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Studies were conducted from 1987 to 1990 to measure the dynamics of sicklepod established at subeconomic threshold populations in a peanut-cotton-corn cropping system. The experimental site had no native populations of sicklepod prior to initiation of the study. Main plots were crops in the rotation sequence plus continuous summer fallow (no crop). Subplots were: sicklepod established in the initial year of the study, sicklepod established every year of the study, and no sicklepod. Sicklepod was established at subeconomic threshold densities to simulate weed survival and seed production in fields where economic thresholds were the basis for weed management decisions. Sicklepod growing alone in fallow plots produced more seed per plant, resulting in significantly more seedlings throughout the study than sicklepod growing with crops. Sicklepod growing in corn produced the fewest seed per plant. Seed produced from subeconomic threshold densities established only in the first year caused 7-, 21-, and 20-fold increases in sicklepod populations during the next three seasons compared to the nontreated control.
Predictions of weed seedling populations from seedbank data should characterize the spatial distribution as well as the composition and abundance of weeds. The spatial distribution of seedbank and seedling populations of common lambsquarters and annual grasses (giant foxtail, large crabgrass, and fall panicum) were described in moldboard plow and no-tillage soybean fields from 1990 to 1993. Spearman rank correlations between seedbank and seedling densities were significant for common lambsquarters in both tillages and all years, but for annual grasses correlations were significant only in no-tillage. Semivariograms showed spatial autocorrelation in seedbank and seedling populations of common lambsquarters in all years in no-till, but less often in the moldboard plow field. Annual grass seed and seedling populations were autocorrelated in the no-till field every year except 1993, and in the moldboard plow field in 1992 and 1993 only. Cross-semivariograms showed spatial continuity between seedbank and seedling population densities in 3 of 4 yr in no-till for common lambsquarters, and in all years of no-till and 1 yr of moldboard plow for annual grasses. Grey-scale field maps of common lambsquarters seedbanks corresponded visually to maps of seedling populations and could have been used to target control efforts, but visual correspondence between annual grass seedbank and seedling maps was poor. Seedbank and seedling mapping may be useful for site-specific management, but additional information is needed to understand the variation in the relationships between these two populations over time and space.
This research was conducted to evaluate the light requirement for redroot and smooth pigweed germination in soil, and how this requirement is affected by germination temperature and seasonal periodicity in seed dormancy. Seed enclosed in nylon mesh bags was buried in the field in December 1993 and 1994 and was recovered throughout the spring and summer of the following year, respectively. Germination was highest with red light or at 30 C. The requirement for red light was more pronounced at 20 vs. 30 C. The saturating fluence of red light was as low as 3 μmol m−2 in buried seed and 1,000 μmol m−2 in unburied control seed, depending on germination temperature. The effect of light and germination temperature pigweed germination also changed throughout the growing season. Our results indicate that light may be a requirement for germination in only the most dormant weed seed in the soil seedbank.
The impact of seed production by subthreshold weed populations on future weed problems has impeded the adoption of integrated pest-management principles for weed management. Studies were conducted in fields with no velvetleaf history to determine how seedbanks and seedling populations change following seed production 1 yr or 5 consecutive yr in plow-disk and no-tillage corn. Cumulative seed production by 0.19 velvetleaf plants m−2 increased in a linear fashion from 1989 to 1994, with annual additions averaging from 330 seeds m−2 for velvetleaf in corn to 2,500 seeds m−2 for velvetleaf without competition from corn. Five-year cumulative seed production was 1,480 seeds m−2 in plow-disk and 1,810 seeds m−2 in no-till corn. In no-till corn, 42 velvetleaf seedlings m−2 emerged the 1st year after the 1989 seed rain, but only 35 seedlings m−2 emerged over the next 4 yr. In plow-disk plots, annual emergence averaged 12 seedlings m−2. Five years after the 1989 seed rain, the proportion of seeds lost to emergence was about 20% in both tillage treatments. Where velvetleaf seeds were allowed to return to the soil every year, cumulative seedling emergence was lower in plow-disk than in no-till corn, with total emergence of 70 and 360 seedlings m−2, respectively, after 5 yr. Seedbank numbers ranged from 10 seeds m−2 5 yr after a single seed rain (290 seeds m−2) by velvetleaf in plow-disk corn to 1,020 seeds m−2 following 5 consecutive yr of seed rain where 12,580 seeds m−2 were returned without corn competition in no-till. Seedbank samples in the fall of the 5th year had 69 to 98% fewer seeds than were accounted for by cumulative seed rain and seedling emergence, with greater apparent seed losses in plow-disk corn than in no-till corn. Over 90% velvetleaf control would be required annually to maintain subthreshold populations for 5 yr following a single seed rain. By comparison, over 95% control would be required annually to maintain subthreshold populations where velvetleaf seed return is permitted each year.
Horsenettle is a deep-rooted perennial weed that cannot be easily controlled by mechanical means or by a single chemical application. A study was conducted at two sites for two consecutive years to identify biological factors that might limit its growth. Insects, nematodes, and plant pathogens were collected from horsenettle growing in bermudagrass pastures. The insects most commonly found included the Colorado potato beetle and the eggplant flea beetle. An unidentified lepidopteron, family Gelechiidae, was found at very low frequency as pupae in hollow leaf chambers constructed at the apices of flowering meristems. Infested apices bore no fruit. Seven genera of nematodes were found in the soil at both sites, but only very low numbers of lesion nematodes were recovered from horsenettle roots, and these had caused little damage. Root rot was observed under wet soil conditions on plants damaged by trampling. A downy mildew was prevalent at both sites in both years in October.
Two strains of crownvetch (Coronilla varia L. # CZRVA) rhizobia were cultured in vitro with various rates of atrazine [6-chloro-N-ethyl-N′-(1-methylethyl)-1,3,5-triazine-2,4-diamine] and bifenox [methyl 5-(2,4-dichlorophenoxy)-2-nitrobenzoate]. Growth, measured turbidimetrically over 48 h, was similar for both strains. Atrazine and bifenox significantly reduced bacterial growth after 14 and 36 h, respectively, only at the highest concentrations tested (463 μM atrazine and 292 μM bifenox). Since growth of crownvetch rhizobia was apparently not affected by rates of atrazine or bifenox above reasonable soil solution concentrations, it is likely that herbicidal effects on nodulation were due to toxicity to the host plant rather than toxicity to these bacteria. In a growth chamber experiment, total nodule activity (TNA) and carbon dioxide exchange rate (CER) were measured simultaneously in an effort to distinguish direct atrazine effects on nodule function from indirect effects due to inhibition of photosynthesis and a resulting decrease in photosynthate supply to nodules. When 5 and 50 mg atrazine per kg soil were applied to intact plants, CER was severely reduced within 24 h, but similar reductions in TNA were not observed until 48 h after treatment. Total nodule activity was reduced similarly by atrazine and defoliation; the application of atrazine to defoliated plants did not inhibit TNA more than did defoliation alone. The data indicate that reductions in crownvetch nodule activity by atrazine are due to inhibition of photosynthesis or other processes rather than direct toxicity to N fixation.
Studies were conducted from 1989 to 1993 in continuous no-tillage and moldboard plow corn fields to describe rates of velvetleaf seed predation with time and with seed density, and to identify principal seed predators. Rates of seed loss from the soil surface averaged 1 to 57% day−1 and were equivalent in the two tillage systems. Predator populations were the same in no-tillage and moldboard plow fields. The predation rate was generally low in winter months, increased in mid-summer, and declined in late summer. In 2 of the 4 yr, predation increased in October and November. The predation rate was described by an exponential decay function of seed density, with high rates of seed loss at low densities and leveling off to a nearly constant level at densities above 600 seeds m−2. Predation was highest where seed access was not restricted, and exclosures of 6.5 and 1.6 cm2 reduced predation up to 15 and 52%, respectively. Mice were important predators in the field. In laboratory feeding studies, the carabid beetle Amara cupreolata, the slugs Arion subfuscus and Deroceras reticulatum, and cutworms (Agrotis ipsilon) consumed imbibed velvetleaf seeds. Amara cupreolata and A. subfuscus were the only predators to damage unimbibed velvetleaf seeds.
Experiments were conducted to test the accuracy of a global positioning system (GPS) in measuring the area of simulated weed patches of varying size and to determine the accuracy in navigating back to particular points in a field. Circular areas of 5, 50, and 500 m2 were established and measured using point and polygon features of a GPS. The GPS estimations of the area of those patches had errors ranging from 7 to 45%, 6 to 15%, and 3 to 6%, respectively, when compared to actual measurements. As patch size increased, errors decreased. A curve describing the relationship between GPS error and patch size had an excellent fit (r2 = 0.92). The error remained the same in all measurements across all patch sizes, but composed a smaller percentage of large patches. The GPS had submeter accuracy in navigation to the correct quadrat 73% of the time, located the correct quadrat 27% of the time, and invariably navigated to within 1.58 m of the correct quadrat. The relationship between patch size and measurement error was applied to natural infestations of hemp dogbane.
Studies were conducted in 1988, 1989, and 1992 in Plains, GA to measure effects of paraquat and alachlor on ‘Florunner’ peanut. Peanut treated with paraquat (0.14 kg ai/ha) plus alachlor (3.4 kg ai/ha) applied at vegetative emergence (VE), or paraquat plus alachlor VE followed by paraquat 28 days after emergence (DAE) were compared with a nontreated control. Both herbicide treatments reduced peanut foliage biomass at 65 DAE in 1989 and 1992. Herbicide treatments did not affect foliage biomass 90 DAE in 1988 and 122 DAE in 1989, but paraquat plus alachlor followed by paraquat reduced foliage biomass at 122 DAE in 1992. Pod biomass, measured at 90 and 65 DAE in 1988 and 1992, respectively, was reduced by herbicides. However, pod biomass did not differ among treatments 122 DAE in 1989 and 1992. Percent reflectance from the peanut canopy measured no effects from herbicides in 1988. However, in 1989 and 1992 herbicides applied sequentially reduced peanut canopy development. Peanut treated with a single herbicide and sequentially took longer to mature. Once optimum maturity was reached, peanut yields were not reduced.
Soils from long-term tillage plots at three locations in Ohio were sampled to determine composition and size of weed seed banks following 25 yr of continuous no-tillage, minimum-tillage, or conventional-tillage corn production. The same herbicide was applied across tillage treatments within each year and an untreated permanent grass sod was sampled for comparison. Seed numbers to a 15-cm depth were highest in the no-tillage treatment in the Crosby silt loam (77 800 m–2) and Wooster silt loam (8400 m–2) soils and in the grass sod (7400 m–2) in a Hoytville silty clay loam soil. Lowest seed numbers were found in conventional-tillage plots in the Wooster soil (400 m–2) and in minimum-tillage plots in the Crosby (2200 m–2) and Hoytville (400 m–2) soils. Concentration of seeds decreased with depth but the effect of tillage on seed depth was not consistent among soil types. Number of weed species was highest in permanent grass sod (10 to 18) and decreased as soil disturbance increased; weed populations were lowest in conventional tillage in the Hoytville soil. Common lambsquarters, pigweeds, and fall panicum were the most commonly found seeds in all soils. Diversity indices indicated that increased soil disturbance resulted in a decrease in species diversity. Weed populations the summer following soil sampling included common lambsquarters, pigweeds, fall panicum, and several species not detected in the seed bank.
Seed dispersal, interacting with environmental disturbance and management across heterogeneous landscapes, results in irregular weed spatial distributions. Describing, predicting, and managing weed populations requires an understanding of how weeds are distributed spatially and the consequences of this distribution for population processes. Semivariograms and kriged maps of weed populations in several fields have helped describe spatial structure, but few generalizations can be drawn except that populations are aggregated at one or more scales. Limited information is available on the effect of weed arrangement, pattern, or field location on weed population processes. Because weeds are neither regular nor uniform in distribution, mean density alone is of limited value in estimating yield loss or describing population dynamics over a whole field. Sampling strategies that account for spatial distribution can increase sampling efficiency. Further research should focus on understanding processes that cause changes in spatial distributions over time to help predict rates of invasion and potential extent of colonization.
Perception of light by phytochrome is a mechanism that triggers weed seed germination in response to soil disturbance. Photoconversion of phytochrome from the red light absorbing form to the active far-red absorbing form depends on hydration of phytochrome. This research was conducted to determine the soil water threshold for the photoinduction of germination by the brief exposure of light that occurs during soil disturbance, and to determine how this threshold is affected by the fluence of the light stimulus and fluence sensitivity of the seed population. Redroot pigweed seedling emergence and germination response to red light (R) was studied for a range of water potentials. Water potential gradients were established by incubating seeds in soils wetted to various water contents, or in polyethylene glycol 8000 (PEG) solutions. After imposing the light treatments, seeds were returned to a fully hydrated state. Seedling emergence in response to R increased as the volumetric water content (θv) of soils increased. At volumetric water contents of 4.0%, R-induced seedling emergence was inhibited 50% compared to photoinduced seedling emergence at the highest soil water contents tested. Attenuation of photoinduction was more pronounced at low vs. high R fluences in freshly imbibed seeds, but was unaffected in seeds that exhibited enhanced fluence sensitivity. In ecosystems where seasonal soil moisture extremes are prevalent, the photoinduction of seed germination may be limited in dry microsites such as surface crusts or under extreme drought conditions.
The objectives of this study were to determine how the timing of weed management treatments in winter wheat stubble affects weed control the following season and to determine if spring herbicide rates in corn can be reduced with appropriately timed stubble management practices. Field studies were conducted at two sites in Ohio between 1993 and 1995. Wheat stubble treatments consisted of glyphosate (0.84 kg ae/ha) plus 2,4-D (0.48 kg ae/ha) applied in July, August, or September, or at all three timings, and a nontreated control. In the following season, spring herbicide treatments consisted of a full rate of atrazine (1.7 kg ai/ha) plus alachlor (2.8 kg ai/ha) preemergence, a half rate of these herbicides, or no spring herbicide treatment. Across all locations, a postharvest treatment of glyphosate plus 2,4-D followed by alachlor plus atrazine at half or full rates in the spring controlled all broadleaf weeds, except giant ragweed, at least 88%. Giant foxtail control at three locations was at least 83% when a postharvest glyphosate plus 2,4-D treatment was followed by spring applications of alachlor plus atrazine at half or full rates. Weed control in treatments without alachlor plus atrazine was variable, although broadleaf control from July and August glyphosate plus 2,4-D applications was greater than from September applications. Where alachlor and atrazine were not applied, August was generally the best timing of herbicide applications to wheat stubble for reducing weed populations the following season.
This research was conducted to determine the environmental conditions necessary for development and expression of very low fluence (VLFR) germination sensitivity, and to determine the roles of type I phytochrome, gibberellic acid (GA) biosynthesis, and nitrate in this process. Redroot pigweed seed was subjected to pregermination incubation regimes of 5, 12, and 23 C for up to 21 d in a −1.2 MPa polyethylene glycol solution in the dark. Development of VLFR sensitivity was most pronounced at 23 C, and induction of germination by the lowest fluence tested (3 μmol m–2) was greater at a germination temperature of 30 than 20 C. In dormant smooth pigweed seed, development of VLFR sensitivity occurred only when seed was subjected to a prechilling treatment to overcome primary dormancy. The instability of the fluence response of seed subjected to pregermination conditions favorable for development of VLFR was consistent with type I phytochrome accumulation and degradation after photoconversion. In redroot pigweed seed, inhibition of GA biosynthesis during pregermination incubation resulted in the partial attenuation of VLFR sensitivity. The addition of 75 ppm nitrate to pregermination and germination media enhanced VLFR sensitivity. Our results suggest that development of VLFR sensitivity is due to cooperative action between type I phytochrome and phytochrome-independent GA biosynthesis. In the soil seedbank, this sensitivity may be augmented in elevated soil nitrate environments.
Florida beggarweed germination in laboratory experiments was highest between 21 and 38 C and at osmotic potentials above −0.2 MPa, with complete germination inhibition below −0.8 MPa. Patterns of seedling emergence in the field corresponded to timing and size of rainfall events in relation to the time of soil disturbance. Smaller percentage total seasonal emergence was observed following large rainfall events where soil had not been disturbed compared with plots recently disturbed, and soil disturbance not followed by rainfall did not promote major weed flushes.
Field studies were conducted in 1990 and 1991 to determine the effects of corn planting date and hairy vetch control method on the efficacy of fall-planted hairy vetch as a weedsuppressive cover crop for no-till corn. Glyphosate controlled hairy vetch when applied at the early bud growth stage (April), but hairy vetch residue provided no weed control compared to the weedy check. Mowing was not an effective means of suppressing hairy vetch at the early bud stage. Untreated hairy vetch reduced weed biomass 96% in 1990 and 58% in 1991 but reduced yield over 76% in April-planted corn. There was no competition of untreated hairy vetch with corn when corn planting was delayed until May or June (mid- or late-bloom growth stages of hairy vetch). Corn planted in May into untreated hairy vetch yielded similarly to corn planted in a no-cover weed-free check.
Studies were conducted in conventional and no-tillage corn in 1990, 1991, and 1992 at Wooster, OH, to measure corn yield and velvetleaf seed production in response to density of early and late emerging velvetleaf, and to estimate economic thresholds. The percent reduction in corn yield fit a hyperbolic function over velvetleaf densities from 1 to 30 plants m2. The percent yield loss and velvetleaf seed production were higher in a warm, wet year (1990) than in a dry (1991) or cold, wet year (1992). The percent corn yield reduction was generally greater in no-tillage than in conventional tillage and from early rather than late emerging velvetleaf. Maximum velvetleaf seed production ranged from about 18,000 seeds m2 for early emerging weeds in no-tillage in 1990 to 100 seeds m2 for late emerging weeds in no-tillage in 1992. The single year economic threshold for early emerging velvetleaf ranged from 0.40 to 14.0 velvetleaf m2 in conventional tillage and 0.13 to 3.13 in no-tillage. Economic thresholds that were predicted using yield goal information deviated from actual thresholds (using actual yields) for a given year by −43 to 30%. Single year economic thresholds were similar in both tillage treatments, but their value for management decisions is questionable due to variation among growing seasons and weed seed production from subthreshold populations.
Weed control, yield, quality, and net return in reduced-cost and standard weed control systems were studied in “Sunbelt runner’ peanuts (Arachis hypogaea L.) planted in a twin-row pattern in 1982 to 85 at Tifton, GA, and 1982 to 84 at Headland, AL. Reduced herbicide rates and/or less expensive herbicides were used to decrease weed control costs. In years and locations where weed populations were low there were no differences in weed control, crop yield, or quality. The lowest cost treatment, which included three applications of paraquat (1,1′-dimethyl-4,4′-bipyridinium ion), caused reduced weed control at both locations in 1982 and reduced yield in 1982 and 1984. None of the systems consistently resulted in the highest weed control, crop yield, or quality. A system including reduced rates of preplant-incorporated herbicides followed by two applications of paraquat performed as well as the standard system but cost about 40% less. Due to low cost and generally high yields this system resulted in consistently high net returns. Results indicate that the potential exists for reducing herbicide inputs without sacrificing yield or quality.
Weed seeds can require an exposure to light for induction of germination. Conducting tillage operations at night and thus preventing the photoinduction of germination has been proposed as a means to reduce weed emergence in agricultural systems. This research was conducted to evaluate night tillage as a weed management option and to determine which tillage operations have the greatest effect on light-mediated recruitment. Weed emergence was evaluated after conducting factorial combinations of day and night moldboard plowing and disking in the springtime from 1992 through 1995. The light environment during disking generally had a slightly greater effect on emergence than the light environment during plowing. Emergence of pigweed species and giant foxtail was, at most, 30 to 55% higher following day vs. night disking. Emergence of other weeds was not affected by the light environment during tillage. We conclude that night tillage may not be a viable approach to weed management due to insufficient reductions in weed emergence associated with night tillage and the high degree of variability in the recruitment response to light conditions during tillage.
Primary physical dormancy caused by seed coat impermeability to water is a major reason for the persistence of velvetleaf in soil seedbanks. Understanding temporal trends in seed dormancy status will help predict potential emergence in the spring. Experiments were begun in 1992 and 1993 to determine the effects of velvetleaf seed maturation time, storage environment, and storage duration on changes in seed dormancy and germination over 20 mo. Seeds buried 1 and 10 cm deep exhibited a 30 to 70% decline in physical dormancy from maturity until winter, little change in dormancy from winter through the following summer, and a further decline the next autumn. The loss of physical dormancy was more rapid for early than for late maturing seeds and more rapid in 1992 than in 1993. Physical dormancy of seeds held at 4 C declined steadily, at a rate of approximately 0.8% per day, over the course of the study. Germination of seeds buried 1 cm averaged 23 to 37% in the first spring after harvest, which was equivalent to 68 to 100% of seeds that had lost physical dormancy over autumn and winter. The percentage of seeds with enforced dormancy reflected the loss of physical dormancy during autumn and the loss of seeds to germination during spring and summer. Additional information on how autumn temperature and moisture conditions influence the pattern of dormancy decline could aid in explaining the variation in velvetleaf infestations over time.