Ethylene gas (C2 H4) was found to stimulate the germination of witchweed [Striga asiatica (L.) O. Kuntze] seeds. Ethylene diffuses greater than 120 cm horizontally from point of injection and more than 90 cm below the soil surface. Rates of 0.42 kg/ha induced germination of witchweed seeds in sandy soil; but 1.1 kg/ha is required on a heavy clay soil. Witchweed seeds respond to ethylene after a period of preconditioning that is necessary to break dormancy. Under field conditions in the Carolinas, maximum response of seeds to ethylene occurs between late April and late July. Field studies indicate a 90% reduction in viable witchweed seeds occurs where a single treatment with ethylene has been applied. The use of ethylene appears now to be a major contributor toward the eventual eradication of witchweed from the United States.

In areas near oil refining centers, as in most of California, petroleum oils are widely used in agriculture. Reference is not made here to the use of petroleum to power and lubricate tractors and other farm machinery, but to its use as an agricultural chemical. There are three typical uses of oils: (1) insecticidal spray oils, (2) various oils as selective and general herbicides, and (3) solvent type oils as carriers for oil soluble agricultural chemicals. In California alone, annually from five to fifteen million gallons of petroleum oil are used for the control of scale, mites, and mealy bug (23). An estimated 10 million gallons of cracked gas oils are used for general weed control on canal banks, road sides, barnyards, and in orchards practicing non-tillage. Much solvent type oil is also used for selective weed control in umbelliferous crops, and for pre-emergence weed control in lettuce, beets, and onions. One large operator alone is using one million gallons annually. Aromatic fractions and fortified heavier oils are used as drying agents on flax, clover, and alfalfa seed crops and as cotton defoliants. DDT and other insecticides are commonly applied as dilute oil emulsions.

Barnyardgrass [Echinochloa crusgalli (L.) Beauv.], duck-salad [Heteranthera limosa (Sw.) Willd.], hemp sesbania [Sesbania exaltata (Raf.) Cory], and northern jointvetch [Aeschynomene virginica (L.) B.S.P.] significantly reduced rice [Oryza sativa L.] yields. Damage to rice increased as populations of barnyardgrass, hemp sesbania, and northern jointvetch increased. Barnyardgrass and duck-salad lowered yields during early season, but hemp sesbania and northern jointvetch decreased yields in late season.

Glyphosate behavior was examined in Italian ryegrass plants from Chile that were sensitive (S) and resistant (R) to this herbicide. In order to explain the resistance to glyphosate, contact angles, spray retention, foliar uptake, herbicide translocation, and target enzyme activity were studied. Contact angles of glyphosate solutions at a field concentration were 40° to 45° on the abaxial surface of R leaves as compared to 70° on S. Glyphosate spray retention by R plants was 35% lower than by S plants. Glyphosate uptake by the abaxial leaf surface of R plants was about 40% lower than that of S plants. In addition, in the R plants more glyphosate migrated to the tip of the treated leaves. The target enzyme in R and S plants was sensitive to the herbicide. Based on these and previous results, it is concluded that resistance in this Italian ryegrass biotype results from lower spray retention, lower foliar uptake from the abaxial leaf surface, and altered translocation pattern. The decreases in spray retention and foliar uptake constitute new mechanisms of glyphosate resistance.

Weed species composition and density were recorded in three identical field experiments established 26 to 30 yr ago in southern Sweden. Each experiment compared three 6-yr crop rotations and four rates of nitrogen application. The rotations differed by having (1) a 2-yr rotational grassland, (2) a 2-yr mixed rotational grassland (legume/grass), or (3) spring wheat followed by fallow. Other crops in the rotations were winter turnip rape, winter wheat, spring oats, and spring barley. Using multivariate analyses, the relative importance of site, crop, crop rotation, and nitrogen application rate on the weed flora was determined. The greatest difference was found between sites, and the second most important factor was crop species. Nitrogen application rate weakly influenced the weed flora, while differences between crop rotations were hardly detectable.

The volatilization, photolysis, microbial degradation, and field persistence of imazethapyr were studied using formulated and 14C-labeled imazethapyr. Volatilization losses from soil were less than 2%. Photodecomposition losses of up to 8% occurred from soil and up to 52% from a glass slide with no soil. Significantly greater photodecomposition occurred with chain-labeled than ring-labeled 14C-imazethapyr. The amount of 14CO2 evolution from soil treated with either ring- or chain-labeled 14C-imazethapyr was not significantly different. The total 14CO2 evolved from the soils ranged from 2.4 to 3.6% of the total 14C-imazethapyr applied to the soil. However, degradation of imazethapyr from the same soils, as determined by high-pressure liquid chromatography, indicated that 62 to 82% of the applied 14C-imazethapyr had been degraded. The degradation rate increased as soil moisture was increased from −100 to −33 kPa. Imazethapyr was more persistent in soil with the higher clay and organic matter content.

Cotton (Gossypium hirsutum L.) grown at two locations in Alabama from 1966 to 1968 produced maximum yields when kept free of annual weeds for approximately 8 weeks after emergence. Weed control for longer periods failed to increase yields. When initial weed control measures were delayed until 6 to 7 weeks after emergence at one location and 4 to 5 weeks at the other location, cotton was not injured. Prolonged weed competition past the critical periods usually resulted in drastic cotton yield reductions. Plant height and stem diameter of cotton were reduced by some of the weed competition treatments.

Several inorganic salts added to the spray solution reduced the herbicidal activity of glyphosate [N-(phosphonomethyl)glycine]. Of the salts tested, iron and aluminum reduced herbicidal activity most. Reductions by calcium and zinc were moderately severe; by magnesium, moderate; by potassium and sodium, none. The reduction in phytotoxicity seems to be caused by cations, but factors other than valence also may be involved.

Reduction in phytotoxicity was related to salt concentration and spray volume. Glyphosate at 0.84 kg/ha in 93 to 374 L/ha distilled water killed test plants. Tap water was an equally effective carrier at 93 and 187 L/ha volumes, but was significantly less effective at the 374 L/ha volume. Activity decreased as the molar concentration of CaCl2 increased to 0.01. Herbicidal activity was eliminated at 0.01 M and 374 L/ha; however, concentrations of CaCl2 exceeding 0.04 M reversed the trend and reacted similarly to a 0.005 M solution, which approximates the minimum hardness of most ground waters in Kansas.

Weed suppression in green pea was evaluated when green pea was planted after a fall planting of rapeseed, white mustard, rye, or wheat had been incorporated into the soil in spring. Tests were conducted at Mount Vernon, WA, in 1994 and 1995. Weed suppression in peas varied between different green manure crops. One month after planting, the highest weed population was in green pea following wheat, whereas the lowest was in green pea following rapeseed. Rye and white mustard suppressed early weeds relative to wheat by 25 and 30%, respectively. However, at harvest, weed density was similar in green pea planted after all green manure crops. Weed suppression improved when cultivation or metribuzin at 0.14 kg ha−1 was used in combination with white mustard, rapeseed, or rye. Pea population was not affected by green manure crops, except for rapeseed, which reduced pea population. In greenhouse experiments, white mustard added to the soil at 20 g per 400 g air dry soil reduced emergence of shepherd's-purse, kochia, and green foxtail by 97, 54, and 49%, respectively. Rapeseed suppressed emergence of shepherd's-purse, kochia, and green foxtail by 76, 25, and 25%, respectively.

The influence of common ragweed (Ambrosia artemisiifolia L.) interference on soybean [Glycine max (L.) Merr. ‘Ransom’] yield was studied in the field utilizing naturally occurring weed populations. The damage-threshold population for a full-season, in-row common ragweed infestation was four weeds/10 m of row, which resulted in an 8% yield loss. Soybeans kept weed-free for 2 weeks or longer after emergence in a dry year produced normal yields, but 4 weeks of weed-free maintenance was required when adequate moisture was available early in the growing season. Soybean yield was not reduced by a natural population of common ragweed if the period of interference was limited to 6 weeks or less after crop emergence. By 8 weeks after emergence, common ragweed height averaged 25 cm taller than soybeans, and the weed canopy intercepted 24% of the photosynthetically active radiation.

Field experiments were conducted in central Iowa to determine the growth of common waterhemp emerging after postemergence herbicide applications in soybean. Common waterhemp survival declined as emergence was delayed in relation to soybean. Ninety percent of plants emerging at approximately the same time as soybean survived, whereas only 13% of plants emerging approximately 50 d after planting (DAP) survived to maturity. Biomass accumulation declined rapidly as emergence was delayed in relation to soybean. Delaying emergence from 14 to 28 DAP resulted in a 50 to 80% reduction in shoot biomass. Common waterhemp emerging 50 DAP produced only 1 to 10% of the biomass of plants emerging at the same time as soybean. Plants emerging with soybean produced approximately 300,000 to 2.3 million seeds plant−1 depending on the location. Fecundity of common waterhemp plants was closely related to biomass accumulation and declined rapidly with delayed emergence. Although common waterhemp emerging after the V4 stage of soybean (40 DAP) are unlikely to affect crop yield because of high mortality levels and reduced growth, these plants may contribute significant seeds to the soil seed bank.

Twenty herbicide treatments were evaluated on conventional-till (plow, disc, and harrow), minimum-till (disc only), and no-till planted soybeans [Glycine max (L.) Merr.] from 1976 through 1978 at the Belleville Research Center in St. Clair County, Illinois. The soil type was a Weir silt loam (Typic Ochraqualf) characterized by poor internal drainage and 1.2% organic matter. Weed population by species, weed control, and soybean population, injury, and yield were obtained. Fall panicum (Panicum dichotomiflorum Michx.) and giant foxtail (Setaria faberi Herrm.) were the dominant species in all tillage systems, exceeding 1 million plants/ha in the conventional and no-till plots. These species and ivyleaf morningglory [Ipomoea hederacea (L.) Jacq.] were the most difficult to control each year. Weed control was the poorest in the no-till plots because of the large size of the weeds at the time of herbicide application, insufficient rainfall following, and because the plots were not cultivated. The soybean population was equal in all tillage systems except in 1976 when the no-till population exceeded that in the other tillage systems. Treatments that included oryzalin (3,5-dinitro-N4,N4-dipropylsulfanilamide) caused 42 and 35% soybean injury in the 1976 minimum and no-till plots, respectively. Postemergence-applied naptalam (N-1-naphthylphthalamic acid) plus dinoseb (2-sec-butyl-4,6-dinitrophenol) caused leaf burn each year that ranged from 5 to 35% but all plants recovered within several weeks of application. The seedbed tillage method and herbicide treatments did not significantly affect soybean yields in 1976 when all the herbicides were effective. No-till yields in 1977 and 1978 were substantially lower than yields in conventional and minimum-till plots because of poor weed control. Soybean yields were 2506, 2466, and 1714 kg/ha in the conventional-till, minimum-till, and no-till plots, respectively, when averaged over the 3 yr and 20 herbicide treatments.

The occurrence of volunteer canola has been increasing in western Canada. The objective of this study was to determine the canola seedbank additions incurred during crop harvest on commercial farms. Over 2 yr, 35 fields of 15 different producers were sampled after harvest using a vacuum cleaner. The canola seeds were separated from the crop residue and soil, and yield loss, 1,000-seed weight, and seedbank additions were determined for each field. Further information for each field was obtained through a producer survey questionnaire. Average yield losses of 107 kg ha−1 or 5.9% of the crop seed yield were observed. This amounted to seedbank additions of approximately 3,000 viable seeds m−2. The yield losses among producers ranged from 3.3 to 9.9% or 9 to 56 times the normal seeding rate of canola. Even with relatively low persistence rates, seedbanks of this magnitude could result in significant volunteer populations for several years, without further seedbank additions from escaped volunteers.

Adsorption of 14C-imazaquin {2-[4,5-dihydro-4-methyl-4-(1-methylethyl)-5-oxo-1H-imidazol-2-yl]-3-quinolinecarboxylic acid} and imazethapyr [2-(4,5-dihydro-4-methyl-4-(1-methylethyl)-5-oxo-1H-imidazol-2-yl)-5-ethyl-3-pyridinecarboxylic acid] to soil increased as soil pH decreased from 8.0 to 3.0 in laboratory studies. Significantly more imazethapyr3 (AC-263,499) than imazaquin was adsorbed at soil pH levels 3.0 and 5.5, while the greatest difference in adsorption behavior between the two herbicides was observed at a soil pH of 5.5. In greenhouse studies, phytotoxicity to corn (Zea mays L.) was greater for imazaquin than AC-263,499 applied at 26 and 53 g ai/ha. There were significant pH by herbicide and pH by rate interactions, but in trend analysis only a small proportion of the corn response (r2 = 0.01 to 0.35) was attributed to increasing soil pH values. In field studies where imazaquin was applied to soil pH levels of 4.2 to 4.8, 5.4 to 5.5, and 5.8 to 6.2, injury to corn across all pH levels decreased as the time delay between herbicide application and corn planting increased. There was no significant effect of soil pH on imazaquin injury to corn planted in July or August. Decreased injury from imazaquin was observed in 1985 on corn planted in June on the soil pH of 5.8 to 6.2. Imazaquin injury was less for June-planted corn in 1984 than in 1985, across all soil pH levels.

Mortality of weed seeds at temperatures of 39, 42, 46, 50, 60, and 70 C was recorded through time under controlled laboratory conditions similar to those of soil solarization for six weed species: annual sowthistle, barnyardgrass, black nightshade, common purslane, London rocket, and tumble pigweed. Time and temperature requirements for thermal death varied considerably among the species studied. Barnyardgrass, London rocket, and annual sowthistle were more susceptible to heat treatment than black nightshade, common purslane, and tumble pigweed. Temperatures of 50 C and above were lethal for seeds of all species. Common purslane seeds were unaffected at 46 C and below, tumble pigweed and barnyardgrass seeds were unaffected at 42 C and below, and black nightshade seeds were unaffected at 39 C. Nonlinear models for mortality as a function of duration of heat treatment were developed for each species at each temperature at which mortality occurred. These models provide an empirical relationship for the construction of field-applicable decision models that could predict the accumulation of time and temperature combinations for effective solarization of weed seeds.

Data containing many variables are often collected in weed science research, but until recently few weed scientists have used multivariate statistical methods to examine such data. Multivariate analysis can be used for both descriptive and predictive modeling. This paper provides an intuitive geometric introduction to the more commonly used and relevant multivariate methods in weed science research, including ordination, discriminant analysis, and canonical analysis. These methods are illustrated using a simple artificial data set consisting of abundance measures of six weed species and two soil variables over 12 sample plots.

Giant ragweed seeds have high nutritional value, consisting of 47% crude protein and 38% crude fat, and may be an important food source for rodent and invertebrate populations in agricultural and early successional ecosystems. We investigated temporal patterns of postdispersal giant ragweed seed predation on the soil surface of a no-tillage cornfield as affected by involucre (seed dispersal unit) size and presence or absence of crop residue. Cage exclusion experiments indicated that rodents and invertebrates were the principal predators of giant ragweed seed, and total predation of involucres over a 12-mo period beginning in November was 88%. Rodents were the greatest predators of giant ragweed involucres during fall and winter, and cumulative predation by February 1 in treatments with rodent access ranged from 39 to 43%. In contrast, giant ragweed involucre predation by invertebrates occurred mainly from May 1 to November 1. When rodent access to involucres was prevented, total involucre predation by invertebrates over a 12-mo period ranged from 57 to 78%. Rodents showed an initial preference for large involucres (> 4.8-mm diameter), and invertebrates preferred small involucres (< 4.8-mm diameter). Involucres covered with corn plant residue underwent less predation by rodents from November to February than uncovered involucres, but residue cover had no effect on seed predation by invertebrates. In a laboratory feeding trial, the carabid Harpalus pensylvanicus preferred seed of smooth pigweed and yellow foxtail to giant ragweed seed, suggesting that giant ragweed seed is an incidental rather than a preferred food source for some carabids. Because giant ragweed exhibits relatively low fecundity and short seed bank persistence, results of this study suggest that postdispersal predation may directly reduce giant ragweed recruitment the next year by reducing new seed bank inputs. However, seed losses from predation alone may be insufficient to maintain giant ragweed populations below economic threshold levels in no-tillage cornfields.

The selection of herbicide-resistant weed populations began with the introduction of synthetic herbicides in the late 1940s. For the first 20 years after introduction, there were limited reported cases of herbicide-resistant weeds. This changed in 1968 with the discovery of triazine-resistant common groundsel. Over the next 15 yr, the cases of herbicide-resistant weeds increased, primarily to triazine herbicides. Although triazine resistance was widespread, the resistant biotypes were highly unfit and were easily controlled with specific alternative herbicides. Weed scientists presumed that this would be the case for future herbicide-resistant cases and thus there was not much concern, although the companies affected by triazine resistance were somewhat active in trying to detect and manage resistance. It was not until the late 1980s with the discovery of resistance to Acetyl Co-A carboxylase (ACCase) and acetolactate synthase (ALS) inhibitors that herbicide resistance attracted much more attention, particularly from industry. The rapid evolution of resistance to these classes of herbicides affected many companies, who responded by first establishing working groups to address resistance to specific classes of herbicides, and then by formation of the Herbicide Resistance Action Committee (HRAC). The goal of these groups, in cooperation with academia and governmental agencies, was to act as a forum for the exchange of information on herbicide-resistance selection and to develop guidelines for managing resistance. Despite these efforts, herbicide resistance continued to increase. The introduction of glyphosate-resistant crops in the 1995 provided a brief respite from herbicide resistance, and farmers rapidly adopted this relatively simple and reliable weed management system based on glyphosate. There were many warnings from academia and some companies that the glyphosate-resistant crop system was not sustainable, but this advice was not heeded. The selection of glyphosate resistant weeds dramatically changed weed management and renewed emphasis on herbicide resistance management. To date, the lesson learned from our experience with herbicide resistance is that no herbicide is invulnerable to selecting for resistant biotypes, and that over-reliance on a weed management system based solely on herbicides is not sustainable. Hopefully we have learned that a diverse weed management program that combines multiple methods is the only system that will work for the long term.

A field study was conducted to determine the effects of giant ragweed emergence time and population density on corn grain yield, giant ragweed seed production, and giant ragweed predispersal seed losses. When weeds and crop emerged concurrently, hyperbolic regression of percent corn yield loss on giant ragweed population densities of 1.7, 6.9, and 13.8 weeds per 10 m2 gave a predicted loss rate of 13.6% for the first weed per 10 m2 in the linear response range at low densities and a maximum yield loss of 90% at high weed densities. Crop yield loss response to weed density was linear when giant ragweed emerged 4 wk after corn, and the regression coefficient indicated a yield loss rate of 1% per unit increase in weed density. A larger proportion of the variation in corn yield loss was explained by weed density (r 2 = 0.99) than by weed biomass (r 2 = 0.81). There was a positive linear relationship between giant ragweed seed production and weed density at each weed emergence time. When giant ragweed emerged with corn, regression equations for 1997 and 1998 gave a predicted seed rain of 146 and 238 seeds m−2 per unit increase in weed density, respectively. In both years when giant ragweed emerged 4 wk after corn, predicted seed rain was 16 seeds m−2 per unit increase in weed density. Viability of total giant ragweed seed was 56 and 38% in 1997 and 1998, respectively, and was not affected by weed emergence time or weed density. Feeding by insect larvae accounted for 13 to 19% of giant ragweed seed viability losses. Granivorous insects infesting giant ragweed seed were identified as a fruit fly (Diptera: Tephritidae), two weevils (Coleoptera: Curculionidae), and a moth (Lepidoptera: Gelechiidae).