A study was conducted near Garden City, KS, under irrigated conditions to determine the effect of full-season Palmer amaranth infestation on corn water use efficiency and light interception in a fully developed corn canopy. Palmer amaranth at densities of 0, 0.5, 1, 2, 4, and 8 plants m−1 was established at corn planting in 1996 and 1997 and at two locations in 1998. Soil water was monitored 240 cm deep in 30-cm increments with a neutron probe each year and at each location every 10 d. Photosynthetic photon flux was measured in 1997 and 1998 by using a circular and a linear quantum sensor for above canopy and in four 50-cm increments for within canopy, respectively. Palmer amaranth reduced corn yield from 11 to 91% as density increased from 0.5 to 8 plants m−1. Water use efficiency of corn declined with increased Palmer amaranth density. Regardless of Palmer amaranth density, soil water extraction was greatest in the top 30 cm of the soil profile. The pattern of corn leaf area distribution was similar across Palmer amaranth densities, with 15, 70 to 75, and 5 to 15% of the total leaf area occurring 1.5 m, 0.5 to 1.5 m, and 0 to 0.5 m above the ground, respectively. In weed-free corn, over 60% of light was intercepted from 0.5 to 1.5 m above the ground. In contrast, in mixed canopies 60 to 80% of light was intercepted 1 m above the ground, where 80% of Palmer amaranth leaf area was concentrated. Under the conditions of this study, water was not a limiting factor. The effect of Palmer amaranth density on total light interception was not significant. However, within each treatment, light interception at different heights differed, emphasizing the importance of evaluating the vertical distribution of light through the canopy to assess the effect of weed height on light competition.

Studies were conducted to identify and characterize different accessions of itchgrass. Seeds were collected in the counties of Aramina, Campinas, Dumont, Igarapava, Jaboticabal, and Ribeirão Preto, all in the state of São Paulo, Brazil. Accessions were characterized based on dimensions of their stomata, stomatal index (SI), and length and width of their seed (caryopses and husk). Chromosome number and length also were determined, and accessions were further differentiated using molecular markers (polymerase chain reaction [PCR]). Itchgrass from Ribeirão Preto had much longer and narrower seeds than those from the other locations, and their husks were longer as well. Accessions had similar SIs, both on the abaxial and adaxial leaf surfaces. Stomata from Campinas and Igarapava accessions were longer and wider, whereas those from Dumont and Ribeirão Preto were similar and smaller than all others. The accession from Ribeirão Preto is diploid (2n = 20); the rest are polyploid, with the total length of chromosomes smaller than all others. These differences were confirmed by molecular differentiation (PCR).

Studies were conducted to determine the extent of full and partitioned interference of two nutsedge species with tomato. For full interference, the crop and the weed were transplanted in the same container. For belowground interference, tomato and either weed species were grown in the same container, but the canopies were separated. For aboveground interference, tomato and nutsedges were grown in separate containers placed adjacently, whereas for the no-interference treatment, tomato and nutsedge plants were grown in individual containers. Full interference by yellow nutsedge was more detrimental to tomato shoot dry weight accumulation (34% reduction) than was full interference by purple nutsedge (28% reduction). Belowground interference by purple nutsedge reduced tomato shoot dry weight (18%) more than did aboveground interference (9%). Yellow nutsedge interference above- or belowground reduced tomato shoot dry weight to a similar extent (19%). The belowground interference of both nutsedges with tomato resulted in deficient concentrations of nitrate in the sap of tomato (> 18% reduction). The growth of purple nutsedge was influenced more strongly by tomato shading than by belowground interference from the crop, whereas yellow nutsedge growth was equally affected by tomato above- and belowground. According to these results, shoot dry weight accumulation in tomato was affected to the same extent by belowground interference from purple and yellow nutsedge, and the higher effect of full interference by yellow nutsedge may be attributed to increased aboveground competition between tomato and yellow nutsedge.

Interspecific hybridization of Palmer amaranth and common waterhemp produce hybrids with unique DNA fragments not found in either parent. The objective of this research was to investigate the mechanisms involved in the formation of the polymorphic fragments. Six novel fragments were cloned and sequenced. Five of the six were significantly similar to plant transposons, the sixth was similar to squamosa promoter–binding proteins from other plant species. Southern blot analysis using one of the novel fragments as probe revealed a consistent pattern of repetitive DNA that was species and biotype specific. These results indicate that transposon-like elements may play an important role in the formation of new fragments in Amaranthus hybrids derived from interspecific hybridization, suggesting that considerable instability of the hybrid genome may occur.

Variation in spring emergence periodicity (both before and after crop seeding) of summer annual weeds is a potentially exploitable attribute that may be applied to weed management in canola. Tillage intensity, which is decreasing in the Great Plains of North America, may influence emergence periodicity of summer annual weeds. Emergence periodicity of common lambsquarters, field pennycress, green foxtail, redroot pigweed, wild buckwheat, wild mustard, and wild oat were monitored during the spring of 2000 in 17 producers' canola fields across southern Manitoba, Canada. The fields represented a region of approximately 2 million ha and included a broad range of soil types, agronomic practices, environmental conditions, and seedbank distributions. Fields were grouped into one of two broad tillage classifications (conventional or conservation). For most species, except redroot pigweed and wild mustard, conservation tillage promoted earlier emergence than conventional tillage in terms of both thermal and chronological time. The differences were significant even though there was only a limited range of tillage intensity for the two tillage classes within this region. Onset of canola crop emergence preceded that of all but one weed species in the conservation-tillage fields and five weed species in the conventional-tillage fields. This suggests that canola seeded in conservation- vs. conventional-tillage systems may have a competitive advantage by way of an earlier relative time of crop emergence. The influence of tillage system on weed emergence periodicity is likely due to the influence of tillage on the vertical origin of weed seedling recruitment because measurements of soil temperature and soil moisture did not help to fully explain the differences in emergence periodicity between tillage systems. The results from this study will facilitate weed control timing decisions in canola and provide validation data for weed emergence models.

Amaranthus species, commonly referred to as “pigweeds,” are among the most troublesome weeds in many crop production systems. Effective control of these species often begins with an understanding of their biological and reproductive characteristics. At two sites in Missouri, six pigweed species (redroot pigweed, common waterhemp, spiny amaranth, tumble pigweed, smooth pigweed, and Palmer amaranth) were established in 60-m rows spaced 1.5 m apart. At biweekly intervals, plant heights and dry weights were recorded for each species; seed numbers were estimated at the end of the growing season. Dry weight of Palmer amaranth was up to 65% greater than those of all other species 2 wk after planting (WAP). Palmer amaranth biomass accumulation remained greater than those of the other species throughout the season and at the end of the season was 1.2- and 2.7-fold greater than those of redroot and tumble pigweed, respectively. Palmer amaranth was approximately 10 cm tall 2 WAP (37% taller than the next tallest species, redroot pigweed) and approximately 24 cm tall 4 WAP (45% taller than redroot pigweed). In contrast, common waterhemp had not emerged 2 WAP, and plant dry weight 4 WAP was approximately 11 and 26% those of Palmer amaranth and redroot pigweed, respectively. Final plant height ranged from 58 (tumble pigweed) to 208 cm (Palmer amaranth). Redroot pigweed, smooth pigweed, common waterhemp, and Palmer amaranth plants each produced over 250,000 seeds plant−1. Spiny amaranth and tumble pigweed produced approximately 114,000 and 50,000 seeds plant−1, respectively. Common waterhemp produced 535 seeds g−1 of total plant dry weight; this seed production was 1.4-, 1.4-, 2.0-, 3.4-, and 3.4-fold greater than those of redroot pigweed, smooth pigweed, Palmer amaranth, tumble pigweed, and spiny amaranth, respectively. Because the timing for many postemergence herbicides depends on weed height, rapid growth shortly after emergence reduces the time frame for optimum control of species such as Palmer amaranth. Delayed emergence also could result in escaped common waterhemp. Escape of only a few plants could result in a rapid increase in seed populations in the soil seed bank and may select for late-emerging biotypes.

Information on responses of weeds to various soil fertility levels is required to develop fertilizer management strategies as components of integrated weed management programs. A controlled environment study was conducted to determine shoot and root growth response of 23 agricultural weeds to N fertilizer applied at 0, 40, 80, 120, 180, or 240 mg kg−1 soil. Wheat and canola were included as control species. Shoot and root growth of all weeds increased with added N, but the magnitude of the response varied greatly among weed species. Many weeds exhibited similar or greater responses in shoot and root biomass to increasing amounts of soil N, compared with wheat or canola. With increasing amounts of N, 15 weed species showed a greater increase in shoot biomass, and 8 species showed a greater increase in root biomass, compared with wheat. Ten weed species exhibited increases in shoot biomass similar to that exhibited by canola, and five weed species showed greater increases in root biomass than did canola, as N dose was increased. All crop and weed species extracted > 80% of available N at low soil N levels. At the highest N dose, 17 of 23 weed species took up similar or greater amounts of soil N than did wheat, and 6 weed species took up N in amounts similar to that taken up by canola. These findings have significant implications as to how soil fertility affects crop–weed competition. The high responsiveness of many weed species to N may be a weakness to be exploited through development of fertilizer management methods that enhance crop competitiveness with weeds.

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.

Crenate broomrape is a major constraint for legume production in Mediterranean and East Asian countries. Resistance to this parasitic weed is scarce in many legumes but is common in chickpea germ plasm. A detailed in vitro study has shown that resistance in chickpea is the result of the combination of at least two mechanisms. First, and most importantly, the two chickpea lines studied have been identified with low rates of stimulant production. Once germination is induced by exogenous applications of the synthetic germination stimulant GR24, thus overcoming the primary resistance mechanism in these lines, a second resistance mechanism is apparent. This is evidenced by a darkening of host cell tissue in contact with the broomrape radicle, leading to failure of establishment, which was frequently observed in the chickpea accessions. Anatomical studies have shown that this apparently “hypersensitive” response does not correspond with the death of host cells in contact with the parasite cells but corresponds to blocking and death of the penetration structures of the parasite.

Five common cocklebur biotypes from southern Minnesota, central Iowa, southern Iowa, and Ohio were grown at the Iowa State University Curtiss Research Farm in 1995 and 1996 to examine intraspecific variations. Maximum plant heights were measured, and anthesis dates and day of bur set were recorded. At the end of the growing season, all plants were excised at the soil surface and weighed. Common cocklebur biotypes did not differ significantly in height. Flowering date was associated strongly with photoperiod and varied little between years within a biotype. But flowering date and bur set date differed among biotypes. The highest dry matter yields occurred in later flowering biotypes.

Laboratory experiments were conducted to determine the effects of optimum temperature, depth of planting, and simulated moisture stress on smellmelon germination and subsequent emergence. Smellmelon germination increased with temperatures ranging from 20 to 30 C, with the highest germination of 83% occurring at 30 C. Germination percentages decreased to 49 and 15% with temperatures of 35 and 45 C, respectively. Emergence from depths of 9 to 15 cm was less than 25% for all temperatures. Highest emergence from depths of 1 to 6 cm occurred with 30 and 35 C. Smellmelon germination was reduced from 81 to 61% when the osmotic potential was reduced to − 0.2 MPa and was further reduced to 48 and 7% with solutions of − 0.4 and − 0.6 MPa, respectively. Field studies were conducted to evaluate smellmelon biomass production in 1999 and 2000 when competing with cotton (Gossypium hirsutum L.). End of season-smellmelon leaf area values were 16,280 and 22,053 cm2 per plant in 1999 and 2000, respectively, or 67 and 117 g of leaf dry weight per plant for 1999 and 2000, respectively. In 1999, smellmelon produced 60 g of stem dry weight per plant, and it increased to 75 g in 2000. Optimum germination temperature for smellmelon coincides with those for many annual crops. These data indicate that smellmelon germination and emergence can occur under a wide variety of environmental conditions. On the basis of biomass production values, smellmelon may also be a very efficient competitor with many cultivated crops.

Common waterhemp is a significant weed problem in Midwestern cropping systems partly because of its potential for multiple emergence events during the growing season. The effects of shade and time of emergence on this weed have not been characterized. In the field, common waterhemp vegetative and reproductive growth were evaluated under different irradiance levels at two emergence times. In full sunlight a common waterhemp plant emerging in late May produced 720 g of biomass and over one million seeds, and a plant emerging in late June produced 350 g of biomass and over 730,000 seeds. Plant biomass and seed production were lower as irradiance levels were decreased to 40, 68, and 99% shade. Mortality was high for common waterhemp grown in 99% shade; however, surviving plants produced some viable seed. Common waterhemp plants grown under reduced irradiance had higher leaf area ratios and lower relative growth rates.

Yellow starthistle root growth was measured under field conditions using a minirhizotron camera system. Roots grew 1.0 to 1.3 cm d−1 and quickly reached the bottom of the 100-cm tube. When plants were grown under 80 and 92% shading, roots grew 45 and 64% slower, respectively, than when plants were grown unshaded. Using a neutron probe, we evaluated the effect of yellow starthistle density on soil moisture depletion to 180 cm in the soil profile. At the end of the growing season, we also measured aboveground biomass and seedhead production. Total plant dry weight and seedhead number at the lowest density (0.6 plants m−2) were 43 and 97% of the maximum values, respectively, suggesting that yellow starthistle is capable of reaching near-maximum yield at low densities. Moisture depletion was density dependent, and plants in low-density plots used more soil moisture from deep in the profile than from shallow soil (30 cm) early in the season. In contrast, yellow starthistle densities > 90 plants m−2 rapidly depleted moisture from all depths in the soil profile by preflowering growth stages. High yellow starthistle density expands the moisture depletion zone and leads to increased shallow moisture depletion. In high-density plots, soil moisture did not recharge, compared with bare-ground plots, after subnormal winter and spring precipitation. These results illustrate the importance of reducing yellow starthistle densities in grassland restoration efforts, where shallow soil moisture is critical to the establishment of seeded perennial grasses or annual forbs and where moderate to deep soil moisture is essential for the establishment and survival of transplanted shrubs and trees.

Field experiments were conducted in 1999 and 2000 in northern Greece to study interference between johnsongrass from seed or rhizomes and three corn hybrids (‘Dunia’, ‘Papea’, and ‘Costanza’). Fresh weight of each corn hybrid was lower with greater duration of johnsongrass interference, and it was lower for corn grown with johnsongrass from rhizomes than from seed. Corn-silage yield and corn-grain yield were 10 to 14% less than weed-free corn yield when grown with johnsongrass plants from rhizomes 4 wk after sowing (WAS) or from seed 6 WAS. Regression analysis indicated that a corn-grain yield loss of 10% was associated with an interference period of 2.6 to 2.7 WAS for johnsongrass from rhizomes and 4.3 to 5.1 WAS for johnsongrass from seed compared with weed-free corn. Corn-silage yield with season-long interference by johnsongrass from rhizomes or from seed was 83 and 62% lower, respectively, than was weed-free corn yield. Similarly, corn-grain yield with season-long interference by johnsongrass from rhizomes or from seed was 88 and 57% lower, respectively, than was the yield from weed-free corn. Corn-ear length of each hybrid was more affected by johnsongrass interference than was 1,000-seed weight. Johnsongrass plants from rhizomes emerged earlier, grew faster, and produced greater fresh weight than did plants from seed. However, fresh weight and stem number of johnsongrass plants from either seed or rhizomes were not affected by corn hybrids.

Field experiments were conducted in 1999 and 2000 to determine if (1) seed predation of redroot pigweed plants occurred in agricultural fields, (2) corn-cropping patterns could be manipulated to influence the quantity of weed seed predation, and (3) alterations in corn canopies affected the microenvironment, possibly influencing predator populations. Corn was planted in standard (75 cm) or narrow (37.5 cm) rows with corn population densities ranging from low to very high (2.5 to 10 plants m−2). The extent of seed predation occurring on terminal weed inflorescences in the treatments was evaluated. Predation levels of redroot pigweed were highly variable spatially and temporally. Coleophora lineapulvella Chambers (Lepidoptera: Coleophoridae) was the dominant predator of redroot pigweed seed. Seed predation was higher in 2000 than in 1999 (P < 0.05). On average, C. lineapulvella larvae attacked 11% of the inflorescences in 2000 and 3% of inflorescences in 1999. The proportion of damaged seeds per attacked inflorescence was as high as 93% in 2000 but only 42% in 1999. Row spacing and corn density did not affect levels of weed seed predation (P > 0.05). But canopies of closely spaced corn increased shading to redroot pigweed plants growing below the canopy, consequently decreasing total weed biomass and seed production.

Growth and reproductive potential with respect to season and photosynthetic gas exchange behavior under elevated (short-term) CO2 at varying temperature, relative humidity (RH), and irradiance levels were investigated in ragweed parthenium (also known as carrot grass and congress grass), a noxious weed in India. Lower values of biomass, relative growth rate, net assimilation rate, crop growth rate, leaf area duration, leaf area index, and numbers of flowers and seeds in winter compared with summer stands showed that ragweed parthenium is greatly suppressed by low temperatures during winter. This was due to constrained vegetative growth, seedling emergence, and seed to flower ratio. The species showed maximum photosynthetic response to temperature at 25 to 35 C, and the net photosynthetic rate was reduced considerably at a low temperature (7 C). These temperatures approximately corresponded to the normal temperatures experienced by summer (25–35 C) and winter (7 C) stands of ragweed parthenium. Elevated CO2 enhanced leaf net photosynthetic efficiency, maximum photosynthetic rate, and water use efficiency (WUE) but decreased the light compensation point for net photosynthesis, stomatal conductance, and transpiration rate. The interactive effects of elevated CO2 and temperature resulted in a decrease in light-limited and light-saturated net photosynthetic rates and WUE. The interactive effects also reduced an elevated CO2-induced decrease in light compensation point relative to elevated CO2 alone. Stomatal conductance was insensitive to photosynthetic photon flux but was greatly influenced by RH. Leaves of the species may show increasing rates of net photosynthesis with a rise in CO2 and temperature. However, excessive increase in transpiration with temperature, especially at 47 C (noon temperature during summer in the plains of northern India), appears to be disadvantageous for the leaves when conservation of water is of prime importance.

In western Canada, little is known about the seedbank ecology of volunteer canola. Therefore, integrated recommendations for the management of this weed are limited. In this study, we investigated the seedbank persistence and seedling recruitment of two spring canola genotype groups with different secondary seed dormancy potentials under contrasting tillage systems. The study was conducted at two locations with different soils in the Mixed Moist Grassland ecoregion of Saskatchewan. A single cohort seedbank was established in 1999 and was followed for 3 yr in successive wheat crops. In a separate laboratory study, the six canola genotypes examined were classified as those with high and those with medium potentials for the development of secondary seed dormancy (HD and MD, respectively). After one, two, and three winters, maximum persistence of 44, 1.4, and 0.2% of the original seedbank was observed among the treatments, respectively. In 2001, HD canola genotypes tended to exhibit 6- to 12-fold greater persistence than MD canola genotypes, indicating lower seedbank mortality in HD canola. Seedling recruitment of HD canola also was higher than MD canola when differences were observed between these genotype groups. Therefore, long-term seedbank persistence of canola can be reduced by growing genotypes with low inherent potential for the development of secondary seed dormancy. The proportion of persisting seeds tended to be higher under conventional tillage than under zero tillage because of lower seedbank mortality, but no clear distinction in seedbank persistence in terms of absolute time could be made between these two tillage systems. Volunteer canola seedling recruitment followed the pattern of a typical summer-annual weed, where seedling emergence was observed only during May and June.

Blackgrass is a common winter annual grass weed in autumn-sown crop rotations in Atlantic European countries. Control with a minimum amount of herbicides in integrated cropping systems would be facilitated by modeling the effect of cropping systems on its demography. To develop the submodel relating weed seed bank to emerged seedlings, laboratory experiments were conducted to analyze and quantify seed germination and shoot elongation. These processes were studied as a function of environmental conditions during seed production (nitrogen availability, water deficit, plant density, and crop) and of seed characteristics (seed weight, harvest date, storage length, and dry-stored/soil-buried). Nonlinear equations relating germination and shoot elongation to time calculated as cumulated degree-days were fitted to the observed germination and shoot elongation data. These were used to estimate parameters for germination proportion and rate, as well as final shoot length and elongation rate. Recently harvested seeds germinated best and fastest when they were collected in spring crops compared with winter crops. Germination proportion and rate increased with seed storage length, especially for seeds collected in winter crops. Midgermination time decreased with seed weight and water deficit during seed production; it increased with nitrogen amounts available to the mother plants. Maximum shoot length increased with seed weight and plant density during seed production. It decreased with nitrogen availability to mother plants and with storage length, irrespective of whether the seeds were dry-stored or buried in soil. Elongation rate was highest for early- and fast-germinating seeds. Time to midelongation increased with maximum shoot length. These germination and preemergence growth models can now be combined with other submodels to develop a blackgrass emergence model.

Laboratory and greenhouse studies were conducted to determine the effect of temperature, pH, water stress, and planting depth on crowfootgrass germination. When treated with constant temperature, crowfootgrass germinated over a range of 15 to 40 C, with the optimum germination occurring at 30 C (42%). Onset, rate, and total germination (94%) were greatest in an alternating 20 and 35 C temperature regime. Germination decreased as pH increased, with greatest germination occurring at pH 4 and 5. Germination was reduced when seed was subjected to water stress, and no germination occurred below −0.8 mPa. Emergence was similar when seed were placed on the soil surface or buried at depths of 0.5 or 1 cm. Germination decreased with burial depth, and no seed emerged from 10 cm. These data suggest that crowfootgrass may emerge later in the season with warmer temperatures and after a precipitation event, and may emerge rapidly. These attributes could contribute to poor control later in the season by soil-applied herbicides or allow crowfootgrass to emerge after final postemergence treatments are made.

An improved understanding of the effects of nitrogen (N) on crop–weed interactions is needed for the development of integrated weed management systems where responsible use of N fertilizers is considered. Field experiments conducted in 1999 and 2000 at two locations in eastern Nebraska quantify the effects of N and increasing duration of weed interference on corn growth and development. A naturally occurring population of weeds was allowed to compete with the corn crop for increasing lengths of time and at three rates of N application (0, 60, and 120 kg N ha−1). Weed interference and withholding applied N increased the time to 50% silking by an average of 3.9 and 2.9 d, respectively. Regardless of treatments, relative growth rates of corn leaf area and biomass were maximized between the V1 and V2 growth stages of corn and increased linearly with N rate but were affected to a lesser extent by weed presence. The improvement in early season corn growth with addition of N resulted in greater leaf area, biomass, and height, which improved the competitive ability of corn against weeds. Reductions in maximum corn leaf area and height due to weed interference usually began earlier and were more extensive at reduced rates of N. Partitioning of biomass to reproductive structures increased with N during reproductive stages, likely contributing to greater harvest indices at the end of the season. Results from this study indicate that the effects of N fertilization on early-season crop growth provided a competitive advantage for corn relative to weeds, thereby increasing the length of time that weeds could compete with a crop before removal was required, but further research is needed to identify mechanisms regarding improved crop tolerance to weeds.