Annual bluegrass (Poa annua L.) populations in turfgrass have evolved resistance to several herbicides in the United States (US), but there has been no confirmed resistance from an agricultural field. Recently, glyphosate failed to control a P. annua population found in a field in a soybean [(Glycine max (L.) Merr.] and rice (Oryza sativa L.) rotation in Poinsett County, Arkansas. The present study focused on determining the sensitivity of a putatively-resistant accession (R1) to glyphosate compared with two susceptible accessions (S1 and S2). The experiments included a dose-response study, 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS) gene copy number and expression analysis, and assessment of mutations in EPSPS. Based on the dose-response analysis, R1 required 1,038 g ae ha-1 of glyphosate to cause 50% biomass reduction, whereas S1 and S2 only required 148.2 g and 145.5 g ae ha-1, respectively. The resistant index (RI) was approximately 7-fold relative to the susceptible accessions. Real-time polymerase chain reaction data revealed at least a 15-fold increase in the EPSPS copy number in R1, along with a higher gene expression. No mutations in EPSPS were found. Gene duplication was identified as the main mechanism conferring resistance in R1. The research presented here reports the first incidence of glyphosate resistance in P. annua from an agronomic field crop situation in the US.

Under greenhouse conditions, annual bluegrass (Poa annua L.), common groundsel (Senecio vulgaris L.), shepherd's purse [Capsella bursa-pastoris (L.) Medicus], small nettle (Urtica urens L.), canola (Brassica napus L. ssp. napus), and pea (Pisum sativa L.) differed in sensitivity to ultraviolet (UV) radiation. Of the weed species, annual bluegrass was the least sensitive; whereas, among the crop species, canola was about sevenfold more sensitive than was pea. The sensitivity of a species to UV radiation was highly dependent upon its stage of development. The study indicates some potential for using UV radiation to control weeds, but the method needs further investigation to unravel the selectivity of the methods and potential health hazards.

Two field studies, a seeded study and a golf course study, were conducted to compare competition among creeping bentgrass, annual bluegrass, and roughstalk bluegrass when subjected to common weed control practices and foliar applications of iron and magnesium. A research site was selected for the seeded study and divided into 10 whole plots receiving irrigation at either 50 or 100% evapotranspiration deficit. Each whole plot was further divided into subplots receiving one of seven treatments: bensulide, ethofumesate, trinexapac-ethyl, foliar Mg, foliar Fe, foliar Mg plus foliar Fe, and control. The site was seeded to a mixture of creeping bentgrass, annual bluegrass, and roughstalk bluegrass in September 1995, and treatments began in March 1996. Annual bluegrass was reduced 29% in plots treated with foliar Fe and 65% in plots treated with foliar Fe plus foliar Mg. Roughstalk bluegrass was significantly reduced in seeded plots treated with foliar iron (50%), plant growth regulator (75%), and foliar iron plus foliar magnesium (100%). Annual bluegrass and roughstalk bluegrass proportions were not affected by irrigation regime. In a second study, the most effective treatment, foliar magnesium plus foliar iron, was tested on a working golf course fairway and on a practice putting green beginning April 1997 and ending November 1997. Treatments on the golf course fairway and practice putting green were ineffective due to the established, perennial nature of the annual bluegrass biotypes on these sites. Further research is required to improve the efficacy of nutritional treatments on these perennials.

Field studies were conducted for 4 yr to investigate the effects of tillage and herbicide programs on weed control and wheat (Triticum aestivum L. ‘Potomac’ in 1981 and ‘Wheeler’ from 1982 to 1984) grain yields in a wheat-soy bean [Glycine max (L.) Merr.] double-crop rotation. Predominant weed species were common chickweed [Stellaria media (L.) Vill. # STEME], corn chamomile (Anthemis arvensis L. # ANTAR), and annual bluegrass (Poa annua L. # POAAN) at the onset of research and cheat (Bromus secalinus L. # BROSE) and soft brome (Bromus mollis L. # BROMO) at the conclusion of the study. Control of all species was excellent with conventional tillage and no-till plus nonselective herbicides but was significantly less with no-till without nonselective herbicides and with minimum tillage. After 4 yr, maximum wheat grain yields were significantly higher in conventional- than in minimum- or no-till systems.

Sethoxydim {2-[1-(ethoxyimino)butyl]-5-[2-(ethylthio)propyl]-3-hydroxy-2-cyclohexen-1-one} is used to control weedy grasses in red fescue (Festuca rubra L. # FESRU and F. rubra ssp. commutata), an important seed crop in Oregon. The sethoxydim rate required to reduce growth rate of 11 cultivars of red fescue and four cultivars of bentgrass (Agrostis tenuis Sibth. # AGSTE and A. stolonifera L. var. palustris # AGSST), a common contaminant in red fescue fields, by 50% (GR50) was determined in greenhouse and laboratory experiments. Field experiments compared only red fescue cv. ‘Pennlawn’ and bentgrass cv. ‘Penncross'. The GR50 values for Pennlawn red fescue in the field, the greenhouse, and the laboratory were 30 kg ai/ha, 15 kg/ha, and 5300 μM, respectively. Penncross bentgrass was approximately 400, 1400, and 12 900 times more susceptible than Pennlawn red fescue in the field, in the greenhouse, and in laboratory experiments, respectively. Plants were at different growth stages in the different experiments. The xylene and surfactant used as a solvent for the technical grade sethoxydim was as toxic to red fescue without sethoxydim as when sethoxydim was included. Cultivars of red fescue differed in their response to sethoxydim, but cultivars of bentgrass did not. The GR50's of other species tested in the greenhouse were >4 kg/ha for annual bluegrass (Poa annua L. # POAAN), 0.22 kg/ha for tall fescue (F. arundinacea Schreb. # FESAR), 0.37 kg/ha for downy brome (Bromus tectorum L. # BROTE), and 0.017 kg/ha for Italian ryegrass (Lolium multiflorum Lam. # LOLMU).

Substantial weed growth often occurs in legume-cereal cover-crop mixes commonly grown on organic vegetable farms. A 2-yr study at the USDA-ARS in Salinas, CA, was conducted to test the effect of zero, one, and two passes with a rotary hoe on weed control in a mixed cover crop of 10% rye, 15% common vetch, 15% purple vetch, 25% peas, and 35% bell bean. Rotary hoeing occurred 14–15 days after planting (DAP) in the one-pass treatment, and 14 and 28 DAP in the two-pass treatment. Rotary hoeing did not affect total cover-crop density or biomass in either year, but reduced rye density and biomass in year 2. One pass reduced total weed density by 69% in year 1 and 49% in year 2. A second pass did not affect weed density in year 1 but reduced weed density an additional 33% in year 2. One pass decreased weed biomass in year 1, whereas two passes were required to reduce weed biomass in year 2. Rotary hoeing reduced seed shed by chickweed and shepherd's-purse seeds, the two predominant weed species, by 80 to 95% in both years. Rotary hoe efficacy depended on weather conditions directly before and after cultivation. The decision to repeat rotary hoeing should be based upon field scouting and weather conditions following the initial pass with the rotary hoe.

Nonfungicidal effects of agricultural fungicides on crop plants have been reported previously; however, there are few reports of nontarget effects of fungicides on weedy species. Field research trials in Oregon demonstrated that the growth of several broadleaf weeds was reduced after multiple applications of the fungicide propiconazole. Greenhouse experiments confirmed that preemergence applications of propiconazole reduced the biomass accumulation of several common broadleaf and grass weeds 15 to 63%. Laboratory experiments were performed on redroot pigweed, the most sensitive species, to examine the effects of propiconazole on germination and early seedling growth. Redroot pigweed germination and total seedling length (root plus shoot) were reduced at propiconazole concentrations above 37 and 0.36 mg/L, respectively. Growth-regulating effects of fungicides such as propiconazole on the germination and early growth of weeds may contribute to integrated weed management, especially when adequate moisture ensures the presence of germinating seeds and small seedlings throughout the growing season.

The annual bluegrass weevil (ABW), Listronotus maculicollis Kirby, is an economically important pest of short-cut turfgrass in Eastern North America. Wide spread insecticide resistance warrants the development of alternative management strategies for this pest. ABW damage typically occurs in areas with a high percentage of annual bluegrass, Poa annua L., the preferred ABW host. Damage to bentgrasses, Agrostis spp., is much rarer and usually less severe. To aid the implementation of host plant resistance as an alternative ABW management strategy we investigated the tolerance of three bentgrass species to ABW feeding. Responses of P. annua, creeping bentgrass, Agrostis stolonifera L., colonial bentgrass, Agrostis capillaris L., and velvet bentgrass, Agrostis canina L., to adult and larval feeding were compared in greenhouse experiments. Grass responses were measured as visual damage, dry weight of the grass stems and leaves, color, density and overall grass quality. To determine possible mechanisms of grass tolerance constitutive fiber and silicon content were also determined. The three bentgrass species tolerated 2–3 times higher numbers of ABW adults and larvae than P. annua before displaying any significant quality decrease. Creeping bentgrass had the lowest damage ratings. ABW infestation caused higher plant yield reduction in P. annua (up to 42%) than in bentgrasses. Observed differences among the grass species in fiber and silicon content in the plant tissue are unlikely to play a role in the resistance of bentgrasses to ABW. Our findings clearly show that A. stolonifera is the best grass species for the implementation of host plant resistance in ABW management.

To predict changes in the range and density of plant species as a consequence of elevated atmospheric CO2, it is essential to characterize the effect of elevated CO2 on key components of plant life history stages such as rate of establishment and maturation of individuals, reproductive output and offspring fitness. We measured vegetative response, phenology, reproductive output and seed quality of four annual C3 plant species, grown from seed to senescence under ambient and elevated (ambient+200 ppm) CO2 and ambient and reduced (33% shade) light. Few previous studies have included all stages of a plant's life as well as characteristics of the next generation. Elevated CO2 had no effect on the vegetative attributes of Cardamine hirsuta, Spergula arvensis or Poa annua whereas Senecio vulgaris produced longer leaves and greater biomass. Both Senecio and Poa had faster maturation times. The vegetative response of Senecio was not translated into increased seed output, although seed mass and carbon[ratio ]nitrogen ratios were significantly increased. By contrast, Poa showed no vegetative response to elevated CO2, but had significantly increased seed production. Thus we found no evidence for a simple translation from vegetative response to elevated CO2 into reproductive response. There was also no consistent light-mediated response to elevated CO2 among the four species. However, the effect of reduced light (33% shade) on vegetative and reproductive output was consistent across the four species and significantly stronger than the effect of elevated CO2. On the basis of this glasshouse study, we predict that Poa would be most likely of the four species to show significant increases in population size and migration potential, as a result of increased reproductive output, under elevated atmospheric CO2. However, this response may be relatively small compared with variation in growth and reproduction as a result of environmental heterogeneity in resources such as light.

Spontaneous hybrids between the two diploids Poa infirma Kunth and P. supina Schrader were obtained at the rate of 0·2%. No reciprocal maternal effect was observed. Almost all hybrids survived, grew normally but were sterile. Hybrids were diploid, and resembled in morphology the tetraploid P. annua L. (annual meadow grass). In their isozymes, they showed additivity of the parental patterns, and were very similar to those found in P. annua populations. The ease with which hybrids were obtained and the similarity of their morphological and isozyme pattern to P. annua suggest that the two diploid species are closely related to the allotetraploid species, and are perhaps its parent species.