Field experiments were conducted from 1997 through 1999 to evaluate interspecific interference between pitted morningglory at 0, 10, 16, and 62 plants m−2 and drill-seeded, glyphosate-resistant soybean as influenced by soybean population and a single glyphosate application of 1.12 kg ai ha−1. Photosynthetic rate of soybean was not influenced by pitted morningglory density or glyphosate use 2 wk after treatment (WAT). Photosynthetic rate of soybean 12 WAT was reduced by 21 and 91% with 62 treated and untreated pitted morningglory plants m−2, respectively, whereas 10 treated and untreated pitted morningglory plants m−2 had no effect on the soybean photosynthetic rate. Pitted morningglory photosynthetic rate 2 and 12 WAT was reduced by 64 and 80%, respectively, when treated with glyphosate. The reduction in the photosynthetic rate of glyphosate-treated pitted morningglory was partially attributed to shading by soybean, whereas untreated plants were fully exposed to sunlight. Glyphosate-treated pitted morningglory at 10 and 16 plants m−2 did not reduce the rate of soybean leaf area index (LAI) accumulation; however, when the density was increased to 62 pitted morningglory plants m−2, soybean LAI decreased from 1.19 to 0.88 for each accumulated 100 growing degree days. Pitted morningglory produced a maximum of 24 million seeds ha−1 in the absence of glyphosate with 217,000 soybean plants ha−1. Pitted morningglory seed production declined with increasing soybean seeding rate in the absence of glyphosate, with a 41% reduction occurring when soybean population increased from 217,000 to 521,000 plants ha−1. Seed production of treated pitted morningglory ranged from 380,000 to 700,000 seeds ha−1. Soybean seed yield was not influenced by pitted morningglory density when treated with glyphosate. Untreated pitted morningglory at 10, 16, and 62 plants m−2 reduced soybean seed yield by 47, 62, and 81%, respectively. Competitiveness of untreated soybean increased with soybean seeding rate, resulting in 22% less yield loss with 521,000 than with 217,000 plants ha−1. Soybean seed yield was not reduced by 10 and 16 glyphosate-treated pitted morningglory plants m−2, but a 9% loss in yield occurred with 62 pitted morningglory plants m−2. Averaged over all pitted morningglory densities, glyphosate-treated pitted morningglory failed to reduce soybean seed yield at each of the three soybean densities. Following a single application of glyphosate, no apparent benefit was noted from increasing the soybean population above 217,000 plants ha−1.

Several seed production fields of the Italian ryegrass cultivar ‘Tetrone’ were destroyed in 1988 by 280 to 350 g ai ha−1 racemic fenoxaprop applied for wild oat control. Because similar rates of fenoxaprop had possessed adequate safety when applied to ‘Oregon common’ Italian ryegrass, suspicion arose that the cultivars differed in tolerance. Seedlings of 21 commonly grown cultivars were screened in the greenhouse at the three-leaf growth stage to determine their fresh weight GR50 for fenoxaprop. The GR50 values for the two most tolerant cultivars, ‘Marshall’ and ‘Torero’, were more than threefold greater than the two most sensitive cultivars, ‘Futaharu’ and ‘Ace’. Cultivars could be separated into sensitive, intermediate, and tolerant groups, but the distribution of the GR50 values appeared to be continuous rather than discrete. Tolerance increased with growth stage, and the average GR50 for tillered plants was 80% higher than that for the two-leaf stage and 41% higher than that for the four-leaf stage seedlings. Cultivars differed slightly in the specific activity of acetyl–coenzyme A carboxylase (ACCase) (EC 6.4.1.2) and in the I50 values for the inhibition by fenoxaprop, but the only clear relationship between these biochemical factors and whole-plant tolerance was a threefold increase in ACCase activity at the tillered stage over that present in the younger seedlings.

Greenhouse studies were conducted to examine the influence of temperature and relative humidity (RH) on the foliar activity of mesotrione on five weed species. Regression analysis was performed allowing for comparison of estimated GR50 (herbicide dose to inhibit growth by 50%) values for each weed response at either temperature (18 or 32 C) or RH level (30 or 85%). Temperature and relative humidity did not influence the response of ivyleaf morningglory, common cocklebur, and velvetleaf to mesotrione markedly. An increase in temperature or relative humidity increased the efficacy of mesotrione on common cocklebur and velvetleaf up to threefold. Conversely, common waterhemp and large crabgrass were six- and sevenfold more susceptible at 18 C than at 32 C, respectively. Common waterhemp and large crabgrass were four- and twofold more susceptible to mesotrione at 85% compared with 30% RH, respectively. The influence of temperature and RH on the efficacy of mesotrione in foliar applications is species dependent and may be an important consideration for field applications.

Glyphosate treatments to glyphosate-resistant (GR) cotton have been associated with poor pollination and increased boll abortion. Anatomical studies were conducted to characterize the effect of glyphosate treatments on the development of male and female reproductive organs of cotton flowers at anthesis. In comparison with nontreated plants, glyphosate applied at both the four-leaf stage postemergence (POST) and at the eight-leaf stage POST directed inhibited the elongation of the staminal column and filament, which increased the distance from the anthers to the receptive stigma tip by 4.9 to 5.7 mm during the first week of flowering. The increased distance from the anthers to the stigma resulted in 42% less pollen deposited on stigmas of glyphosate-treated plants than in nontreated plants. Moreover, pollen from glyphosate-treated plants showed numerous morphological abnormalities. Transmission electron microscopy showed the presence of large vacuoles, numerous starch grains, and less organized pockets of the endoplasmic reticulum containing fewer ribosomes in pollen from glyphosate-treated plants than from nontreated plants. Pollen development in glyphosate-treated plants is likely inhibited or aborted at the vacuolate microspore and vacuolate microgamete stages of microgametogenesis, resulting in immature pollen at anthesis. Although stigmas from glyphosate-treated plants were 1.2 to 1.4 mm longer than those from nontreated plants, no other anatomical differences in stigmas were visibly evident. The presence of the GR 5-enolpyruvylshikimate-3-phosphate synthase (CP4-EPSPS) enzyme from Agrobacterium sp. strain CP4 was quantified in reproductive and vegetative tissues using enzyme-linked immunosorbent assay. The content of CP4-EPSPS in the stigma, anther, preanthesis floral bud (square), and flower petals was significantly less than that in the vegetative leaf tissue. Glyphosate effects on the male reproductive development resulting in poor pollen deposition on the stigma, as well as production of aborted pollen with reduced viability, provide a likely explanation for reports of increased boll abortion and pollination problems in glyphosate-treated GR cotton.

Field experiments were conducted at Fayetteville, AR, in 1997, 1998, and 1999 to evaluate the degree of interspecific interference between drill-seeded, glyphosate-resistant soybean and hemp sesbania as influenced by soybean population, hemp sesbania density, and a single glyphosate application. Soybean was planted at 247,000, 430,000, and 618,000 seed ha−1 with early-season emergence of 217,000, 371,000, and 521,000 plants ha−1. Hemp sesbania densities were 0, 4, 10, and 16 plants m−2 in combination with 0 and 1.12 kg ai ha−1 glyphosate applied at the V4 to V6 soybean growth stage. Untreated hemp sesbania produced a maximum of 49 million seed ha−1 with 217,000 soybean ha−1, whereas seed production of glyphosate-treated hemp sesbania ranged from 2.8 to 0.6 million seed ha−1 with 217,000 and 521,000 soybean ha−1, respectively. Soybean seed yield was reduced 43% by 16 untreated hemp sesbania m−2, while glyphosate-treated hemp sesbania did not reduce seed yield. Averaged over all untreated hemp sesbania densities, soybean yield loss was reduced from 44 to 22% by increasing the soybean population from 217,000 to 521,000 plants ha−1. Because of the absence of soybean yield loss at 521,000 soybean ha−1, this density appears beneficial when using a single application of glyphosate to provide season-long weed control. However, a single glyphosate application in combination with a high soybean density did not completely prevent hemp sesbania seed production, and the seeding rate required to achieve a stand of 521,000 plants ha−1 would not be affordable; thus, alternate management methods targeting hemp sesbania must be employed to prevent an increase in the number of seed in the soil seedbank and to prevent a potential shift in the weed spectrum.

Soybean and annual weed competition studies were conducted over 2 yr in Balcarce, Argentina, to define the critical period of weed control and to compare yield losses in soybean, when weeds competed in the total cropping area (row plus furrow) or only in the furrow. In the first year, broad-leaf weeds were more abundant than annual grasses, but in the second year, grasses were more abundant. Critical weed-free periods required to prevent yield losses of 2.5 and 10% were long and consistent for both years and for both types of competition, lasting between 50 and 61 d after crop emergence (DAE) or the V8 to V9 and R2 to R3 soybean growth stages, respectively. No differences in predicted yield for both places of competition was found when weeds grew after the V4 or later crop growth stages (35 DAE). The critical periods of weed removal to prevent equivalent yield losses were different in both years. Varying between 1 DAE in 1986–1987 and 30 DAE in 1988–1989, for 2.5% yield losses, corresponding to growth stages between V0 and V3 to V4, respectively. For yield losses lower than 10%, the critical periods of weed removal varied between 15 DAE in 1986–1987 and 35 DAE in 1988–1989, corresponding to the V2 and V4 crop growth stages, respectively. Nonsignificant differences in predicted yield were found between places of competition, if weeds were removed before V4 (35 DAE), maintaining yield losses lower than 20%.

As a follow-up to greenhouse studies, acetolactate synthase (ALS) (EC 4.1.3.18) was extracted from one imidazolinone (IMI)-susceptible (S) and three IMI-resistant (R1, R2, and R3) smooth pigweed populations, and activity was assayed in the presence of imazethapyr, chlorimuron, thifensulfuron, and pyrithiobac. ALS inhibitor concentrations, required to reduce enzyme activity by a specified percentage compared with the untreated control (Ip), were determined for each herbicide using regression analysis, and resistance ratios were calculated from these values. An I50 value of >35 μM imazethapyr was calculated for all R populations compared with a value of 3.4 μM for the S population. With chlorimuron, thifensulfuron, and pyrithiobac data sets, pairwise comparisons of regression coefficients were used to determine significant differences between regression lines. Using this technique, it was established that ALS from R3 was more sensitive than ALS from S to inhibition by chlorimuron and thifensulfuron. Also, ALS from R2 and R3 displayed increased sensitivity to pyrithiobac compared with ALS extracted from the S population. We have confirmed enzyme-level resistance to imazethapyr in all R populations and have documented negative cross-resistance in some R populations to ALS inhibitors other than imazethapyr.

Site of uptake, absorption, translocation, and metabolism of ethofumesate were studied to determine the differential responses of annual bluegrass, creeping bentgrass, and perennial ryegrass. Using barriers to isolate foliage or soil, the primary site of ethofumesate absorption was found to be the foliage. Symptoms resulting from foliar and over-the-top spray applications of ethofumesate to annual bluegrass included stunting of growth, epinasty, swelling of the crown, darker green color, and fusing of expanding leaf blades. Averaged over the course of the experiment, annual bluegrass, creeping bentgrass, and perennial ryegrass absorbed 33, 11, and 13% of ethofumesate applied. No ethofumesate was detected distally from the treated leaf, but overall translocation of 14C 14 d after treatment (DAT) was 2.2, 5.8, and 2.2% of the absorbed 14C for annual bluegrass, creeping bentgrass, and perennial ryegrass, respectively. Ethofumesate was rapidly metabolized in each species with half-lives ranging from 4.5 to 7 d. The three grasses had metabolized, on average, 67% of the absorbed ethofumesate 14 DAT.

Glyphosate is a broad-spectrum herbicide that has been used extensively for more than 20 yr. The first glyphosate-resistant weed biotype appeared in 1996; it involved a rigid ryegrass population from Australia that exhibited an LD50 value approximately 10-fold higher than that of sensitive biotypes. We have characterized gene expression levels and glyphosate sensitivity of 5-enolpyruvylshikimate 3-phosphate synthase (EPSPS), the target enzyme for glyphosate inhibition, in sensitive and resistant lines derived from this population. Restriction fragment length polymorphism analyses were also performed to examine the distribution of EPSPS gene variants and the gene copy number. A two- to threefold increase in basal EPSPS messenger RNA (mRNA) and enzyme activity levels was observed in the most resistant lines analyzed; however, differences among lines in the sensitivity of EPSPS to glyphosate were not apparent. Induction of EPSPS was observed within 48 h after application of 1.5 kg ae ha−1 of glyphosate. This was reflected in elevated levels of both EPSPS mRNA and enzyme activity. Similarly, 3-deoxy-D-arabino-heptulosonate 7-phosphate synthase mRNA levels increased after glyphosate treatment; however, basal and induced transcript levels were comparable for sensitive and resistant lines in this case. The restriction fragment length polymorphism analyses showed no evidence for gene amplification or cosegregation of a specific EPSPS gene variant with glyphosate resistance. EPSPS expression in lines exhibiting an intermediate level of resistance was indistinguishable from that in glyphosate-sensitive lines, suggesting that the mechanism could, at least in part, be non–target-based.

We characterized the size and species composition of the weed seedbank after 35 yr of continuous crop rotation and tillage system treatments at two locations in Ohio. Spring seedbanks were monitored during 1997, 1998, and 1999 in continuous corn (CCC), corn–soybean (CS), and corn–oats–hay (COH) rotations in moldboard plow (MP), chisel plow (CP), and no-tillage (NT) plots where the same herbicide was used for a given crop each growing season. There were 47 species at Wooster and 45 species at Hoytville, with 37 species occurring at both locations in all 3 yr. Crop rotation was a more important determinant of seed density than was tillage system. Seed density was highest in NT and generally declined as tillage intensity increased. Seeds accumulated near the surface (0 to 5 cm) in NT but were uniformly distributed with depth in other tillage systems. At both locations there was a significant interaction between tillage and rotation for estimates of the total seed density. Seed density was highest in NT-CCC, with 26,850 seeds m−2 at Wooster and 8,680 seeds m−2 at Hoytville. At Wooster total seed density in CCC plots was 45 and 60% lower than in COH plots for CP and MP. In NT the total seed density was 40% greater in CCC than in COH. At Hoytville total seed density in CCC plots was 72% lower than in COH plots that were CP or MP, whereas seed density was 45% higher in CCC than in COH plots that were in an NT system. There were more significant differences in seedbank density for any given species for crop rotation than for tillage treatments. Seed densities of three broadleaves (shepherd's-purse, Pennsylvania smartweed, and corn speedwell) at Wooster and four broadleaves (yellow woodsorrel, redroot pigweed, Pennsylvania smartweed, and spotted spurge) at Hoytville were more abundant in COH (140 to 630 seeds m−2) than in CS (10 to 270 seeds m−2) or CCC (< 1 to 60 seeds m−2), regardless of the tillage system. At both locations Pennsylvania smartweed seeds were more abundant in COH (260 and 630 seeds m−2) than in other rotations (10 to 20 seeds m−2). Relative importance (RI) values, based on relative density and relative frequency of each species, were lower in CS than in CCC for common lambsquarters and five other weeds at Wooster; RI of giant foxtail was 80% lower in COH than in CCC at Hoytville. The data show how species composition and abundance change in response to crop and soil management. The results can help to determine how complex plant communities are “assembled” from a pool of species by specific constraints or filters.

Jointed goatgrass is most commonly described as a winter annual species. However, it has been observed to produce spikes in spring crops, apparently without being exposed to vernalizing conditions. A controlled environment study was conducted to determine the reproductive response of jointed goatgrass plants grown from seeds of fall- and spring-emerging parent plants to various vernalization durations. Winter wheat was included as a control. Winter wheat spikelet production was dependent on vernalization, and the number of spikes per plant was 10-fold greater if the plants were exposed to 4 C for 10 wk. In contrast, jointed goatgrass spike production without vernalization remained as high as 50% of that produced by plants exposed to 10 wk of vernalization conditions. Jointed goatgrass is thus not as dependent on vernalization for reproduction as the comparative winter wheat standard. Apparently, jointed goatgrass is more a facultative rather than an obligate winter annual. Rotating to a spring-seeded crop should not be expected to completely prevent jointed goatgrass seed production. Fields rotated to spring wheat to eliminate jointed goatgrass seed production should be monitored, and jointed goatgrass should be hand pulled or otherwise controlled to ensure zero seed production.

Experiments were conducted to determine the inheritance of resistance in a Wisconsin accession of eastern black nightshade to acetolactate synthase (ALS) inhibitors. ALS-inhibitor–susceptible (S) and ALS-inhibitor–resistant (R) plants were crossed (S × R), and inheritance was characterized in F1 and F2 generations. Inheritance was characterized further in progeny of reciprocal crosses (R × S) and backcrosses (BCs) (S × F1 and R × F1). In dose–response experiments, three- to four-leaved F1 plants were intermediate in response to imazethapyr compared with parent R and S plants. The imazethapyr ED50 value (the effective dose that reduced shoot dry biomass by 50% compared with nontreated plants) was 99.6 ± 13.6, 32.5 ± 8.9, 24.5 ± 3.3, and 0.6 ± 0.1 g ae ha−1 for R, F1 (R × S), F1 (S × R), and S plants, respectively. Similarly, the response of in vivo ALS activity in F1 plants to imazethapyr was intermediate compared with that of parent R and S plants. To differentiate among phenotypes in F2 generations and in BC generations, the response of three- to four-leaved plants to imazethapyr applied in a single dose was scored 21 d after treatment as R (no imazethapyr symptomology), S (total plant necrosis), or intermediate (I, severely stunted plants with chlorotic and twisted leaves at the apical meristem). R, I, and S phenotypes segregated in a 1:2:1 ratio in the F2 generation. S and I phenotypes segregated in a 1:1 ratio in progeny from F1 BCs to the S parent. Similarly, R and I phenotypes segregated in a 1:1 ratio in progeny from BCs to the R parent. Eastern black nightshade resistance to ALS inhibitors is associated with a single, nuclear, incompletely dominant allele, which codes for an insensitive form of the target ALS enzyme.

Experiments were conducted in a growth chamber to study alachlor and metolachlor metabolism in soil and corn and to determine if alachlor and metolachlor and their metabolites are exuded from corn roots to the growth medium. Alachlor was more readily absorbed by corn than was metolachlor. The absorption of alachlor and metolachlor was 72 and 55%, respectively, 10 d after seedling emergence (DAE). Alachlor and metolachlor were rapidly metabolized in corn, although metabolism rates were higher with metolachlor than with alachlor. Ten similar alachlor metabolites were detected in roots and shoots. In addition, two metabolites were detected only in the shoots, and one metabolite was detected only in the roots. Metolachlor metabolism in corn produced fewer metabolites than did alachlor metabolism. At 5 DAE, 10 and 9 metabolites were detected in shoots and roots, respectively. The metabolism of alachlor and metolachlor in soil showed patterns similar to the metabolism in corn but produced fewer metabolites. One unique alachlor metabolite appeared in soil but not in corn. Roots of corn seedlings treated with 14C-alachlor or 14C-metolachlor released significant amounts of radioactivity to the surrounding growth medium 5 d after treatment. Plants treated with alachlor released more radioactivity than did plants treated with metolachlor.

Wild oat continues to reduce spring wheat yields and profits despite the wide spread use of herbicides. Further reductions in the occurrence of wild oat could be achieved with the development of competitive cropping systems. Field studies were conducted to investigate the effects of wheat seed size and seeding rate on wild oat demographic processes under a range of wild oat densities. Spring wheat competitiveness increased as seed size and seeding rate increased, significantly reducing wild oat biomass and seed production. Averaged across all other factors, spring wheat plants derived from large seed reduced wild oat panicle numbers 15% and biomass and seed production 25% compared with small seed. Increasing spring wheat seeding rate from 175 to 280 plants m−2 reduced the number of panicles 10% and wild oat biomass and seed production 20%. The combined effect of large seed plus increased seeding rate reduced wild oat biomass and seed production 45%. Results demonstrate that the use of large seed size and increased seeding rates can improve wheat competitiveness and provide an effective means to reduce wild oat biomass and seed production.

Velvetleaf growth and canopy architecture were compared under a range of light conditions representative of competitive and noncompetitive environments typical of irrigated Mediterranean-type agroecosystems. Velvetleaf biomass and seed production exceeded those reported in the literature. Plants grown in full light produced 1,370 g dry weight and 44,200 seeds per plant and showed low relative variability. Velvetleaf grown with corn was reduced to 21 g dry weight and 349 seeds per plant, and had high relative variability for biomass and seed numbers. Velvetleaf grown with kidney bean, intraspecific neighbors, or under shadecloth had dry weights and seed numbers that were intermediate to plants grown in full light or with corn. Relative growth rate (RGR), net assimilation rate (NAR), and leaf area ratio (LAR) were assessed utilizing Richards functions, which were fitted to the primary biomass and leaf area data by weighted regression. RGR was highest for all plants early in the season, but declined later. Dynamics of NAR and LAR appeared to be correlated with increased self-shading, shading by neighbors, leaf age, and shedding of lower canopy leaves. Dynamics of specific leaf area corresponded with light availability such that the leaves exposed to full light were thicker than those exposed to shade. The branches of plants in all treatments had random azimuths and the foliage area density was concentrated along the perimeter of the plant's canopy. Velvetleaf increased the canopy radius through extensive branching when exposed to full sunlight. Leaf area distribution and branching patterns resulted in leaf area indices of less than 1.0. Leaves maintained a perpendicular angle to the sun throughout the day, but this depended on whether leaves received a consistent directional signal from the sun and not necessarily on whether they received a high-intensity signal. When shaded, the allocation of dry matter went primarily to the stem tissue, which increased the height rather than the girth of the plants. There was a 10- to 20-d delay for allocations to seed in the case of shaded plants relative to those grown in full sunlight. In brief, velvetleaf had a wide range of growth and canopy responses to a variety of light availabilities and it should have little difficulty in becoming fully established in the irrigated agroecosystems of Mediterranean-type regions.

Field studies were conducted in northern Greece, in 1997 and 1998, to investigate the effect of nitrogen fertilization and red rice density on interference between red rice and two rice cultivars (Thaibonnet, Ariette). Interference between rice and red rice began 3 wk after rice emergence, but was not affected by the increasing nitrogen rate from 100 to 150 kg N ha–1. Dry weight of both rice cultivars was proportionally reduced with increasing red rice interference duration and density, but dry weight of Thaibonnet was reduced more than that of Ariette. At harvest, grain yield of Thaibonnet was reduced by 58% because of the occurrence of 40 red rice plants m–2, whereas that of Ariette was reduced by 46%. Red rice interference affected panicle number more than 1,000 grain weight in both rice cultivars. The reduction of all yield components was greater in Thaibonnet than in Ariette. Dry weight and stem or panicle number of red rice plants grown with either of the two rice cultivars increased with increasing red rice density and were greater most of the time when grown with Thaibonnet than with Ariette. Ten weeks after rice emergence, red rice plants were 14 and 35 cm taller than the Ariette and Thaibonnet plants, respectively. Shattering of red rice plants ranged from 63 to 79% and was greater grown with Thaibonnet than with Ariette, but it was not affected by nitrogen fertilization and red rice density.

Wild oat, a troublesome weed in small grain cereals, infests about 11 million ha of cropland in the United States. Diversifying cereal production with alternative crops, such as yellow mustard and canola, provides flexible cropping systems, decreases production risks, and may allow more effective weed suppression. A greenhouse study was conducted to assess the competitive ability of yellow mustard and canola with wild oat in 1999 and 2000, using replacement series interference experiments to relate the results to plant development stages. Yellow mustard, regardless of its proportion in mixture, reduced aboveground biomass of wild oat 33 to 66%, leaf and tiller number 34 to 36%, and panicle production 58% compared with wild oat in monoculture. Canola did not affect wild oat biomass in mixtures. Yellow mustard per plant biomass in 2000 and inflorescence production in 1999 decreased 30 and 20% with increased density of yellow mustard in mixtures. Yellow mustard biomass was not affected by the addition of wild oat to the mixture, indicating the greater importance of intraspecific competition between yellow mustard relative to interspecific competition with wild oat. Canola per plant biomass was affected more by interspecific competition with wild oat than by intraspecific competition. A second greenhouse experiment was conducted to compare plant height and biomass accumulation by the three species over 7 wk. Yellow mustard had the greatest biomass accumulation and plant elongation rate, followed by canola and wild oat. The greater competitive ability of yellow mustard with wild oat, compared with canola, is likely associated with the rapid growth and canopy elevation of yellow mustard.

Integration of two or more methods in a weed control strategy may produce a positive interaction. In this study, sequential applications of sublethal rates of 2,4-D and the plant pathogen Sclerotinia minor were assessed for integrated control of common dandelion. S. minor was prepared as a granular treatment of fungal-colonized barley grits. Treatments of 2,4-D (25 or 50% of the recommended field rate) and S. minor treatments (20, 40, or 60 g m–2 rate) were applied alone or sequentially with a 3 wk interval. Fourteen days after inoculation (DAI), sequential applications of either rate of 2,4-D with 40 or 60 g m–2 of S. minor caused greater damage than either treatment alone (P = 0.05). By 21 and 28 DAI, control from 60 g m–2 of S. minor alone was equivalent to any of the sequential treatments (P = 0.05). At all assessment times, the combination of either rate of 2,4-D and 20 or 40 g m–2 of S. minor caused damage equivalent to or greater than that caused by 60 g m–2 of S. minor alone (P = 0.05). According to Colby's test for interactions, 19 of 24 assessments of the sequential treatments were synergistic. Therefore, sequential treatments of sublethal rates of 2,4-D and S. minor can interact positively to increase damage. This synergistic interaction may reduce the rate of either component required for adequate levels of control, possibly decreasing the cost or volume of use of herbicides in traditional weed control strategies.

Two separate field experiments were conducted to quantify the degree of plant-to-plant outcrossing and pollen-mediated gene flow (PMGF) in wild oat. The purpose of the study was to determine the extent to which pollen movement could contribute to the spread of herbicide resistance in this species. In both experiments, an acetyl-CoA carboxylase inhibitor–resistant (R) wild oat genotype (UM1) was used as the pollen donor and a susceptible (S) genotype (UM5) was used as the pollen receptor. Hybrid progeny resulting from a cross between UM1 and UM5 were identified using the herbicide resistance trait as a marker. In the plant-to-plant outcrossing experiment, single UM5 plants were closely surrounded by 20 homozygous R UM1 plants in hills. By screening seed from the S parent for resistance, outcrossing was determined to range from 0 to 12.3%, with a mean of 5.2% over 10 hills. In the PMGF experiment, single homozygous R UM1 plants were surrounded by UM5 plants arranged in a hexagonal pattern at low and high densities (total of 19 and 37 wild oat plants m−2), growing within spring wheat and flax crops. In the wheat crop, mean wild oat outcrossing was 0.08 and 0.05% at low and high densities, respectively. In the less competitive flax, corresponding outcrossing values were 0.07 and 0.16% at low and high densities, respectively. Distance from the pollen source was a significant factor only for the high-density planting arrangement in flax. Up to 77 R hybrid seeds were recovered from 6 m2 in the PMGF experiment, indicating that PMGF contributes to the evolution of resistance in wild oat populations. However, the contribution of pollen movement to resistance evolution and the spread of resistance in wild oat populations would be relatively small when compared with R seed production and dispersal from a resistant plant.

EDITOR'S NOTE: This manuscript was reviewed by six colleagues whose recommendations varied widely. Lack of repetition was a major concern. The authors address the problem in the last paragraph of the results section. Factors favoring publication included the worldwide importance of wild oats, the minimal data on gene flow in the species, and the fact that the results are consistent with those of other studies cited in this manuscript. The points raised by reviewers who did not favor publication, especially the role of the environment in pollen production and viability, are acknowledged.

R. L. Zimdahl, Editor

Knowing the distribution of weed seedlings in farmer-managed fields could help researchers develop reliable distribution maps for site-specific weed management. With a knowledge of the spatial arrangement of a weed population, cost effective sampling programs and management strategies can be designed, so inputs can be selected and applied to specific field areas where management is warranted. In 1997 and 1998, weeds were sampled at 612 to 682 sites in two center pivot irrigated corn fields (71 and 53 ha) in eastern Colorado. Weeds were enumerated when corn reached the two-leaf, four-leaf, and physiological maturity stages in a 76.2- by 76.2-m grid, a random-directed grid where sites were established at intervals of 76.2 m, and a star configuration based on a 7.62- by 7.62-m grid within three 23,225 m2 areas. Directional correlograms were calculated for 0, 30, 60, 90, 120, and 150° from the crop row. Fifteen weed species were observed across fields. Spatial dependence occurred in 7 of the 93 samples (a collection of sampling units for a particular weed species that was detected within a field at a particular sampling time and year) for populations of field sandbur, pigweed species, nightshade species, and common lambsquarters. Correlogram analysis indicated that 18 to 72% of the variation in sample density was a result of spatial dependence over a geographic distance not exceeding 5 to 363 m among the examined data. Because of the lack of spatial correlation for weed seedling distributions in these eastern Colorado corn fields, interpolated density maps should be based on grid sizes (separation distances) less than 7.62 m for weed seedling infestations.