Experiments were established in 1982 to study the loss of viability of spotted knapweed seeds (achenes) in soil. Greater than 50 and 25% of buried seeds remained viable but dormant after 5 and 8 yr at two locations, respectively. Decline in seed viability was also measured in two natural seedbanks. Although the soil seed reserve decreased by 95% over a 7-yr period, approximately 400 000 viable seeds per ha remained, indicating that spotted knapweed seeds will last for many years in soil.

Destructive growth analysis of field replacement series experiments with mayweed chamomile and dry field peas was used to determine the competitive relationship between the two species. Mayweed chamomile produced similar amounts of leaf area and dry weight per plant in a dry year and a wet year. On the other hand, dry field peas produced 20% more leaf area and 100% more dry weight per plant in the wet year compared to the dry year. Height, leaf area, and dry weight of peas reached maximum between bloom and pod-set, and then declined. Height and dry weight of mayweed chamomile increased steadily throughout the growing season. Mayweed chamomile leaf area reached a maximum at the beginning of flower stem elongation. Initially, the relative growth rate of mayweed chamomile was about three times greater than the rate for peas, but by 40 to 48 d after planting, rates were similar for both species. Relative yields and relative crowding coefficients for dry weight showed that peas were 3 to 20 times more aggressive than mayweed chamomile. Results of these experiments show that mayweed chamomile is a weak competitor against peas, but because it continues to grow after peas senesce, it could interfere with crop harvest.

Triazine-resistant (TR) kochia control with herbicides was evaluated in the greenhouse and in the field on ridge-till and ecofallow corn. In the greenhouse, bromoxynil, dicamba, linuron, and paraquat applied POST controlled TR kochia > 86%. Control of TR kochia with 2,4-D ester at 0.3 kg ae ha−1 varied with kochia biotype. In three ridge-till fields in 1985, atrazine plus fluorochloridone at 2.2 plus 0.6 kg ai ha−1 applied before planting plus a layby cultivation was the only treatment that reduced TR kochia biomass to < 750 kg ha−1. In 1986, atrazine plus dicamba at 2.2 plus 0.3 kg ha−1, atrazine plus fluorochloridone at 2.2 plus 0.3 or 0.6 kg ha−1 applied EPOST; atrazine at 2.2 kg ha−1 applied EPOST followed by an MPOST application of bromoxynil at 0.3 or 0.4 kg ha−1, or dicamba plus 2,4-D ester at 0.3 plus 0.3 kg ha−1; and an MPOST application of atrazine plus dicamba at 1.3 plus 0.5 kg ha−1, atrazine plus pyridate at 1.1 plus 1.0 kg ai ha−1, or pyridate plus cyanazine at 1.0 plus 1.7 kg ai ha−1 followed by a layby cultivation reduced kochia biomass to < 400 kg ha−1. In ecofallow corn, the only herbicide treatment that consistently controlled TR kochia > 95% 21 to 30 d after application was pyridate plus fluorochloridone at 1.0 plus 0.6 kg ha−1. Other treatments were less effective because kochia was too large or additional kochia emerged after application. When added to atrazine plus cyanazine, paraquat at 0.3 kg ai ha−1 plus dicamba was more effective than paraquat plus 2,4-D ester or paraquat alone in controlling TR kochia.

Adaxial and abaxial epidermal surfaces of johnsongrass leaves were studied to determine which cells contribute to leaf microroughness. Cork-silica cell (CSC) pairs, three types of prickles, macrohairs, bicellular trichomes, stomata, and ordinary short and long epidermal cells were found and described. CSC pairs made up about 22% of all cells and probably contribute more to microroughness than any other single type because each cork cell produces 11 ± 3 wax filaments that are up to 100 μm long. Bicellular trichomes represented 4 to 5% of the total cells but decreased leaf roughness by secreting a type of mucilage that covers microscopic wax crystals. Stomatal complexes comprised 15 to 18% of all cells and contributed to leaf roughness because they are slightly recessed below the leaf surface. Long prickles occur primarily over veins and represent less than 1% of all cells. Small prickles were present primarily on adaxial surfaces and represent only 3% of all cells. Macrohairs were the largest appendages, 237 ± 104 μm, but they represent far less than 1% of all cells and occur primarily over veins. Ordinary short cells comprised 6 to 13% of all cells. Long cells were most common (41%) of all cells. Short and long cells contribute to leaf roughness because the surface is often convex. A typical johnsongrass leaf may contain more than 25 million appendages on each surface that increase the roughness already caused by epicuticular wax crystals.

Leaf development and expansion are important factors in determining the outcome of crop-weed interference. The comparative effects of temperature and corn canopy-induced shading on the rate of leaf appearance (RLA) of redroot pigweed were quantified in this study. Growth cabinet results indicated a linear increase in RLA with increased temperature. Weed RLA was predicted utilizing both this function and field temperature data. The ratio of observed to predicted RLA of redroot pigweed grown in field experiments decreased in 1990 and 1991 as shading increased with increased corn density and delayed weed planting date. Results indicated that RLA is substantially affected by canopy-induced shading in addition to temperature.

Resistance to aryloxyphenoxypropionate and cyclohexanedione herbicides was identified in four wild oat populations from western Canada. Populations UM1, UM2, and UM3 originated from northwestern Manitoba and UM33 from south-central Saskatchewan. Field histories indicated that these populations were exposed to repeated applications of diclofop-methyl and sethoxydim over the previous 10 yr. The populations differed in their levels and patterns of cross-resistance to these and five other acetyl-CoA carboxylase inhibitors (ACCase inhibitors). UM1, UM2, and UM3 were resistant to diclofop-methyl, fenoxaprop-p-ethyl, and sethoxydim. In contrast, UM33 was resistant to the aryloxyphenoxy propionate herbicides but not to sethoxydim. The dose of sethoxydim required to reduce growth of UM1 by 50% was 150 times greater than for a susceptible population (UM5) or UM33 based on shoot dry matter reductions 21 d after treatment. This population differed from UM2 and UM3 that had R/S ratios of less than 10. In the field UM1 also exhibited a very high level of resistance to sethoxydim. In contrast to susceptible plants that were killed at the recommended dosage, shoot dry matter of resistant plants treated at eight times the recommended dosage was reduced by only 27%. In growth chamber experiments none of the four populations was cross-resistant to herbicides from five different chemical families.

Field experiments were conducted in 1990 and 1991 at Lethbridge, Alberta, to determine if narrower row spacings and increased plant densities than commonly practiced would improve safflower's competitive ability with weeds. A factorial set of treatments of safflower at two row spacings of 11 and 22 cm and six densities ranging from 10 to 192 plants m−2 grown weed free or infested with green foxtail (Setaria viridis (L.) Beauv.). Decreasing safflower row spacing from 22 to 11 cm slightly improved competition with green foxtail, but increasing safflower density had a much greater effect. Weed-free safflower biomass and seed yield plateaued at 70 and 84 plants m−2 in 1990 and 1991, respectively. However, safflower infested with 500 plants m−2 of green foxtail increased in biomass and seed production up to 100 plants m−2 in 1990 and 156 plants m−2 in 1991. At these high densities, weedy safflower yielded less than weed-free safflower but three to four times more than achieved at lower stand densities. Concurrently, high safflower density reduced green foxtail biomass up to 72% and seed yield up to 85%. Dense safflower stands developed a closed canopy early in the season and shaded green foxtail more than thin stands.

Field experiments were conducted at the Elora Research Station, Elora, Ontario, in 1988 and 1989 to quantify the inherent competitive ability of three white bean cultivars: OAC Gryphon, OAC Sprint, and OAC Laser growing with a naturally occurring population of annual weeds. An attempt was made to increase the competitive ability of these cultivars by altering row spacing and seeding density. Uncontrolled populations of weeds reduced white bean yields by 70%. OAC Gryphon and OAC Laser reduced weed biomass by 10 to 35% compared to OAC Sprint. The ability of cultivars to reduce weed biomass was further enhanced in medium and narrow rows compared to traditional wide rows. Cultivar, row spacing, and seeding density combinations which maximized leaf area index when grown under weedy conditions also had significantly less weed biomass. However, cultivar selection, row spacing, and seeding density did not reduce weed density. A significant negative correlation was observed between weed biomass accumulation and final yield of white bean. For each kg ha−1 increase in weed biomass the corresponding white bean yield loss averaged 0.380 kg ha−1. Season-long weed competition significantly reduced total number of pods per plant, number of seeds per pod, and 100-seed weight.

Formation and distribution of 14C-atrazine degradation products in the top 120 cm of soil were determined over 16 mo under field conditions in an Estherville sandy loam. After 16 mo, 78% of applied 14C was still present in the soil. By 2 mo after treatment (MAT), 14C had moved to the 30- to 40-cm depth; however, movement to depths greater than 40 cm was not observed. Greater than 98% of the 14C remaining in the soil profile after 16 mo was in the top 20 cm. Twenty-seven percent of the 14C applied was atrazine 16 MAT. Atrazine was the predominant 14C-compound in soil below 10 cm. Hydroxyatrazine (HA) was the major degradation product in the top 10 cm of soil. The proportion of 14C as HA in the top 10 cm increased from 15% 2 MAT to 37% 16 MAT. Deethylatrazine (DEA) was the predominant degradation product at the 10- to 30-cm depth and accounted for up to 23% of the 14C present in the 10- to 20-cm depth. Deisopropylatrazine (DIA) accounted for less than 6% of the radioactivity recovered at any soil depth. The proportion of DEA and DIA increased while the proportion of HA decreased as soil depth increased, indicating that DEA and DIA are more mobile in soil than HA. Detection of HA at depths greater than 10 cm appears to be due to in situ degradation of atrazine previously moved to that soil depth. The large amount of 14C remaining in the soil 16 MAT suggests that a large pool of atrazine and its degradation products are present in the soil for an extended period following application and have the potential to contaminate ground water.

Interference between redstem filaree and round-leaved mallow was studied under controlled environmental conditions. Each species was grown in monoculture at densities of 2, 4, 8, and 12 plants per 20-cm-diam pot and in mixtures at all possible combinations of these densities. Leaf area per plant was similar for both species but round-leaved mallow grew taller and produced more shoot biomass than redstem filaree when each was grown in monoculture. Mixed culture responses varied with the proportion density of each species. A reciprocal yield model was tested and modified to account for this significant density interaction. When grown in mixture, round-leaved mallow usually gained in leaf area and shoot biomass at the expense of redstem filaree indicating that it was the superior competitor. Calculated competition ratios indicate that round-leaved mallow was about twice as competitive as redstem filaree under the growing conditions of this study.

Adverse changes in weed communities are a limiting factor for the adoption of conservation tillage practices. Predictions of an increased association of annual and perennial grasses, perennial dicot weeds, wind-disseminated species, and volunteer crops as weeds, and decreased association of annual dicot weeds in reduced-tillage systems were tested. Field experiments involving zero-, minimum-, and conventional-tillage systems were conducted in Saskatchewan from 1986 to 1990 at Ituna and Waldron, and from 1986 to 1988 at Tadmore. Weed community composition was analyzed for years 1988 to 1990 by canonical discriminant analysis. An increased association of perennial and annual grasses with zero tillage did not generally occur. Wind-dispersed species and volunteer crops were associated with reduced tillage and summer annual dicots with conventional tillage, but exceptions occurred. Species responded differently among sites or within a site over time. Within the time frame of this research, changes in weed communities were influenced more by location and year than by tillage systems, indicating fluctuational rather than directional or consistent changes in community composition.

Transpirations of cotton and velvetleaf plants competing for water and light were estimated with heat balance stem flow gauges in a controlled environment. Cotton and velvetleaf plants were grown together in pots and monitored during fluctuations in soil water, irradiance, air temperature, and humidity. Shading from other plants imposed light competition on velvetleaf or cotton. Velvetleaf used significantly more water than cotton given ample water and light. Transpiration by both species was reduced in dry soil, but velvetleaf transpiration was reduced primarily by leaf abscision, while cotton leaves did not abscise. Both cotton and velvetleaf plants transpired significantly less when shaded. Leaf water potentials indicated that shaded plants experienced reduced evaporative demand suggesting that shading reduced plant stress in dry soil. Competition for water was strongly influenced by competition for light.

Field studies determined flowering pattern, seed production, and somatic polymorphism of pitted morningglory, common cocklebur, and sicklepod under interspecific and no interference with soybean. Flowering patterns differed between years but were similar under both interferences. Interspecific interference reduced flower and seed numbers 85 to 99%. Rainfall had little effect on flowering, but photoperiod and temperature responses were evident for all weed species. The production equations for seed number per plant from dry weight were quadratic for pitted morningglory and common cocklebur but linear for sicklepod. All species showed somatic polymorphism; however, differences in seed germination and viability were not consistent between years.

An experiment was established at three sites to determine the longevity and distribution of rhizome buds of two quackgrass biotypes in the soil profile under two tillage systems (no-till or fall-plow). The experiments were established in May 1987 at sites initially free of quackgrass. Rhizome segments with three nodes were planted 2 to 5 cm deep to give an initial density of 21 buds m-2. Production of new buds was prevented in subsequent years. Rhizome bud populations were sampled every 6 mo between October 1987 and October 1989. Buds were produced until snow cover, indicating the need to control quackgrass until late fall to prevent a bud population increase. Under no-till conditions, 94% of quackgrass buds occurred in the top 10 cm of soil while 68 and 19% were found below 10 and 20 cm, respectively, in the fall-plow treatments. A control program against quackgrass should aim at keeping the majority of rhizomes in the top 10 cm in order to facilitate control by having most shoots emerging at the same time. There was no statistically significant difference in bud longevity between biotypes, sites, and tillage systems. Viability of the rhizome buds declined rapidly during the first year. In most cases it took 2 yr for the bud populations to reach extinction although 0.6% of buds of one biotype survived as long as 30 mo in one of the three sites. Therefore, a minimum of 2 yr of total control would be required to eradicate quackgrass from most locations.

A pot bioassay, based on root growth of pregerminated corn, was used to evaluate factors influencing field persistence of chlorsulfuron, metsulfuron, triasulfuron, and tribenuron, which were applied preemergence at 0, 10, 20, and 40 g ai ha−1 to wheat grown in three soils that differed in texture (sandy loam, sandy clay loam, and silty clay loam) and pH (7.9, 4.7, and 7.6). Residual activity and leaching of all herbicides in all soils increased with increasing rate of application, with the exception of tribenuron which showed practically no residual activity and leaching in sandy clay loam soil. Sunflower sown 4 mo after tribenuron application in all soils was not injured by any rate used but was significantly affected by the other herbicides. Lentil and sugarbeet also were affected by all herbicides in all soils. These three crops sown 8 mo after herbicide application were not affected by any herbicide used in the sandy clay loam soil but were injured by chlorsulfuron, triasulfuron, and metsulfuron in the sandy loam soil. Only lentil and sugarbeet were injured by chlorsulfuron in the silty clay loam soil.

Shoot growth and morphology of sicklepod responded to distance of sicklepod from the soybean row and relative time of emergence of crop and weed. Sicklepod was added to soybean as either uniform stands of 26 individuals m−2 or as widely spaced individuals. Sicklepod shoot height, number of main-stem nodes, number of primary branches, and shoot dry weight were decreased at 12 wk after emergence when plants were situated closer to a soybean row. Sicklepod that emerged 7 d later than soybean were shorter statured and had fewer main-stem nodes. Virtually all sicklepod overtopped the soybean, but proximity to the soybean row or later emergence reduced shoot dry weight up to 60%. A doubling in branches originating at lower main-stem nodes on sicklepod 25 cm from the row, compared to those in the row (upper branch numbers were unchanged), was associated with a 67% increase in leaves. Sixty to 90% of lower branches reached the upper canopy and thus contributed to the number of leaves above the soybean. Early-season effects of soybean interference on sicklepod reduced lower branches and, as a consequence, leaf production above the soybean late in the season, when sicklepod has the greatest impact on soybean yield.

A leaching test conducted in field lysimeters for the purpose of pesticide registration is evaluated, particularly in terms of factors such as the effects of soil type, variability in leaching between replicate lysimeters, and simulation of worst-case scenarios. Two herbicides, dichlorprop and bentazon, were chosen as test compounds due to their documented high mobility in laboratory tests. Four different soil types (sand, loam, clay, peat) and two irrigation treatments were included. Both herbicides were applied at rates representing normal doses (1.6 and 0.6 kg ai ha−1 of dichlorprop and bentazon, respectively). 36Cl was also applied to sand and clay lysimeters to follow the pattern of water movement. Leaching of dichlorprop for the varying soil type/treatment combinations ranged from 0.02 to 1.8% of the amount applied. Leaching losses of bentazon reached up to 0.07% of that applied. Leaching of both herbicides was greater mostly in clay monoliths than in sand monoliths, which was explained in terms of macropore flow. A more effective macropore flow was also suggested to be the main reason why more dichlorprop leached in clay and peat monoliths treated with a small water input. Detectable, and in some cases large, concentrations of dichlorprop were found in the first drainage water in early autumn in all soil/treatment combinations, indicating the occurrence of preferential flow in all soils tested, including sand. A rapid breakthrough of 36Cl was also found in clay and low-irrigation input sand, providing additional confirmation of the role of preferential flow processes in these soils. It is concluded that field mobility tests for pesticide registration are a necessary complement to measurements of physical/chemical properties of a compound and that these should be performed in a range of soil types, including at least one structured soil. Other factors identified to be of importance when evaluating lysimeter studies such as this were the analytical detection limits of the pesticides and the need for replication.

Velvetleaf is difficult to control in corn and soybean and the seed can persist in soil for many years. Seven cultural and tillage practices were established in 1974 on a site heavily infested with velvetleaf to determine the time required to eradicate velvetleaf seed from the soil. A rapid decline in velvetleaf seed population in the top 23 cm of soil occurred during the first 5 yr of this study. In the fifth year, the chemical fallow and continuous alfalfa treatments had 37 and 56% of the original velvetleaf seed population remaining, respectively. In the 17th year, soils in these treatments that had received no tillage since study initiation still contained 15 and 25% of the original velvetleaf seed population, respectively. Systems involving moldboard plowing with continuous-tillage fallow, continuous cropping of corn or oat, or an annual corn and soybean rotation had a more rapid decline in the velvetleaf seed population in soil compared to the chemical fallow and continuous alfalfa treatment. After 17 yr, soil in any system that had received at least one moldboard plowing per year still contained 1 to 3 million velvetleaf seed ha−1, which is only 0.8 to 2.5% of the initial viable seed population. Nearly 100% of the seed remaining in the soil in the 17th year for all treatments was still viable.

In experiments conducted under controlled conditions, glyphosate-induced inhibition of rhizome bud growth in quackgrass was reduced by increasing the nitrogen (N) concentration in the nutrient solution from 10.5 to 210 mg L−1 either 2 or 4 d before foliar application of the herbicide, and for 7 d after the herbicide treatment. The additional N reduced the glyphosate-induced inhibition of rhizome growth on the intact plant at the lowest glyphosate dosage (2.8 μg per plant) but had no significant effect at higher dosages. Both the 2- and 4-d high N treatments significantly increased the length, dry weight, and water content of the lateral buds by the time the herbicide was applied. They also increased bud growth on the glyphosate-treated plants during the 7 d following the herbicide application and promoted release of the buds from inhibition during a subsequent bioassay of their regenerative growth on excised, single-node rhizome segments. These effects of N supply interacted significantly with glyphosate dosage, bud position on the rhizome, and duration of incubation. It is postulated that bud growth response to increased N supply may reduce the inhibiting effect of the herbicide by diluting the glyphosate concentration in the bud, or that the additional N may counteract the toxic action of glyphosate on amino acid metabolism or protein synthesis.

Effects of proso millet interference with irrigated dry beans were evaluated in Nebraska over a 2-yr period. Dry bean yield reduction ranged from 12 to 31% from a wild proso millet density of 10 plants m-2. As density increased, dry bean yield reduction could be predicted with a rectangular hyperbola regression model. Ten wild proso millet plants m-2 growing with dry beans produced 14 780 to 21 420 seed m-2. Dry bean yields were reduced 41 and 11% in 1990 and 1991, respectively, when wild proso millet removal was delayed 6 wk after dry bean planting. Four weeks of weed-free maintenance were sufficient to provide dry bean yields comparable to plots kept weed free all season.