Field and laboratory studies were conducted to evaluate the efficacy of several herbicides applied after small-grain harvest on Russian thistle (Salsola iberica Sennen and Pau # SASKR) control and subsequent seed germination. Best postharvest control of Russian thistle was with chlorsulfuron {2-chloro-N-[[(4-methoxy-6-methyl-1,3,5-triazin-2-yl)amino] carbonyl] benzenesulfonamide} treatments, paraquat (1,1′-dimethyl-4,4′-bipyridinium ion), and bromoxynil (3,5-dibromo-4-hydroxybenzonitrile) plus metribuzin [4-amino-6-(1,1-dimethylethyl)-3-(methylthio)-1,2,4-triazin-5(4H)-one], where control ranged from 73 to 96%. During the summer-fallow year, chlorsulfuron and paraquat decreased Russian thistle population and biomass compared to the untreated control. Germination of large seeds from plants treated with paraquat and both rates of chlorsulfuron was reduced at least 64% compared to seeds of similar size from untreated plants.

Studies were conducted to determine the usefulness of HOE-39866 (HOE-00661) in chemical fallow systems on the Canadian prairies. HOE-39866 at 0.5 to 1.0 kg ai/ha controlled Russian thistle, kochia, green foxtail, wild oats, and wheat comparable to paraquat, glyphosate, and glyphosate plus the isopropylamine salt of 2,4-D. However, control of barley with HOE-39866 was unacceptable. HOE-39866 was compatible in tank mixtures with ammonium sulfate, paraquat, chlorsulfuron, and metsulfuron. Ammonium sulfate improved weed control when HOE-39866 was applied at 0.25 kg/ha but not at 0.75 kg/ha. Adding paraquat at 0.07 to 0.21 kg ai/ha to HOE-39866 improved control of grass species over HOE-39866 alone. Adding chlorsulfuron and metsulfuron to HOE-39866 provided greater initial control of certain species as well as residual control of many weeds. HOE-39866 alone or in conjunction with other herbicides is an alternative to the herbicides used in chemical fallow systems.

Field studies were conducted to determine suitability of flurtamone alone and in tank mixture with ethalfluralin for selective control of weeds in safflower. Flurtamone at 0.6 to 0.9 kg ha−1 controlled field pennycress, kochia, and Russian thistle. Flurtamone and ethalfluralin were compatible in tank mixture and provided comparable or superior control of wild oat, kochia, and field pennycress than either herbicide alone. Plant height, date of flowering, seed yield, and quality data indicated that safflower tolerated flurtamone alone up to 1.6 kg ha−1 and flurtamone tank mixed with ethalfluralin at rates as high as 1.1 plus 1.1 kg tar1, respectively. Safflower yield response to these herbicides was positive and reflected level of weed control attained with various treatments.

Greenhouse and field experiments were conducted to evaluate the efficacy of chlorsulfuron {2-chloro-N-[[(4-methoxy-6-methyl-1,3,5-triazin-2-yl)amino] carbonyl] benzenesulfonamide} on Russian thistle (Salsola iberica Sennen and Pau # SASKR) in a spring wheat (Triticum aestivum L.) summer-fallow rotation. In the greenhouse, when chlorsulfuron was applied to Russian thistle at three stages of growth, shoot dry weight was reduced similarly with either 4.4 g ai/ha plus surfactant or 140 g ai/ha plus surfactant. In the field, postharvest control (1981) of Russian thistle with 17.5 to 140 g/ha of chlorsulfuron ranged from 35 to 60% without surfactant and from 76 to 93% with surfactant. During the summer-fallow year (1982), Russian thistle treated in the preflowering stage of growth was not effectively controlled with 17.5 or 35 g/ha. In contrast, residual control from postharvest treatments 1 yr after application ranged from 90 to 100%. Chlorsulfuron applied in the crop (1983) controlled Russian thistle throughout the growing season. All treatments of chlorsulfuron applied in 1981, 1982, and 1983 increased wheat yield at least 75%.

Imazethapyr, alone and in combination with other herbicides, was applied PPI, PRE, and POST to pinto beans to determine weed control and selectivity to the crop. All of the herbicides improved pinto bean yield as compared with the unweeded control. Imazethapyr applied PPI and POST provided excellent control of black nightshade, kochia, Russian thistle, prostrate pigweed, and redroot pigweed. Barnyardgrass control with imazethapyr ranged from 58 to 96% and increased to 98% or greater when imazethapyr was combined with metolachlor, pendimethalin, trifluralin, or EPTC. Pinto bean yield was not reduced from any herbicide treatment compared to the handweeded control.

The effect of Russian thistle density and duration of interference on spring wheat was investigated in two 3-yr field studies. In the density study, the best fit regression equation predicted a loss in spring wheat yield of 0.5, 0.5, and 0.6% in 1983, 1984, and 1985, respectively, for each percent of the total biomass contributed by Russian thistle. In 1985, when rainfall was 46% below normal, the highest weed density produced greater than 70% of the total plant biomass and reduced yields more than 50%. In contrast, in 1984 when rainfall was 65% above normal, the highest weed density produced less than 20% of the total plant biomass and reduced yields 11%. In the duration study, the predicted yield loss was 2.3 and 0.7% for 1983 and 1984, respectively, for each week of interference. In 1985, yield was not significantly reduced until after 6 weeks of interference. In both studies, spikes/m of row had the highest correlation with yield and were affected the most by weed interference.

The DNA sequence of a 196 base pair (bp) region of the acetolactate synthase (ALS) genes of three weed species, kochia, prickly lettuce, and Russian thistle, was determined. This region encompasses the coding sequence for Domain A, a region of the amino acid sequence previously demonstrated to play a pivotal role in conferring resistance to herbicides that inhibit ALS. The Domain A DNA sequence from a chlorsulfuron-resistant (R) prickly lettuce biotype from Idaho differed from that of a chlorsulfuron-susceptible (S) biotype by a single point mutation, which substituted a histidine for a proline. The Domain A DNA sequence from an R kochia biotype from Kansas also differed from that of an S biotype by a single point mutation in the same proline codon. This point mutation, however, conferred substitution of threonine for proline. Two different ALS-homologous sequences were isolated from an R biotype of Russian thistle. Neither sequence encoded amino acid substitutions in Domain A that differed from the consensus S sequence. The DNA sequence variation among the R and S kochia biotypes was used to characterize six Ada County, Idaho, kochia collections for correlation between phenotypic chlorsulfuron susceptibility and restriction digest patterns (RFLPs) of polymerase chain reaction amplification products. Most collections showed excellent correspondence between the RFLP patterns and the phenotypic response to chlorsulfuron application. However, one entirely R collection had the RFLP pattern of the S biotype, suggesting that resistance was not due to mutation in the proline codon.

Clomazone is used in the Central Great Plains for weed control during fallow in a winter wheat-fallow rotation. Improved precipitation storage during non-crop periods has stimulated new crop rotations such as winter wheat-corn or proso millet-fallow. The objective of this study was to determine if clomazone applied in the fall after winter wheat harvest would injure succeeding spring-planted crops. Greenhouse studies indicated crop tolerance to clomazone was in the order of safflower > corn > proso millet > barley > winter wheat. Clomazone did not affect grain yields of safflower, corn, or proso millet grown at two field sites with different soil textures (silt loam and sandy loam), nor germination of seed from treated plants of these crops. A no-till production system with clomazone increased grain yields for all crops compared to the conventional system where tillage replaced clomazone for fallow weed control.

A long-term study was conducted at Lethbridge, Alberta, to determine the response of weed populations to various crop rotations and tillage treatments. Weed density and species composition differed with rotation, tillage, and date of sampling within years. Fewer weeds were found in winter wheat-fallow than continuous winter wheat, winter wheat-lentil, or winter wheat-canola rotations. A dense infestation of downy brome developed in the continuous winter wheat rotation. In all rotations, more weeds were present in zero tillage plots than in either minimum or conventional tillage plots. Dandelion and perennial sowthistle densities increased slightly over years in the minimum and zero tillage treatments. Flixweed, field pennycress, wild buckwheat, and common lambsquarters densities decreased in zero till but densities of downy brome, redroot pigweed, and Russian thistle increased. Russian thistle was not well controlled with trifluralin and its density increased in the winter wheat-canola rotation.

Under irrigated field conditions at Prosser, WA, Russian thistle [Salsola kali (L.) var. tenuifolia Tausch. ♯3 SASKR] began to emerge by mid-March each year and usually had completed its emergence by May 1. Common lambsquarters (Chenopodium album L. ♯ CHEAL) was the next to appear, usually during the third or fourth week of March. Barnyardgrass [Echinochloa crus-galli (L.) Beauv. ♯ ECHCG], redroot pigweed (Amaranthus retroflexus L. ♯ AMARE), hairy nightshade (Solarium sarrachoides Sendt. ♯ SOLSA), black nightshade (5. nigrum L. ♯ SOLNI), eastern black nightshade (S. ptycanthum Dun.), and cutleaf nightshade (S. triflorum Nutt. ♯ SOLTR) generally began to emerge during the first 2 weeks of April and emergence generally peaked mid-April to mid-May. Russian thistle and cutleaf nightshade had the most restricted emergence patterns, whereas seedlings of common lambsquarters, redroot pigweed, hairy nightshade, and black nightshade emerged each month from April through September. Shallow tillage at monthly intervals increased the overall emergence of common lambsquarters, redroot pigweed, black nightshade, and eastern black nightshade; decreased the emergence of barnyardgrass; and had no effect on the emergence of Russian thistle, cutleaf nightshade, or hairy nightshade.

Ten herbicide treatments were evaluated for early season control of three broadleaf weeds and effects on marketable potato yields in low-organic, coarse-textured soil. All treatments controlled prostrate pigweed 100%. Trifluralin with metolachlor or with EPTC did not control kochia well. Pendimethalin alone or with EPTC controlled Russian thistle poorly, and produced the lowest marketable tuber yields. Fluorochloridone at 0.6 kg ai/ha caused chlorosis and reduced potato yield 11%. Treatments with metribuzin tended to have high potato yields.

The repeated use of sulfonylurea (SU) herbicides to control broadleaf weeds in wheat fields and right-of-ways has selected for herbicide-resistant Russian thistle populations. A survey was conducted in 1991 and 1992 to ascertain the relative occurrence of SU-resistant Russian thistle in eastern Washington state. The 55 574 km2 survey area was divided into 149 equal sample areas. All sample areas were surveyed for Russian thistle and seed was collected from plants in 86 sample areas. No Russian thistle was found in the center of the remaining 63 sample areas. Seeds were collected, by plant, from 30 plants at each site. Site samples were tested in the greenhouse for resistance or susceptibility to chlorsulfuron. Populations that were either homogeneous or heterogeneous for chlorsulfuron-resistance were found in 70% of the sample areas and all of the plants were susceptible in 30% of the sample areas.

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.

The first significant cases of herbicide-resistant weed populations were to the triazines in the 1970s. In the last 10 years there has been an increase in the number of weed populations that have become resistant to an array of herbicides. In some of these cases, like rigid ryegrass in Australia, a multitude of resistant biotypes has evolved with different mechanisms of resistance. If the present trend continues, the number of herbicides effective on certain weed species may diminish rapidly. To counteract this trend, industry has organized a number of intercompany working groups to specifically address the development of resistance and to implement plans to manage resistance. University and extension along with industry personnel across the world have begun educating growers on resistance management. However, this effort needs to be intensified to find new solutions for controlling weeds through the use of integrated weed management practices that incorporate new and established herbicides with cultural, mechanical, and biological control methods. The challenge is to develop cost effective, environmentally sustainable programs for weed control while maintaining the present efficiency in food and fiber production so that needs of an ever expanding human population can be met.

Producers in the northern Great Plains are exploring alternative crop rotations, with the goal of replacing spring wheat-fallow. We characterized the weed associations occurring with tillage system and nitrogen level in two rotations, spring wheat (SW)-fallow (F) and SW-winter wheat (WW)-sunflower (SUN). Weed density was measured 10 yr after initiation of the study. With both rotations, weed community density was highest with no-till. For SW-F, green foxtail, yellow foxtail, and fairy candelabra comprised 99% of the weed community, whereas 13 species were observed in SW-WW-SUN. Fairy candelabra, a rangeland species, was observed only in the no-till system of SW-F. In SW-WW-SUN, no-till favored kochia, Russian thistle, and foxtails, whereas common lambsquarters and annual sowthistle were more common in tilled systems. Nitrogen fertilizer increased crop competitiveness in SW-WW-SUN with no-till, subsequently reducing weed density. Cultural strategies that disrupt weed associations will aid producers in managing weeds.

Weeds spread through movement of seeds and vegetative reproductive propagules. Pollen movement can spread weedy traits, such as herbicide resistance, between related weed and crop species. Weed seeds can spread short or long distances by natural plant dehiscence mechanisms, wind, water, animals, and man&s activities. This symposium paper is a practical review of short-distance spread of weed seeds in and between nearby arable fields and noncrop lands, examining some of the causes of spread and subsequent effects on crop production. Pollen movement, as it affects the spread of herbicide resistance, also is considered a component of short-distance weed spread. Specific weed management options can be used to reduce man-caused weed seed spread within and between nearby fields, thus reducing potential crop yield losses. Long-term management will be more difficult for weed seed spread by natural dispersal mechanisms.

The effectiveness of oxyfluorfen and bromoxynil for Russian thistle control in onion was measured in a 2-yr study conducted in the high desert region of southern California. Oxyfluorfen, bromoxynil, or combinations of the two applied one at the 2-leaf growth stage of onion did not control Russian thistle. When a first application was followed 14 days later with a second herbicide application, a first application of oxyfluorfen plus bromoxynil was most effective. Oxyfluorfen applied alone as a second application was least effective, but bromoxynil alone plus oxyfluorfen improved control. Adding petroleum oil to oxyfluorfen plus bromoxynil improved Russian thistle control.

A 2-yr field study was conducted to measure the growth and development of Russian thistle (Salsola iberica Sennen and Pau # SASKR) in the growing crops of winter and spring wheat (Triticum aestivum L.) and after harvest of these crops. In herbicide-free conditions, few Russian thistle seedlings emerged in winter wheat. Only 50% of these plants survived compared to 92 and 95% survival in spring wheat and crop-free treatments, respectively. Compared to growth in the crop-free treatment, both wheat types suppressed oven-dry weight, height, and width of Russian thistle plants during the crop-growing season and after crop harvest. During the crop-growing season, winter wheat suppressed Russian thistle height and width more than spring wheat. After crop harvest, oven-dry weight of Russian thistle plants grown in winter wheat stubble was suppressed 75% compared to plants grown in spring wheat stubble. Russian thistle plants grown in crop-free, spring wheat, and winter wheat treatments produced 152 100, 17 400, and 4 600 seeds/plant, respectively.

Russian thistle plant movement and seed dispersal were studied in 1991 and 1992 by placing Russian thistle plants in the center of wheat fields in eastern Washington. Three adjacent site treatments, with 24 plants on each site, were used each year; wheat stubble, summerfallow planted to winter wheat, and a “stationary” site. Plants in the “stationary” site were anchored to the ground to prevent tumbling. Plants in the stubble and summerfallow sites were allowed to tumble naturally. Individual plant movement was monitored and recorded weekly by satellite global positioning systems technology. Average estimated seed number per plant at the beginning of the experiment was 57,400 in 1991 and 66,000 in 1992. The direction plants moved correlated highly with wind direction. Some plants moved a maximum distance of 4069 m in 6 wks, while other plants moved only 60 m because of variable winds and being compressed by snow or frozen into wheat stubble. Average percentage seed loss in 1991 and 1992 for stationary plants was 15 and 26%, and for tumbling plants was 48 and 66%, respectively.

2,4-D diethanolamine control of kochia was influenced by the source of water used as the spray carrier. Sodium bicarbonate in one water source related to the reduced kochia control. Sodium bicarbonate in a water spray carrier antagonized 2,4-D control of all plant species evaluated. Antagonism increased with the tolerance of a species to 2,4-D, concentration of sodium bicarbonate, and spray volume. Sodium bicarbonate antagonism of 2,4-D phytotoxicity was greatest with low temperature and relative humidity.