In-crop Herbicide Research Articles

Effects of reduced pesticide use on winter wheat production in the Canadian Prairies

Winter wheat (Triticum aestivum L.) is a highly competitive crop with the potential to enhance on-farm revenue and reduce reliance on crop protection inputs. The adoption of winter wheat in the Canadian Prairies has varied significantly over recent decades; however, improved weed and disease management practices could facilitate stability of hectares cultivated. To assess current and alternative pesticide management practices, we conducted a study across 15 site-years at four locations over four years (2018-2022). Experimental treatments included pre-plant weed management (glyphosate vs. glyphosate mixed with pyroxasulfone+carfentrazone-ethyl), in-crop weed management (no in-crop herbicide vs. fall-applied 2,4-D vs. fall-applied 2,4-D+spring-applied site-year-specific herbicides) and in-crop fungicide management (no in-crop fungicide vs. one prothioconazole+tebuconazole application at ZGS60 vs. two prothioconazole+tebuconazole applications at ZGS32 and ZGS60). Pre-plant glyphosate alone and glyphosate tank-mixed with pyroxasulfone+carfentrazone-ethyl exhibited comparable effects on grain yield, quality parameters, and agronomic characteristics. In-crop weed management had no significant influence on these factors compared to the no in-crop herbicide control, suggesting that in-crop herbicide applications are unnecessary due to the high competitiveness of winter wheat against weeds. However, disease mitigation was prudent as single and double application of fungicide increased grain yield while maintaining grain protein concentration levels. A high-yielding, stable system for optimal grain yield typically required pre-plant weed management coupled with two fungicide applications. These observations confirm herbicide inputs can be reduced in a winter wheat cropping system but disease pressure requires careful cultivar selection with an emphasis on disease resistance as multiple applications of fungicides were needed to optimize grain yield.

Quantifying changes in the environmental impact of in-crop herbicide use in Saskatchewan, Canada

Abstract The sustainable management of herbicides is critical to modern agriculture and the environment. This article examines the evolution and environmental implications of herbicide use in Saskatchewan, Canada, agriculture. It quantifies changes in herbicide use and their environmental impacts by analyzing farm-level herbicide use data from 1991 to 1994 and from 2016 to 2019 through the environmental impact quotient. Results confirm significant reductions in both environmental and toxicological impacts of herbicides used, underlining the pivotal shift from tillage-based weed control to herbicide-resistant cropping systems. The environmental impact of the top five herbicides (glufosinate, glyphosate, clethodim, imazamox, and 2,4-D) used from 2016 to 2019 is 65% lower than that for those herbicides (MCPA, 2,4-D, bromoxynil, diclofop-methyl, and trifluralin) used from 1991 to 1994, with a 45% reduction in the active ingredient applied per acre. Despite increased herbicide use due to more crop acres being seeded, the findings highlight a marked improvement in the sustainability of herbicide use, affirming the importance of technological advancements in agriculture. This research contributes valuable insights into long-term trends in herbicide use, offering a practical framework for informed decisions aligning with sustainable agricultural practices as well as reduced biodiversity impacts.

Using integrated weed management systems to manage herbicide-resistant weeds in the Canadian Prairies

Although herbicides have been a dominant and effective weed control strategy for decades in Western Canada, herbicide resistance and the lack of new modes of action have resulted in weed management challenges. Integrated weed management strategies have been shown to be successful in controlling certain weed species that are problematic in cropping systems. The objective of this study was to investigate integrated weed management strategies that have been successful on individual species to determine their applicability to a multiple weed species that may coexist in a field. In addition, harvest weed seed control was incorporated into these integrated weed management strategies to determine its impact in western Canadian cropping systems. A 5-year rotational study was conducted from 2016 to 2020 at Beaverlodge, Lacombe, and Lethbridge, AB; Scott and Saskatoon, SK; and Carman, MB, that incorporated integrated weed management strategies such as rotational crop diversity (including winter annuals and perennials), increased seeding rates, crop silaging, chaff collection, and with or without in-crop herbicides. This research confirmed success in managing some species of weeds such as wild oat when increased seeding rates, 2 years of early cut silage barley, and competitive winter cereals were incorporated into a cropping system, even when no in-crop herbicides were applied. However, some weed growth morphologies (e.g., twining weeds) or life cycles (e.g., facultative winter annuals) were not managed successfully with this combination of strategies. Chaff collection provided incremental weed control benefits but did not serve as a replacement for herbicidal weed control. Weed densities had an apparent impact on the success of these integrated weed management strategies, suggesting that the sooner they are adopted, the more likely they are to be successful at maintaining or reducing weed densities. This study not only showed the ability to reduce reliance on herbicides with strategies that can be effective in Western Canada but also highlighted the need for further understanding of different weed species and their responses to integrated weed management strategies, as well as the complexity of managing a weed community with integrated weed management.

Response of dicamba-resistant Palmer amaranth and cotton to malathion applied in conjunction with dicamba

Abstract Cotton and soybean growers were offered new technologies in 2016, expanding in-crop herbicide options to include dicamba or 2,4-D. Within 3 yr of commercialization, dicamba use in these crops increased 10-fold, and growers began to report Palmer amaranth escapes in dicamba-tolerant production systems in western Tennessee. In 2020, Palmer amaranth seed was collected from eight Tennessee locations where growers witnessed poor control following dicamba. Greenhouse experiments were conducted to evaluate the response of these Palmer amaranth populations to dicamba. In 2021, field experiments were conducted on two tentative dicamba-susceptible populations in Georgia, on three confirmed dicamba-resistant populations in Tennessee, and on a tentative dicamba-susceptible population in Texas to evaluate cotton response following dicamba and to examine if malathion insecticide (a cytochrome P450 inhibitor) would improve weed control and not reduce cotton yield when applied in conjunction with dicamba. Palmer amaranth populations collected in 2020 survived dicamba in the greenhouse at 1, 2, and 4 times the labeled rate. Five Palmer amaranth populations exhibited 15% to 26% survival to the labeled dicamba rate (560 g ha−1) in the greenhouse. These findings were reinforced in the field when research on three of those populations in 2021 showed 55% control with the labeled dicamba rate and 69% control with 2 times the labeled rate. This demonstrates that the dicamba resistance allele or alleles were passed between generations. This result was not consistent in the Macon County, GA, or Worth County, GA, locations, where malathion improved dicamba control of 15- to 38-cm-tall Palmer amaranth. Cotton injury was observed when malathion was applied in combination with dicamba. These results further document the evolution of dicamba-resistant Palmer amaranth in Tennessee. Moreover, the nonreversal of resistance phenotype by malathion may suggest that the resistance mechanism is something other than metabolism.

Impact of integrated weed management practices on cotton economics and Palmer amaranth (Amaranthus palmeri) populations

AbstractThe threat of herbicide-resistant weed species, such as Palmer amaranth, has driven the development of robust weed management programs that rely on more than chemicals for weed control. Previous research has shown that zero-tolerance weed thresholds, cover crops, deep tillage, and diverse herbicide programs are effective strategies for controlling Palmer amaranth. Unfortunately, research investigating the integration of all four of these weed management strategies in a system is lacking. To better leverage these integrated weed management strategies in cotton production systems, a long-term study was initiated in fall 2018 near Marianna, AR, with zero tolerance, deep tillage, a cereal rye cover crop, and either a dicamba or non-dicamba in-crop herbicide program as factors. Results found that total Palmer amaranth emergence was reduced 76% as the result of deep tillage in 2019 and, in the absence of a zero-tolerance strategy, 73% in 2020. In the absence of a zero-tolerance strategy, the combination of a non–cover crop strategy and dicamba herbicide program decreased total Palmer amaranth emergence by 73%, while the combination of a cover crop strategy and dicamba herbicide program decreased total Palmer amaranth emergence by 78% compared to the combination of a cover crop and non-dicamba herbicide program. Under a zero-tolerance strategy in 2019, tillage reduced cotton yield by 12% and partial returns by US$370 ha−1. In 2020, tillage reduced cotton yield by 14% and partial returns of US$371 ha−1 under a non-zero-tolerance strategy, while a 12% yield reduction and a US$260 ha−1 decrease in partial returns were observed under a zero-tolerance strategy. In 2019, the non-dicamba program resulted in greater partial returns than the dicamba in-crop program because of greater yield and lower program costs. However, in 2020, partial returns were greater for the dicamba in-crop herbicide program owing to greater yields achieved by this program.

Evaluation of panicle removal methods and crop topping applications as supplemental tools for wild oat (Avena fatua) management

AbstractIncreased frequency and occurrence of herbicide-resistant biotypes heightens the need for alternative wild oat management strategies. This study aimed to exploit the height differential between wild oat and crops by targeting wild oat between panicle emergence and seed shed timing. Two field studies were conducted either in Lacombe, AB, or Lacombe, AB and Saskatoon, SK, from 2015 to 2017. In the first study, we compared panicle removal methods: hand clipping, use of a hedge trimmer, and a selective herbicide crop topping application to a weedy check and an industry standard in-crop herbicide application in wheat. These treatments were tested early (at panicle emergence), late (at initiation of seed shed), or in combination at one location over 3 yr. In the second study, we investigated optimal timing of panicle removal via a hedge trimmer with weekly removals in comparison to a weedy check in wheat and lentil. This study was conducted at two locations, Lacombe, AB, and Saskatoon, SK, over 3 yr. Among all the tested methods, the early crop topping treatment consistently had the largest impact on wild oat density, dockage, seedbank, and subsequent year crop yield. The early (at panicle emergence) or combination of early and late (at initiation of seed shed) treatments tended to reduce wild oat populations the following season the most compared to the late treatments. Subsequent wild oat populations were not influenced by panicle removal timing, but only by crop and location interactions. Panicle removal timing did significantly affect wild oat dockage in the year of treatment, but no consistent optimal timing could be identified. However, the two studies together highlight additional questions to be investigated, as well as the opportunity to manage wild oat seedbank inputs at the panicle emergence stage of the wild oat lifecycle.

Cultural weed management practices shorten the critical weed-free period for soybean grown in the Northern Great Plains

Abstract Soybean [Glycine max (L.) Merr.] has recently become a popular rotational crop in the Canadian Northern Great Plains where herbicide-resistant (HR) soybean cultivars have been widely adopted. Intense reliance on herbicides has contributed to the development of HR weeds in soybean and other crops. Cultural weed management practices reduce the need for herbicides and lower the selection pressure for HR weed biotypes by improving the competitiveness of the crop. The effects of two row spacings, three target densities, and three cultivars on the critical weed-free period (CWFP) in soybean were evaluated as three separate experiments in southern Manitoba. In the row-spacing experiment, soybean grown in narrow rows shortened the CWFP by up to three soybean developmental stages at site-years with increased weed pressure. In the target density experiment, low-density soybean stands lengthened the CWFP by one soybean developmental stage compared with higher-density soybean stands. The effect of soybean cultivar varied among locations, yet tended to be consistent within location over the 2-yr study, suggesting that competitive ability in these soybean cultivars was linked to edaphic and/or environmental factors. Generally, the cultivar with the shortest days to maturity, which also had the shortest stature, consistently had a longer CWFP. Each of these cultural practices were effective at reducing the need for in-crop herbicide applications.

Insecticide Seed Treatments Reduced Crop Injury from Flumioxazin, Chlorsulfuron, Saflufenacil, Pyroxasulfone, and Flumioxazin + Pyroxasulfone + Chlorimuron in Soybean

With increased instances of weed resistance to applications of postemergence herbicides, the use of soil-applied herbicides that offer residual activity is becoming popular. Unfortunately, under some conditions, the use of residual herbicides can result in unintentional injury to crops. However, there are a number of ways to reduce these risks, including the use of in-crop herbicide safeners. Based on previous research conducted on rice, the potential may exist for crop injury from certain soil-applied herbicides to be reduced (safened) in seeds treated with insecticides. Field trials were conducted in Marianna, Arkansas, in 2015 and 2016, and near Colt, Arkansas, in 2016, to explore this possibility in soybean. Soybean seeds were treated with the insecticide thiamethoxam and subsequently the herbicides metribuzin, saflufenacil, pyroxasulfone, sulfentrazone, chlorimuron, flumioxazin, flumioxazin + pyroxasulfone + chlorimuron, mesotrione, and chlorsulfuron were applied immediately after planting. Of the nine herbicides evaluated, the insecticide reduced crop injury for flumioxazin, chlorsulfuron, saflufenacil, pyroxasulfone, and flumioxazin + pyroxasulfone + chlorimuron. The highest degree of injury reduction was seen 1 week after emergence (WAE) at Marianna, where injury from flumioxazin + pyroxasulfone + chlorimuron was reduced from 15% to 5%. Based on the results from this study, the insecticide seed treatment thiamethoxam may have the potential to safen soybean to applications of some soil-applied herbicides.

Altering the competitiveness of tame oat (Avena sativa L.) versus wild oat (Avena fatua L.) with phosphorus and seeding rate

Currently, no in-crop herbicide is registered to control wild oat (Avena fatua L.) in tame oat (Avena sativa L.). Wild oat must be controlled in tame oat using other agronomic practices. The objective of this research was to determine if side-banded phosphorus (P) in combination with seeding rate would increase the competitiveness of tame oat with wild oat, increasing yield and quality. An experiment was conducted from 2003–2005 at Indian Head, SK. The experimental design was a strip-plot design with four replications. The strips were low and high wild oat density. A two-way factorial, seeding rate (150, 250, 350, and 450 plants m−2), and P rate (0, 15, and 30 kg P2O5 ha−1) were seeded across the strips. Phosphorus affected seed density, grain yield, oat biomass, and wild oat fecundity. Seeding rate affected most of the measured variables and interacted with wild oat and year. The application of P increased the competiveness of oat by increasing crop biomass by 7.6% and grain yield by 3.4% and decreasing wild oat seed from 1.26% to 0.76% in the harvested grain. Wild oat decreased grain yield by 23% in 2003, 4.4% in 2004, and 11% in 2005. Increasing the seeding rate increased grain yield by 5% when wild oat was present. Wild oat did not interfere with the uptake of side-banded P. Producers need to use both P fertilization and higher seeding rates to improve the competitiveness of tame oat and the management of wild oat in tame oat.

Integrating Herbicide Programs with Harvest Weed Seed Control and Other Fall Management Practices for the Control of Glyphosate-Resistant Palmer Amaranth (Amaranthus palmeri)

A large-plot field experiment was conducted at Keiser, AR, from fall of 2010 through fall of 2013 to understand to what extent soybean in-crop herbicide programs and postharvest fall management practices impact Palmer amaranth population density and seed production over three growing seasons. The effect of POST-only (glyphosate-only) or PRE followed by (fb) POST (glyphosate or glufosinate) + residual herbicide treatments were evaluated alone and in combination with postharvest management options of soybean residue spreading or soil incorporation, use of cover crops, windrowing with/without burning, and residue removal. Significant differences were observed between fall management practices on Palmer amaranth population density each fall. The use of cover crops and residue collection and removal fb the incorporation of crop residues into soil during the formation of beds were the most effective practices in reducing Palmer amaranth population. In contrast, the effects of fall management practices on Palmer amaranth seed production were inconsistent among years. The inclusion of a PRE herbicide application into the herbicide program significantly reduced Palmer amaranth population density and subsequent seed production each year when compared to the glyphosate-only program. Additionally, the glufosinate-containing residual program was superior to the glyphosate-containing residual program in reducing Palmer amaranth seed production. PRE fb POST herbicides resulted in significant decreases in the Palmer amaranth population density and seed production compared to POST application of glyphosate alone for all fall management practices, including the no-till practice. This study demonstrated that crop residue management such as chaff removal from the field, the use of cover crops, or seed incorporation during bed formation in combination with an effective PRE plus POST residual herbicide program is important for optimizing in-season management of Palmer amaranth and subsequently reducing the population density, which has a profound impact on lessening the risk for herbicide resistance and the consistency and effectiveness of future weed management efforts.

Cropping systems and the prevalence of giant ragweed (Ambrosia trifida): From the 1950’s to present

Over the course of the past two decades, giant ragweed (Ambrosia trifida L.) has become an important weed of arable lands in many parts of the United States and Canada. Its shift from a primarily ruderal weed to one commonly found in row crop production has coincided with advancements in maize and soybean production practices and the associated fluctuations in the area seeded to each of these major crops. A field trial was conducted over three years to explore how cropping systems practices representative of distinct eras of maize and soybean production (i.e., the 1950s, 1980s and 2000s) influenced giant ragweed’s ability to grow, develop and successfully reproduce within a crop field. Maize hybrids and soybean cultivars bred in each era were seeded in separate split plot designs with era of production practices as the whole plot and time of giant ragweed transplanting (i.e., at crop emergence or 14 days after) as the split plot. Results of this study indicate that historic changes brought about through maize breeding and shifts in cropping systems practices have increased the capacity of this crop to suppress the growth and reproduction of giant ragweed. Conversely, the phenotypic shift of soybean from a vine to a more upright row crop, coupled with the relative lack of change in production practices, has had little impact on its weed suppressive capacity. Giant ragweed’s ability to overtop soybean, even when placed at a competitive disadvantage by delayed emergence, creates a situation where the control of giant ragweed seed production is almost exclusively dependent on the availability and efficacy of in-crop herbicide options. In contrast, there is a clear potential for the suppression of giant ragweed fecundity when modern maize hybrids, planted at high plant population densities, are provided a 14d weed-free window following emergence.

Late-Season Grass Weed Management with In-Crop and Post-Harvest Herbicides in Twin-Row Glyphosate-Resistant Soybean

Emergence of grasses late in the season has become a problem in glyphosate-resistant (GR) soybean production in the southern US. A 3-yr field study was conducted from 2011 to 2013 at Stoneville, MS to determine efficacy of post-harvest and pyroxasulfone-based in-crop herbicides on late-season grasses and yield in twin-row glyphosate-resistant soybean. Experiments were conducted in a split-plot arrangement of treatments in a randomized complete block design with fall herbicides (with and without pendimethalin at 1.12 kg ai ha-1 and paraquatat 0.84 kg ai ha-1) as main plots and in-crop herbicides as subplots with four replications. The six in-crop herbicide programs were: glyphosate applied early postemergence (EPOST) at 0.84 kg·aeha-1 followed by (fb) glyphosate late postemergence (LPOST) at 0.84 kg·ha-1 with and without pyroxasulfone preemergence (PRE) applied at 0.18 kg ai ha-1, pyroxasulfone PRE fb glyphosate at 0.84 kg·ha-1 LPOST or glyphosate at 0.84 kg·ha-1 + S-metolachlor at 1.68 kg ai ha-1 EPOST, pyroxasulfone PRE fb S-meto- lachlor at 1.12 kg·ha-1 + fomesafen at 0.27 kg ai ha-1 EPOST fb clethodim at 0.14 kg ai ha-1, and a no-herbicide control. Browntop millet, Digitaria spp., and junglerice densities at 2 weeks after LPOST, grass weed dry biomass at harvest, and soybean yield were similar regardless of post- harvest herbicides in all three years. At 2 weeks after LPOST, browntop millet, Digitaria spp. and junglerice densities were greatly reduced in all five in-crop herbicide treatments compared with no herbicide plot in all three years. Grass weed dry biomass in no-herbicide plots was 3346, 6136, and 6916 kg·ha-1 in 2011, 2012, and 2013, respectively and the five herbicide treatments reduced grass weed dry biomass by at least 87%, 84%, and 99% in 2011, 2012, and 2013, respectively. Soybean yield was higher with all five in-crop herbicide treatments compared to no herbicide control in all three years. These results indicate that browntop millet, Digitaria spp., and junglerice infestations can be reduced with pyroxasulfone-based in-crop herbicide programs in twin-row GR soybean.

Wide row spacing for deep-furrow planting of winter wheat

A tillage-based winter wheat (Triticum aestivum L.)-summer fallow rotation is practiced on 1.56 million cropland hectares in the low-precipitation (<300mm annual) region of the Inland Pacific Northwest of the United States (PNW). Farmers use deep-furrow drills with rows spaced 40–45-cm apart to plant winter wheat (WW) as deep as 20cm below the soil surface to reach moisture in summer fallow (SF). Conservation tillage methods have been successfully developed that preserve ample residue during SF to control wind erosion, but existing drills cannot pass through heavy residue without plugging; thus farmers are reluctant to adopt conservation-tillage practices. We conducted field experiments over 3 years at three sites using the same number of seeds row−1 (8 site years) and same number of seeds ha−1 (3 site years) with row spacing of 40, 45, 50, 55, 60, and 80cm and measured effects on grain yield, grain yield components, straw production, and weed dynamics. With same number of seeds row−1 (seeding rate declined as row spacing widened) the highest average grain and straw yield was achieved with the 40 and 45-cm spacing with gradual decline as row spacing widened due to fewer spikes unit area−1 (SPU) and despite increased kernels spike−1 (KPS). Kernel weight (KW) was not a factor. With same number of seeds ha−1 (more seeds row−1 as row spacing widened) there were no overall differences in SPU, KPS, KW, and straw production among treatments and only a slight grain yield reduction at the two widest spacing treatments. Weeds were not an agronomic problem with any spacing treatment due to timely and effective in-crop herbicide application although weed dry biomass did increase slightly as row spacing widened. Our research suggests that row spacing for WW production in the dryland PNW can be widened to at least 50cm and most likely 55cm to facilitate conservation-tillage farming with equal grain and straw production compared to narrower row spacing currently used by farmers.

Intensive cropping systems select for greater seed dormancy and increased herbicide resistance levels in Lolium rigidum (annual ryegrass).

Lolium rigidum (annual ryegrass) is a widespread annual crop weed that has evolved high levels of resistance to selective herbicides. Anecdotal evidence suggests that intensive cropping also leads to higher seed dormancy in L. rigidum. This was quantified by measuring dormancy levels in L. rigidum populations collected from paired sites (one with nil to low cropping intensity, the other intensively cropped) located throughout the Western Australian grain belt. Populations from non-cropped fields or those with low cropping intensity showed higher and faster germination than populations from fields with a medium- or high-intensity cropping regime. Resistance to selective herbicides was also higher in the medium- and high-intensity cropping fields than in the low-intensity cropping fields. High-intensity cropping systems are likely to impose greater selection pressures for seed dormancy and selective herbicide resistance, because late-emerging seedlings avoid preplanting weed control practices (tillage and non-selective herbicide application) but are exposed to selective in-crop herbicides.

Site-specific wild oat (Avena fatua L.) management

Beckie, H. J. and Shirriff, S. 2012. Site-specific wild oat ( Avena fatua L.) management. Can. J. Plant Sci. 92: 923–931. Variation in soil properties, such as soil moisture, across a hummocky landscape may influence wild oat emergence and growth. To evaluate wild oat emergence, growth, and management according to landscape position, a study was conducted from 2006 to 2010 in a hummocky field in the semiarid Moist Mixed Grassland ecoregion of Saskatchewan. The hypothesis tested was that wild oat emergence and growth would be greater in lower than upper slope positions under normal or dry early growing season conditions. Three herbicide treatments were imposed on the same plots each year of a 2-yr canola (Brassica napus L.) – wheat (Triticum aestivum L.) sequence: (1) nontreated (weedy) control; (2) herbicide application to upper and lower slope positions (i.e., full or blanket application); and (3) herbicide application to lower slope position only. Slope position affected crop and weed densities before in-crop herbicide application in years with dry spring growing conditions. Site-specific wild oat herbicide application in hummocky fields in semiarid regions may be justified based on results of wild oat control averaged across slope position. In year 2 of the crop sequence (wheat), overall (i.e., lower and upper slope) wild oat control based on density, biomass, and dockage (i.e., seed return) was similar between site-specific and full herbicide treatment in 2 of 3 yr. Because economic thresholds have not been widely adopted by growers in managing wild oat, site-specific treatment in years when conditions warrant may be an appropriate compromise between no application and blanket herbicide application.

The Effect of Rotation and In-Crop Weed Management on the Germinable Weed Seedbank after 10 Years

Agricultural production systems that reduce the use of in-crop herbicides could greatly reduce risks of environmental damage and the development of herbicide-resistant weeds. Few studies have investigated the long-term effects of in-crop herbicide omissions on weed seedbank community size and structure. A crop-rotation study was sampled 10 yr after a strictly annual rotation and an annual/perennial rotation were exposed to different in-crop herbicide omission treatments. In-crop herbicides were applied either in all annual crops (control), omitted from oats only, or omitted from both flax and oats. Seedbank densities were greatest when in-crop herbicides were omitted from flax and oats, and this treatment also reduced crop yield. Shannon-Wiener diversity differed among crops in the annual crop rotation and among herbicide omission treatments in the perennial rotation. Herbicide omissions changed the weed-community structure in flax and in wheat and canola crops in the annual rotation enough to warrant alternate control methods in some treatments. The magnitude of the effects on the seedbank parameters depended largely on the competitive ability of the crop in which herbicides were omitted. No yield response to omitting herbicides in oats indicated that standard weed management practices have reduced weed populations below yield-loss thresholds.

Environmental Effects on the Relative Competitive Ability of Canola and Small-Grain Cereals in a Direct-Seeded System

Growing crops that exhibit a high level of competition with weeds increases opportunities to practice integrated weed management and reduce herbicide inputs. The recent development and market dominance of hybrid canola cultivars provides an opportunity to reassess the relative competitive ability of canola cultivars with small-grain cereals. Direct-seeded (no-till) experiments were conducted at five western Canada locations from 2006 to 2008 to compare the competitive ability of canola cultivars vs. small-grain cereals. The relative competitive ability of the species and cultivars was determined by assessing monocot and dicot weed biomass at different times throughout the growing season as well as oat (simulated weed) seed production. Under most conditions, but especially under warm and relatively dry environments, barley cultivars had the greatest relative competitive ability. Rye and triticale were also highly competitive species under most environmental conditions. Canada Prairie Spring Red wheat and Canada Western Red Spring wheat cultivars usually were the least competitive cereal crops, but there were exceptions in some environments. Canola hybrids were more competitive than open-pollinated canola cultivars. More importantly, under cool, low growing degree day conditions, canola hybrids were as competitive as barley, especially with dicot weeds. Under most conditions, hybrid canola growers on the Canadian Prairies are well advised to avoid the additional selection pressure inherent with a second in-crop herbicide application. Combining competitive cultivars of any species with optimal agronomic practices that facilitate crop health will enhance cropping system sustainability and allow growers to extend the life of their valuable herbicide tools.

A Sustainable Management Package to Improve Winter Wheat Production and Competition with Weeds

Advances in cultivar development and the demand for winter wheat (Triticum aestivum L.) as an ethanol feedstock has increased winter wheat acreage across the Canadian Prairies. A sustainable production package is required to maintain this renewed interest. Experiments were established in 2002–2004 at Lethbridge and Lacombe, AB, to determine cultivar, seeding rate, and herbicide effects on weed competition and crop yield. Treatments included a factorial combination of four contrasting cultivars (Radiant, CDC Osprey, CDC Falcon, and CDC Ptarmigan), three seeding rates (300, 450, and 600 seeds m−2), and two herbicide treatments (fall only or fall plus a spring in‐crop herbicide). CDC Ptarmigan yield was higher (12%) than the other cultivars. This was expected as CDC Ptarmigan has higher yield potential, but its ability to maintain high yields in the presence of weeds was unexpected. The yield of CDC Falcon and Radiant was similar, but CDC Falcon had inferior weed competitive ability, as shown by yield differences, relative to the other three cultivars. CDC Osprey yielded less than the other cultivars. Grain yield was reduced when planted at 600 seeds m−2 by 4%, but weed biomass was less (40%). Spring in‐crop herbicide application reduced weed biomass, but the extra application did not improve grain yield. These results suggest winter wheat yields can be maintained without added inputs of spring herbicides, and greater stability of yield, winter survival, and competitiveness will usually occur with increased seeding rates.

Pre-emergence mechanical weed control in field pea (Pisum sativum L.)

Producers who wish to adopt organic or pesticide-free crop production are seeking alternatives to herbicides. A field experiment was conducted at Scott, SK, from 1999 to 2001 to determine the combination of seeding date (early-May, mid-May, and late-May) and weed management system (shallow-seeded untreated, shallow-seeded with pre-emergence tillage, shallow-seeded herbicide, deep-seeded untreated, deep-seeded with pre-emergence tillage, and deep-seeded herbicide) that would optimize yield of field pea grown without the use of herbicides. The shallow and deep seeded pre-emergence tillage treatments were a 2.5-cm seeding depth followed by two sequential harrow passes and a 7.5-cm seeding depth followed by two sequential rod-weeder passes, respectively. None of the weed management systems had a significant effect on field pea density. The herbicide treatments were the only treatments that had a significant effect on weed density as the application timing was more synchronous with weed emergence than were the tillage treatments. The pre-emergence tillage treatments reduced weed biomass when seeding was delayed until mid-May following weed emergence. Deep seeding followed by pre-emergence rod-weeding was more efficacious in reducing weed fresh weight than shallow seeding followed by pre-emergence harrowing; however, they both provided similar yield responses at the mid-May seeding date. Delayed seeding until late May resulted in a 30 to 35% reduction in field pea yield compared with early or mid-May seeding, respectively. Early-seeded field peas followed by an in-crop herbicide application resulted in the highest field pea yields with the best seeding date by pre-emergence tillage combination achieving 81% of these yields. A strategy for field pea producers who choose not to use herbicides would be to delay seeding until some weeds emerge and seed at a 7.5-cm depth followed by two sequential rod-weeding passes prior to crop emergence. Key words: Field pea, seeding date, seed depth, pre-emergence tillage

In-Crop and Autumn-Applied Glyphosate Reduced Purple Nutsedge (Cyperus rotundus) Density in No-Till Glyphosate-Resistant Corn and Soybean

A 3-yr field study was conducted from 2005 to 2007 at Stoneville, MS, to determine efficacy of in-crop and autumn-applied glyphosate on purple nutsedge density and yield of no-till glyphosate-resistant (GR) corn and GR soybean. Separate experiments were conducted in GR corn and GR soybean in areas maintained under a no-tillage system after the autumn of 2004. Each experiment was conducted in a split-plot arrangement of treatments in a randomized complete-block design with and without autumn application of glyphosate at 1.68 kg ae/ha as main plots and in-crop herbicide application (glyphosate- and nonglyphosate-based programs) as subplots with three replications. In GR corn, glyphosate applied in the autumn reduced purple nutsedge density by 40 to 67% compared with no glyphosate during 3 yr. In GR corn, glyphosate applied in-crop reduced purple nutsedge shoot density by 48% in 2005, 92% in 2006, and 100% in 2007 compared with no herbicide. However, GR corn yields were unaffected by either in-crop or autumn-applied glyphosate. In GR soybean, glyphosate applied in the autumn reduced purple nutsedge shoot density by 64 to 83% compared with no glyphosate during 3 yr. Glyphosate applied in-crop in GR soybean reduced purple nutsedge density by 81% in 2005 and by 100% in 2006 and 2007 compared with no herbicide. GR soybean yields were similar in 2005, but yields were 34 and 18% higher in 2006 and 2007, respectively, with autumn-applied glyphosate compared with no glyphosate. GR soybean yields were higher with glyphosate applied in-crop compared with no herbicide in 2 of 3 yr. These results indicate that purple nutsedge density could be reduced with glyphosate applied in-crop in no-till GR corn and GR soybean. In addition, autumn-applied glyphosate was effective in reducing purple nutsedge populations following harvest of crops and could be an effective purple nutsedge management strategy regardless of GR trait.