Daniel A. Ball

Ecological fitness of acetolactate synthase inhibitor–resistant and –susceptible downy brome (Bromus tectorum) biotypes

Abstract Studies were conducted to determine the relative fitness and competitive ability of an acetolactate synthase (ALS) inhibitor–resistant (R) downy brome biotype compared with a susceptible (S) biotype. In previous research, the mechanism of resistance was determined to be an altered ALS enzyme. Seed germination of the R biotype was compared with that of the S biotype at 5, 15, and 25 C. There were no different germination characteristics between R and S biotypes at 15 and 25 C. However, the R biotype germinated 27 h earlier than the S biotype and had reached over 60% germination when the S biotype initially germinated at 5 C. Under noncompetitive greenhouse conditions, growth of the R biotype was similar to that of the S biotype on the basis of shoot dry weight, leaf area, and plant height. Seed production of the R biotype was 83%, when compared with the S biotype, but seeds of the R biotype were larger than those of the S biotype. Replacement series experiments were conducted in the greenhouse to determine the relative competitive ability of R and S biotypes. No difference in competitive ability was observed between R and S biotypes on the basis of shoot dry weight, leaf area, or plant height. Thus, it appears that ALS-resistance trait is not associated with growth penalty in either noncompetitive or competitive conditions. In the absence of ALS inhibitors, these results suggest that the R biotype would remain at a similar frequency in a population of R and S biotypes. Nomenclature: Downy brome, Bromus tectorum L. BROTE.

Biological Attributes of Rattail Fescue (Vulpia myuros)

Abstract Control of rattail fescue, a winter annual grass, can be difficult in spring or winter wheat. Although rattail fescue is not a new weed species in the Pacific Northwest, occurrences have been increasing in circumstances where soil disturbances are minimized, such as in direct-seed cropping systems. To develop integrated management strategies for rattail fescue, information is needed on the longevity of seed viability in the soil, the presence of seed dormancy, vernalization requirements, and optimal environmental conditions for seed germination and establishment under field conditions. Controlled experiments on the biology of rattail fescue indicated that newly mature seed required an afterripening period of 1 to 12 mo to obtain high levels of seed germination, depending on germination temperature. Maximum seed germination was observed at constant day/night temperatures of approximately 20 C from thermogradient plate studies. Germination tests from seed burial studies indicated that a majority of buried seed was not viable after 2 to 3 yr. Field-grown rattail fescue plants required vernalization to produce panicles and germinable seed. A short afterripening period, cool germination temperature, and vernalization requirements support the classification of rattail fescue as a winter annual. This information will facilitate development of rattail fescue management systems, including crop rotations and various control tactics such as tillage or herbicide application timing during fallow periods. Nomenclature: Rattail fescue, Vulpia myuros (L.) K.C. Gmel. VLPMY, wheat, Triticum aestivum L.

Effect of Imazamox Soil Persistence on Dryland Rotational Crops1

Imazamox is an imidazolinone herbicide being developed for weed control in imidazolinone-resistant wheat (IMI-wheat) cultivars and various legume crops. In a series of studies conducted under a range of dryland cropping environments in the Pacific Northwest United States, imazamox applied to IMI-wheat or pea injured barley and canola grown 1 yr after imazamox treatment in low-rainfall, low–soil pH locations of Oregon. Injury was not observed in higher rainfall locations near Pullman, WA. Non–herbicide-resistant wheat planted 1 yr after IMI-wheat treated with imazamox was not injured. Of particular concern for imazamox carryover are low-rainfall areas with low-pH soils. Reduced soil moisture appears to limit imazamox degradation. Imazamox sorption is reduced in low-pH soils, which increases its bioavailability, thereby increasing the potential for injury to rotational crops such as barley, canola, and spring wheat. Nomenclature: Imazamox; barley, Hordeum vulgare L.; canola, Brassica napa L.; pea, Pisum sativum L.; wheat, Triticum aestivum L. Additional index words: Barley, canola, carryover, Clearfield™, herbicide-resistant wheat, pea. Abbreviations: IMI-wheat, imidazolinone-resistant wheat; PNW, Pacific Northwest.

Vernalization response of plants grown from spikelets of spring and fall cohorts of jointed goatgrass

Abstract Jointed goatgrass is most commonly described as a winter annual species. However, it has been observed to produce spikes in spring crops, apparently without being exposed to vernalizing conditions. A controlled environment study was conducted to determine the reproductive response of jointed goatgrass plants grown from seeds of fall- and spring-emerging parent plants to various vernalization durations. Winter wheat was included as a control. Winter wheat spikelet production was dependent on vernalization, and the number of spikes per plant was 10-fold greater if the plants were exposed to 4 C for 10 wk. In contrast, jointed goatgrass spike production without vernalization remained as high as 50% of that produced by plants exposed to 10 wk of vernalization conditions. Jointed goatgrass is thus not as dependent on vernalization for reproduction as the comparative winter wheat standard. Apparently, jointed goatgrass is more a facultative rather than an obligate winter annual. Rotating to a spring-seeded crop should not be expected to completely prevent jointed goatgrass seed production. Fields rotated to spring wheat to eliminate jointed goatgrass seed production should be monitored, and jointed goatgrass should be hand pulled or otherwise controlled to ensure zero seed production. Nomenclature: Jointed goatgrass, Aegilops cylindrica L. AEGCY; winter wheat, Triticum aestivum L. ‘Madsen’.

Predicting timing of downy brome (Bromus tectorum) seed production using growing degree days

Abstract Downy brome in dryland winter wheat presents a major constraint to the adoption of reduced tillage cropping systems in the Pacific Northwest of the United States. Effective suppression of downy brome during fallow periods depletes seed in the soil and reduces infestations in subsequent winter wheat crops. Delayed tillage operations or delayed herbicide applications in the spring increase the risk for production of viable downy brome seed during fallow periods. In a series of studies, downy brome panicles were sequentially sampled at Pendleton, OR, and Pullman, WA, in 1996 and 1997, and at nine locations around the winter wheat growing region of the western United States in 1999 and 2001. Cumulative growing degree days (GDD) were calculated using local, daily maximum, and minimum air temperature data. A simple GDD model based on the formula GDD = (daily maximum temperature [C] + daily minimum temperature [C])/2, with a base temperature of 0 C and a starting point of January 1, was used to calculate cumulative GDD values for panicle sampling dates. Number of seed germinating per collected panicle was recorded in greenhouse germination tests. Estimations of degree days required for production of viable downy brome seed were made using nonlinear regression of germination on GDD. The GDD value at which viable seed can be found on plants (i.e., when seed germination > 0) was of interest. Estimates of the GDD values at which viable seed could be found in the three studies ranged from 582 GDD at Bozeman, MT, to 1,287 GDD at Stillwater, OK, with a group of GDD values for Pendleton and Pullman around 1,000. Variation in seed-set GDD among locations may be attributed to differing climatic conditions that control vernalization at the various locations or to differences in vernalization requirements among downy brome biotypes (or both). Nomenclature: Downy brome, Bromus tectorum L. BROTE; winter wheat, Triticum aestivum L. TRZAW.

Herbicide Resistance in Jointed Goatgrass (Aegilops cylindrica): Simulated Responses to Agronomic Practices1

A population model was constructed to simulate the development of imazamox-resistant jointed goatgrass (AEGCY) in imazamox-resistant (Clearfield™) wheat. The model computed changes in the surface and in the buried AEGCY seed banks for both resistant and susceptible biotypes. Simulations started with an initial density of 1,000 susceptible and zero resistant seeds/m2 in each seed bank. Simulation of continuous, no-till Clearfield wheat production resulted in rapid development of resistant AEGCY without hybridization with wheat and in extremely rapid resistance development with hybridization. In less than 10 yr, the resistant population was growing exponentially in both simulations. Adding a fallow year with tillage into the simulated rotation did not substantially slow down the appearance of resistance but did delay the rate of resistant population increase by several orders of magnitude over 10 yr. Alternating Clearfield and a nonresistant winter wheat in combination with fallowing prevented the establishment of a significant resistant AEGCY population and prevented the susceptible seed population from increasing exponentially. These projections suggest that imazamox-resistant wheat can be a tool for managing AEGCY populations especially if combined with rotations that include fallow and crops other than Clearfield winter wheat. Nomenclature: Imazamox; jointed goatgrass, Aegilops cylindrica Host #3 AEGCY; winter wheat, Triticum aestivum L. Clearfield™. Additional index words: Crop rotation, pollen flow, population model, resistance management. Abbreviations: ALS, acetolactate synthase; BSB, buried seed bank; DSP, local seed dispersal; EST, established plant; MAT, mature plant; PRD, seed production; SDL, seedling; SSB, surface seed bank.

Accase-inhibitor Herbicide Resistance in Downy Brome (Bromus Tectorum) in Oregon

Abstract In spring 2005, a downy brome population with possible resistance to fluazifop-P, an acetyl-CoA carboxylase (ACCase) inhibitor (group 1) herbicide was found in a commercial creeping red fescue seed production field, near La Grande, OR, where fluazifop-P had been used to control downy brome repeatedly over 7 yr. Greenhouse experiments were conducted to confirm resistance to a number of group 1 herbicides. The suspected resistant downy brome accession was tested for resistance to fluazifop-P and tested for cross-resistance to other aryloxyphenoxy propionate (APP) and cyclohexanedione (CHD) herbicides, including quizalofop-P, sethoxydim, and clethodim. Data recorded included plant-mortality counts and biomass. Tests revealed that the La Grande downy brome accession was highly resistant to fluazifop-P and sethoxydim at all tested rates. The La Grande accession suffered no mortality from fluazifop-P or sethoxydim treatments up to the maximum tested rate of eight times (8×) the labeled recommendation. The La Grande accession was resistant to quizalofop-P and clethodim at the labeled rate or less but was susceptible to application rates higher than the labeled rate. The control downy brome accession was completely susceptible to fluazifop-P, quizalofop-P, and clethodim at all rates and exhibited increasing susceptibility with increasing sethoxydim rate. This pattern of cross-resistance differs from that of a previously reported case of ACCase resistance in downy brome. Nomenclature: Clethodim fluazifop-P quizalofop-P sethoxydim downy brome, Bromus tectorum L. BROTE creeping red fescue, Festuca rubra L.

Herbicide-Resistant Grass Weed Development in Imidazolinone-Resistant Wheat: Weed Biology and Herbicide Rotation'

A general life cycle model was modified to demonstrate how agronomic practices and weed biology factors affect the rate of appearance of herbicide-resistant downy brome, jointed goatgrass, and wild oat in Pacific Northwest wheat cropping systems. The model suggests herbicide rotation strategies for cropping systems that include imidazolinone-resistant wheat as a weed management tool. Simulation of continuous annual imidazolinone-resistant winter wheat and imazamox herbicide use resulted in the resistant soil seed banks of downy brome, jointed goatgrass, and wild oat surpassing their susceptible soil seed banks in 5, 7, and 10 yr, respectively. Reducing the initial seed bank density of downy brome before beginning a rotation that includes imidazolinone-resistant winter wheat reduces the likelihood of selecting for herbicide-resistant biotypes. The best simulated management option for reducing the total jointed goatgrass soil seed bank in low-precipitation areas is an imidazolinone-resistant winter wheat–fallow rotation. Rotations that include winter and spring crops and rotations that include non–group 2 herbicides minimize herbicide resistance selection pressure and reduce the wild oat soil seed bank. Nomenclature: Imazamox; downy brome, Bromus tectorum L. # BROTE; jointed goatgrass, Aegilops cylindrica Host #3 AEGCY; wild oat, Avena fatua L. # AVEFA; winter wheat, Triticum aestivum L. Clearfield™. Additional index words: Crop rotation, population model, resistance management. Abbreviations: ALS, acetolactate synthase; PNW, Pacific Northwest.

Spring-germinating jointed goatgrass (Aegilops cylindrica) produces viable spikelets in spring-seeded wheat

Abstract The most common strategy recommended for management of jointed goatgrass infestations is to rotate from winter wheat to a spring crop for several years. A field study was conducted at three locations in 1998 and 1999 to determine the effects of spring seeding date on the ability of jointed goatgrass to flower and produce viable seed in the presence or absence of spring wheat and to determine the effect of jointed goatgrass competition and crop seeding date on spring wheat grain yield. Spring wheat was seeded on four dates at each location in both hand-sown and natural jointed goatgrass infestations. Jointed goatgrass plants from hand-sown spikelets flowered and developed spikelets on all seeding dates except the last; viable seed was produced on the two earliest seeding dates. Jointed goatgrass plant densities from natural infestations were from 1 to 12 plants m−2, and spikelet production ranged from 0 to 480 spikelets m−2. Natural jointed goatgrass infestations produced spikelets containing viable seed on all seeding dates at one location in 1998, the driest location. Spring wheat yield was not affected by jointed goatgrass competition; however, jointed goatgrass spikelet production was reduced by spring wheat competition compared with that of monoculture jointed goatgrass. The last seeding date of spring wheat was associated with 51% less crop yield compared with the recommended seeding date. The decision to manage jointed goatgrass infestations with a spring crop rotation should consider delayed seeding dates to minimize viable spikelet production by spring-germinating jointed goatgrass; however, the cost of this decision may include grain yield reduction. Nomenclature: Jointed goatgrass, Aegilops cylindrica Host. AEGCY; spring wheat, Triticum aestivum L. ‘Penewawa’.

Afterripening Requirements and Optimal Germination Temperatures for Nuttall's Alkaligrass (Puccinellia nuttalliana) and Weeping Alkaligrass (Puccinellia distans)

Abstract In the Grande Ronde Valley of eastern Oregon, two perennial grass species in the genus Puccinellia, weeping alkaligrass and Nuttalls alkaligrass, are weeds of Kentucky bluegrass grass-seed production fields. Weeping alkaligrass is introduced from Eurasia, whereas Nuttalls alkaligrass is native to the region. These two species were studied to determine dormancy attributes and optimal temperature conditions for seed germination. Results from the current studies indicate that both species have a high level of embryonic dormancy immediately following seed harvest, which is primarily eliminated through dry storage (afterripening) and an incubation temperature of 20 C. Following adequate afterripening, a prechill treatment of 5 d at 5 C had an inconsistent effect on germination of weeping alkaligrass (P  =  0.012 in 2002, 0.156 in 2003) and improved germination of Nuttalls alkaligrass over both years (P < 0.0001). The afterripening requirement for weeping alkaligrass was more than 90 d, whereas Nuttalls alkaligrass required more than 180 d. Following adequate afterripening, both species had rapid and well-synchronized germination at fluctuating day/night temperatures of 30/10 C given unlimited moisture conditions. Given these results, it is unlikely that seeds of either species would germinate in eastern Oregon during the summer months. The data predict a long viability period under dry storage for both species. Weeping alkaligrass and Nuttalls alkaligrass should exhibit a rapid, well-synchronized germination in the spring as observed in the field. Nomenclature: Nuttalls alkaligrass, Puccinellia nuttalliana Hitchc; weeping alkaligrass, Puccinellia distans (L.) Parl; Kentucky bluegrass, Poa pratensis L.