As an extension of weed threshold models in which crop losses are based on weed density, an alternative model for grass weeds in cereal crops is proposed that incorporates the theoretical importance of selection for herbicide resistance, initial weed population frequency, and weed seed dispersal. Simulations suggest optimum weed population levels (thresholds) for maintaining genotypes that are susceptible to control practices and which minimize crop yield reductions. Weed population frequency, in combination with dispersal and competitive traits may determine optimum weed management strategies/Model simulations indicate that understanding how agricultural practices select for “weedy” traits (e.g. herbicide resistance, competitive ability, dispersal potential) may be important in determining weed density thresholds.

The influence of temperature, soil moisture, and light intensity on the tolerance of spring wheat, durum wheat, and spring barley to imazamethabenz and safened fenoxaprop-p-ethyl was determined in growth chamber experiments. Imazamethabenz at 500 g/ha reduced plant height and shoot fresh weight of durum and spring wheats grown under prolonged 70% shade. The height of durum and spring wheats was reduced by imazamethabenz under a low temperature regime. Compared to the plants grown under standard conditions, the shoot weight of spring and durum wheat plants was not reduced by imazamethabenz under drought, or high and low temperature conditions. Barley tolerance to imazamethabenz was not affected by environment. All three spring cereals were tolerant to fenoxaprop-p-ethyl plus a safener at 92 g/ha; and such tolerance was not greatly affected by various environments examined.

Integrated weed management practices, such as crop rotation and increased seeding rates, potentially improve weed management. Yet, few studies compare competitive interactions of weeds with different crops. This research quantified the impact of Persian darnel on spring wheat, canola, and sunflower yield across different seeding rates. Increasing crop density increased yield when Persian darnel affected crop yield early in physiological development. Crop yield loss was estimated to reach 83, 70, and 57% for spring wheat, canola, and sunflower, respectively, at high Persian darnel densities. Persian darnel reduced spring wheat yield by limiting the number of tillers per plant and seed per tiller; reduced canola yield by limiting the number of branches per plant, pods per branch, and seed per pod; and reduced sunflower yield by limiting the number of seed per plant. Persian darnel affected crop growth early in physiological development, indicating that interspecific interference occurred early in the growing season. Cultural and resource management aimed at reducing Persian darnel impact on resource availability and crop yield components will reduce Persian darnel impact on crop yield.

Development of genetically modified (GM) wheat has raised concerns about the movement and persistence of transgenes in agroecosystems and the ability of growers to segregate GM from conventional wheat. Wheat as a crop has been studied extensively but the population biology of volunteer wheat is not well characterized. Artificial seed bank studies were conducted in western Canada to provide baseline data on volunteer wheat seed persistence. Seed from two cultivars of Canadian western red spring wheat, ‘AC Splendor’ and ‘AC Superb’, were buried in mesh bags at three depths (0, 2, and 15 cm) in two different environments in the fall of 2003 and 2004. In addition, in 2004, ‘AC Superb’ seed were separated into small and large seed lots and buried with a medium seed lot to examine the influence of seed size on seed bank persistence. Seeds were withdrawn at intervals to assess seed germination and viability and regression analysis conducted on the viable seed at each sample period, after burial. Seed viability was variable within years and sites, and declined exponentially over time. In the spring, approximately 6 mo after initiation, viable seed ranged from 1 to 43%. With the exception of a single site and year, seeds on the soil surface persisted significantly longer than buried seeds and increasing burial depth accelerated loss of viability. The maximum viability of wheat seeds at 0, 2, and 15 cm depth in the spring following planting was 43, 7, and 2%, respectively. The extinction of viability for 99% (EX99) of the seed was estimated from regression analysis. The EX99 values of seeds buried at 0, 2, and 15 cm ranged from 493 to 1,114, 319 to 654, and 175 to 352 d after planting (DAP), respectively, with the exception of one site in 2003 where burial depths were not different and all had an EX99 value of 456 DAP. Seed size and cultivar did not significantly affect persistence, with the exception of one site in 2003 where the difference in EX99 values was 20 DAP. The rapid loss of seed viability limits temporal gene flow via volunteers in years following a wheat crop. Results provide data on spring wheat biology to aid in Canadian environmental biosafety assessments of GM wheat and will be incorporated into a mechanistic model to predict wheat gene flow on the Canadian prairies.

Yield loss was related to weed species composition and density in permanent plots, recorded several weeks after sowing of spring cereals in southern Sweden. A range of agronomic situations was included by experimentally varying fertilizer application and sowing density in 33 field trials in different locations during 3 yr. Direct gradient analysis, using yield loss as the sole predictor, arranged weed community composition in the spring along a gradient of small to large yield losses. Yield loss could be explained, to some extent, by the species composition in the spring. Species associated with situations with large losses were hempnettle and wild radish, whereas several benign species were identified based on their association with lack of yield loss. The results suggest that possible predictive tools using spring species composition would be improved if they also considered soil type and seed rate. Some agronomically important weed species were not identified as associated with yield loss when assessed in terms of their abundance in the spring, which may limit the possibilities of basing management decisions on weed plant density in the spring.

Glyphosate-tolerant spring wheat currently is being developed and most likely will be the first major genetically engineered crop to be marketed and grown in several areas of the northern Great Plains of the United States. The public has expressed concerns about environmental risks from glyphosate-tolerant wheat. Replacement of traditional herbicide active ingredients with glyphosate in a glyphosate-tolerant spring wheat system may alter ecological risks associated with weed management. The objective of this study was to use a Tier 1 quantitative risk assessment methodology to compare ecological risks for 16 herbicide active ingredients used in spring wheat. The herbicide active ingredients included 2,4-D, bromoxynil, clodinafop, clopyralid, dicamba, fenoxaprop, flucarbazone, glyphosate, MCPA, metsulfuron, thifensulfuron, tralkoxydim, triallate, triasulfuron, tribenuron, and trifluralin. We compared the relative risks of these herbicides to glyphosate to provide an indication of the effect of glyphosate when it is used in a glyphosate-tolerant spring wheat system. Ecological receptors and effects evaluated were avian (acute dietary risk), wild mammal (acute dietary risk), aquatic vertebrates (acute risk), aquatic invertebrates (acute risk), aquatic plants (acute risk), nontarget terrestrial plants (seedling emergence and vegetative vigor), and groundwater exposure. Ecological risks were assessed by integrating toxicity and exposure, primarily using the risk quotient method. Ecological risks for the 15 herbicides relative to glyphosate were highly variable. For risks to duckweed, green algae, groundwater, and nontarget plant seedling emergence, glyphosate had less relative risk than most other active ingredients. The differences in relative risks were most pronounced when glyphosate was compared with herbicides currently widely used on spring wheat.

The potential for future commercialization of glyphosate-resistant wheat necessitates evaluation of agronomic merits of this technology. Experiments were established to evaluate glyphosate-resistant wheat and weed responses to glyphosate rate, application timing, and tank mixtures. Glyphosate at 1,680 g/ha did not injure wheat. Wheat response to glyphosate applied to one- to three- or three- to five-leaf wheat was not different from that of untreated wheat. Wheat was injured more from glyphosate plus thifensulfuron or glyphosate plus dicamba than from individual herbicides at one of six locations, but grain yield was not affected by glyphosate tank mixtures. Glyphosate application timing did not affect control of wild oat or common lambsquarters 56 d after treatment. Glyphosate when applied to one- to three-leaf wheat provided better control of wild buckwheat than later glyphosate application, whereas glyphosate applied to three- to five-leaf wheat provided the best control of green and yellow foxtail, redroot pigweed, and Canada thistle. Weed control with glyphosate tended to be better than with conventional herbicides, and wheat treated with glyphosate produced approximately 10% more grain than wheat treated with conventional herbicide tank mixes.

Producers are interested in tactics for managing crop residues when growing corn after spring wheat. We compared five systems of managing spring wheat residues: conventional tillage, no-till, strip-till, cover crop (hairy vetch) with no-till, and cover crop with strip-till following spring wheat. Conventional tillage consisted of chisel plowing and disking, whereas strip-till consisted of tilling a 15-cm band centered on corn rows, which were spaced 76 cm apart. Plots were split into weed-free and weed-infested subplots. Grain yield in weed-free conditions did not differ among treatments. However, weed-free yield was nearly 40% greater than weed-infested corn in conventional tillage. In contrast, weeds reduced yield only 15% with strip-till. Weed density and biomass was twofold greater with conventional tillage compared with the no-till and strip-till treatments. Weed seedlings also emerged earlier with conventional tillage. Increased weed tolerance with strip-till may be related to fertilizer placement. Corn growth and tolerance to weeds in no-till systems may be improved if a starter fertilizer is placed in the seed furrow.

Dry pea improves corn yield and tolerance to weed interference compared with soybean, spring wheat, or canola as preceding crops. To understand this synergy between dry pea and corn, growth and nutrient concentration of corn were examined following dry pea or soybean in sequence. Each corn plot was split into weed-free and weed-infested subplots, with foxtail millet established at one density to provide uniform weed interference. Compared with soybean, dry pea improved corn grain yield 10% in weed-free conditions and corn tolerance to weed interference more than twofold. Dry pea synergy to corn in weed-free conditions was not related to differences in corn development, height, or nutrient status of corn seedlings. When foxtail millet was present, dry pea increased corn height and rate of development late in the growing season compared with soybean. Improved corn tolerance to weed interference was not related to seedling emergence or growth of foxtail millet, as these parameters did not vary with preceding crop. Other biological factors must be involved in dry pea synergy to corn.

The increasing occurrence of herbicide resistance, along with no new herbicide modes of action developed in over 30 yr, have increased the need for nonherbicidal weed management strategies and tactics. Harvest weed seed control (HWSC) practices have been successfully adopted in Australia to manage problematic weeds. For HWSC to be effective, a high proportion of weed seeds must be retained on the plant at crop maturity. This 2-yr (2014, 2015) study evaluated seed shatter of wild oat, green foxtail, wild mustard, and cleavers in both an early (field pea) and late (spring wheat) maturity crop in field experiments at Scott, Saskatchewan. Seed shatter was assessed using shatter trays collected once a week during crop ripening stage, as well as at two crop maturation or harvest stages (swathing, direct-combining). Seed shatter differed among weed species, but was similar between crops at maturity: ca. 30% for wild oat, 5% for cleavers, < 2% for wild mustard, and < 1% for green foxtail. Overall, seed shatter of wild oat occurred sooner and at greater levels during the growing season compared with the other weed species. Viability of both shattered and plant-retained seeds was relatively high for all species. The small amount of seed shatter of cleavers, wild mustard, and green foxtail suggests that these species may be suitable candidates for HWSC. Due to the amount and timing of wild oat seed shatter, HWSC may not reduce population abundance of this grassy weed.

Crop rotations, particularly those that include legumes, often result in improved soil quality and crop yields. A study was conducted to confirm the presence and persistence of the residual effects of crop rotation and weed management on a test crop and weeds in three tillage systems (moldboard plow [MP]; chisel plow [CP]; no tillage [NT]). Rotation (spring barley monoculture and spring barley–red clover rotation) and weed management (intensive, moderate, minimum) treatments, initiated in 1987, were terminated, and a test crop of spring wheat was grown in 1995 and 1996. Tillage treatments were maintained throughout. Multivariate analysis showed that weed communities were more affected by treatment termination in the rotation NT treatment with minimum weed management than in all other treatments. The former treatment was dominated by perennial broadleaf weeds but sustained adequate wheat yields (3.3 Mg ha−1) compared with the monoculture (1.0 Mg ha−1) one year after termination. Weed communities in CP and MP plots were less affected by treatment termination. Yet, changes in herbicide use at termination caused the virtual elimination of quackgrass from tilled plots and allowed field pennycress to become ubiquitous across treatments. Residual effects from crop rotation were more important than those from weed management in increasing wheat yields in tilled systems. Differences in wheat yield in NT systems 2 yr after treatment termination were attributed to residual effects from previous weed management rather than from crop rotation. Beneficial effects of crop rotation and weed management may persist for 2 yr but will vary according to tillage system.

Crop diversity may improve tolerance to weed interference and reduce the need for herbicides. This experiment measured weed interference in corn as affected by the preceding crop in two studies. The first study, based on interference of the resident weed community, compared dry pea, soybean, canola, and spring wheat for effect on corn tolerance to weeds. Prominent weeds were green and yellow foxtail. The second study examined corn tolerance to a uniform infestation of foxtail millet as affected by dry pea, soybean, spring wheat, and corn as preceding crops. Each plot was split into weed-free and weed-infested subplots in both studies. Corn was most tolerant to weed interference following dry pea; compared with soybean, dry pea improved corn tolerance more than twofold. Corn also yielded the highest in weed-free conditions following dry pea; compared across 4 yr, corn yielded 7 to 23% more following dry pea than following either soybean or spring wheat. Crop diversity has helped producers reduce herbicide inputs in the Great Plains and may provide an additional benefit of reducing weed impact on crop yield.

A field trial was conducted for 6 yr (1998 through 2003) at Scottsbluff, NE, to measure weed shifts following multiple applications of two rates of glyphosate or alternating glyphosate with nonglyphosate treatments in continuous corn or in a crop rotation of corn, sugarbeet, and spring wheat with all three crops resistant to glyphosate. After 6 yr, plant densities of common lambsquarters, redroot pigweed, hairy nightshade, and common purslane increased in the crop-rotation treatment compared with continuous corn. There were four weed control subplot treatments consisting of two in-crop applications of glyphosate at 0.4 or 0.8 kg ae/ha each spring, alternating two applications of glyphosate at 0.8 kg/ha one year with a nonglyphosate treatment the next year, or a nonglyphosate treatment each year. The composition of the weed population averaged across all four treatments shifted from kochia and wild proso millet to predominately common lambsquarters. After 3 yr of using glyphosate at 0.4 kg/ha twice each year, common lambsquarters density increased compared with that in the 0.8 kg/ha rate of glyphosate or alternating glyphosate treatments. By the sixth year, the density of common lambsquarters in the glyphosate at 0.4 kg/ha treatment had increased to the extent that corn grain yield was reduced 43% compared with corn grain yield in the 0.8 kg/ha glyphosate treatment. Using glyphosate at either rate for 6 yr decreased the densities of kochia, wild proso millet, and longspine sandbur, did not alter densities of redroot pigweed and green foxtail, and increased the density of hairy nightshade. In the low-rate treatment of glyphosate, the number of common lambsquarters seeds in the seed bank were 134 seeds/kg soil in 1998, declined to 15 seeds/kg by 2002, but began to increase in 2003 as the densities of plants not controlled by glyphosate increased.

The adoption of precision technologies that spatially register measurements using global positioning systems (GPS) greatly facilitates conducting large-scale on-farm research by farmers. On-farm experiments that utilize producer equipment include variations in agronomic practices that occur in situations where we want to predict the effect of inputs on yield. The domain of inference for such on-farm studies therefore more closely matches that desired by researchers. To investigate the feasibility of on-farm research using GPS, a study was conducted to evaluate the potential benefit of site-specific weed management. The study utilized producer-maintained field-scale equipment on four Montana farms in dryland spring wheat production. Paired site-specific and whole-field herbicide treatment areas were established in 0.9 to 1.9-ha blocks using consultant weed maps and a geographic information system (GIS). Yield was unaffected by herbicide treatment strategy (site-specific or broadcast). Minimal detectable yield differences were evaluated for the experimental design (0.2 T ha−1). Net returns increased when the percentage of field infested by wild oat decreased. Visual ratings of wild oat density taken at harvest indicated no difference in wild oat control between treatments in two of four site-years. This research suggests that producer-owned equipment can be used to compare treatments, but the accuracy and subsequent power of such comparisons are likely to be low.

The advent of site-specific weed management has generated research aimed at predicting weed spatial distributions from existing weed maps or correlations with soil properties and edaphic factors. Forecasting the spatial distribution of annual weeds requires knowledge of fecundity, dispersal, management, and suitable habitat distribution. We hypothesized that wild oat habitat was limited by field-scale heterogeneity in plant-available water. We eliminated seed number and dispersal limitations by seeding wild oat in areas with and without historical wild oat patches in three similarly managed spring wheat fields that differed in soil properties and wild oat infestations and were situated within a 160-km radius of Great Falls, MT. Wild oat habitat was quantified by wild oat leaf area growth rate, mature shoot biomass, seeds produced per plant, biomass water use efficiency, and competitive ratio with spring wheat. Soil texture and plot elevation correlated with existing wild oat patch areas in individual fields, but no site properties consistently correlated with wild oat patch areas in all three fields. Soil water use (SWU) and almost all habitat-defining variables for wild oat were similar between historic patch and nonpatch areas. Wild oat grew and produced seed regardless of existing patch boundaries and field-scale heterogeneity in SWU. This research suggested that (1) wild oat habitat may be unlimited in cereal grain cropping systems of the Northern Great Plains and (2) soil properties are a poor predictor of weed distribution for a generalist such as wild oat.

As a weed, wheat has recently gained greater profile. Determining wheat persistence in cropping systems will facilitate the development of effective volunteer wheat management strategies. In October of 2000, glyphosate-resistant (GR) spring wheat seeds were scattered on plots at eight western Canada sites. From 2001 to 2003, the plots were seeded to a canola–barley–field-pea rotation or a fallow–barley–fallow rotation, with five seeding systems involving seeding dates and soil disturbance levels, and monitored for wheat plant density. Herbicides and tillage (in fallow systems) were used to ensure that no wheat plants produced seed. Seeding systems with greater levels of soil disturbance usually had greater wheat densities. Volunteer wheat densities at 2 (2002) and 3 (2003) yr after seed dispersal were close to zero but still detectable at most locations. At the end of 2003, viable wheat seeds were not detected in the soil seed bank at any location. The majority of wheat seedlings were recruited in the year following seed dispersal (2001) at the in-crop, prespray (PRES) interval. At the PRES interval in 2001, across all locations and treatments, wheat density averaged 2.6 plants m−2. At the preplanting interval (PREP), overall wheat density averaged only 0.2 plants m−2. By restricting density data to include only continuous cropping, low-disturbance direct-seeding (LDS) systems, the latter mean dropped below 0.1 plants m−2. Only at one site were preplanting GR wheat densities sufficient (4.2 plants m−2) to justify a preseeding herbicide treatment in addition to glyphosate in LDS systems. Overall volunteer wheat recruitment at all spring and summer intervals in the continuous cropping rotation in 2001 was 1.7% (3.3 plants m−2). Despite the fact that volunteer wheat has become more common in the central and northern Great Plains, there is little evidence from this study to suggest that its persistence will be a major agronomic problem.

Persian darnel control options are limited and unmanaged populations can cause substantial crop yield loss. Integrating crop diversification and higher crop seeding rates into a cropping system might improve Persian darnel management. Field experiments were conducted to determine the effect of different crops and increased crop seeding rates on Persian darnel fecundity. Persian darnel produced up to 2,800 seeds per plant and 53,000 seeds/m2 when grown without competition. Increasing crop density reduced Persian darnel tillers per plant, seed weight, and fecundity. Increasing crop density reduced Persian darnel fecundity 0.4 to 0.2% per spring wheat plant, 0.6 to 0.1% per canola plant, and 16 to 8% per sunflower plant. Persian darnel fecundity was impacted the greatest by reduced weed seedling establishment, which was caused by crop competition and seeding sunflower late in the spring preceded by a nonselective herbicide application. Results indicated delaying the seeding of spring crops or including a late-seeded warm season crop, like sunflower or safflower, in the cropping system is an effective weed management tool for reducing Persian darnel fecundity.