Field experiments were conducted in 1992 and 1993 to characterize the weed seed bank, to determine the influence of moldboard plowing and secondary soil disturbance on the emergence pattern of weeds, and to measure weed seed bank depletion by emergence in a long-term moldboard plow corn cropping system. Viable seeds of common lambsquarters, redroot pigweed, and each of 10 other species accounted for about 85, about 9, and less than 1%, respectively, of the total weed species in the seed bank. A negative binomial distribution described the spatial distribution of viable seeds of 10 species, but not of common lambsquarters or of redroot pigweed. Decreased density of seeds among species was associated with increased aggregation. Secondary soil disturbance increased the rate and magnitude of common lambs quarters emergence in 1992 but did not influence emergence in 1993. Secondary soil disturbance did not influence the magnitude and rate of emergence of redroot pigweed or velvetleaf. Whereas cumulative growing degree days from April through July were similar between years, the amount of rainfall was about 50% less in 1992 than in 1993. Secondary soil disturbance may have increased common lambsquarters emergence by increasing the availability of soil moisture and improving conditions for seed germination during the dry year. Even though seed bank depletion by seedling emergence was relatively low for all species, secondary soil disturbance in creased seed bank depletion of common lambsquarters and redroot pigweed about 7- and 3-fold, respectively, in 1992. Seasonal variation in the amount of rainfall may have influenced the effect of soil disturbance on emergence and seed bank depletion of common lambsquarters, which is the most abundant species in the weed community.

Peak germination and emergence of common lambsquarters usually occur in early to mid-spring, but both processes can occur during summer and fall. Seeds produced by different common lambsquarters cohorts (seedlings that emerge at nearly the same time) may vary in dormancy status, response to environmental conditions, and response to management factors. Therefore, experiments were conducted to determine the influence of different cohorts on common lambsquarters demography. Field experiments determined plant density, biomass, and seed production of different common lambsquarters cohorts within a crop-free community of annual weed species that included redroot pigweed, giant foxtail, and velvetleaf. Common lambsquarters plant density and aboveground biomass were greater for a mid-May cohort than for early June, late June, mid-July, or early August cohorts, but seed production of the mid-May and early June cohorts did not differ (about 192,000 seeds m−2) and was greater than that of other cohorts (111,500 seeds m−2 or less). In the laboratory, percent germination prior to stratification (exposure of seeds to low temperatures) was less for seeds harvested from early May and late May cohorts (≥ 9%) than those of mid-June or early July cohorts (≤ 75%). After stratification in the field, percent emergence (seedlings per number of planted seeds) and mean emergence time were similar among early May, late May, mid-June, and early July cohort seed sources, and were not influenced by shallow burial in soil. These results suggest that recruitment from seeds produced by different common lambsquarters cohorts is similar, but proportional to the number of seeds produced by each cohort.

Greenhouse and laboratory studies were conducted to determine the effects of dicamba, atrazine, and bentazon on efficacy, foliar absorption, and translocation of MON 12000 or CGA-152005 applied to velvetleaf. The efficacy of MON 12000, CGA-152005, and a combination of CGA-152005 plus primisulfuron applied at 4.5 g ai ha−1 was similar when applied alone or with 140 g ha−1 of dicamba. However, applying these herbicides in combination with 840 or 560 g ha−1 of atrazine or bentazon, respectively, reduced velvetleaf control. Increasing the rate of MON 12000, CGA-152005, or the combination of CGA-152005 plus primisulfuron to 9 g ai ha−1 or replacing crop oil concentrate (COC) with methylated seed oil (MSO) increased velvetleaf control of the atrazine and bentazon combinations but not to levels equal to these herbicides applied alone. Dicamba had no effect on the foliar absorption and translocation of 14C from MON 12000 or CGA-152005. Atrazine had little effect on foliar absorption of 14C from MON 12000 or CGA-152005, but bentazon reduced the foliar absorption of 14C from MON 12000. Replacing COC with MSO increased the foliar absorption of 14C from MON 12000 or CGA-152005 applied alone or with dicamba or atrazine, but not with bentazon. Translocation of 14C from MON 12000 or CGA-152005 out of the treated leaves was 11 and 12%, respectively, averaged across adjuvants and sampling times. These values were reduced to an average of 3 to 4% for both MON 12000 and CGA-152005 when applied in combination with atrazine or bentazon. The majority of 14C from MON 12000 or CGA-152005 was translocated acropetally. Atrazine and bentazon significantly reduced the acropetal translocation of 14C from MON 12000 at 24 and 72 h and for CGA-152005 at 12, 24, and 72 h. The physiological basis for the observed antagonism of MON 12000 and CGA-152005 by atrazine and bentazon appears to be due to reductions in acropetal translocation of MON 12000 and CGA-152005 to velvetleaf meristems.

Research was conducted to determine the optimum population and row spacing for corn production and for suppressing velvetleaf growth and seed production. Corn was grown in a factorial arrangement of three populations targeted at 64,000 (1 ×), 96,000 (1.5 ×), or 128,000 (2 ×) plants ha−1 and two row spacings of 38 or 76 cm. Influences on corn were determined in weed-free plots, and influences on velvetleaf were determined for target plants established at 1.5–m intervals along the center of corn interrows. Four velvetleaf plantings were made at weekly intervals beginning at corn planting. Corn row spacing had little influence on corn or velvetleaf. Corn yield exhibited a parabolic response to population with a maximum of approximately 90,000 plants ha−1 in one year, no response to population in another year, and a linear decline with increasing population in a dry year. Velvetleaf seed production was reduced 69 to 94% by the 1.5 × population and 99% by the 2 × population compared to the standard 1 × population when velvetleaf emerged with corn. Velvetleaf seed production was eliminated when velvetleaf emerged at or later than corn leaf stages 3, 5, and 6 for corn populations of 2 ×, 1.5 ×, and 1 ×, respectively. Reduced velvetleaf seed production was correlated with lower positioning of plants in the corn canopy and reduced light availability. Results suggest that higher corn populations could aid integrated weed management strategies by reducing seed production and limiting the build-up of weed populations.

The critical period for weed control (CPWC) identifies the phase of the crop growth cycle when weed interference results in unacceptable yield losses; however, the effect of planting date on CPWC is not well understood. Field studies were conducted in 2004 and 2005 at Urbana, IL, to determine CPWC in sweet corn for early May (EARLY) and late-June (LATE) planting dates. A quantitative series of treatments of both increasing duration of interference and length of weed-free period were imposed within each planting-date main plot. The beginning and end of the CPWC, based on 5% loss of marketable ear mass, was determined by fitting logistic and Gompertz equations to the relative yield data representing increasing duration of weed interference and weed-free periods, respectively. Weed interference stressed the crop more quickly and to a greater extent in EARLY, relative to LATE. At a 5% yield-loss level, duration of weed interference for 160 and 662 growing-degree days (GDD) from crop emergence marked the beginning of the CPWC for EARLY and LATE, respectively. When maintained weed-free for 320 and 134 GDD, weeds emerging later caused yield losses of less than 5% for EARLY and LATE, respectively. Weed densities exceeded 85 plants m−2 for the duration of the experiments and predominant species included barnyardgrass, common lambsquarters, common purslane, redroot pigweed, and velvetleaf. Weed canopy height and total aboveground weed biomass were 300% and 500% higher, respectively, for EARLY compared with LATE. Interactions between planting date and CPWC indicate the need to consider planting date in the optimization of integrated weed management systems for sweet corn. In this study, weed management in mid-June–planted sweet corn could have been less intensive than early May–planted corn, reducing herbicide use and risk of herbicide carryover to sensitive rotation crops.

Field studies were conducted in 2007 and 2008 at seven sites in Ohio, Indiana, and Illinois to determine the effect of PRE herbicide and POST application timing on weed control and yield of glyphosate-resistant corn. Levels of PRE herbicide included none; low—atrazine; medium—atrazine and metolachlor; and high—atrazine, mesotrione, and metolachlor. Glyphosate was applied POST when corn was 30 cm tall, or 1 or 2 wk later. Common lambsquarters, giant foxtail, and giant ragweed infested at least six of the seven sites, and other weed species occurred at two to three sites. Control of weeds at the time of POST application ranged from 48 to 91%, 58 to 99%, and 87 to 100% for the low, medium, and high levels of PRE herbicide, respectively, averaged over POST application timing. Control of giant foxtail and redroot pigweed decreased by about 20% between the second and third POST timing, averaged over PRE herbicide, but control of other weeds was similar among timings. Late-season control of common ragweed, velvetleaf, common lambsquarters, and Pennsylvania smartweed exceeded 90%, regardless of PRE herbicide or POST timing. Control of redroot pigweed, ivyleaf morningglory, and giant ragweed was as low as 74, 67, and 83%, respectively, but the high level of PRE herbicide resulted in 90 to 97% control of these weeds. An interaction between PRE herbicide and POST timing for late-season control of giant foxtail, tall waterhemp, and yellow nutsedge reflected the more effective control among POST timings from the higher levels of PRE herbicide. The overall trend in this study was for more effective weed control in PRE/POST herbicide programs with more comprehensive PRE herbicides that have substantial activity on both grass and broadleaf weeds. Highest yield occurred where the PRE treatment consisted of a two- or three-way combination of herbicides applied at 50% of the recommended rate or higher. Yield was reduced at all POST timings with atrazine alone or in the absence of PRE herbicide.

To improve understanding of relationships between crop diversity, weed management practices, and weed population dynamics, we used data from a field experiment and matrix models to examine how contrasting crop rotations affect velvetleaf. We compared a 2-yr rotation system (corn–soybean) managed with conventional rates of herbicides with a 4-yr rotation (corn–soybean–triticale + alfalfa–alfalfa) that received 82% less herbicide. In November 2002, a pulse of velvetleaf seeds (500 seeds m−2) was added to 7- by 7-m areas within replicate plots of each crop phase–rotation system combination. Velvetleaf seed, seedling, and reproductive adult population densities, seed production, and seed losses to predators were measured during the next year. Velvetleaf seed production was greater in the 4-yr rotation than in the 2-yr rotation (460 vs. 16 seeds m−2). Averaged over 12 sampling periods from late May to mid-November 2003, loss of velvetleaf seeds to predators also was greater in the 4-yr rotation than in the 2-yr rotation (32 vs. 17% per 2 d). Modeling analyses indicated that velvetleaf density in the 4-yr rotation should decline if cumulative losses of seeds produced in the soybean phase exceeded 40%. Achieving such a level of predation appears possible, given the observed rates of velvetleaf seed predation. In addition, no tillage occurs in the 4-yr rotation for 26 mo after soybean harvest, thus favoring seed exposure on the soil surface to predators. Models that included estimates of seed predation indicated that to prevent increases in velvetleaf density, weed control efficacy in soybean must be ≥ 93% in the 2-yr rotation, but could drop to 86% in the 4-yr rotation. These results support the hypothesis that diverse rotations that exploit multiple stress and mortality factors, including weed seed predation, can contribute to effective weed suppression with less reliance on herbicides.

Experiments investigating mortality in the soil seedbank are aided by using only seeds that are initially viable and capable of remaining ungerminated (hereafter “persistent seeds”). However, seed mortality experiments often use heterogeneous populations containing persistent, nonviable, and germinable individuals. In this investigation we developed and compared nondestructive tests for isolating persistent seeds of two weed species characterized by physical seed dormancy (dormancy imposed by a water-impermeable seed coat): velvetleaf and ivyleaf morningglory. Individual seeds were weighed, steeped in water (hereafter “steepate”) for 48 h, and then assayed for imbibition. These seeds were then subjected to persistence assays conducted under controlled conditions (60 d in hydrated soil under 25/15 C day/night temperatures, 14-h photoperiod). Persistent seeds were less likely to imbibe and more likely to produce steepates with low electrical conductivity compared with germinable and nonviable seeds. For velvetleaf, persistent seeds were best segregated by comparing changes in steepate conductivity during 4 to 48 h of soaking, with the corresponding classification and regression tree (CART) model making few false discoveries (false discovery rate for persistence; FDRp = 8.6%, n = 93) and many true positive classifications (true positive rate for persistence; TPRp = 100%, n = 85). For ivyleaf morningglory, both a change in steepate conductivity from 4 to 48 h of soaking and imbibition status after soaking accurately separated persistent seeds (accuracy measures of corresponding CART models: FDRp = 5.6%, n = 150; TPRp = 100%, n = 142). Thus, for species with physical seed dormancy, we recommend use of steepate conductivity and imbibition status after soaking for isolation of persistent seeds. These seeds can then be used to optimize experiments on mortality in the soil seedbank. Nondestructive tests for isolating persistent seeds of species characterized by physiological seed dormancy require further research.

The nitrogen (N) response of competing plants may be affected by photosynthetically active radiation (PAR) availability and maximum potential growth rate, which determine N requirements. The responses of two crop (corn and soybean) and six weed species (common lambsquarters, common waterhemp, giant foxtail, velvetleaf, wild mustard, and woolly cupgrass) in low and high (150 and 450 μmol m−2 s−1) PAR levels to daily fertilization with either low or high (0.2 or 7.5 mM) NH4NO3 levels were studied. Leaf area of all species responded positively to N by 8 d after emergence (DAE) when grown in high PAR; in low PAR, most species did not respond until 11 DAE. Dry weight and leaf area of all species at 18 DAE were greater with high than with low N. These responses to high N were also greater in high than in low PAR for all species. Dry weights with high N were up to 100% greater in low PAR and up to 700% greater in high PAR than dry weights with low N. These responses suggest that low PAR reduced the benefit of N to the plants. The regression of relative growth rate (RGR) with high N to RGR with low N had a slope that was less than unity (β = 0.79), indicating that species with a higher RGR with high N experienced greater decreases in RGR with low N. Similarly, the sensitivity (change in RGR) of plants grown with high and low N was positively related to RGR with high N. RGR differences among crop and weed species may be related to differences in N requirement that could be exploited for weed management. RGR and seed size were negatively correlated, which may explain previous observations that small-seeded weeds were more sensitive to environmental stress.

Greenhouse studies were conducted to determine the response of common lambsquarters and velvetleaf to glyphosate applied alone or with 20 g L−1 of ammonium sulfate (AMS). Minimal response of common lambsquarters to glyphosate plus AMS was observed. The GR50 values for velvetleaf decreased dramatically from 451 to 92 g ha−1 for glyphosate applied alone and glyphosate plus AMS, respectively. The addition of AMS did not affect foliar absorption of 14C-glyphosate in common lambsquarters but increased absorption in velvetleaf. A twofold increase in translocation, as a percentage of total 14C-glyphosate absorbed, occurred in velvetleaf with the addition of AMS. Increased control of velvetleaf with glyphosate plus AMS may be partially explained by greater glyphosate absorption and translocation. Increased translocation of glyphosate applied with AMS in velvetleaf was an indirect effect of greater foliar uptake as well as greater partitioning of glyphosate out of the treated leaf.

Experiments were conducted to determine the efficacy, absorption, and translocation of glyphosate, glufosinate, and imazethapyr with selected weed species. In the greenhouse glyphosate, glufosinate, and imazethapyr were applied at 0.25, 0.5, and 1 times their label rates of 1,121, 410, and 70 g ha−1, respectively, on 10- to 15-cm black nightshade, common waterhemp, eastern black nightshade, field bindweed, giant ragweed, ivyleaf morningglory, prairie cupgrass, velvetleaf, and yellow nutsedge. Glyphosate applied at the 1-time rate caused injury greater than or similar to injury from the 1-time rate of glufosinate or imazethapyr on black nightshade, common waterhemp, eastern black nightshade, field bindweed, giant ragweed, prairie cupgrass, and velvetleaf. The 1-time rate of glufosinate injured ivyleaf morningglory and yellow nutsedge more than did the 1-time rate of glyphosate or imazethapyr. Under field conditions glyphosate caused the greatest injury to common waterhemp, prairie cupgrass, and velvetleaf across plant growth stages. Giant ragweed and ivyleaf morningglory injury was more dependent on growth stage, with the 15- and 30-cm growth stages more susceptible to glyphosate than to glufosinate or imazethapyr. Differential response of these weed species may be caused by differences in herbicide translocation. Glyphosate was translocated more in both giant ragweed and ivyleaf morningglory, and these species were injured more by glyphosate than by glufosinate or imazethapyr at the larger growth stages.

Recent interest in describing the spatial distribution of weeds and studying their association with site properties has increased the use of interpolation to estimate weed seedling density from spatially referenced data. In addition, farmers and consultants adopting elements of site-specific farming practices are using interpolation methods for mapping weed densities as well as soil properties. This study was conducted to compare the performance of four interpolation methods, namely inverse-distance weighting (IDW), ordinary point kriging (OPK), minimum surface curvature (MC), and multiquadric radial basis function (MUL), with respect to their ability to map weed-seedling densities. These methods were evaluated on data from four weed species, velvetleaf, hemp dogbane, common sunflower, and foxtail species, of contrasting biology and infestation levels in corn and soybean production fields in Nebraska. Mean absolute difference (MAD) and root mean square (RMS) between the observed point sample data and the estimated weed seedling density surfaces were used to evaluate the performance of the interpolation methods. Four neighborhood search types were compared within each interpolation method, and Search3 (12 to 16 neighboring locations) generated an interpolated surface with the smallest MAD and RMS indicating the highest precision. IDW with a power coefficient of p = 4 gave the smallest MAD and RMS, as did a test with an elliptical search and no anisotropy. The level of precision of all four interpolation methods was very poor for weed species with low infestation levels (> 75% of field weed-free; MAD ranged from 100 to 187% of the observed mean density), whereas precision was improved for weed species with high infestation levels (< 25% of field weed-free; MAD ranged from 45 to 85%). No single interpolator appears to be more precise than another. Implications of this study indicate that grid sample spacing and quadrat size are more important than the specific interpolation method chosen.

Velvetleaf has been a major concern of Southern cotton growers, yet information on its competitiveness and seed production in cotton is lacking. Experiments were conducted in 1997 and 1998 at the Central Crops Research Station in Clayton, NC, to evaluate density-dependent effects of velvetleaf interference and seed production dynamics in conventional tillage cotton. Velvetleaf at densities of up to 3.5 plants m−1 of row did not influence cotton height until at least 4 wk after planting. Velvetleaf height increased as plant density increased throughout the season in 1997, but it was not affected until 9 wk after planting in 1998. Because of differences in environmental conditions, velvetleaf and cotton achieved maximum height later in 1998 than in 1997; however, velvetleaf seed production and cotton yields were higher in 1998 than in 1997 regardless of velvetleaf density. Velvetleaf density had no effect on the fresh weight, dry weight, and stem diameter of velvetleaf plants in 1997. But in 1998, all these parameters decreased significantly with increasing velvetleaf density. Velvetleaf seed production in 1998 was nearly twice that in 1997. Averaged over velvetleaf densities, the greatest number of seed were produced between nodes 6 and 20 in 1997 and between nodes 1 and 10 in 1998. In both years, cotton yield loss increased with velvetleaf density. Maximum yield loss was 84% at 3.5 velvetleaf plants m−1 of row. Yield losses of 5 and 10% were caused by 0.2 and 0.4 velvetleaf plants m−1 of row (1,930 and 4,110 plants ha−1), respectively, in 1997 and by 0.03 and 0.08 velvetleaf plants m−1 of row (360 and 850 plants ha−1), respectively, in 1998. To understand better the applicability of these results, we hypothesized that environmental variation caused differences in measured responses between 1997 and 1998. Therefore, kriging methods were used to fit correlations between observed rainfall and growing degree days (GDD) each year at the experiment site. Results based on climate data from 4 yr at 110 sites indicated that inference space was higher for GDD than for rainfall. The conditions observed at the experiment site in 1997 were deemed most appropriate for the recommendations made in the surrounding area.

Trials were carried out to investigate the effects of seed burial depth on seedling emergence rate of 20 weed species. Marked depth-mediated variation in emergence ability of the different species was observed, together with a general pattern of decreasing emergence with increasing soil depth. At 10 cm, only johnsongrass, velvetleaf, catchweed bedstraw, and cutleaf geranium emerged, albeit only in limited numbers. Species most severely inhibited by burial depth were buckhorn plantain, large crabgrass, common purslane, chickweed, and corn spurry, none of which emerged from beyond 6 cm. In all species, depth-mediated inhibition was found to be sigmoidal (polynomial regression). In addition, the number of seedlings and rate of seedling emergence decreased when depth of burial increased. The depth at which the number of emerged seedlings was halved varied by species and ranged from 3.6 cm for common purslane and chickweed to 7 cm for velvetleaf and catchweed bedstraw. Excessive burial depth generally induced dormancy (in roughly 85% of cases) rather than suicide germination. A close inverse relation (second-degree equation) between seed unit weight and depth-mediated inhibition was observed. The physiological involvement of depth inhibition in seed bank ecology is discussed.

Field and laboratory studies were conducted to evaluate herbicide efficacy, absorption, and translocation of cloransulam within broadleaf weeds. Control of morningglory species and velvetleaf with cloransulam was dependent upon application rate and timing. A reduced rate of cloransulam (9 g ai ha−1) was as effective as the labeled rate (18 g ha−1), when applications were targeted to small to midsize morningglory and velvetleaf. Prickly sida, hemp sesbania, and sicklepod were suppressed by cloransulam. Contour maps predicted accurately weed control for all species except tall morningglory. Cloransulam absorption and translocation provided some information about tolerance mechanisms. Susceptible species, entireleaf morningglory and velvetleaf, both rapidly absorbed 14C-cloransulam. Absorption in the more tolerant velvetleaf was 20% lower than entireleaf morningglory at all harvest times. Absorption of 14C-cloransulam in prickly sida was only 26% 6 h after treatment, and absorption did not increase with time. Differences in cloransulam absorption and translocation partially explained differences in susceptibility among some weed species but not others.

Field studies were conducted in 1999, 2000, and 2001 to evaluate weed control and cotton response from PRE herbicides followed by (fb) trifloxysulfuron POST. In the first study, trifloxysulfuron at 3.8, 7.5, or 15 gai/ha was applied POST with or without pendimethalin at 690 gai/ha applied PRE in a factorial treatment arrangement. Visible crop injury occurred after all trifloxysulfuron applications, but injury was not affected by application of pendimethalin PRE. Cotton injury was 19 to 22% 7 d after POST treatment (DAT) from trifloxysulfuron at 3.8 to 15 g/ha but was 5 to 12% 28 DAT. Trifloxysulfuron controlled smooth pigweed, common ragweed, and common cocklebur, but spurred anoda, large crabgrass, goosegrass, and stinkgrass were not controlled by trifloxysulfuron. Morningglory species (tall morningglory, ivyleaf morningglory, and pitted morningglory) control with trifloxysulfuron at 7.5 and 15 g/ha was at least 79%, whereas velvetleaf was controlled 66% over all years. In a second study, clomazone, pendimethalin, pendimethalin plus fluometuron, pyrithiobac, or flumioxazin were applied PRE fb 7.5 g/ha trifloxysulfuron POST. Cotton injury from PRE herbicides fb trifloxysulfuron was 13 to 39% 7 DAT. Spurred anoda control exceeded 54% only with treatments that included flumioxazin or pyrithiobac PRE. Common lambsquarters, common cocklebur, and morningglory species were controlled at least 75% with all treatments that included trifloxysulfuron POST, whereas pendimethalin and clomazone usually controlled annual grasses. In both studies, the application of pendimethalin PRE controlled annual grass species and improved control of smooth pigweed and common lambsquarters over that controlled by trifloxysulfuron POST without a PRE herbicide.

Base temperature (Tb) for leaf appearance and the thermal time interval between appearance of successive leaves (phyllochron) were determined for six common weed species from a series of growth chamber experiments. Mean leaf appearance Tb values for giant ragweed, velvetleaf, redroot pigweed, large crabgrass, woolly cupgrass, and wild-proso millet were 1.3, 8.0, 8.5, 4.5, 2.2, and 5.1 C, respectively, and mean phyllochron values were 37.2, 34.4, 17.3, 42.2, 65.2, and 34.2 growing degree-days per leaf, respectively. Phyllochron values increased slightly with mean temperature for each weed species. To our knowledge, these are the first reported leaf appearance Tb and phyllochron values for giant ragweed, woolly cupgrass, and wild-proso millet. The results confirm leaf appearance Tb and phyllochron values reported previously for velvetleaf and redroot pigweed. However, large crabgrass leaf appearance Tb and phyllochron values in our study varied somewhat from those reported previously. The results provide information needed for parameterization of plant growth models that predict leaf development based on thermal time.

Field experiments were conducted in 1998 and 1999 at Five Points, CA, in the San Joaquin Valley under irrigated conditions to study competition between four commonly grown tomato cultivars and velvetleaf and to identify cultivar characteristics associated with greater tolerance to velvetleaf. The effect of velvetleaf competition varied with both year and tomato cultivar. When grown with 5 velvetleaf plants m−1 of row, marketable yield of tomato was reduced 8% in 1998 and 60% in 1999 for cultivar H8892 and 58% in 1998 and 80% in 1999 for cultivar H9661, compared to cultivars grown in monoculture. Across velvetleaf densities, height of tomato cultivars was not reduced compared to that of cultivars grown in monoculture. However in 1999, canopy width of tomato cultivars grown with velvetleaf was less than that of cultivars grown in monoculture. At early stages of growth, the leaf area index of tomato cultivars grown with velvetleaf was less than that of cultivars grown in monoculture. Crop growth rate and aboveground dry biomass of tomato cultivars grown with velvetleaf were generally less than those of cultivars grown in monoculture. Yield loss at high weed density was similar among cultivars, whereas yield loss at low weed density varied among cultivars. Cultivar tolerance to velvetleaf varied with year. However, cultivar H8892 had low yield loss and cultivar H9661 high yield loss at low weed density in 1998 and 1999. For cultivar H8892, leaf area expansion rate was also among the greatest for both years.

The root exudates produced by sorghums contain a biologically active constituent known as sorgoleone. Seven sorghum accessions were evaluated for their exudate components. Except for johnsongrass, which yielded 14.8 mg root exudate/g fresh root wt, sorghum accessions consistently yielded approximately 2 mg root exudate/g fresh root wt. Exudates contained four to six major components, with sorgoleone being the major component (> 85%). Three-dimensional structure analysis was performed to further characterize sorgoleone's mode of action. These studies indicated that sorgoleone required about half the amount of free energy (493.8 kcal/mol) compared to plastoquinone (895.3 kcal/mol) to dock into the QB-binding site of the photosystem II complex of higher plants. Light, cryo-scanning, and transmission electron microscopy were utilized in an attempt to identify the cellular location of root exudate production. From the ultrastructure analysis, it is clear that exudate is being produced in the root hairs and being deposited between the plasmalemma and cell wall. The exact manufacturing and transport mechanism of the root exudate is still unclear. Studies were also conducted on sorgoleone's soil persistence and soil activity. Soil impregnated with sorgoleone had activity against a number of plant species. Recovery rates of sorgoleone-impregnated soil ranged from 85% after 1 h to 45% after 24 h. Growth reduction of 9 14-d-old weed species was observed with foliar applications of sorgoleone.

Field and greenhouse experiments were conducted to evaluate the efficacy and safening of metsulfuron applied with dicamba, 2,4-D, clopyralid, and fluroxypyr with and without nonionic surfactant. Greenhouse data showed that 2,4-D and dicamba, but not fluroxypyr, safened grain sorghum from metsulfuron injury. In the field study, grain sorghum injury from metsulfuron was decreased when tank mixed with 2,4-D or dicamba but not when tank mixed with clopyralid or fluroxypyr. Tank mixes of 2,4-D or dicamba with metsulfuron did not reduce ivyleaf morningglory or velvetleaf control. At 4 wk after treatment (WAT), ivyleaf morningglory was controlled 95, 84, 59, and 91%, and velvetleaf was controlled 88, 82, 78, and 95% when metsulfuron was tank mixed with 2,4-D, dicamba, clopyralid, and fluroxypyr, respectively. In a separate field study, differential grain sorghum hybrid responses to a tank mix of metsulfuron + 2,4-D was examined. In general, a tank mix of metsulfuron and 2,4-D caused visible injury to all hybrids at 1 and 2 WAT, but grain sorghum recovered and most hybrids appeared normal at the end of the growing season. Differential hybrid responses to metsulfuron + 2,4-D were observed at 1 and 2 WAT in 2000 and 4 WAT in 2001. The most susceptible hybrid was ‘Mycogen 1506’, and the least susceptible hybrids were ‘NK KS-310’ and ‘Pioneer 87G57’. This study demonstrates the potential for 2,4-D or dicamba to safen metsulfuron injury of sorghum without compromising weed control.