Weed emergence time and the longevity of weed seeds within the soil play an important role in implementing a timely and effective weed control program. In this study, the seed longevity and emergence pattern of wild oat (Avena fatua L.) and sterile oat [Avena sterilis ssp. ludoviciana (Durieu) Gillet & Magne] were monitored in field conditions. Fresh seeds of A. fatua and A. ludoviciana were placed into nylon bags (50 seeds per bag in three replications for three locations in eastern Australia: Gatton, Narrabri, and St George) and buried at depths of 0, 2, and 10 cm in November 2017. Bags were exhumed at 6-mo intervals over 30 mo to evaluate seed germination, viability, and decay components. The seed decay component of A. fatua and A. ludoviciana followed an exponential pattern. On both the surface and at the 10-cm burial depth, 50% of the seeds of A. fatua and A. ludoviciana had decayed by 6 mo. The seeds of A. fatua persisted longer at 2-cm depth than at other depths, particularly at St George, where 90% of the seeds decayed after the 30-mo study. However, at Gatton and Narrabri, 90% of the seeds of A. fatua at this depth had decayed after 18 mo of burial in the soil. In the emergence pattern experiment (2017 to 2019), the emergence of A. fatua and A. ludoviciana from different burial depths was also studied. The emergence of A. fatua and A. ludoviciana was greater from 2-cm (29% to 36%) and 5-cm (18% to 43%) soil depths compared with the surface (5% to 10%) and 10-cm (3-9%) soil depth. Avena ludoviciana emerged earlier (2,253 growing degree days [GDD]; March 14, 2018) than A. fatua (3,364 GDD; May 23, 2018). Both species exhibited high emergence between May to June 2018, and the last cohort of each species was observed in October 2018. The highest seedling emergence occurred at the start of the winter season (May), which emphasizes the need for early PRE weed control such as tillage, herbicide application, and cover crops to ensure crops are planted in a clean seedbed. The continued emergence of these weeds into the spring season (October) emphasizes the need for extended periods of A. fatua and A. ludoviciana management. The results also suggest that management strategies that can control all emerged seedlings over 2 yr and restrict seed rain in the field could lead to complete control of Avena spp. in the field.

Glyphosate-resistant (GR) crops are produced over large areas in North America. A study was conducted at six western Canada research sites to determine seed date and tillage system effects on weed populations in GR spring wheat and canola cropping systems from 2000 to 2003. Four-year wheat–canola–wheat–pea rotations were devised with varying levels of GR crops in the rotation. Weed populations were determined at pre– and post–in-crop herbicide application intervals in 2000 and 2003. Early seeding led to higher and more variable in-crop wild oat and wild buckwheat populations. High frequencies of in-crop glyphosate wheat in the rotation usually improved weed management and reduced weed density and variability. Canonical discriminant analysis (CDA) across all locations revealed that by 2003, green foxtail, redroot pigweed, sowthistle spp., wild buckwheat, and wild oat, all associated with the rotation lacking in-crop glyphosate. Similar CDA analyses for individual locations indicated specific weeds were associated with 3 yr of in-crop glyphosate (Canada thistle at Brandon, henbit at Lacombe, and volunteer wheat, volunteer canola, and round-leaved mallow at Lethbridge). However, only henbit at Lacombe and volunteer wheat at Lethbridge occurred at significant densities. Although excellent weed control was attained in rotations containing a high frequency of GR crops, the merits of more integrated approaches to weed management and crop production should also be considered. Overall, rotations including GR spring wheat did not significantly increase short-term weed management risks in conventional tillage or low soil-disturbance direct-seeding systems.

Barnyardgrass (BYG) has been the most frequently reported troublesome weed in rice because it is an aggressive invader, is difficult to control, and reduces yields significantly. A replacement series study was conducted to determine how a naturally suppressive cultivar (T65∗2/TN 1; ‘PI 312777’), a nonsuppressive cultivar (‘Lemont’), and an F3 cross between the two (‘PI 312777 × Lemont’) would interfere with BYG in the southern United States. The rice cultivars did not differentially affect BYG height. The PI 312777 produced more tillers and greater shoot dry weight but was only moderately competitive (relative yield [RY]) or aggressive (relative crowding coefficient) against BYG. Competitiveness at 2:2 rice and BYG mixture proportion and replacement series illustrations on RYs for the number of tillers and shoot dry weights for the three rice cultivars indicated that PI 312777 suppressed BYG growth relatively better than the other two cultivars. Plant-for-plant, PI 312777 was more competitive than Lemont. PI 312777 × Lemont suppressed BYG relatively less than did PI 312777 and therefore would require significant genetic improvements before it is suitable for commercial use in a reduced herbicide production system.

Grain sorghum (Sorghum vulgare Pers.) was grown in combination with varying densities and durations of yellow foxtail (Setaria glauca (L.) Beauv.) during 1963-1965 at Manhattan, Kansas. Weed yield and competitive influence were greatest during the year of above-average rainfall and supplemental N fertility. Reduction in sorghum grain yield was due principally to a reduction in number of seeds per head. A 16-inch weed band on the crop row reduced soil moisture more than any other density treatment, but lesser weed densities reduced soil moisture below that of weed-free plots.

Herbicide effects on vesicular-arbuscular mycorrhizal fungi were studied in greenhouse and field tests. In field plots treated with a bromacil (5-bromo-3-sec-butyl-6-methyluracil) and diuron (3-[3,4-dichlorophenyl]-1,1-dimethylurea) mixture, fungus infection was similar to levels present in cultivated plots. A slight reduction of infection occurred in roots of grove trees in a simazine (2-chloro-4,6-bis [ethylamino]-s-triazine) plus paraquat (1,1′-dimethyl-4,4′-bipyridinium ion)-treated plot compared to a cultivated plot. In the greenhouse studies, diuron, bromacil, and trifluralin (α,α,α-trifluoro-2,6-dinitro-N,N-dipropyl-p-toluidine) had no apparent effect on Glomus etunicatus Becker and Gerd. and plant growth. Paraquat, PPG 844 {1-(carboethoxy) ethyl 5-[2′-chloro-4′-(trifluoromethyl) phenoxy]-2-nitrobenzoate} and simazine adversely affected plant growth and the fungus.

Annual applications of three nitrogen (N) rates, two broadleaf herbicides, and six annual-grass herbicides in Kentucky bluegrass (Poa pratensis L.) turf were made from 1974 to 1981. With each increment increase in N (2, 4, and 6 kg N/200 m2), the percentage cover of crabgrass (Digitaria spp.) and number of common dandelion (Taraxacum officinale Weber # TAROF) plants were reduced when compared with the next lower rate. There was an interaction between crabgrass control by the annual-grass herbicides and fertilizer rates. When highly effective herbicides were used, there were no differences in crabgrass control related to N rate. When less effective herbicides or no annual-grass herbicides were used, there were great changes directly related to N rate. Each increment of increased N rate increased the turf quality rating. The broadleaf herbicides resulted in differential quality ratings, although differences were relatively small. Highest quality ratings were for plots treated with the three most effective annual-grass herbicides. Residues in the ppb range of some herbicides were detected in soils sampled about a year after the eighth annual herbicide treatments, but residues were below the phytotoxic level and therefore would have had no effect on control of weeds.

The high near-infrared reflectance (0.76 to 0.90 μm) of Big Bend loco and Wooton loco contributed significantly to their orange-red and red image tonal responses, respectively, on color-infrared aerial photographs making them distinguishable from associated vegetation and soil. Big Bend loco could also be distinguished on color-infrared and near-infrared black-and-white video imagery where it had distinct red and whitish tonal responses, respectively. Computer analyses of photographic and videographic images showed that Big Bend loco and Wooton loco populations could be quantified from other landscape features. A global positioning system was integrated with the video imagery that permitted latitude-longitude coordinates to appear on each image. The latitude-longitude data were integrated with a geographical information system to map Big Bend loco populations.

Primary root elongation of prickly sida (Sida spinosa L.) at pH 5.5 was similar to that at pH 6.5 while sicklepod (Cassia obtusifolia L.) was reduced to 62% and tall morningglory [Ipomoea purpurea (L.) Roth] to 87% of that at pH 6.5. At pH 5.1, root elongation of prickly sida was reduced to 77% of that at pH 5.5. Some elongation was observed in the pH range of 5.1 to 6.5 for all species. Maximum primary root growth of tall morningglory, sicklepod, and prickly sida occurred at approximately 32° which was similar to that for cotton (Gossypium hirsutum L. ‘Empire’). Temperature as high as 39° did not significantly reduce root elongation of sicklepod. Tall morningglory root growth was reduced approximately 50% by 0.2 ppm of trifluralin (α,α,α-trifluro-2,6-dinitro-N,N-dipropyl-p-toluidine). In contrast, sicklepod and prickly sida root growth was reduced only slightly by 1 ppm of trifluralin.

Greenhouse and field experiments (at Rosemount and Roseau, Minnesota) were conducted to characterize the growth and development of 10 quackgrass [Agropyron repens (L.) Beauv.] biotypes selected near Roseau, Minnesota. The biotypes were highly variable for all traits studied. Eight of the biotypes had flag leaves approximately 1 cm wide, but two biotypes had flag leaves approximately 1.5 cm wide. Four biotypes were glaucous and blue-green in color, and the other biotypes varied from light to dark green and had varying leaf pubescence. In the field, plant height varied from 59 to 79 cm, and rhizome length varied from 105 to 135 cm. Spike production varied from 4 to 62 spikes per plant, and mid-season rhizome bud production varied from 246 to 1,211 buds, suggesting that the biotypes varied in their reproductive potential. Shoot dry weight varied from 48 to 362 g per plant, and daughter shoot production varied from 7 to 235 shoots per plant. The quackgrass near Roseau, Minnesota, appears to be made up of several distinct biotypes.

Experiments were conducted from 1992 through 1994 to determine the effect of 0 to 12 Mg ha−1 of surface wheat residues (SWR) on giant foxtail density and crown node length, and soybean yield. Giant foxtail density decreased as SWR increased from 0 to 12 Mg ha−1. SWR of 6 to 12 Mg ha−1 reduced giant foxtail density by 2 to 50 % compared to bare soil. The crown node of giant foxtail was 2 cm above the soil surface with 12 Mg ha−1 of SWR. Frost in 1992 injured soybean more than weeds in plots with SWR while soybean in soil with no SWR was not injured. In absence of frost in 1993 and 1994, yield of weedy soybean increased 20 to 29%, respectively, with the increase of SWR from 0 to 6 Mg ha−1. In weed-free plots, soybean yield was similar across all SWR levels. These results confirm the hypothesis that high levels of SWR increased soybean yield in weedy plots because of decreased giant foxtail infestation.

The effects of the antidote CGA-154281, available soil water (ASW), and soil temperature on corn injury from preemergence applications of metolachlor were evaluated in field and growth chamber studies. In field studies, metolachlor at rates of 5.6, 8.4, and 11.2 kg ha–1 caused corn injury when there was sufficient ASW before corn emergence to activate the herbicide. Injury was prevented when CGA-154281 was applied with metolachlor (30:1, metolachlor:CGA-154281 by wt). The effects of surface-soil ASW, soil temperature, and CGA-154281 on corn tolerance to metolachlor were further evaluated in growth chamber studies. Corn injury from metolachlor was more severe when the surface soil was wet for 5 days immediately after herbicide treatment than when the surface-soil was dry. Corn injury from metolachlor also was greater when corn was grown under cool temperatures (21/13 C, day/night) than when grown under warm temperatures (30/21 C, day/night). CGA-154281 conferred protection against metolachlor injury regardless of surface soil ASW or growth temperature.

Modeling is increasingly being used as a tool for the evaluation of the environmental fate of pesticides. Sorption, leaching, degradation, and volatilization are some of the processes being integrated through the use of simulation modeling techniques. Several research programs are focusing their attention on such issues (16, 17, 18, 32, 35), with regulatory agencies involved in management of pesticides also taking a modeling approach (3, 7). Because of the extreme complexity of agroecosystems, it is obvious that the use of simulation models will continue to be the most expeditious, reliable, and cost-effective means of integrating the various processes acting upon a pesticide to determine its fate. For example, modeling will help to summarize and interpret efficacy trials and will provide the vehicle for transferring experimental results to unstudied situations, such as the potential environmental fate of an applied herbicide. However, proper development, testing, and responsible use of a modeling approach must be based upon a thorough, comprehensive understanding of interdependent and dynamic natural processes.

The concept of controlled release of herbicides has created in some workers expectations beyond possibility. In this theoretical paper, we examine the results to be expected based on a simplistic model. The purpose of the development is that it serve as a guide to experimentation and as a stimulant to such experimentation by revealing the possible advantages which may occur.

Response of grain sorghum [Sorghum bicolor (L.) Moench. ‘BR 64’] to norflurazon [4-chloro-5-(methylamino)-2-(3-(trifluoromethyl)phenyl)-3(2H)-pyridazinone] and fluridone {1-methyl-3-phenyl-5-[3-(trifluoromethyl) phenyl]-4(1H)pyridinone} and the effect of previous treatment on the persistence of herbicide activity in five soils were determined in the greenhouse. Relative phytotoxicity of fluridone in the soils was ordered as Greenville sandy clay loam > Appling coarse sandy loam > Dothan loamy sand = Rome gravelly clay loam > Bradson clay loam. Phytotoxicity of norflurazon was ordered as Greenville = Appling = Dothan > Rome > Bradson. Fluridone was more phytotoxic than norflurazon. Field soil that has been previously treated with norflurazon or fluridone (500 days earlier at 1.7 kg ai/ha) still had low concentrations of each herbicide present at the time of retreatment in the greenhouse. Previous treatment of norflurazon did not affect the rate of decline in activity in any of the soils compared to previously nontreated soils except for Appling. Significantly higher concentrations of norflurazon remained in all previously treated soils 297 days after treatment (DAT) compared to previously nontreated soil. Residual carryover of norflurazon was greater in the Bradson soil (2.8% O.M.) than soils having less than 2% O.M. Fluridone dissipation was significantly faster in previously treated soil than in previously nontreated soil, indicating enhanced microbial degradation of the herbicide.

Insufficient weed control is a major constraint to adoption of reduced-tillage practices for organic grain production. Tillage, cover crop management, and crop planting date are factors that influence emergence periodicity and growth potential of important weed species in these systems. We assessed two hairy vetch cover crop management practices, disk-kill and roll-kill, across a range of corn planting dates from early May to late June in three experiments in Beltsville, MD. Patterns of seed dormancy, emergence, and early weed growth were determined for overseeded populations of common ragweed, giant foxtail, and smooth pigweed, three important species in the Mid-Atlantic states that represent early to late emergence. Common ragweed emergence was lowest and dormancy was highest of the three species across all planting dates. Giant foxtail emergence was higher than the other species in roll-killed hairy vetch and included a significant number of seeds that germinated before rolling operations in late June. Smooth pigweed had the highest emergence and lowest dormancy in disk-killed hairy vetch in June. Individual giant foxtail plant weight was higher in roll-killed than disk-killed hairy vetch in 2 of 3 yr, whereas that of smooth pigweed plants was higher in disk-killed than roll-killed vetch in 2 of 3 yr. Giant foxtail was the dominant species in roll-killed hairy vetch (averaged 79% of total weed biomass at corn silking), probably because of early germination and establishment before rolling operations. Smooth pigweed was the dominant species in disk-killed hairy vetch at June planting dates (averaged 77% of total weed biomass), probably because of high growth rates under warm conditions in tilled soil. This research demonstrated that cover crop management practices and the timing of planting operations can shift the dominant species of weed communities in organic farming systems and must be considered in long-term weed management planning.

Hot water extracts of quackgrass (Agropyron repens (L.) Beauv.) rhizomes and leaves have been reported by Kommedahl et al. and Le Tourneau et al. to inhibit seed germination and growth of various crop plants. Experiments were performed to determine in which organs the toxic material is located, whether it is the same substance in all organs, and to compare toxin production in two clonal lines of quackgrass. In addition, experiments were performed to determine whether soil in which quackgrass had been growing was toxic and to determine if toxic material is present in young seedlings.

A bioassay using red beet root length indicated an increase in imazethapyr bioactivity in Berryland sand (BLS) as soil pH increased from 3.7 to 6.5. Increasing pH above 6.5 had no effect on imazethapyr bioactivity. The lowest imazethapyr concentrations detected by the bioassay at pH 6.5 were 0.5, 1, 2.5, and 5 to 10 μg kg−1 in acidwashed quartz sand, BLS, Aura loamy sand, and muck soil, respectively. After application of 0.07 kg ae ha−1 with 12.5 mm of simulated rainfall at 3-d intervals, imazethapyr remained in the 0 to 15 cm soil in BLS soil columns. Under this rainfall regime, the herbicide moved 30 cm deep 5 mo after application, but residues decreased significantly in surface soil. Imazethapyr bioactivity was highest and mobility was lowest at low soil temperature (10 C) and low moisture (4 to 6% w/w). Imazethapyr movement was monitored successfully using the red beet root length bioassay, and an herbicide residue map was developed to show the pattern of imazethapyr bioactivity in soil columns.

The photosynthetic capacities of three species of dodder, largeseed dodder (Cuscuta indecora, Choisy), field dodder (Cuscuta campestris, Yuncker), and small-seeded dodder (Cuscuta approximata, Bab.) were studied by radioactive carbon dioxide uptake in light and darkness, and by manometric methods. Both methods showed that all three species were able to carry on a small amount of photosynthesis. The photosynthetic capacity was highest in the seedling, and decreased as the dodder became parasitic.

In studying herbicides in relation to a complex weed like nut grass (Cyperus rotundus L.), it soon becomes clear that high efficiency is attained only if the herbicide is able to penetrate and translocate readily to all parts of the plant, still retaining sufficient toxicity to kill every one of the vital organs.

Greenhouse and laboratory experiments were conducted on common waterhemp and soil collected from 131 soybean fields in Missouri that contained late-season common waterhemp escapes. The objectives of these experiments were to determine the effects of soil sterilization on glyphosate-resistant (GR) and -susceptible (GS) common waterhemp survival, to determine the effects of soil sterilization and glyphosate treatment on infection of GR and GS common waterhemp biotypes by Fusarium spp., and to determine the soil microbial abundance and diversity in soils collected from soybean fields with differences in common waterhemp biotypes and herbicide and crop rotation histories. Common waterhemp biotypes were treated with 1.7 kg glyphosate ae ha−1 or left untreated once plants reached approximately 15 cm in height. Common waterhemp survival was visually assessed at 21 d after glyphosate treatment (21 DAT). To determine Fusarium infection frequency, a single intact common waterhemp root was harvested from each treatment at 0, 3, 7, 14, and 21 DAT and surface sterilized, and 10 to 15–mm common waterhemp root sections were plated on Komada culture medium. After 14 d incubation, fungal colonies were selected from colonized roots and maintained on potato dextrose agar medium amended with antibiotics before identification. Speciation of Fusarium isolates was conducted through microscopic examination of fungal characters and confirmed by sequencing and analysis of ribosomal DNA. Soil samples from 131 different collections were subjected to phospholipid fatty acid (PLFA) analysis and were conducted utilizing gas chromatography to determine the soil microbial community abundance and structure. Common waterhemp plants grown in sterile soils had the highest common waterhemp survival, regardless of biotype. After treatment with glyphosate, survival of GS common waterhemp grown in nonsterile soil was only 29% 21 DAT, whereas survival of GS common waterhemp grown in nonsterile soil was only 10%. Similarly, GR common waterhemp survival was reduced from 83 to 61% following treatment with glyphosate when grown in nonsterile compared to sterile soil. Fusarium spp. were recovered from only 12% of the assayed roots (223 treatments with Fusarium out of a total 1,920 treatments). The greatest occurrence of Fusarium root infection in both GR and GS common waterhemp occurred in nonsterile soils following a glyphosate treatment. Few differences in total PLFA were observed in field soil collected from locations with either GR or GS common waterhemp, and regardless of herbicide or crop history. This research supports previous findings that plant species are more sensitive to glyphosate in nonsterile than sterile soils and indicates glyphosate may predispose plants to soil-borne phytopathogens. This research also suggests that continuous use of glyphosate does not significantly affect soil microbial abundance or diversity.