effect on the excised cotyledons. Benzyladenine did not reverse this inhibition. The effect of endothall resembled the inhibition caused by actinomycin D, possibly indicating that endothall interfered with ribonucleic acid metabolism. Bromoxynil inhibited the proteolytic enzyme(s) development in the cotyledons of intact embryos but not in the excised cotyledons. This inhibition was partly overcome by benzyladenine. These results suggest that bromoxynil not only interferes with the hormonal control mechanism but also proteinase synthesis.

Poa annua control with norflurazon was different between two field trials in 1999. At the field site with suspected diuron-resistant P. annua, control was less than at the site with known diuron-susceptible P. annua. Diuron-resistant and -susceptible biotypes of P. annua were subjected to a range of norflurazon rates in the greenhouse. The diuron-resistant P. annua biotype (R/S : of 2.5) was collected in 1995 from a Lolium perenne L. seed production field near Tangent, OR. The R/S : for norflurazon in these experiments was 4.8; thus, confirming that diuron-resistant P. annua is resistant to norflurazon.

Potential crop yield loss due to early-season weed competition is an important risk associated with postemergence weed management programs. WeedSOFT is a weed management decision support system that has the potential to greatly reduce such risk. Previous research has shown that weed emergence time can greatly affect the accuracy of corn yield loss predictions by WeedSOFT, but our understanding of its predictive accuracy for soybean yield loss as affected by weed emergence time is limited. We conducted experiments at several sites across the Midwestern United States to assess accuracy of WeedSOFT predictions of soybean yield loss associated with mixed-species weed communities established at emergence (VE), cotyledon (VC), first-node (V1), or third-node (V3) soybean. Weed communities across research sites consisted mostly of annual grass species and moderately competitive annual broadleaf species. Soybean yield loss occurred in seven of nine site-years for weed communities established at VE soybean, four site-years for weed communities established at VC soybean, and one site-year for weed communities established at V1 soybean. No soybean yield loss was associated with weed communities established at the V3 stage. Nonlinear regression analyses of predicted and observed soybean yield data pooled over site-years showed that predicted yields were less than observed yields at all soybean growth stages, indicating overestimation of soybean yield loss. Pearson correlation analyses indicated that yield loss functions overestimated the competitive ability of high densities of giant and yellow foxtail with soybean, indicating that adjustments to competitive index values or yield loss function parameters for these species may improve soybean yield loss prediction accuracy and increase the usefulness of WeedSOFT as a weed management decision support system.

Palmer amaranth is the most economically damaging glyphosate-resistant (GR) weed in the southern United States. An understanding of the basic biology, including relative growth and competitiveness of GR and glyphosate-susceptible (GS) Palmer amaranth phenotypes from a segregating population collected from the same geographical location, may yield information helpful in the management of resistant populations. A segregating population of Palmer amaranth collected in North Carolina during 2010 was used as a plant source for both GR and GS traits. Research was conducted in the greenhouse to compare the following: level of resistance and shikimate accumulation in GR and GS phenotypes following glyphosate application; interference from GR and GS phenotypes on early-season vegetative growth of corn, cotton, and peanut; effect of various durations of imposed drought stress on GR and GS phenotypes; and response of GR and GS phenotypes to POST-applied herbicides. The GR50 (glyphosate rate providing 50% reduction in shoot dry biomass) was 17 times greater with the GR phenotype compared with the GS phenotype. Shikimate accumulated in both GR and GS phenotypes following glyphosate application, but greater concentrations were found in GS plants. The GR and GS phenotypes responded similarly when subjected to drought stress; grown with corn, cotton, and peanut; or treated with 2,4-D, atrazine, dicamba, fomesafen, glufosinate, paraquat, tembotrione, and thifensulfuron. These results indicate that in the absence of glyphosate selection pressure, resistance to glyphosate does not influence the growth and competitiveness of GR and GS Palmer amaranth phenotypes collected from the same geographical location.

New arylpicolinate herbicide chemistry under development for rice, aquatic weed management, and other uses was evaluated using five aquatic plants. The herbicide 4-amino-3-chloro-6-(4-chloro-2-fluoro-3-methoxyphenyl)-5-fluoro-pyridine-2-benzyl ester—also identified as XDE-848 BE or SX-1552 (proposed International Organization for Standardization common name in review; active tradename RinskorTM)—and its acid form (XDE-848 acid or SX-1552A) were evaluated on three dicots: (1) Eurasian watermilfoil (EWM), (2) megalodonta, and (3) crested floating heart (CFH), and two monocots: (1) hydrilla and (2) elodea. A small-scale Organization for Economic Cooperation and Development (OECD) protocol developed using EWM for registration studies was utilized. EWM and megalodonta were also evaluated in larger-scale mesocosms for comparison. In-water concentrations between 0.01 and 243 μg ai L−1 as SX-1552 or SX-1552A were applied under static conditions for 14 (growth chamber) or 28 d (mesocosm). EWM was susceptible to both SX-1552 and SX-1552A, with dry-weight 50% effective concentration (EC50) values of 0.11 and 0.23 μg ai L−1 under growth chamber conditions. Megalodonta had EC50 values of 11.3 and 14.5 μg ai L−1 for the SX-1552 and SX-1552A. CFH was more sensitive to SX-1552 (EC50 = 5.6 μg ai L−1) than to SX-1552A (EC50 = 23.9 μg ai L−1). Hydrilla had EC50 values of 1.4 and 2.5 μg ai L−1, whereas elodea was more tolerant, with EC50 values of 6.9 and 13.1 μg ai L−1 for SX-1552 and SX-1552A, respectively. For EWM mesocosm trials, EC50 values for SX-1552 and 1552A were 0.12 μg ai L−1 and 0.58 μg ai L−1, whereas the megalodonta EC50 was 6.1 μg ai L−1. Activity of SX-1552 on EWM, hydrilla, and CFH merits continued investigation for selective aquatic weed control properties. Results suggest that the OECD protocol can be used to screen activity of herbicides for multiple aquatic plant species.

Differences in degree of seed dormancy were shown to exist both among and within the four varieties of wild oats (Avena fatua L. spp. fatua (L.) Thell.) occurring in natural infestations throughout southwestern Alberta. Among the varieties, pilosissima was the most dormant, intermedia was more dormant than vilis, and glabrata possessed about the same dormancy as intermedia. Dormancy was overcome more rapidly under dry storage in vilis and glabrata than in intermedia and pilosissima. Seed dormancy of most collections made in 1959 differed from that of collections made at the same locations in 1962, being both higher and lower by as much as 40% in either of the varieties vilis or intermedia. In seeds from plants grown as transplants at three sites, dormancy differed within varieties, but there was no consistent pattern of difference evident among sites.

Fine fescues (Festuca spp.) are cool-season grasses used in low-maintenance turf areas. Mesotrione is a PRE and early-POST herbicide used during establishment of most cool-season turfgrasses, excluding fine fescues. Currently, efforts are being made to breed for increased tolerance to mesotrione in fine fescues to enhance weed control during establishment. This study was conducted to evaluate the association of foliar and root uptake of [14C]mesotrione with the tolerance of three lines each of Chewings fescue [Festuca rubra ssp. commutata Gaudin; syn. F. rubra ssp. fallax (Thuill.) Nyman], hard fescue [Festuca trachyphylla (Hack.) Hack.], and strong creeping red fescue (Festuca rubra L. ssp. rubra) lines. From a rate-titration experiment, the hierarchical rank of species for mesotrione tolerance from highest to lowest was: hard > Chewings > strong creeping red fescue. The hierarchical rank of species for foliar uptake from highest to lowest was: Chewings > strong creeping red > hard fescue. Translocation of foliar-absorbed 14C was not associated with differential tolerance levels of the three species. Root absorption was comparable among species, but differences between lines were detected within the species. The most susceptible lines of Chewings and strong creeping red fescue exhibited greater root uptake than lines with greater tolerance. Hard fescue translocated the least amount of root-absorbed radioactivity to shoots, while Chewings and strong creeping red fescues were comparable.

Weeds compete with crops for soil moisture, along with other resources, which can impact the germination, growth, and seed production of weeds; however, this impact has not been systematically recorded and synthesized across diverse studies. To address this knowledge gap, a global meta-analysis was conducted using 1,196 paired observations from 86 published articles assessing the effect of water stress on weed germination, growth characteristics, and seed production. These studies were conducted and published during 1970 through 2020 across four continents (Asia, Australia, Europe, and North America). Imposed water stress was expressed as solution osmotic potential (ψsolution), soil water potential (ψsoil), or soil moisture as percent field capacity. Meta-analysis revealed that water stress inhibits weed germination, growth, and seed production, and the quantitative response intensified with increasing water stress. A ψsolution greater than −0.8 MPa completely inhibits germination of both grass and broadleaf weeds. A ψsolution from −0.09 to −0.32 MPa reduces weed germination by 50% compared with the unstressed condition. Moderate soil water stress, equivalent to 30% to 60% field capacity, inhibits growth characteristics (branches or tillers per plant, leaf area, leaves per plant, plant height, root, and shoot biomass) by 33% and weed seed production by 50%. Severe soil water stress, below 30% field capacity, inhibits weed growth by 51% and seed production by 88%. Although water stress inhibits weed growth, it does not entirely suppress the ability to germinate, grow, and produce seeds, resulting in weed seedbank accumulation. This creates management challenges for producers, because weed seeds can survive in the soil for many years, depending on weed species and environmental conditions. Quantitative information compiled in this meta-analysis can be instrumental to model the weeds’ multidimensional responses to water stress and designing integrated weed management strategies for reducing the weed seedbank.

Natural dissemination of johnsongrass seeds as well as the effect of combine harvesting on this process were studied in corn fields. The estimation of natural dispersal was carried out by two different methods, collecting seeds throughout the season using seed traps and sampling soil–surface seed abundance before harvest using a vacuum device. Both methods showed the same dispersal pattern. A minimum of 84.6% was dispersed in the first 2 m from the focus and a maximum of 1.6% was dispersed beyond the first 5 m. An average of 76.3% of these dispersed seeds were lost or buried after shedding but before harvest. Seed dispersal by the combine harvester was estimated from the difference between soil–surface seed abundance in the same sites pre and postharvest. Although the quantity of seeds dispersed by the combine was similar to those dispersed by natural factors, dispersal distances were significantly higher. Around 90% of the dispersed seeds were found in the first 5 m forward and backward of the combine direction from the infestation source, and 1.6% of the seeds were found beyond 22 m forward and 10 m backward of the combine direction from the infestation source. A large proportion of the seeds dispersed were dormant or not viable. It is concluded that the major role of sexual reproduction in johnsongrass population dynamics may be to spread the risks, promoting dispersal in time and space.

The interactions between copper (Cu++) and cyanide (CN−) with 1,1′-dimethyl-4,4′-bipyridinium ion (paraquat) on phytotoxicity to Chlorella pyrenoidosa Chick, IAC 251, were determined by measuring chlorophyll absorbance at 434 and 670 mμ. Cu++ inhibited phytotoxicity and CN− accelerated paraquat activity with highly significant mutually antagonistic interactions occurring between Cu++ (2 ppm) and CN− (16 ppm) at paraquat levels to 2 ppm.

Chinese sprangletop, a problematic weed in aerobic rice fields, has developed resistance to various groups of herbicides. In search of natural herbicides, the phytotoxic effects of napiergrass extracts on Chinese sprangletop were investigated. Phytotoxicity-directed extraction and fractionation of the culm plus leaves of napiergrass led to the isolation and identification of three major compounds: 2,4-di-tert-butylphenol (2,4-DTBP), cis-9-octadecenoic methyl ester (methyl oleate), and phthalic acid, mono-(2-ethylhexyl) ester (MEHP). These compounds showed different degrees of inhibition against the tested bioassay species. 2,4-DTBP was the most potent of the three compounds and completely inhibited the germination of Chinese sprangletop at the concentration of 0.5 g L−1, followed by MEHP, where 62% inhibition of germination was obtained. In contrast, methyl oleate showed only slight inhibition (< 10%) of germination. A soil bioassay further demonstrated that 2,4-DTBP is a strong inhibitor of root growth and completely prevented root growth of Chinese sprangletop at an application rate as low as 0.60 kg ai ha−1. Under aerobic conditions, 2,4-DTBP at 2.4 kg ai ha−1 reduced the emergence and shoot fresh weight of Chinese sprangletop by more than 60%, with negligible effect on root and shoot growth of aerobic rice seedlings, suggesting that 2.4 kg ai ha−1 2,4-DTBP is the most suitable rate to control Chinese sprangletop without injuring rice seedlings. Reduction in shoot height of rice plant was evident at 0 d after sowing across 2,4-DTBP rates. However, the rice plants became less susceptible with increasing growth stages. The present findings imply that 2,4-DTBP may potentially be developed as a PRE soil-applied natural herbicide for control of Chinese sprangletop and perhaps other weeds in aerobic rice system.

A water-soluble linseed oil formulation was compared with emulsifiable linseed oil as an additive to bentazon [3-isopropyl-1H-2,1,3-benzothiadiazin-(4)3H-one 2,2-dioxide] for redroot pigweed (Amaranthus retroflexus L.) control. A water-soluble linseed oil formulation as an additive to bentazon controlled redroot pigweed better than did an emulsifiable linseed oil additive. Further, the water-soluble linseed oil additive enhanced redroot pigweed control with bentazon more than the emulsifiable linseed oil when a simulated rainfall followed application, or when humidity was low (30 to 45%) or spray volume high (170 to 340 L/ha). Redroot pigweed control with bentazon plus the water-soluble linseed oil additive followed by simulated rainfall in 4 h was similar to control with bentazon applied alone without a simulated rainfall.