Variation in interference relationships have been shown for a number of crop-weed associations and may have an important effect on the implementation of decision support systems for weed management. Multiyear field experiments were conducted at eight locations to determine the stability of corn-foxtail interference relationships across years and locations. Two coefficients (I and A) of a rectangular hyperbola equation were estimated for each data set using nonlinear regression procedures. The I and A coefficients represent percent corn yield loss as foxtail density approaches zero and maximum percent corn yield loss, respectively. The coefficient I was stable across years at two locations and varied across years at four locations. Maximum yield loss (A) varied between years at one location. Both coefficients varied among locations. Although 3 to 4 foxtail plants m−-1 row was a conservative estimate of the single-year economic threshold (Tc ) of foxtail density, variation in I and A resulted in a large variation in Tc . Therefore, the utility of using common coefficient estimates to predict future crop yield loss from foxtail interference between years or among locations within a region is limited.

Data on the spread of invasive weeds into arid western lands are used to evaluate the environmental and economic importance of controlling invasive weed infestations early. Variable rate and constant rate infestation expansion paths are estimated. The implications of variable vs. constant infestation growth rates for projecting both biophysical and economic effects are illustrated. The projections derived from both constant and variable growth rate expansion paths support the contention that it is expedient to control new infestations early.

The influence of ultraviolet-B (UV-B) radiation on seedling growth and leaf surface characteristics of important rangeland weeds Cynoglossum officinale L. (houndstongue), Centaurea diffusa Lam. (diffuse knapweed), and Tragopogon pratensis L. (meadow salsify) was investigated in a greenhouse by exposing seedlings to 0, 7, or 11 kJ m−2 d−1 of biologically effective UV-B radiation. UV-B radiation reduced leaf, stem, and root fresh weights, leaf area, and leaf: shoot ratio and increased shoot dry matter content, specific leaf weight, and leaf greenness of C. officinale seedlings. Leaf, stem, and root fresh weight, and leaf area of C. diffusa seedlings decreased, whereas shoot dry matter content, specific leaf weight, leaf: shoot ratio, and leaf greenness were unaffected. T. pratensis seedlings exposed to UV-B radiation had diminished leaf greenness, but no change was observed in leaf or stem fresh weight, leaf area, or leaf: shoot ratio. Uniseriate trichome abundance in C. officinale seedlings increased and trichome orientation changed in response to UV-B exposure. UV-B treatments decreased uniseriate trichome abundance in C. diffusa; glandular trichome abundance and the ratio of glandular: uniseriate trichomes were not affected. T. pratensis seedlings showed no change in the amount of epicuticular wax on the adaxial leaf surface in response to UV-B irradiation. Shoot dry weight was not influenced by removal of epicuticular wax prior to UV-B exposure. Results suggest that epicuticular wax is not an important factor in acclimation of T. pratensis seedlings to UV-B radiation. Susceptibility to UV-B–induced damage was greatest for C. officinale seedlings and least for T. pratensis seedlings. Modification of plant morphology or reduction in growth induced by enhanced UV-B radiation may influence competitive relationships between rangeland weeds and their associated forage species.

Herbicide resistance has been reported in several Amaranthus species throughout the U.S. Because evidence exists of interspecies hybridization in some species of this genus, this study was conducted to determine whether acetolactate synthase (ALS)-inhibiting herbicide resistance could be transferred from Amaranthus palmeri to Amaranthus rudis through interspecific crosses. Plants of each species were grown in a growth chamber, and controlled interspecies crosses were made between ALS-resistant and -susceptible plants. A total of 15 putative hybrid plants were produced from an estimated 10,000 cross-pollinated flowers. Analysis of restriction enzyme digests of the ALS gene in which a single base substitution confers resistance inferred that herbicide resistance had been transferred from a resistant male A. rudis to the hybrid plant. Offspring of hybrid plants, backcrossed to the susceptible parent, survived herbicide treatment, demonstrating that herbicide resistance was transferred between species. DNA analysis also was performed using the amplified fragment length polymorphism (AFLP) technique between parental and putative hybrid plants. Several unique bands were found only in the hybrid.

Some species of the genus Echinochloa are troublesome weeds in rice fields. The taxonomy of this genus leads to confusion in many cases due to its great morphological diversity. Because of the differential sensitivity to the herbicide quinclorac shown by Echinochloa spp., it was necessary to assess the botanical and molecular characterization of this weed. Echinochloa colonum, E. oryzoides, and E. oryzicola were very susceptible to quinclorac treatment; by contrast, E. crus-galli and E. hispidula showed some degree of natural tolerance. Physiological and molecular results agreed with the botanical classification of the genus Echinochloa in Flora Europea. The importance of these results is due to yield losses produced by the infestation of Echinochloa and the need for a strategy for Echinochloa management depending on the distribution of the Echinochloa species.

Field experiments were conducted in 1997 and in 1998 to determine the effects of density and time of emergence on Eriochloa villosa growth and seed production in Zea mays. E. villosa was transplanted at four densities (3, 9, 27, and 81 plants m−2) to simulate emergence at four Z. mays growth stages (VE, V2, V5, and V10). Compared to E. villosa plants that emerged with Z. mays plants, total above-ground E. villosa biomass at maturity of plants grown at 3 plants m−2 was reduced by 54, 97, and 99% when emergence was delayed until the V2, V5, and V10 stages of Z. mays, respectively, in 1997. In 1998, total aboveground E. villosa biomass at maturity was reduced by 70, 87, and 99% when emergence was delayed until the V2, V5, and V10 stages of Z. mays, respectively. E. villosa aboveground vegetative biomass per plant at maturity was linearly related to seed production per plant in each year. E. villosa seed production m−2 decreased nonlinearly as density decreased and time of emergence was delayed. Based on estimated model parameters, maximum seed production was 57,100 and 12,700 seeds m−2 in 1997 and 1998, respectively. Within time of emergence, E. villosa density did not affect seed mass per seed, however, seed mass of late-emerging cohorts was less than that of early-emerging cohorts. Time of weed emergence relative to the crop was a very important factor in determining biomass and seed production. Results suggest that late-emerging plants may not be very important to long-term management of E. villosa.

Near isolines of ‘Nugaines’ winter Triticum aestivum that differed in height were planted with and without Aegilops cylindrica to determine the effect of plant height on competition against A. cylindrica. The isolines had either reduced height gene Rht 1, Rht 2, Rht 1 plus Rht 2, or neither Rht genes and averaged 79, 77, 51, and 101 cm tall, respectively, when grown with or without competition from A. cylindrica. Plants with fewer reduced height genes had the faster rates of height and weight gain, which are important traits for enhanced competitiveness. When growing in competition with A. cylindrica, the shortest isoline allowed the greatest amount of A. cylindrica seed production but did not have the lowest T. aestivum yield. However, when compared to the A. cylindrica-free control, the shortest isoline had the greatest percent yield loss. The tallest isoline reduced A. cylindrica seed production the most, and T. aestivum yield reduction due to A. cylindrica on a percent basis was the least when averaged over 2 yr. When competing against A. cylindrica, the tallest isoline did not always have the largest yield and yield parameters, and the shortest isoline did not always have the smallest yield and yield parameters. There is a cost to the T. aestivum plant to produce extra stem biomass that may reduce yield potential of taller plants and reduce the advantage gained by being taller than the surrounding weeds.

Greenhouse studies in 1995 and 1996 examined the response of yellow and purple nutsedge to inoculation with increasing densities of southern root-knot nematodes. Yellow and purple nutsedge root and shoot weight, numbers of leaves and tubers produced, and tuber weight were unaffected across 17 nematode inoculum densities that ranged from 0 to 20,000 eggs per 15-cm pot, four times the maximum nematode density recorded under field conditions in New Mexico. Hence, yellow and purple nutsedge do not exhibit a classic threshold response to root-knot nematodes. Moreover, the results suggest that the relationship between these nematodes and perennial nutsedges is an example of a positive biological interaction. The relationship between root-knot nematodes and purple nutsedge appears to be one of commensalism, because while the nematodes reproduced effectively, purple nutsedge reproduction was not related to final nematode populations. The relationship between yellow nutsedge and root-knot nematodes appears to be a mutually beneficial one, because yellow nutsedge tuber number and weight and root weight increased as final nematode populations increased. However, while both nutsedges were unaffected in the absence of the association, root-knot nematodes cannot survive without a host plant.

Earlier studies on the source of signals controlling correlative inhibition of root buds (underground adventitious buds located on the lateral roots) in Euphorbia esula indicated that either growing meristems (apical or axillary buds) or fully expanded leaves could prevent root buds from breaking quiescence. An investigation of the production and transport requirements of the leaf-derived signal is described. As few as three leaves remaining on budless stems greatly reduced the growth of (but not the number of growing) root buds. Also, light and CO2 fixation were necessary for the leaf effects on root bud growth, but not necessary for correlative inhibition imposed by growing axillary buds. Treatment of plants with Ametryn induced root bud growth on budless plants but not on plants with intact axillary buds. The polar auxin transport inhibitor N-1-naphthylphthalamic acid prevented transmission or the signal from growing axillary buds, but it had only a minor effect on the transmission of the leaf-derived signal. Treatment of plants with gibberellic acid (GA) induced growth of root buds under otherwise noninducing conditions to some extent in all plants. However, the greatest effects of GA were on plants with intact leaves (meristemless/budless and meristemless). GA had no significant effect on root bud quiescence under conditions that induced root bud growth.

Information on the influence of temperature on growth of spring Triticum aestivum L. (wheat) relative to Kochia scoparia (L.) Schrad. (kochia) and Salsola iberica Sennen and Pau (Russian thistle) could lead to more efficient weed management practices. An experiment was conducted to determine the growth of spring T. aestivum, K. scoparia, and S. iberica at 15, 23, and 30 C in growth chambers. Fresh weight at 15 C was greater for spring T. aestivum than for K. scoparia and S. iberica. Conversely, fresh weights at 23 or 30 C were greater for K. scoparia and S. iberica than for spring T. aestivum. Growth response to temperature is consistent with the physiological characteristics for photosynthesis of spring T. aestivum, a C3 plant, and K. scoparia and S. iberica, which are C4 plants. The results indicate that cool temperatures that usually occur for early-seeded spring T. aestivum would give spring T. aestivum the competitive advantage over K. scoparia or S. iberica. Additionally, growing degree day (GDD) equations for height development of K. scoparia and S. iberica were determined and could be used to determine the optimum time for postemergence herbicide application.

Four field experiments were conducted to measure the effects of seven Ipomoea hederacea (L.) Jacq. densities on Gossypium hirsutum L. lint yield, stripper-harvest efficiency, and fiber properties. The seven densities were 0, 2, 4, 6, 8, 10, and 12 weeds 10 m−1 of row. Data were used to develop prediction models to compare with those previously constructed that used fewer experiments and fewer weed densities in this range. Gossypium hirsutum lint yield in kilograms per hectare and as a percentage of the weed-free control best fit a linear regression model. Gossypium hirsutum lint yield reductions for each increase of one weed 10 m−1 of row ranged from 30.7 to 36.2 kg ha−1 at Chickasha and from 35.4 to 36.4 kg ha−1 at Perkins. Lint yield reductions for each weed 10 m−1 of row ranged from 3.8 to 6.9% at Chickasha and from 3.9 to 6.0% at Perkins. All plots could be mechanically stripper harvested, except for the 12-weed density at Chickasha in 1994 and the 10- and 12-weed densities at Perkins in 1996. Harvest efficiencies were not significantly different in any experiment. The only fiber properties to display significant differences were micronaire and strength at Chickasha in 1994. Prediction models calculated herein were highly similar to those previously constructed.

In a semiarid Mediterranean site in central Spain, field experiments were conducted on a Calcic Haploxeralf (noncalcic brown soil), which had been managed with three crop rotations and two tillage systems (no-tillage and conventional tillage) since 1987. The crop rotations consisted of barley→vetch, barley→sunflower, and a barley monoculture. The study took place in two growing seasons (1992–1994) to assess the effects of management practices on the weed seedbank. During this period, spring weed control was not carried out in winter crops. In the no-tillage system, there was a significant increase in the number of seeds of different weed species: anacyclus, common purslane, corn poppy, knotted hedge-parsley, mouse-ear cress, spring whitlowgrass, tumble pigweed, venus-comb, and Veronica triphyllos. Conversely, the presence of prostrate knotweed and wild radish was highest in plots under conventional tillage. These results suggest large differences in the weed seedbank as a consequence of different soil conditions among tillage systems, but also the necessity of spring weed control when a no-tillage system is used. With regard to crop rotations, the number of seeds of knotted hedge-parsley, mouse-ear cress, and spring whitlowgrass was greater in the plots under the barley→vetch rotation. Common lambsquarters dominated in the plots under the barley→sunflower rotation, whereas venus-comb was the most frequent weed in the barley monoculture. Larger and more diverse weed populations developed in the barley→vetch rotation rather than in the barley→sunflower rotation or the barley monoculture.

Our objective was to identify traits in winter wheat important to competitiveness against jointed goatgrass, measured as increased wheat yields and reduced jointed goatgrass seed production. Jointed goatgrass is an important winter annual grass weed that cannot be controlled selectively in winter wheat. Seven cultivars of soft white winter wheat were grown with and without competition from jointed goatgrass over two growing seasons. Measurements of numerous traits of winter wheat and jointed goatgrass were recorded throughout each growing season. The data were analyzed using path analysis with latent variables to determine which traits most enhanced competitiveness. In a drier year, increased rate of height development was important in maintaining wheat yields when wheat was growing in competition with jointed goatgrass. Increased rate of height development also was an important trait in reducing jointed goatgrass seed production. In a wet year compared to a dry year, the number of wheat heads per plant, the rate of water use, and weight gain were positively correlated to maintaining winter wheat yields. Jointed goatgrass seed production in the wet year was reduced overall compared to the dry year, but from the cultivars tested, there were no traits identified that were critical in enhancing this loss of seed production. This study suggests that cultivars with greater height development rates will be more competitive when growing in fields infested with jointed goatgrass.

We studied the effect of tillage systems (no-till, chisel, and moldboard plow) and the presence or absence of Zea mays L. (corn) on soil temperature, moisture, and, subsequently, the emergence phenology and density of Chenopodium album L. (common lambsquarters) at two sites (Elora and Woodstock) from 1993 to 1995. The tillage system affected the phenology of C. album seedling emergence only in 1995. In that year, more days were required to reach 80% cumulative seedling emergence in no-till than in the chisel or moldboard plow treatments. The delay in obtaining 80% cumulative emergence was attributed to a dry period from days 159 to 177 at Elora and from days 155 and 176 at Woodstock. The presence or absence of Z. mays did not affect soil temperatures, soil moisture, or C. album seedling emergence phenologies. Chenopodium album seedling density was influenced by tillage and environmental conditions. Large variations in seedling density were attributed to environmental conditions. The presence or absence of Z. mays did not affect C. album seedling density.

The development of problematic weed populations is a concern in western Canadian fields where canola and pea are grown in a 4-yr sequence with spring cereal grains. Weed densities were examined at a site near Melfort, Saskatchewan, Canada, from 1994 to 1997 in seven zero-till managed crop rotations. Four rotations that included canola, pea, or flax in at least 3 of 4 yr (HBF: high broadleaf–crop frequency) were compared with three rotations that included broadleaf crops grown in 2 of 4 yr (LBF: low broadleaf–crop frequency). Spring wheat and barley were the cereal crops in rotation. Residual (postherbicide application) weed density for each weed species in a given year was summed across all phases for each rotation to reflect the overall weed infestation. Four annual broadleaf weed species were most abundant in 1996 and a second group of three species, having a variety of reproductive strategies, became progressively less abundant as the study progressed. The difference between the HBF and LBF rotations for the density of these species varied and was most prominent in years when environmental conditions were conducive for their growth. More frequent applications of ethafluralin, with its residual weed control, best explained why wild oat and catchweed bedstraw generally were less abundant in the HBF rotations. Of particular interest was the 8 plants m−-2 greater density of dandelion and perennial sowthistle in the HBF vs. LBF rotations in the last year of the study. It is thought that the limited herbicide options for the control of these species could present a future problem if they continued to develop in the HBF rotations. Differences in herbicide use between the HBF and LBF rotations were considered the primary factor controlling the rotation effects on weed density.

A greenhouse experiment compared the vegetative growth in pure cultures and mixtures of winter Triticum aestivum cultivar ‘Mit’ and Lolium multiflorum cultivar ‘Marshall’ in continuously watered controls and drought treatments. Control L. multiflorum in pure culture 14 wk after planting produced more leaf area, tillers, and dry weights of stem and root than control T. aestivum in pure culture. The greater seed size, larger initial leaf area, and height allowed T. aestivum to produce greater final leaf area and dry stem weight in control mixtures than L. multiflorum. Watering following drought shifted the relative performance of the two species in pure cultures and mixtures compared to controls. The ability of T. aestivum to maintain a greater leaf expansion rate during drought and a greater leaf area afterward than L, multiflorum allowed T. aestivum to attain greater growth than L. multiflorum in pure cultures exposed to temporary drought followed by watering. Conversely, drought and its relief enhanced the relative competitiveness of L. multiflorum compared to controls in mixtures with T. aestivum. During 4 wk of watering following the drought, L. multiflorum in mixtures grew vigorously and was similar to T. aestivum in all measures except in height and dry stem weight. Thus, L. multiflorum was similar in root growth with T. aestivum in control and drought mixtures and had its aboveground competitiveness amplified by the cycle of drought and watering in this study. There was no evidence of an allelopathic interaction between the two species.

Glycine max fields with Sesbania exaltata and Senna obtusifolia were selected to evaluate weed effects on insecticide spray droplet deposition within the crop canopy and soybean looper (Pseudoplusia includens) mortality. Weedy and weed-free treatments with and without thiodicarb insecticide application were included. Soybean looper populations in plots with S. exaltata and S. obtusiolia were approximately half those found in weed-free plots. Thiodicarb reduced soybean looper larval survival in the absence of weeds or when weeds were present in three of four experiments. Dye-sensitive cards placed in the top, middle, and bottom thirds of the G. max canopy showed droplet coverage to be highest on cards placed in the top of the canopy. Weeds reduced insecticide coverage in the top of the G. max canopy 26 to 43%. Thiodicarb deposition within the middle and bottom levels of the canopy was not reduced by weeds. Leaflets from the top, middle, and bottom thirds of the crop canopy were collected immediately after insecticide application to bioassay for insecticide deposition. For both years, mortality of soybean looper larvae on leaflets collected from the middle or top of weedy and weed-free G. max treated with thiodicarb was equivalent. Assuming that an economic return can be expected, soybean looper management may not need to be altered when moderate infestations of S. exaltata and S. obtusifolia are present in G. max in late season.

A spatially explicit metapopulation neighborhood model was extended to encompass the seed bank dynamics of the annual weed Alopecurus myosuroides Huds. (blackgrass), growing in U.K. winter wheat crops established after noninversion tillage. The effects of the initial spatial pattern of infestation, herbicide, and combine harvesting on seed bank densities and on expected yield losses of the infested crop were studied within a 634 m2 area. In the absence of herbicide, all seed bank populations were large and typical of values in the literature; those with patchy initial distributions spread quickly over the entire field. The effects of intraspecific competition ensured that even after 10 yr, the average seed bank density from three patterns with the same initial density consistently retained the same rank order: the initially uniform pattern consistently ranked largest; a patchily distributed moderate infestation ranked next; a more patchily distributed heavy infestation ranked least. Expected grain yield losses sometimes exceeded 40%. With the introduction of a herbicide, seed densities declined exponentially, but the rank order with regard to spatial pattern remained. Relatively dense patches occasionally persisted for longer than 10 yr. The economic threshold for treatment was achieved within 2 and 5 yr, depending on the infestation pattern, but the model predicted that it would take many more than 10 yr before infestations could be completely eliminated. The effects of harvesting by combine were modeled. In all cases, the speed of the spread of infestation along a row appeared to be largely due to the combine rather than to natural dispersal or other cultivation practices. The proportion of seeds removed by the combine was an important determinant of the number of years required to drive the population below the economic threshold and of the probability of long-term eradication of the weed metapopulation.

Laboratory studies were conducted to describe germination and seedling elongation of Ambrosia artemisiifolia L. (common ragweed) seed. The germination process was tested for the interaction of temperature and water potential across eight thermo-periods (7.5, 12.5, 17.5, 22.5, 27.5, 32.5, 37.5, and 42.5 C) and 12 water potentials (0, −0.03, −0.06, −0.1, −0.2, −0.4, −0.6, −0.9, −1.2, −1.5, −1.8, and −2.1 mPa). The rate of seedling shoot and radicle elongation was described as a function of temperature and tested for eight day: night temperature treatments (10: 5, 15 : 10, 20 : 15, 25 : 20, 30 : 25, 35 : 30, 40 : 35, and 45 : 40 C). The rate of germination was mathematically modeled by a Weibull function. Probit analysis was used to determine the cardinal temperatures (base, optimum, and maximum) and base water potential (αb ). The base temperature (T b), optimum temperature (T opt), maximum temperature (T max), and αb for A. artemisiifolia germination were estimated as 3.6, 30.9, and 40 C and −0.8 mPa, respectively. The rates of shoot and radicle elongation were described by regression models. The T b , T opt, and T max for shoot and radicle elongation were estimated as 7.7 and 5.1, 29.5 and 31.4, and 43.0 and 44.3 C, respectively. A mathematical model describing the process of A. artemisiifolia seed germination in terms of hydrothermal time (θHT) was derived. The θHT model described the phenology of A. artemisiifolia seed germination using a single curve generated from the relationship of temperature and water potential. This model can help in predicting germination and emergence of A. artemisiifolia under field conditions.

Field studies were conducted near Tifton, GA, from 1995 to 1997 to measure Cyperus esculentus (yellow nutsedge) interference with ‘Fancipack’ Cucumis sativus (cucumber) using a response prediction experiment with a natural infestation of C. esculentus. Cucumis sativus was direct-seeded each year. Plots (1.8 by 1.8 m) were established immediately after crop emergence. Cyperus esculentus plants were counted 2 wk after crop emergence in each plot, at which time four weed-free plots were randomly established. C. esculentus densities ranged from 0 to 955 plants m−2. Total yield, plant biomass, and C. sativus stand were regressed against C. esculentus density and biomass. Regression analysis showed a 5% reduction in C. sativus yield with a C esculentus infestation of approximately 15 plants m−2. A uniform C. sativus stand maximized its competitive ability and minimized interference from C. esculentus. Cyperus esculentus was more competitive and reduced C. sativus yields when C. sativus stands were low and nonuniform.