Shortawn foxtail is a competitive annual grass weed widely spread in east, south-central, and southwest China and parts of the Yellow River basin. One shortawn foxtail population (JSQT-1) resistant to fenoxaprop was identified in Jiangyan, Jiangsu province. Whole-plant experiments determined that the resistant population conferred high-level resistance to fenoxaprop (93-fold), clodinafop (21-fold), sethoxydim (107-fold), mesosulfuron (41-fold), and pyroxsulam (12-fold); moderate-level resistance to haloxyfop (8-fold), clethodim (9-fold), and pinoxaden (8-fold), and no resistance to isoproturon. Molecular analyses confirmed that the Ile-1781-Leu mutation was present in the resistant population. A dCAPS marker was used to detect the Ile-1781-Leu mutation. All 97 plants of the resistant population analyzed were homozygous mutants at the 1781 position. Our study established the first case of fenoxaprop resistance in shortawn foxtail, determined cross resistance to other herbicides, and elucidated that the molecular basis of resistance resulted from, at least partly, an Ile to Leu mutation at amino acid position 1781 in the plastid ACCase.

Microsomes (100,000 g pellet containing mixed membrane fractions but primarily endoplasmic reticulum) were isolated from shoots of corn, shattercane, and woolly cupgrass grown from naphthalic anhydride treated or untreated seed to determine if metabolism of bentazon, nicosulfuron, and primisulfuron could be demonstrated in the preparations. Corn is tolerant of all three herbicides, shattercane is tolerant of bentazon, and woolly cupgrass is tolerant of bentazon and primisulfuron. Naphthalic anhydride treatment was required for detectable bentazon, nicosulfuron, and primisulfuron hydroxylation in corn microsomes and for bentazon hydroxylation in woolly cupgrass microsomes. Bentazon hydroxylation was low, but detectable, in microsomes from shattercane shoots without naphthalic anhydride treatment. Naphthalic anhydride-treated corn microsomes hydroxylated 292, 120, and 52 pmol mg−1 protein min−1 of bentazon, nicosulfuron, and primisulfuron, respectively. Primisulfuron (19 pmol mg−1 protein min−1), but not nicosulfuron, was hydroxylated in woolly cupgrass microsomes. Neither nicosulfuron nor primisulfuron was hydroxylated in shattercane microsomes. Bentazon and primisulfuron inhibited nicosulfuron hydroxylation in corn microsomes. Bentazon, but not nicosulfuron, also inhibited primisulfuron hydroxylation in the corn microsomes. This indicates that the three herbicides can interact at the same cytochrome P-450(s) in corn. Primisulfuron hydroxylation was not inhibited by either bentazon or nicosulfuron in woolly cupgrass microsomes. This suggests that the cytochrome P-450(s) for primisulfuron hydroxylation are different between corn and woolly cupgrass. Also, bentazon hydroxylation in corn and shattercane microsomes was inhibited by the cytochrome P-450 inhibitor tetcyclasis, while that in woolly cupgrass was not. Again, this suggests a difference in the cytochrome P-450(s) responsible for bentazon metabolism among the species. Although absolute conclusions comparing in vitro microsomal activities to whole plant herbicide tolerance cannot be made because it is unknown whether the same cytochrome P-450(s) are studied in microsomes from naphthalic anhydride-treated tissue as are responsible for in vivo herbicide metabolism, there was a broad correlation between metabolism of a particular herbicide in microsomes of a species and the species' tolerance of that herbicide.

The primary defense against seed mortality, the seed coat, is maternally derived. Hence, weed seed mortality in the soil seedbank is likely to be influenced by the maternal environment and genetics. We hypothesized that seed accessions from contrasting maternal environments (seed lots) exhibit different rates of seed mortality and that the relative differences among seed lots remain consistent across burial environments. Velvetleaf and giant foxtail annual seed mortality rates were studied in field experiments in Hickory Corners, MI, and Wooster, OH, using seed lots collected from the same locations. Seeds enclosed within mesh bags and unenclosed seeds (“seeded cores”) exhibited similar levels of seedbank persistence (r = 0.90, P < 0.001) and seed mortality (r = 0.65, P = 0.006). Annual seed mortality rates ranged from 16 to 56% and 27 to 91% for seed lots of velvetleaf and giant foxtail, respectively. Relative differences among velvetleaf seed lots were consistent across burial environments in both years, whereas giant foxtail differences were consistent in only 1 of 2 yr. The relative ranks among velvetleaf seed lots varied between years, indicating that maternal environment may have influenced seed persistence more than seed-lot genetics. Within years, variation in seed mortality was predicted by changes in soil moisture in the burial environment (R2 = 0.47, P < 0.001 for velvetleaf; R2 = 0.34, P = 0.007 for giant foxtail). Accelerated seed mortality was associated with moist soils (soil water potential = −6 kPa for velvetleaf, −7 kPa for giant foxtail). These results suggest that agronomic practices affecting the maternal environment and moisture levels in the soil seedbank may promote weed seed mortality in the soil seedbank.

Three major hypotheses were examined in this study: (1) the density of summer annual weeds is reduced in crop rotation systems that include winter wheat compared to those with strictly summer annual crops, (2) the integration of a red clover in cropping systems reduces weed seedbank densities, and (3) changes in weed seedbanks due to crop rotation system have greater impact on future crops that are managed with cultivation alone, compared to those managed with herbicides. To test these hypotheses, five 3-yr rotation sequences were examined in central New York state: continuous field corn (FC); field corn with red clover (FC + CL); field corn–oats–wheat (FC/O/W); sweet corn–peas–wheat (SC/P/W), and SC/P/W with red clover (SC/P/W + CL). In the fourth year, sweet corn, snap beans, and cabbage were planted in subplots with three levels of weed management as sub-subplots: cultivation alone, reduced-rate herbicides (1/2×), and full-rate herbicides (1×). The trial was carried out in two separate cycles, from 1997 to 2000 (cycle 1) and from 1998 to 2001 (cycle 2). Crop rotations with strictly summer annual crops (FC) did not result in consistently higher weed seedbank densities of summer annual weeds compared to rotations involving winter wheat (FC/O/W; SC/P/W; SC/P/W + CL). Integration of red clover in continuous field corn resulted in higher weed seedbanks (cycle 1) or emergence (cycle 2) of several summer annual weeds compared to field corn alone. In contrast, integration of red clover in the SC/P/W rotation led to a 96% reduction in seedbank density of winter annuals in cycle 1, although this effect was not detected in cycle 2. Observed changes in weed seedbank density and emergence due to crop rotation resulted in increased weed biomass in the final year in only one case (sweet corn, cycle 2), and did not result in detectable differences in crop yields. In contrast, final year weed management had a strong effect on weed biomass and yield; cultivation alone resulted in yield losses for sweet corn (32 to 34%) and cabbage (0 to 7%), but not snap beans compared to either 1/2× or 1× herbicides.

Research was conducted to determine picloram and aminopyralid sorption in five soils and three clay minerals and to determine if the potential for off-target movement of aminopyralid in soil is less than that of picloram. Nearly all sorption of picloram and aminopyralid occurred between 0 and 8 h, and the maximum theoretical sorption of picloram and aminopyralid were 10.3 and 15.2%, respectively. Freundlich distribution coefficients (K f) for picloram ranged from 0.12 in a Cecil sandy loam to 0.81 in an Arredondo fine sand, while K f values for aminopyralid ranged from 0.35 in a Cecil sandy loam to 0.96 in an Arredondo fine sand. Furthermore, K f values of aminopyralid were higher than those of picloram in all soils tested. K f values of picloram in clay minerals were 0.25 (kaolinite), 1.17 (bentonite), and 1,016.4 (montmorillonite), and those of aminopyralid were 5.63 (kaolinite), 2.29 (bentonite), and 608.90 (montmorillonite). It was concluded that soil sorption of aminopyralid was greater than that of picloram and that the potential for off-target movement of aminopyralid is less than that of picloram.

Exotic annual weeds on western U.S. rangelands, including yellow starthistle, owe at least part of their invasion success to prolific seed production. Occurrence of flowering in relation to soil moisture and development of competing vegetation heavily influences reproductive output. Yellow starthistle flowers much later than associated Mediterranean invaders, which avoid summer drought by early flowering and senescence. This study used successive planting dates and four densities to investigate the influence of temperature, photoperiod, and intraspecific competition on reproductive phenology in a population of yellow starthistle from Lewiston, ID. A base temperature for seed germination of 2 C was determined by experiments at constant temperatures. Under the conditions of this study, density, photoperiod, and vernalization did not regulate reproductive development, as measured by onset of bud, flowering, and achene dispersal stages. A thermal time model adequately predicted phenology in this population of yellow starthistle. Plants emerging between October and July required about 1,240 degree days for 50% of the plants to reach bud stage, and an additional 500 and 900 degree days to flowering and achene dispersal, respectively. The relatively long time from emergence to mature achenes in this species influences management options such as prescribed fire, livestock grazing, and management of competing vegetation. Insensitiviry to photoperiod and lack of a vernalization requirement allow late germinating plants to reproduce if moisture is adequate.

Pronamide [3,5-dichloro-N-(1,1-dimethyl-2-propynyl)benzamide] was most phytotoxic to oat (Avena sativa L.) when placed in the seed zone and to quackgrass (Agropyron repens (L.) Beauv.), when placed in the rhizome zone. Inhibition of part of the buds by pronamide on detached quackgrass rhizome sections did not influence other buds. Foliar applications of pronamide were not phytotoxic to established oat or quackgrass. Foliar-applied 14C-pronamide showed little uptake by quackgrass. Application to the roots of established plants showed rapid root uptake and movement to the foliage. 14C-pronamide was rapidly absorbed by oat seedlings for the first 0.5 hr and subsequently at a much slower rate. Over 60% of the pronamide taken up after 1 hr was exchangeable. Pronamide was adsorbed to the cell walls of treated roots, but little was associated with nuclear, mitochondrial, microsomal, or soluble protein fractions. Pronamide was adsorbed to varying degrees depending on soil type. Adsorption was more highly correlated with soil organic matter content than with cation exchange capacity, clay content, or pH.

Autumn-planted sugarcane was seeded with itchgrass in early March to give 1 plant per 30.5 cm of row length (1.8 plants m−2). In three of four experiments, itchgrass was allowed to reseed and develop naturally in the second or third-yr ratoon crops. In the first-yr crop, itchgrass stands developed by May 15 and itchgrass was removed after 30, 60, or 180 d of interference. After 30 d interference, on about June 15, itchgrass biomass ranged from 200 to 2700 kg ha−1 in four experiments and reduction in sugar yield averaged 7%. After 60 d interference, on July 15, the biomass ranged from 1400 to 2900 kg ha−1 in two experiments and reduction in sugar yield averaged 17%. Itchgrass removed at harvest in November, after 180 d interference, reduced sugar yield by 19% in four experiments. Itchgrass infestations removed in the second-yr crop on May 1, June 1, and November 15 harvest reduced sugar yield in one experiment by 3, 11, and 72%, respectively, compared to a 2-yr weed-free control. Itchgrass interference reduced sugar yield primarily by reducing stalk population even though full-season interference increased sugar concentration of juice by 2 to 10%. These studies indicate that itchgrass must be removed from sugarcane well before June 15 (30 d of interference) under Louisiana growing conditions to prevent a rapid increase in itchgrass biomass and loss in sugar yield. However, sugarcane stands and yield recovered almost completely when maintained weed free in the second-yr crop following full-season itchgrass interference in the first-yr crop.

Selective herbicides that will control certain broadleaved but not grassy species or vice versa have been in use for a number of years, but recently selective herbicides have been developed that are toxic to a greater number of weed species. Such an herbicide is 2–chloro–4,6–bis(ethylamino)–s–triazine (simazine) which selectively controls virtually all grassy and broadleaved annual weeds in corn and in some horticultural and woody species. This approaches the ultimate in selective weed control since a farmer usually wants to grow one plant species and eliminate all others.

Field studies were conducted in 1987 and 1988 to evaluate the control and competitiveness of common lambsquarters in soybeans when present at four densities and when removed at four time intervals. Common lambsquarters were removed by hand or treated with a postemergence herbicide application of bentazon plus acifluorfen. Common lambsquarters could remain 10 weeks following soybean emergence before a 20% soybean yield reduction occurred when weeds were removed by hand. When treatment was attempted with postemergence herbicides, a 20% yield reduction occurred in 1987 if application was not made prior to 5 weeks after emergence. In 1988, all postemergence herbicide applications failed to control lambsquarters due to drought conditions. The postemergence herbicide's degree of control was reflected in the number of remaining lambsquarters plants. Common lambsquarters seed production and germination was not influenced by postemergence herbicide application. There was a strong correlation between dry weight of uncontrolled common lambsquarters plants and seed produced per plant.

The toxicity to bluegill sunfish (Lepomis macrochirus, Rafinesque) of 3 to 21 commercial formulations of 2,4-dichlorophenoxyacetic acid (2,4-D), 2,4,5-trichlorophenoxyacetic acid (2,4,5-T), 2-(2,4-dichlorophenoxy)propionic acid [2-(2,4-DP)], and 2-(2,4,5-trichlorophenoxy)propionic acid (silvex) was determined. The variations among formulations of the same herbicide were much greater than those among different herbicides. Variations in toxicity of a single formulation were also noted. Formulations of each herbicide were reported as either safe or unsafe for aquatic applications. Amine salts were generally less toxic to fish than esters. The formulation selected for use should be assayed prior to field application, using the receiving water.

Canada thistle and perennial sowthistle have been given several names by various authors. However, it appears that the Canada thistle used in this study is generally called Cirsium arvense (L.) Scop. and the perennial sowthistle was a mixture of Sonchus arvensis L. and S. uliginosus Bieb. The two species of sowthistle are very similar. However, S. arvensis has “long glandular-hispid peduncles and involucres covered with lead colored glandular phyllaries” and a chromosome complement of 54, while S. uliginosus has “slenderly turbinate to cylindric pale involucres and glaborous white-margined phyllaries” and a chromosome complement of 36.

Acetyl-coenzyme A carboxylase (ACCase)–resistant Italian ryegrass is one of the most difficult-to-control weeds in United States wheat-production systems. Seed was collected from a suspected ACCase-resistant Italian ryegrass population in a winter wheat field with a history of ACCase-inhibitor herbicide use. This study investigated cross-resistance patterns in this Italian ryegrass population. Resistance was identified to the commercial dose of the ACCase herbicides pinoxaden, clethodim, sethoxydim, and clodinafop. Partial chloroplastic ACCase sequences revealed aspartate-to-glycine or isoleucine-to-asparagine substitutions at positions 2078 or 2041 in individuals of the resistant population. This is the first report, to our knowledge, of Asp-2078-Gly and Ile-2041-Asn substitutions in ACCase-resistant Italian ryegrass in the United States. Associating the occurrence of resistance alleles with resistance to specific active ingredients provides a better understanding of ACCase cross-resistance in Italian ryegrass and possibly options for its control.

The mesquite problem on southern Arizona ranges has been comprehensively described by Parker and Martin (9), and certain ecological relationships of this noxious tree, insofar as are known, have recently been considered by Glendening and Paulsen (5). Although a serious problem only in the states of Arizona, New Mexico, and Texas, at least one of the three common varieties of Prosopis juliflora now may be found in each of eight southwestern states (2). Its progressive invasion is unquestioned.

The controlled release of metribuzin [4-amino-6-(1,1-dimethylethyl)-3-(methylthio)-1,2,4-triazin-5(4H)-one] and alachlor [2-chloro-N-(2,6-diethylphenyl)-N-(methoxymethyl)acetamide] from pine kraft lignin was studied using 14C-labeled herbicides and 3H-labeled lignin with soil thin-layer chromatography. NB-5203-58 series and PC940 series kraft lignins adsorbed and desorbed metribuzin as soil plates were developed with water. PC671, a formulated kraft lignin control release product, did not provide controlled release with metribuzin. NB-5203-58B and PC940C were the most effective of their respective series of kraft lignins. PC940C also provided controlled release of alachlor.

Studies were conducted in North Carolina to determine if johnsongrass [Sorghum halepense (L.) Pers.] rhizomes in the soil exude or produce substances that can affect soybean [Glycine max (L.) Merr.] seedling growth. Fresh and dry weights of soybean seedlings were dependent on the concentration of rhizomes present in the soil before soil samples were used for soybean growth, and on the time of year when soil samples were collected. In studies on rhizome residues, soybean seedling dry weight decreased as percent of dry, decayed rhizomes in the soil increased. For each rhizome concentration, soybean growth inhibition decreased as decay time increased. Fresh and dry weights of soybean plants were reduced by diluted fresh rhizome extract used for irrigation once a week for 3 weeks. Since no rhizomes were present in most of the soils during soybean growth and since nutrient concentrations, organic matter, and pH were not different among the soils used, it is suggested that johnsongrass rhizomes living or decaying in the soil exude, contain, or produce substances that exhibit allelopathic characteristics to soybean growth.

Interference studies utilizing natural infestations of broadleaf signalgrass [Brachiaria platyphylla (Griseb.) Nash] in peanuts [Arachis hypogaea (L.) ‘Florigiant′] were conducted at two sites in 1978 and repeated at the same sites in 1979. Full-season interference from broadleaf signalgrass populations of 8, 16, and 1050 plants/10 m of row reduced seed yields 14, 28, and 69%, respectively. Linear regression analysis showed that slightly less than 4 plants/10 m of row significantly reduced yields. Broadleaf signalgrass interference with peanuts for 6 weeks or less after planting did not reduce seed yields; however, interference for 8 weeks or longer after planting did. Peanuts kept weed-free for 6 weeks or more after planting had seed yields equal to peanuts kept weed-free all season. Full-season broadleaf signalgrass interference reduced mature peanut forage yields 64%, but peanut interference only reduced broadleaf signalgrass forage yield 10%.

Biological agents, including fungal pathogens, are being considered to control weeds such as dandelion. Compatibility of a fungal pathogen, Phoma herbarum, with nonformulated 2,4-D was tested in vitro. Moderate to high concentrations (828 to 2,451 ppm) of 2,4-D reduced fungal colony diameter by 50% or less, suggesting that practical concentrations may be chosen for tank mixes. Under growth room and field conditions, integration of 2,4-D and P. herbarum produced enhanced control of dandelion. In general, synergistic responses to sublethal doses of 2,4-D followed 4, 8, or 16 d later by P. herbarum inoculation were found. Synergistic interactions, as determined by the Colby method, ranged as high as 62% greater than expected for an additive interaction. In general, dandelion response was reduced under field conditions, suggesting that environmental conditions or other factors play a role in efficacy. Integration of chemical and biological agents shows potential for enhanced weed control.

Two leaf-spotting pathogens, [Bipolaris setariae (Saw.)] and [Piricularia grisea (Cke.) Sacc.], were isolated from severely infected goosegrass plants. Pathogenicity tests conducted at 28 C showed that both fungi were 100% effective in infecting goosegrass when given a 72-h dew period. Dew period temperature and duration requirements were tested by inoculating 2-week-old plants with conidial suspensions of each fungus and incubating them in dew chambers (100% relative humidity). Disease index increased as dew period duration increased for both fungi and at all temperatures tested. Infection occurred at all temperatures with an optimum of 24 to 28 C for B. setariae and 28 C for P. grisea. In host range tests, representative plants of the Fabaceae, Malvaceae, Poaceae, and Solanaceae were inoculated with suspensions of either 20 000 or 60 000 spores/ml of each fungus and placed in growth chambers at 28 C. Infection was limited to members of the Poaceae. Sorghum showed a hypersensitive response to B. setariae. Both cultivars of corn developed light symptoms in response to both fungi at 20 000 and 60 000 spores/ml.

The herbicide 3,6-dichloropicolinic acid was more effective than 2,4,5-T [(2,4,5-trichlorophenoxy)acetic acid] or equal-ratio combinations of 2,4,5-T with picloram (4-amino-3,5,6-trichloropicolinic acid) or dicamba (3,6-dichloro-o-anisic acid) when aerially applied as broadcast sprays for control of honey mesquite [Prosopis juliflora (Swartz) DC. var. glandulosa (Torr.) Cockerell]. Applied at 0.56 kg/ha, 3,6-dichloropicolinic acid controlled 60 and 68% of the honey mesquite at two locations; whereas, the mixture of 2,4,5-T and picloram controlled only 21 and 30% of the honey mesquite at the same rate of application. The herbicide 3,6-dichloropicolinic acid was equally effective when applied alone or in 1:1 mixtures with 2,4,5-T, picloram, or triclopyr {[(3,5,6-trichloro-2-pyridinyl)oxy] acetic acid} at equivalent rates. Applying 3,6-dichloropicolinic acid at 1.12 kg/ha resulted in an absolute increase in mortality of 26% over the 0.56-kg/ha rate.