An intensive survey of two farmer-managed corn and soybean fields in eastern Nebraska was conducted to investigate parametric sequential sampling of weed seedling populations using a multistage procedure to estimate k, of the negative binomial distribution. k is a nonspatial aggregation parameter related to the variance at a given mean value. Mean weed seedling density ranged from 0.18 to 3.11 plants 0.38 m−2 (linear meter of crop row) based on 806 sampling locations. The average value of k, derived from 200 multistage estimation procedures, ranged from 0.17 to 0.32. A sequential sampling plan was developed with the goal of estimating the mean with a coefficient of variation (CV) of 10, 20, 30, and 40% of the sample mean. A sampling plan was also constructed to estimate the mean within a specified distance H of the true mean (H(x̄)= 0.10, 0.50 and 1.0 plants 0.38 m−2) with 80, 85, and 90% confidence. Estimating mean weed seedling density within a specified CV of the true mean CV(x̄) using parametric sequential sampling techniques was superior to estimating the mean within a specified distance (H(x̄)) of the true mean when considering the frequency of sampling and probability of error, especially at intermediate k values. At a k: value of 0.32 and 0.25, the difference between the actual CV(x̄) obtained from sampling and the CV(x̄) specified by the sampler was minimal. However, the accuracy of weed seedling density estimates was reduced with decreasing k values below 0.25, especially as the specified CV(x̄) increased.

Seasonal changes in germination behaviour of meadow salsify seeds buried at different depths, and survivorship of seed and plant populations were investigated. Freshly matured seeds were mixed with sterilized soil, placed into nylon mesh bags, and buried at three soil depths in a meadow salsify-infested rangeland. Two germination peaks were observed: one in the fall, 2 to 3 mo after burial, and a second the following spring. Burial depth affected seed dormancy. The number of seeds with enforced dormancy, and innate or induced dormancy, increased with burial depth in the 1989 cohort, and the number with enforced dormancy increased in the 1990 cohort. Burial depth also affected seed survivorship. The survivorship curves for 1989 and 1990 seed cohorts were not different as indicated by the Logrank test. The seed population was nearly depleted after 9 to 10 mo of burial; less than 3 % of the buried seeds remained viable after 13 mo. About 50% of the plants monitored in a natural meadow salsify population in 1990 died before winter, and an additional 35% died during winter. The mortality of the remaining plants during the second growing season was very low. Only 12% of the 1990 cohort remained in October 1991. Mortality of meadow salsify plants was density-independent.

Weed seed return and seedbank composition, with particular reference to common lambsquarters, were studied in four tillage systems established on a site near Fingal, Ontario. The tillage treatments were moldboard plow, chisel plow, ridge-till, and no-till. The cropping system was a cornsoybean rotation. Tillage effects on weed population composition were assessed after all weed control measures had been implemented. More than 60% of the weed seedbank was concentrated in the upper 5 cm of soil in chisel plow and no-till. The seedbank of the moldboard plow system was more uniformly distributed over depth and larger than the other systems. Common lambsquarters comprised more than 50% of the seedbank in all systems except ridge-till, but only dominated the aboveground weed population in chisel plow. Seedbank populations of common lambsquarters with moldboard plowing were greater than those with ridge-till and no-till, and chisel plow seedbank populations were greater than those in ridge-till. Chisel and moldboard plow systems generally had higher aboveground plant populations of common lambsquarters than the other two systems. Seed production per plant by common lambsquarters was equivalent among the four systems, but estimated seed production per unit area was higher in moldboard plow and chisel plow systems than in the other systems. Populations of common lambsquarters and similar species may produce more seeds and persist in moldboard plow and chisel plow systems; these weeds may produce fewer seeds per unit area and be easier to manage in no-till and ridge-till systems.

Ivyleaf morningglory weed-free period, weedy period, and density studies were conducted in 1993 and 1994 to determine the effects of soil-applied imazaquin and chlorimuron plus metribuzin on ivyleaf morningglory interference in soybean. A weed-free period of 4 wk was needed to maintain soybean yield equivalent to weed-free season long when herbicides were not used, but treatment with either chlorimuron plus metribuzin or imazaquin maintained yield comparable to weed-free at all intervals. Ivyleaf morningglory biomass was reduced by both herbicides across all weed-free periods. The period nontreated ivyleaf morningglory plants were tolerated without yield loss was only 2 wk, but this interval was extended to 8 wk after treatment with chlorimuron plus metribuzin or imazaquin. Without herbicides, soybean yield was reduced by four ivyleaf morningglory per m of row, but the highest population evaluated in this study (eight per m of row) did not reduce soybean yield after treatment with chlorimuron plus metribuzin or imazaquin. Ivyleaf morningglory biomass was reduced at all densities after treatment with either herbicide.

Photographic techniques and the point-frame transect method were used to study areal growth of kikuyugrass grown in the field without competition in 1990 and 1991. The effect of cutting height on plant extension was also examined in 1991. There were no differences in sward area between eight selections planted in 1990, except on the first measurement date, but selections differed in height and number of primary and secondary stolons. Expansion was irregular in all but one selection, and the patch shape of all irregular selections was defined by the direction and number of the earliest buds to sprout from the stolons. Only two of the eight selections were planted in 1991. Cutting height affected extension from the plot center, linear plant frequency, and plant biomass. The lowest height of cut, 2.5 cm, had the least extension and frequency, the higher cut, 5 cm, was intermediate, and the unmowed plots had the highest extension and frequency. There were differences between selections in number of nodes per unit area, which resulted in differences in response to mowing. Results from a greenhouse clipping experiment showed that although root weight of stolon sections used as propagules was reduced by defoliation, starch content was not affected. The spreading ability of kikuyugrass appears to be based on its ability to branch from its stolons presumably due to reallocation of resources. Based on these results, close mowing could slow the process of kikuyugrass invasion into other turf species but is inadequate to control it.

Seeds (achenes) of fringed sage were collected in central Saskatchewan in 1987, 1990, and 1991, and studies were conducted to determine the effects of imbibition time before brief or prolonged exposure to red (R) and far-red (FR) light on germination. A majority of the seeds required light for germination, but a substantial proportion germinated in darkness. Phytochrome controlled germination, and its activity was influenced by the duration of imbibition and by seed collections. At least 8 h of imbibition were required before phytochrome was activated by R light. The stimulation of germination could be reversed by FR light, and this suppression subsequently could be reversed by exposure to R light. Variable light requirements for germination of fringed sage can be met following large- or small-scale disturbances or in relatively undisturbed conditions.

Narrower corn row widths, higher crop densities, and interrow cultivation may be used as part of integrated weed management (IWM). During a three-year study, we tested whether these factors affected corn growth, development and grain yield at final harvest, and weed biomass when weeds were late-emerging (after the three-leaf stage of corn). Increasing corn density from 7 to 10 plants m−2 or decreasing row width from 75 to 50 cm significantly increased corn leaf area index (LAI), and reduced photosynthetic photon flux density (PPFD) available for a mixture of weed species located below the corn canopy. Narrower rows and higher corn density significantly reduced biomass of late-emerging weeds. Corn yields increased significantly (10 to 15%) only when narrower rows were used. Intraspecific competition between corn plants in the higher density significantly reduced early corn growth and offset any gain in yield from reduced weed competition. In comparison to plots where late-emerging weeds grew uncontrolled, interrow cultivation did not decrease biomass of late-emerging weeds, hence did not increase corn yield. We recommend using narrower row widths to reduce weed competition and increase corn yield. Increased corn densities had no net yield benefit. Early-season weed management is crucial as the later-emerging weeds were less important in terms of their effect on yield.

Field experiments were established in Greece from 1990 to 1993 to study the effect of littleseed and short-spiked canarygrass density on wheat and barley yield, as well as the influence of littleseed canarygrass removal time on wheat yield. Competitive ability of both canarygrass species in wheat was similar, though littleseed canarygrass showed faster growth rate and formed more panicles than short-spiked canarygrass. Grain yield of wheat was not significantly affected by the presence of 76 plants m−2 of either canarygrass species, but it was reduced by 36 to 39% by the presence of 304 canarygrass plants m−2. None of the canarygrass species at any density had any effect on grain yield of wheat when cold and dry conditions prevailed during the early stages of growth. Grain yield of barley was not affected by any density of either canarygrass species. Growth and consequently panicle number of both canarygrass species were severely reduced by the interference of barley. The littleseed canarygrass removal time study indicated that emergence of this weed was completed by the middle of February. Grain yield of wheat grown with 150 littleseed canarygrass plants m−2 was not affected even when these plants were removed early in April. However, the presence of these canarygrass plants until harvest reduced grain yield of wheat by 23 to 28%.

Greenhouse experiments were conducted to assess the storage of nonstructural carbohydrates in the stems and roots of honey mesquite and their mobilization and utilization following defoliation by hand or clopyralid. All leaves were removed by hand from one group of plants while leaving stems intact. Another group received a foliar spray of clopyralid at a sublethal acid equivalent rate of 0.07 kg ha−1. Samplings at treatment on day 0 and at 14, 28, 42, and 58 d after treatment measured leaf area; dry weight of leaves, stems, and roots; and grams and percent total nonstructural carbohydrates in stems and roots. Controls accumulated dry weight and grams total nonstructural carbohydrates in stems and roots, while maintaining a constant percent total nonstructural carbohydrates throughout the experiment. Percent and grams total nonstructural carbohydrates in stems and roots of hand defoliated plants decreased by about one-half from day 0 to 14 as new leaf growth occurred, but increased and returned to pretreatment levels on day 28. A similiar, though slower, pattern of decline and recovery of total nonstructural carbohydrates in stems and roots occurred in response to defoliation by clopyralid. These results showed that the nonstructural carbohydrates in stems and roots of honey mesquite were important sources of carbohydrates to support new leaf regrowth following defoliation by hand or herbicide.

The effect of Canada thistle on major yield components of spring wheat was characterized using path coefficient analysis, which provides insight concerning which yield components are most sensitive to Canada thistle competition. Increasing Canada thistle density decreased wheat stand in each of three years. Canada thistle also reduced spikes per plant and seed per spike to varying extents depending on year, but Canada thistle had comparatively little effect on wheat seed weight per 1000 seed. In the path coefficient model proposed, Canada thistle was assumed to reduce yield by directly reducing wheat yield components and through them, indirectly reducing yield. Path coefficient correlation analysis showed that Canada thistle reduced spring wheat yield chiefly by indirect effects of decreasing wheat density, the earliest formed yield component. Canada thistle reduced wheat density which, in turn, reduced wheat yield. These data suggest that Canada thistle must be controlled either before or shortly after wheat emergence if detrimental effects on wheat yield are to be minimized.

The growth and seed return of auxin herbicide resistant (R) wild mustard was compared to that of a susceptible (S) biotype in wheat in the field. In the absence of herbicide, the S biotype accumulated shoot dry matter more quickly than the R biotype throughout most of the growing season. However, in only one of the two years did the S biotype set substantially more seed than the R biotype (3120 versus 2520 seeds plant−1). The recommended dosage of 2,4-D for wild mustard control (420 g ai ha−1) killed all S plants in both years of the study, and severely inhibited growth and seed return of R plants. Shoot dry matter accumulation and seed return of treated R plants were reduced 75 to 90% compared to the untreated control. However, at a density of 20 plants m−2 R seed return was still very high; 9000 and 5700 seeds m−2 in 1992 and 1993, respectively. The recommended dosage of dicamba (300 g ha−1) did not inhibit the growth and seed return of either S or R wild mustard to the same extent as 2,4-D. Dicamba at 300 g ha−1 reduced S shoot dry matter and seed return 80 to 90%, while R shoot dry matter and seed return was reduced 60 to 65%. The results of this study indicate a very high selection pressure for R wild mustard at recommended dosages of 2,4-D. Despite a high selection pressure, and considering the long history of phenoxy herbicide usage on the Prairies, the relatively rare occurrence of phenoxy herbicide resistant weeds implies that the frequency of resistant individuals is very low. From a mathematical model it was determined that the frequency of R wild mustard in an unselected population may be in the order of 10−30.

Competitive relationships between johnsongrass, smooth pigweed, and soybean were examined in 1986 and 1987, and the adequacy of an additive response model (ARM) and product response model (PRM) in predicting yield reductions due to multispecies weed populations was assessed. A severe drought reduced soybean yields statewide in 1986, whereas ideal conditions for soybean production were experienced in 1987. Averaged over monospecific weed densities of 1, 2, 4, and 8 plants 4.6 m−1 of row, smooth pigweed intercepted 2.5 and 1.8 times more light than johnsongrass in 1986 and 1987, respectively. In multispecies populations having either 4 or 8 smooth pigweed plants 4.6 m−1 of row, light interception by johnsongrass was negligible. Averaged over monospecific weed densities, smooth pigweed produced 5160 and 1760 kg ha−1, while johnsongrass produced 1530 and 450 kg ha−1 dry weight in 1986 and 1987, respectively. In multispecies populations, smooth pigweed contributed more than 80% of the total weed biomass. As monospecific johnsongrass density increased, reductions in soybean seed yield were linear, whereas exponential response patterns adequately characterized reductions in soybean seed yield due to smooth pigweed interference. Based on calibrated monospecific responses, the ARM and PRM generally projected higher soybean seed yield reductions for multispecies weed populations than were observed. When crop production conditions were favorable and competitive effects of weeds were low, both models adequately predicted soybean seed yield reductions. When dry conditions unfavorable for crop production existed, the PRM best accommodated the interactive effects between johnsongrass, smooth pigweed, and soybean.

LAB 145 138 (LAB) was evaluated as a safener to improve sweet corn tolerance to nicosulfuron applied POST alone or with terbufos applied in the planting furrow or bentazon applied POST. To ensure enhanced injury for experimental purposes, nicosulfuron was applied at twice the registered rate alone or mixed with bentazon at the six- to seven-leaf growth stage of corn previously treated with the highest labeled rate of terbufos 15 G formulation. LAB applied as a seed treatment (ST) or POST at the two- to three-, four- to five-, or six- to seven-leaf growth stages reduced height reduction and yield loss from nicosulfuron applied POST in combination with terbufos applied in-furrow. LAB applied POST at the four- to five-leaf growth stage was most effective in preventing injury from this treatment, with yield reduced only 8% compared with 54% from the nicosulfuron and terbufos treatment. LAB applied POST at the eight- to nine-leaf growth stage did not alleviate injury. With the nicosulfuron, terbufos, and bentazon combination, LAB applied POST at the three- to four- or six- to seven-leaf growth stages decreased height reduction and yield loss caused by this combination, with LAB at the three- to four-leaf growth stage being most effective.

Experiments were conducted to determine the response of purple nutsedge tuber sprouting to diurnally alternating temperature. These experiments compared the response to alternating and constant temperatures and determined the effect of the amplitude of alternation and time of exposure to the maximum temperature. Tuber sprouting was more rapid and complete with alternating temperatures than with constant temperatures. Increasing temperature fluctuation from 0 to 6 C for 12 h daily linearly increased total tuber sprouting. As little as 30 min exposure to high temperature per day provided nearly the same level of sprouting as a 12 h alternating temperature cycle. This phenomenon should be considered when conducting studies to describe tuber temperature responses or when predicting tuber sprouting and emergence.

Experiments were conducted from 1990 to 1994 to investigate the density-dependent effects of rhizome and seedling johnsongrass on the magnitude of full-season interference in field corn. Corn grain yield was decreased as rhizome johnsongrass density increased. A maximum density of 3 rhizome johnsongrass plants 9.8 m−1 of row was considered critical to avoid yield losses above 5% of full-season weed-free corn. Interference of johnsongrass plants originating from seed and growing at densities up to 128 plants 9.8 m−1 of row did not affect grain yield. Seedling johnsongrass plants growing at a constant density of 9.8 plants m−1 of row did not affect yield from corn planted at five densities.

Laboratory studies were conducted to determine the effect of various factors on germination of tropical soda apple. Tropical soda apple seed were found to be moderately photoblastic with 30% germination occurring in the dark. Germination increased in response to green (545 nm) and red light (650 nm) to 75 and 66%, respectively, indicating phytochrome regulation. No germination occurred in response to blue (450 nm) and far-red light (750 nm). Germination increased from 4 to 64% between 10 to 30 C, but no germination was found at 5 and 40 C. Maximum germination occurred at 30 C. A second degree polynomial (Y= −48.7 + 7.9X – 0.2X2, R2 = 0.76) best fit the data. Seed germinated over a broad range of pH (2 to 14) with the highest germination occurring at pH 8 and no germination occurring at pH 2 and 14 (Y = −63.3 + 30.5X – 1.9X2, R2 = 0.95). Seed germination declined as osmotic potential increased from 0 to −1.0 MPa (Y = 76.7 – 12.9X + 5.2X2, R2 = 0.87). Seedling emergence was maximum from depth of 3 to 6 cm, but no seedling emergence occurred when seed were planted below 12 cm (Y = 10.2 + 17.8X – 1.6X2, R2 = 0.93). Optimum germination occurred at a depth of 5.6 cm. Mechanical and sulfuric acid scarification increased the rate of germination but not the overall percentage. Tap water or hot water increased the rate of germination 26% and KNO3, GA3, or ethephon increased it 53%. Tropical soda apple seed germinated in response to variable environmental and edaphic conditions which could allow establishment in diverse ecosystems.

Field studies in conventional and vertical-tilled soybeans were conducted in 1992/93 and 1993/94 at four locations in Argentina to compare the efficacy of imazethapyr and haloxyfop for johnsongrass control. Control was evaluated by measuring frequency of johnsongrass-infested areas at the beginning of the 1992/93 and 1993/94 growing seasons. Relative johnsongrass leaf area and number of rhizome phytomers within infested areas also was evaluated. Herbicide application reduced frequency of infested areas in the 1992/93 growing season. Frequency of infested areas increased in check plots. No differences were found among herbicide treatments, suggesting that the effect of imazethapyr was as high as that observed for specific graminicides. Reduction in frequency of infested areas caused by herbicide treatments was accompanied to some extent by a decrease in intensity of infestation within infested patches assessed by measuring johnsongrass relative leaf area. Phytomer population levels, on the other hand, were reduced only after two consecutive years of herbicide application. No differences were found among herbicide treatments in terms of impact on both measures of weed abundance. Johnsongrass responded similarly to all treatments regardless of tillage system.

This study documented an undescribed aspect of medusahead germination behavior. Fibrous hairs were consistently observed growing from the coleorhizal surface of germinating medusahead seeds. Three medusahead accessions were investigated to determine temperature effects on coleorhizal hair growth and germination. Coleorhizal hair emergence at 8 C was first noted at 72 h. By 24 h at 18 C, 15 to 74% of seeds had hairs, and at 28 C, hairs were visible on 75 to 94% of seeds. Germination at 18 and 28 C was 80% at 96 h; germination at 8 C did not exceed 80% until 168 h. Less than 4% of germinated seeds were without coleorhizal hairs. Appearance of coleorhizal hairs confirmed that germinating medusahead embryos were active at least 72 h before germination was complete. These data affirm coleorhizal hairs are a normal part of medusahead germination biology. Coleorhizal hairs probably aid early phases of medusahead seedling establishment.

Weed control decision rules were derived for the application of postemergence herbicides to control pigweed species in soybean. Field experiments were conducted at two locations in 1992 and 1993 to evaluate soybean-pigweed interference. A damage function was determined that related yield loss to time of pigweed emergence, density, and soybean weed-free yield. A control function described pigweed species response to variable doses of imazethapyr and thifensulfuron. The integration of these two functions formed the basis of an economic model used to derive two weed control decision rules, the biologist's “threshold weed density” and the economist's “optimal dose.” Time of weed emergence had a more significant role than weed density in the economic model. Later-emerging pigweed caused less yield loss and therefore, decision rules lead to overuse of herbicides if emergence time is not considered. The selected herbicide dose influenced the outcome of the control function. Depending on the desired level of weed control, a herbicide could be chosen to either eradicate the escaped weed species (label or biologically-effective doses) or reduce the growth of the weed species and thereby offset interference (optimal dose). The development of a biologically-effective dose by weed species matrix was recommended. Decision rules should not be utilized as an exclusive weed management strategy but rather as a component of an integrated weed management program.

Wild poinsettia is a serious weed in several crops, including peanut, grown in the southeastern United States. A study was conducted over 3 yr at Jay, FL, to characterize the growth and development of wild poinsettia grown from seed collected at Plains, GA; Marianna, FL; and Baton Rouge, LA. Seedlings from each selection were transplanted to the field and were grown either alone or in competition with peanut. Observations made throughout the growing season indicated that the Louisiana selection flowered later, grew to a larger size, produced more leaf area and biomass, and caused greater light attenuation and peanut yield reduction than the other two selections. The Georgia selection produced the smallest plants, least leaf area and biomass, and was least competitive with peanut. The Florida selection was intermediate for these parameters. Wild poinsettia dry biomass production was reduced by 78 to 83% when grown with peanut compared with monoculture wild poinsettia.