Winter Annual Grass Weeds Research Articles

Sensitivity of nonresistant winter wheat to quizalofop-P-ethyl in central Oklahoma and Kansas

AbstractThe CoAXium® Production System includes a new herbicide-resistant wheat (AXigen®) that allows for fall and/or spring postemergence (POST) applications of quizalofop-P-ethyl (QPE) for control of winter annual grass weeds. As area planted with AXigen® wheat increases, so will the use of QPE herbicide, and with this comes an increased chance for physical drift, tank contamination, or misapplication to nearby sensitive plants. A total of eight field studies were conducted at four locations during the 2018–2019 and 2019–2020 growing seasons to understand the response of nonresistant wheat when exposed to various rates of QPE herbicide. Five rates of QPE were evaluated: 1× (92 g ai ha−1), 1/10×, 1/50×, 1/100×, and 1/200×. Treatments of QPE were applied in the fall (2- to 3-leaf wheat) or in the spring (3- to 4-tiller wheat). Results indicated an interaction between application timing and QPE rate on grain yield for half of the site-years. The 1× rate resulted in complete or near complete grain yield loss regardless of application timing. However, QPE at the 1/10× rate resulted in yield loss ranging from 0% to 41% when fall-applied, whereas spring application resulted in 80% to 100% yield loss. For site-years when only the main effect of QPE rate was significant, 86% to 100% yield loss was observed following exposure to QPE at the 1/10× and 1× rates. For all site-years, it was infrequent that significant yield reductions were observed following the three lowest rates of QPE. If the two highest QPE rates were considered to represent tank contamination or misapplication and the three lowest rates physical drift, we can assume that physical drift of QPE to non-AXigen® wheat is not of major concern if proper application guidelines are followed. Conversely, tank contamination and misapplication should be carefully considered by growers who have planted both AXigen® and non-AXigen® wheat varieties.

Winter annual grass control and crop safety in quizalofop‐resistant wheat cultivars

AbstractWinter annual grass species such as jointed goatgrass (Aegilops cylindrica Host), downy brome (Bromus tectorum L.), and feral rye (Secale cereale L.) negatively affect winter wheat (Triticum aestivum L.) yields. To generate a novel in‐crop selective herbicide trait for winter annual grass control, mutagenesis was used to generate multiple winter wheat lines resistant to the acetyl co‐A carboxylase (ACCase) inhibitor quizalofop p‐ethyl (QPE). Field trials were performed to assess crop safety for wheat lines with one or two homoeologous mutations and to assess winter annual grass control with QPE treatments. Crop safety parameters including visual injury rating, plant height, grain yield, and kernel weight were evaluated following QPE treatment at multiple growth stages. The susceptible winter wheat cultivar Hatcher displayed 100% crop injury when treated at tillering in early spring or at jointing in late spring. Single‐gene lines generally had crop injury and yield loss, especially at the jointing application timing. The two‐mutation cultivars Incline AX and LCS Fusion AX had low to no injury or yield loss from QPE applications at all timings and no injury from high rate QPE applications (93 and 185 g ha−1) in either the fall or the spring. Control of downy brome and feral rye was highest (92–99%), while jointed goatgrass control ranged from a low of 73% at 93 g a.i. ha−1 to a high of 98% at 109 a.i. ha−1 QPE. Our data indicate that the targeted winter annual grass weeds are controlled by QPE at rates that have acceptable crop safety for two‐mutation QPE‐resistant wheat cultivars.

Feral rye control in quizalofop‐resistant wheat in central Great Plains

AbstractFeral rye (Secale cereale L.) is one of the most troublesome winter annual grass weed species in semi‐arid winter wheat (Triticum aestivum L.) production systems across the central Great Plains (CGP). Lack of effective herbicide options further complicates the selective control of feral rye in winter wheat. The main objectives of this research were: (a) determine the sensitivity response of 10 feral rye populations collected from Kansas wheat fields to quizalofop‐p‐ethyl in a dose‐response assay, and (b) to determine the effectiveness of various rates and application timings (fall or spring) of quizalofop‐p‐ethyl for feral rye control in quizalofop‐resistant winter wheat in the CGP. Dose‐response assays indicated that all tested feral rye populations were highly sensitive to quizalofop‐p‐ethyl with GR90 values (doses of quizalofop‐p‐ethyl needed for 90% reductions in shoot biomass at 3 wk after treatment) ranging from 33 to 72 g a.i. ha−1. A field study conducted in Colorado, Kansas, and Oklahoma for a total of 5 site‐years indicated that quizalofop‐p‐ethyl applied at ≥77 g a.i. ha−1 in the fall or spring provided season‐long feral rye control at 92–100% in quizalofop‐resistant winter wheat. Irrespective of tested rates and application timings, a quizalofop‐p‐ethyl treatment protected winter wheat yield loss of 17–27% due to reduced feral rye competition compared to the nontreated weedy check in Colorado and Oklahoma. Results suggested that quizalofop‐p‐ethyl at labelled use rates (77–92 g a.i. ha−1) applied in fall or spring can provide effective feral rye control in quizalofop‐resistant winter wheat.

Beyond Bird Feed: Proso Millet for Human Health and Environment

Domesticated in 8000–10,000 BP in northern China, proso millet (Panicum miliaceum L.) is the best adaptive rotational crop for semiarid central High Plains of the USA, where average annual precipitation is 356–407 mm. Proso millet has multiple benefits when consumed as human food. Proso millet is rich in minerals, dietary fiber, polyphenols, vitamins and proteins. It is gluten-free and therefore, ideal for the gluten intolerant people. Proso millet contains high lecithin which supports the neural health system. It is rich in vitamins (niacin, B-complex vitamins, folic acid), minerals (P, Ca, Zn, Fe) and essential amino acids (methionine and cysteine). It has a low glycemic index and reduces the risk of type-2 diabetes. Unfortunately, in the USA, it is mostly considered as bird feed, whereas it is mainly used as human food in many other countries. Besides human health benefits, proso millet has an impeccable environmental benefit. Proso millet possesses many unique characteristics (e.g., drought tolerance, short-growing season) which makes it a promising rotational crop for winter wheat-based dryland farming systems. Proso millet provides the most economical production system when used in a two years wheat/summer fallow cropping system in semiarid High Plains of the USA. It helps in controlling winter annual grass weeds, managing disease and insect pressure and preserving deep soil moisture for wheat. Proso millet can also be used as a rotational crop with corn or sorghum owing to its tolerance for atrazine, the primary herbicide used in corn and sorghum production systems. Proso millet certainly is a climate-smart, gluten-free, ancient, and small grain cereal, which is healthy to humans and the environment. The main challenge is to expand the proso millet market beyond bird feed into the human food industry. To overcome the challenge, unique proso millet varieties for human food and ready-to-use multiple food products must be developed. This requires successful collaboration among experts from diverse disciplines such as breeders, geneticists, food chemists and food industry partners.

Jointed Goatgrass Biomass and Spikelet Production Increases in No‐till Winter Wheat

Core Ideas Initial spikelet density affects jointed goatgrass populations in subsequent years. Jointed goatgrass density can quickly increase in consecutive no‐till winter wheat crops. Maintaining low spikelet densities is critical for jointed goatgrass management. Jointed goatgrass (Aegilops cylindrica Host) remains a troublesome winter annual grass weed in winter wheat (Triticum aestivum L.) production systems in the western US. A study involving two field experiments (1992–1994 and 1993–1995) was conducted near Pullman, WA to quantify the effect of initial jointed goatgrass spikelet density on jointed goatgrass biomass and spikelet production in the first and second year of consecutive no‐till winter wheat crops. Initial jointed goatgrass densities were established by placing jointed goatgrass spikelets on the soil surface in 10.8 sq ft (1 sq m) quadrats at target densities of 2, 4, 8, 16, 32, 64, and 128 spikelets/10.8 sq ft. Precipitation varied greatly between the three winter wheat growing seasons and influenced the increase of jointed goatgrass biomass and spikelet production between the first and second year of both field experiments. Despite seasonal precipitation differences, the increase in jointed goatgrass biomass and spikelet production from Year 1 to Year 2 was directly related to initial spikelet density in both experiments. Although keeping jointed goatgrass densities low may slow the rate of increase in jointed goatgrass density from year‐to‐year compared to greater jointed goatgrass densities, even at low densities, jointed goatgrass has the potential to quickly increase population density in consecutive no‐till winter wheat crops.

Germination of Winter Annual Grass Weeds under a Range of Temperatures and Water Potentials

Silky windgrass and annual bluegrass are among the most troublesome weeds in northern European winter crops, while problems with rattail fescue have been especially linked to direct-drilling practices. This study investigated the germination patterns of silky windgrass, annual bluegrass, and rattail fescue in multiple water potentials and temperature regimes. Temperature and water potential effects were similar between silky windgrass and rattail fescue, but differed from annual bluegrass. The three grass weeds were able to germinate under low water potential (−1.0 MPa), although water potentials ≤−0.25 MPa strongly delayed their germination. Silky windgrass and rattail fescue seeds were able to germinate at 1 C, while the minimum temperature for annual bluegrass germination was 5 C. Germination of silky windgrass and rattail fescue was very similar across temperature and water potentials, which implies similar emergence flushes under field conditions, allowing management interventions to follow the same scheme.

Lack of Host Specialization on Winter Annual Grasses in the Fungal Seed Bank Pathogen Pyrenophora semeniperda.

Generalist plant pathogens may have wide host ranges, but many exhibit varying degrees of host specialization, with multiple pathogen races that have narrower host ranges. These races are often genetically distinct, with each race causing highest disease incidence on its host of origin. We examined host specialization in the seed pathogen Pyrenophora semeniperda by reciprocally inoculating pathogen strains from Bromus tectorum and from four other winter annual grass weeds (Bromus diandrus, Bromus rubens, Bromus arvensis and Taeniatherum caput-medusae) onto dormant seeds of B. tectorum and each alternate host. We found that host species varied in resistance and pathogen strains varied in aggressiveness, but there was no evidence for host specialization. Most variation in aggressiveness was among strains within populations and was expressed similarly on both hosts, resulting in a positive correlation between strain-level disease incidence on B. tectorum and on the alternate host. In spite of this lack of host specialization, we detected weak but significant population genetic structure as a function of host species using two neutral marker systems that yielded similar results. This genetic structure is most likely due to founder effects, as the pathogen is known to be dispersed with host seeds. All host species were highly susceptible to their own pathogen races. Tolerance to infection (i.e., the ability to germinate even when infected and thereby avoid seed mortality) increased as a function of seed germination rate, which in turn increased as dormancy was lost. Pyrenophora semeniperda apparently does not require host specialization to fully exploit these winter annual grass species, which share many life history features that make them ideal hosts for this pathogen.

Feral Rye (Secale cereale) Control in Winter Canola in the Pacific Northwest

In the Pacific Northwest (PNW), feral rye is a predominant winter annual grass weed in the low-rainfall region where a winter wheat–summer fallow rotation has been practiced for more than 130 yr. Recently, producers began including winter canola in their rotation, which provides additional herbicide options to control feral rye. A 3-yr study was conducted in Washington to determine the efficacy of clethodim, quizalofop, and glyphosate on feral rye control and winter canola yield. During the first year of the study herbicides were applied in the spring, and they increased canola yield and decreased feral rye biomass, density, and seed production similarly when compared with the nontreated control. During the last 2 yr of the study, split applications of quizalofop and glyphosate were the most effective treatments for controlling rye (> 95%) and increasing canola yield. In general, clethodim was less effective than both quizalofop and glyphosate in controlling feral rye. Results from this study indicate that quizalofop in conventional or glyphosate-resistant winter canola and glyphosate in glyphosate-resistant winter canola can effectively control feral rye.

Competitive ability of rotational crops with weeds in dryland organic wheat production systems

AbstractWhile demand continues to grow and prices for organic grains have remained almost double those of conventional grains, few growers in the dryland wheat production region of Eastern Washington produce organic grain. Growers have cited weed control constraints as a top factor preventing adoption of organic production practices. In organic systems, inherent competitive ability of crops is very important in managing and preventing weed infestations. The objective of this study was to identify crop species that could reduce weed pressure and compete with perennial weeds in a wheat (Triticum aestivum) rotation. To assess weed suppressive ability of alternative rotational crops, relative total weed biomass and relative biomass of two perennial and three annual weed species were examined over 4 years in three organic cropping systems where winter wheat was in rotation with alternative crops, as part of a long-term study examining dryland organic wheat production. Three years of continuous alfalfa (Medicago sativa) production reduced Canada thistle (Cirsium arvense) biomass during the alfalfa production phase, and reduced Canada thistle biomass in winter wheat following alfalfa, compared with wheat following winter pea (Pisum sativum). Alfalfa was competitive with wild oat (Avena fatua), though it competed poorly with winter annual grass weeds. Spring barley (Hordeum vulgare), when established successfully, suppressed jointed goatgrass (Aegilops cylindrica) more than winter triticale (xTriticosecale), winter wheat, and spring wheat, and demonstrated some competitiveness toward field bindweed (Convolvulus arvensis). Winter triticale competed poorly with field bindweed, but suppressed Canada thistle and downy brome (Bromus tectorum). All alternative rotational crops contained lower weed biomass than winter pea, the crop typically rotated with wheat in the region. Organic and conventional growers could gain benefits in perennial and winter annual weed suppression by incorporating alternative rotational crops into wheat rotations.

Resistance Allele Movement between Imazamox-Resistant Wheat and Jointed Goatgrass (Aegilops cylindrica) in Eastern Oregon Wheat Fields

Imazamox-resistant wheat varieties carry theImi1allele, which confers resistance to the imidazolinone (IMI) herbicide imazamox. This resistance trait allows the selective control of jointed goatgrass, a difficult-to-control winter annual grass weed. Allele movement between IMI-resistant wheat and jointed goatgrass may occur via hybridization and backcross events. Hybrids (F1) of IMI-resistant wheat and jointed goatgrass were identified in 2008 in a commercial wheat field in Eastern Oregon. In 2009 and 2010, surveys were conducted in Eastern Oregon to determine the prevalence of theImi1allele in wheat × jointed goatgrass hybrids. Using polymerase chain reaction assays we detected the presence of theImi1allele. A total of 128 sites were surveyed over the 2 yr. Of 1,548 plants sampled, 1,100 were positive for theImi1alelle and of those, 1,087 were heterozygous and 13 were homozygous for the allele. We assessed hybrid yield components and how these components varied across the sampled sites. The association between the proportion of IMI-resistant hybrids and the area or management practice in the commercial fields was determined. Nonagricultural sites or production of IMI-resistant wheat in consecutive years were two factors associated with a greater proportion of IMI-resistant hybrids. Our results demonstrate that theImi1allele is moving from IMI-resistant wheat to jointed goatgrass, producing resistant hybrids and backcross plants. This is the first report of natural occurrence of IMI-resistant backcross plants in commercial wheat fields. Therefore, it is important to implement field management practices that reduce IMI-resistant hybrid production and to effectively manage nonagricultural areas with jointed goatgrass infestations to prevent introgression of the IMI-resistance allele.

Integrated Weed Management Systems Identified for Jointed Goatgrass (Aegilops cylindrica) in the Pacific Northwest

Jointed goatgrass is an invasive winter annual grass weed that is a particular problem in the low to intermediate rainfall zones of the Pacific Northwest (PNW). For the most part, single-component research has been the focus of previous jointed goatgrass studies. In 1996, an integrated cropping systems study for the management of jointed goatgrass was initiated in Washington, Idaho, and Oregon in the traditional winter wheat (WW)–fallow (F) region of the PNW. The study evaluated eight integrated weed management (IWM) systems that included combinations of either a one-time stubble burn (B) or a no-burn (NB) treatment, a rotation of either WW–F–WW or spring wheat (SW)–F–WW, and either a standard (S) or an integrated (I) practice of planting winter wheat. This study is the first, to our knowledge, to evaluate and identify complete IWM systems for jointed goatgrass control in winter wheat. At the Idaho location, in a very low weed density, no IWM system was identified that consistently had the highest yield, reduced grain dockage, and reduced weed densities. However, successful IWM systems for jointed goatgrass management were identified as weed populations increased. At the Washington location, in a moderate population of jointed goatgrass, the best IWM system based on the above responses was the B:SW–F–WW:S system. At the Washington site, this system was better than the integrated planting system because the competitive winter wheat variety did not perform well in drought conditions during the second year of winter wheat. At the Oregon site, a location with a high weed density, the system B:SW–F–WW:I produced consistently higher grain yields, reduced grain dockage, and reduced jointed goatgrass densities. These integrated systems, if adopted by PNW growers in the wheat–fallow area, would increase farm profits by decreasing dockage, decreasing farm inputs, and reducing herbicide resistance in jointed goatgrass.

Diverse no-till irrigated crop rotations instead of burning and plowing continuous wheat

Field burning of residue is a traditional management tool for irrigated wheat ( Triticum aestivum L.) production in the Inland Pacific Northwest of the United States (PNW) that can result in reduced air quality. A 6-year no-till field experiment to evaluate two complete cycles of a 3-year irrigated crop rotation of winter wheat–spring barley ( Hordeum vulgare L.)–winter canola ( Brassica napus L.) was sown (i) directly into standing residue of the previous crop, (ii) after mechanical removal of residue and, (iii) after burning of residue. The traditional practice of continuous annual winter wheat sown after burning residue and inverting the topsoil with a moldboard plow was included as a check treatment. Over-winter precipitation storage efficiency (PSE) was markedly improved when residue was not burned or burned and plowed after grain harvest. Grain yield of winter wheat trended higher in all no-till residue management treatments compared to the check treatment. Average grain yields of spring barley and canola were not significantly different among the no-till residue management treatments. Winter canola failed in 5 of 6 years due to a combination of a newly identified Rhizoctonia damping-off disease caused by Rhizoctonia solani AG-2-1 and cold temperatures that necessitated replanting to spring canola. Six-year average net returns over total costs were statistically equal over all four systems. All systems lost from $358 to $396 ha −1. Soil organic carbon (SOC) increased linearly each year with no-till at the 0–5 cm depth and accumulated at a slower rate at the 5–10 cm depth. Take-all of wheat caused by Gaeumannomyces graminis var. tritici was most severe in continuous annual winter wheat. The incidence and severity of Rhizoctonia on roots of wheat and inoculum of R. solani AG-8, was highest in the no-till treatments, but there was no grain yield loss due to this disease in any treatment. Residue management method had no consistent effect on Rhizoctonia root rot on barley. The annual winter grass downy brome ( Bromus tectorum L.) was problematic for winter wheat in the standing and mechanically removed residue treatments, but was controlled in the no-till residue burned and the burn and plow check. Another winter annual grass weed, rattail fescue ( Vulpia myuros L.), infested all no-till treatments. This was the first comprehensive and multidisciplinary no-till irrigated crop rotation study conducted in the Pacific Northwest.

Control of Rattail Fescue (Vulpia Myuros) in Winter Wheat

Rattail fescue, a winter annual grass weed, has been increasing in Pacific Northwest (PNW) dryland cereal producing areas. Although rattail fescue is not a new weed species in the PNW, its incidence is expanding rapidly in circumstances where soil disturbances are minimized such as in direct seed systems. Options for effective rattail fescue control in winter wheat cropping systems have not been adequately investigated and need to be developed. Rattail fescue control with herbicide treatments was investigated in imidazolinone-resistant winter wheat using imazamox and other herbicides. Across multiple sites and two growing seasons, crop injury from herbicide treatments was minor to negligible with some exceptions. Treatments containing imazamox or mesosulfuron produced minor, transient winter wheat crop injury at some locations in some years. With the exception of flufenacet applied preemergence (PRE), control of rattail fescue in wheat was variable with single herbicide applications, but improved with sequential herbicide treatments. Rattail fescue biomass was greatly reduced by several treatments especially those containing flufenacet or from sequential herbicide application. Crop yield varied among sites due to growing season precipitation, and in some cases from rattail fescue control or herbicide related crop injury.Nomenclature: diuron, flufenacet, imazamox, mesosulfuron, pendimethalin, sulfosulfuron, rattail fescue, Vulpia myuros (L.) K.C. Gmel. VLPMY, winter wheat, Triticum aestivum L. ‘ClearFirst’®

Cropping systems alter weed seed banks in Pacific Northwest semi-arid wheat region

Arable land weed seed banks are dynamic and reflect cropping history, current management, and environment. Changes in crop rotation and tillage system can alter weed seed density and species composition. In the semi-arid region of the Pacific Northwest, USA, no-till spring cropping is being studied as an alternative to the traditional winter wheat (Triticum aestivum L.)/dust-mulch fallow (WWF) rotation. Weed seed bank density and species composition were assessed during the first 6 years of an ongoing cropping system study comparing WWF with three no-till rotations; spring wheat (Triticum aestivum L.)/chemical fallow (SWF), continuous spring wheat (CSW), and spring wheat/spring barley (Hordeum vulgare L.) (SWSB). Soil cores were collected at depths of 0–8, 8–15, and 15–23cm in all plots during August each year following crop harvest. Weed seeds were washed from the soil, dried, and germinated in a glasshouse. Weed species most associated with the 0–8cm depth was Bromus tectorum L., the major winter annual grass weed in WWF. Species most associated with 8–15cm depth was Chenopodium leptophyllum (Moq.) Nut. ex S. Wats, a native warm season broadleaf weed that may have long seed bank persistence. An initial high density of B. tectorum was reduced with no-till spring crops and in WWF with intensive management strategies. In comparison an initial low weed seed density of B. tectorum remained low with no-till but increased in WWF with less management. Broadleaf weed species did not become management problems in no-till; however, seed bank weed shifts occurred where winter annual broadleaf species remained following reduction of high densities of B. tectorum. Summer annual broadleaf weed seeds such as C. leptophyllum and Salsola tragus L. were present but not at high densities. Summer annual grass weed seeds were not present and are not typical in this region. In this research, no-till spring cereal based systems did not result in an increase in total seed density at the soil surface. Results from this research show that no-till spring crop rotations are effective at controlling winter annual grass weeds as well as broadleaf weeds normally associated with WWF.

Management effects on the incidence of jointed goatgrass inhibitory rhizobacteria

Jointed goatgrass (Aegilops cylindrica Host., JGG), a winter annual grass weed, is a major threat to small grain production in the United States. It infests an estimated 2 million hectares in the US and causes annual losses of $145 million in crop yield and quality. Selective herbicides are available only for certain herbicide-resistant crop varieties and alternative management practices are tillage, burning, or rotation to less profitable crops. We evaluated bacteria that were isolated from winter wheat (Triticum aestivum L., WW), downy brome (Bromus tectorum L., DB), and JGG roots to determine their potential as biological control agents for JGG. The bacteria were sequentially screened in laboratory bioassay, growth chamber and field studies for their effect on JGG and WW root growth. In laboratory studies, 1236 isolates (50%) of the 2450 naturally occurring soil bacteria tested were inhibitory to JGG seedling growth. Only 76 isolates (3.1%) inhibited JGG, but not WW in the in vitro bioassays. In the growth chamber, only 7 (0.3%) of the isolates inhibited JGG growth and not WW. In field studies, 4 of the 7 isolates selected after bioassay and growth chamber tests inhibited JGG more than 30%. Conventional tillage systems maintained higher populations of inhibitory bacteria than no-tillage systems. The number of bacteria inhibitory to JGG was greater in the spring sampling and at the highest level of nitrogen fertilization. Crop rotation had little effect on the incidence of jointed goatgrass inhibitory bacteria. Different JGG accessions from the western US were diverse in their response to inhibitory bacteria. Weed-suppressive bacteria isolated from plant roots and soil have the potential to be used as biological control agents for JGG.

Introgression of a Strawbreaker Foot Rot Resistance Gene from Winter Wheat into Jointed Goatgrass

Strawbreaker foot rot (SFR) [Pseudocercosporella herpotrichoides(Fron) Deighton] is a disease of winter wheat (Triticum aestivumL.) in many wheat growing regions in the world. Resistance to SFR can be conferred by a single dominant gene (Pch1) fromAegilops ventricosaTausch that was transferred onto chromosome 7D of wheat. ‘Madsen’, a hexaploid winter bread wheat, carriesPch1and is highly resistant to SFR. Jointed goatgrass (Ae. cylindricaHost.) is a winter annual grass weed. Wheat and jointed goatgrass have the D genome in common and have been found to hybridize and backcross under field conditions. Since SFR resistance in winter wheat is controlled byPch1on the D genome, it is theoretically possible for resistance to be transferred to jointed goatgrass via backcrossing. A SFR resistant jointed goatgrass population would potentially have an ecological advantage in the presence of the disease. To evaluate the likelihood of gene introgression, Madsen, ‘Stephens’ (a SFR susceptible winter wheat), three jointed goatgrass accessions, and 15 artificially produced backcross progenies (BC2S2) were inoculated with SFR. The percentage of infection in Stephens, the joined goatgrass accessions, and the backcross progenies was 80% or greater except for one BC2S2progeny that had only 20% infection. Madsen had 0% infection. The presence ofPch1in the BC2S2progeny was confirmed using a biochemical marker linked to the resistance gene. These results show that a SFR resistance gene from winter wheat can be transferred to jointed goatgrass.

Response of Selected Hard Red Wheat Lines to Imazamox as Affected by Number and Location of Resistance Genes, Parental Background, and Growth Habit

ABSTRACTImidazolinone‐resistant (IR) wheat (Triticum aestivum L.) was released for commercial production in portions of the USA in 2002 and has provided growers with a new technology to selectively control winter annual grass weeds. Imidazolinone herbicides inhibit acetolactate synthase (ALS) in susceptible plants; however, IR wheat has an altered target site which confers resistance to these herbicides. The mutation‐derived resistance trait of most commercially available IR winter wheat cultivars is located on the D‐genome; however, winter and spring wheat cultivars with the resistance trait on the A, B, or D genome or on multiple genomes are currently under development. Four groups of near‐isoline wheat with spring or winter growth habit and resistance genes on the B, D, or both B and D genomes were compared for whole plant and ALS enzyme response to imazamox. Biomass accumulation after treatment was similar among B‐ and D‐ genome resistant winter wheat biotypes and was always higher than B‐ and D‐genome resistant spring wheat biotypes. D‐genome resistant spring wheat was more resistant than B‐genome resistant spring wheat and the two‐gene resistant spring wheat had an additive level of tolerance to imazamox compared with single‐gene resistant spring wheat. Growth habit (spring vs. winter) did not affect in vitro ALS activity among B‐ or among D‐genome resistant cultivars; however, D‐genome resistant cultivars had significantly higher in vitro ALS activity in the presence of imazamox compared with B‐genome resistant cultivars regardless of growth habit. D‐genome resistance appears to provide greater tolerance to imazamox compared with B‐genome resistance; however, multiple‐genome resistance likely will be required to consistently avoid crop injury in spring wheat from labeled U.S. rates. Although ALS extracted from winter wheat and spring wheat responded similarly to imazamox, whole plant responses demonstrates that tolerance is affected by factors other than resistance gene location.

Sustainable grazing systems for the Central Tablelands of New South Wales. 1. Agronomic implications of vegetation - environment associations within a naturalised temperate perennial grassland

Temperate perennial grass-based pastures dominate the high rainfall zone of south-eastern Australia and support a major livestock production industry. This area has experienced a recent change in overall pasture condition, however, typified by a reduction in the abundance of perennial grasses and an increasingly prominent winter-annual grass weed component. Improving the condition and productivity of these pastures can be achieved by improved management but this requires better knowledge of the interactions between management options and pasture species composition and of the interaction between pasture vegetation and the complex effects of a heterogeneous landscape. This paper reports the results of an intensive survey of a 60-ha paddock that was designed to identify the species present, determine their patterns of distribution and examine the relationships between pasture vegetation and the environment. The survey of species present in late summer was supplemented by the identification of seedlings that later emerged from extracted soil cores and by soil physical and chemical analyses. Data were analysed using ordination and interpreted with GIS software so that topographic features could be considered. The most frequently identified taxa were Hypochaeris radicata, Austrodanthonia spp. and Bothriochloa spp. (in late summer) and Vulpia spp., Bromus molliformis and Trifolium subterraneum (winter-annual species). Austrodanthonia spp. were commonly found on the drier ridges and more acid soils with lower phosphate levels. These were also the areas dominated in spring by Vulpia spp. and were generally lower in plant species richness overall. The most species-rich areas occurred downslope where soil fertility was higher and less moisture stress was presumably experienced. The measured environmental factors explained a substantial proportion of the variation in the vegetation dataset, which underlined the importance of considering landscape effects in the management of typical tablelands pastures.

Effects of seed production and storage conditions on blackgrass (Alopecurus myosuroides) germination and shoot elongation

Blackgrass is a common winter annual grass weed in autumn-sown crop rotations in Atlantic European countries. Control with a minimum amount of herbicides in integrated cropping systems would be facilitated by modeling the effect of cropping systems on its demography. To develop the submodel relating weed seed bank to emerged seedlings, laboratory experiments were conducted to analyze and quantify seed germination and shoot elongation. These processes were studied as a function of environmental conditions during seed production (nitrogen availability, water deficit, plant density, and crop) and of seed characteristics (seed weight, harvest date, storage length, and dry-stored/soil-buried). Nonlinear equations relating germination and shoot elongation to time calculated as cumulated degree-days were fitted to the observed germination and shoot elongation data. These were used to estimate parameters for germination proportion and rate, as well as final shoot length and elongation rate. Recently harvested seeds germinated best and fastest when they were collected in spring crops compared with winter crops. Germination proportion and rate increased with seed storage length, especially for seeds collected in winter crops. Midgermination time decreased with seed weight and water deficit during seed production; it increased with nitrogen amounts available to the mother plants. Maximum shoot length increased with seed weight and plant density during seed production. It decreased with nitrogen availability to mother plants and with storage length, irrespective of whether the seeds were dry-stored or buried in soil. Elongation rate was highest for early- and fast-germinating seeds. Time to midelongation increased with maximum shoot length. These germination and preemergence growth models can now be combined with other submodels to develop a blackgrass emergence model.

Absorption, translocation, and metabolism of imazamox in jointed goatgrass and feral rye

Abstract Wheat cultivars resistant to imazamox will facilitate selective chemical control of many winter annual grass weeds, including jointed goatgrass, downy brome, and feral rye. These three weed species respond differently to imazamox postemergence treatments with feral rye, demonstrating more tolerance than jointed goatgrass or downy brome; therefore, growth chamber studies were conducted to evaluate imazamox absorption in all three weed species and translocation and metabolism in jointed goatgrass and feral rye. Adding nonionic surfactant (NIS) or methylated seed oil increased absorption in jointed goatgrass and feral rye but not in downy brome, compared to imazamox applied alone. Imazamox applied with NIS and urea ammonium nitrate resulted in the highest absorption in each species: 97, 91, and 92% of applied 14C for jointed goatgrass, downy brome, and feral rye, respectively, 48 h after treatment (HAT). Imazamox translocation from the treated leaf was similar for jointed goatgrass and feral rye acr...