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.

Studies were conducted to determine the effects of tillage level on weed control and yield of asparagus grown on a sandy loam soil under sprinkler irrigation. Without herbicides, horseweed and kochia populations increased in no-till asparagus production over a 3-yr period, compared to asparagus that was rototilled either once in the spring or once in the spring and again at layby. Without herbicides, hairy nightshade populations increased most when asparagus was rototilled both in the spring and at layby. Volunteer asparagus was controlled best when herbicides and tillage were combined in the spring and again at layby. Common groundsel was controlled without herbicides by rototilling both in the spring and at layby. Diuron at 1.4 kg ai/ha plus prodiamine at 1.4 kg/ha or metribuzin at 0.9 kg/ha plus norflurazon at 2 kg/ha controlled all weeds under all three tillage levels when split-applied in the spring and at layby. Tillage level had no effect on total asparagus spear number or weight if tillage was performed in mid March before asparagus emergence.

The first significant cases of herbicide-resistant weed populations were to the triazines in the 1970s. In the last 10 years there has been an increase in the number of weed populations that have become resistant to an array of herbicides. In some of these cases, like rigid ryegrass in Australia, a multitude of resistant biotypes has evolved with different mechanisms of resistance. If the present trend continues, the number of herbicides effective on certain weed species may diminish rapidly. To counteract this trend, industry has organized a number of intercompany working groups to specifically address the development of resistance and to implement plans to manage resistance. University and extension along with industry personnel across the world have begun educating growers on resistance management. However, this effort needs to be intensified to find new solutions for controlling weeds through the use of integrated weed management practices that incorporate new and established herbicides with cultural, mechanical, and biological control methods. The challenge is to develop cost effective, environmentally sustainable programs for weed control while maintaining the present efficiency in food and fiber production so that needs of an ever expanding human population can be met.

The cross-resistance of triazine-resistant biotypes of smooth pigweed (Amaranthus hybridus L. # AMACH), common lambsquarters (Chenopodium album L. # CHEAL), common groundsel (Senecio vulgaris L. # SENVU), and the crop canola (Brassica napus L. var. Atratower) to a selection of herbicides was evaluated at both the whole plant and chloroplast level. The triazine-resistant biotypes of all four species showed a similar pattern of cross-resistance, suggesting that a similar mutation had occurred in each species. The four triazine-resistant biotypes were resistant to injury from atrazine [6-chloro-N-ethyl-N′-(1-methylethyl)-1,3,5-triazine-2,4-diamine], bromacil [5-bromo-6-methyl-3-(1-methylpropyl)-2,4-(1H,3H)pyrimidinedione], and pyrazon [5-amino-4-chloro-2-phenyl-3(2H)-pyridazinone] and were slightly resistant to buthidazole {3-[5-(1,1-dimethylethyl)-1,3,4-thiadiazol-2-yl]-4-hydroxy-1-methyl-2-imidazolidinone}. The triazine-resistant biotypes were more sensitive to dinoseb [2-(1-methylpropyl)-4,6-dinitrophenol]. Triazine-resistant smooth pigweed showed resistance to cyanazine {2-[[4-chloro-6-(ethylamino)-1,3,5-triazin-2-yl] amino]-2-methylpropanenitrile} and metribuzin [4-amino-6-(1,1-dimethylethyl)-3-(methylthio)-1,2,4-triazin-5(4H)-one] with slight resistance to linuron [N′-(3,4-dichlorophenyl)-N-methoxy-N-methylurea] and desmedipham {ethyl [3-[[(phenylamino)carbony] oxy] phenyl] carbamate}. There was little or no resistance to diuron [N′-(3,4-dichlorophenyl)-N,N-dimethylurea], bromoxynil (3,5-dibromo-4-hydroxybenzonitrile), bentazon [3-(1-methylethyl)-(1H)-2,1,3-benzothiadiazin-4(3H)-one 2,2-dioxide], or dicamba (3,6-dichloro-2-methoxybenzoic acid). Parallel studies at the chloroplast level indicated that the degree of resistance to inhibition of photosynthetic electron transport was highly correlated with the degree of resistance to herbicidal injury. This correlation indicates that atrazine, cyanazine, metribuzin, pyrazon, bromacil, linuron, desmedipham, and buthidazole cause plant injury by inhibition of photosynthesis. This correlation also indicates that triazine resistance and cross-resistance at the whole plant level is due to decreased sensitivity at the level of photosynthetic electron transport. Cross-resistance to numerous additional herbicides was evaluated on isolated chloroplast thylakoid membranes and these results are discussed. 14C-atrazine was displaced from thylakoid membranes by several herbicides, indicating that these herbicides compete for a common binding site.

A study was conducted to evaluate efficacy of 17 PRE herbicides on seven weed species that are important competitors in container-grown ornamentals. Treatments included napropamide, metolachlor, acifluorfen, lactofen, oxyfluorfen, oryzalin, pendimethalin, prodiamine, trifluralin, isoxaben, oxadiazon, simazine and combination products. Control was evaluated by counting weed seedlings 42, 202, and 249 d after treatment. Some treatments provided nearly total control of less than 1.0 seedling per container at each rating over two years. Those treatments were: common groundsel—diphenyl ethers, isoxaben, oxadiazon, oxyfluorfen + oryzalin, and oxyfluorfen + pendimethalin; prostrate spurge—pendimethalin, prodiamine, prodiamine-ABD, trifluralin, oxadiazon, isoxaben + oryzalin, and methazole + prodiamine; common chickweed—isoxaben and isoxaben + oryzalin; hairy bittercress—oxyfluorfen, pendimethalin, prodiamine, prodiamine-ABE, oxadiazon, oxyfluorfen + oryzalin, oxyfluorfen + pendimethalin, and methazole + prodiamine; common ragweed—oxyfluorfen, prodiamine-ABD, methazole + prodiamine; Virginia pepperweed—lactofen, oxyfluorfen, oryzalin, pendimethalin, prodiamine, prodiamine-ABD, isoxaben, oxadiazon, oxyfluorfen + oryzalin, oryzalin + pendimethalin, isoxaben + oryzalin, and methazole + prodiamine; horseweed—oxyfluorfen, dinitroanilines, oxadiazon, oxyfluorfen + oryzalin, oxyfluorfen + pendimethalin, isoxaben + oryzalin, and methazole + prodiamine.

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.

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.