Six corn hybrids were studied in field experiments in 1989 and 1990 to identify hybrids and corn plant characteristics that may be valuable in systems using reduced levels of weed control. Four weed control treatments (no-control, cultivation-only, band herbicide-plus-cultivation, and broadcast herbicide) represented main plots and maize hybrids were subplots. Medium-season hybrids with differences in height, early-season vigor, and leafiness were used. There were significant differences among hybrids in leaf angle, leaf width, leaf number, plant height, leaf area index (LAI), plant dry matter (DM) and grain and stover yields. Lower yielding hybrids had grain yields that ranged from 87 to 91% of the highest yielding hybrid. Aboveground corn characteristics were not correlated with weed numbers, weed cover, or weed biomass. A significant interaction between hybrid and weed control for grain yields was observed in 1989, suggesting that some hybrids are more competitive when weed pressure is high.

Greenhouse experiments were established to investigate the effects of tank-mixing glyphosate, paraquat, or glufosinate with metribuzin plus chlorimuron, imazaquin, or metribuzin on entireleaf morningglory, sicklepod, and johnsongrass control. Antagonism was the most frequent interaction, and usually occurred when the lower rates of non-selective foliar-active herbicides were used in tank mixtures with selective soil-active herbicides. Antagonism occurred on all species when 180 g ai/ha paraquat was tank-mixed with 90 g ai/ha metribuzin plus 15 g ai/ha chlorimuron. When the rates of non-selective herbicide were increased, antagonism was usually overcome. Antagonism also occurred on entireleaf morningglory control when 210 g ai/ha glyphosate was tank-mixed with 90 g/ha metribuzin plus 15 g/ha chlorimuron or 36 g ai/ha imazaquin. When lower rates of paraquat or glufosinate were tank-mixed with 210 g/ha metribuzin, antagonism also occurred. Less antagonism was noted with glufosinate.

Canada thistle control in no-tillage corn was studied in Western Maryland from 1990 to 1992. The best treatment at planting for controlling Canada thistle was a tank mixture of 2240 g/ha glyphosate plus 560 g/ha 2,4-D. Clopyralid applied at 210 and 280 g/ha alone and 106 g/ha tank mixed with 560 g/ha 2,4-D effectively controlled Canada thistle in no-tillage corn (85 to 96%). Nicosulfuron applied at 35 g/ha and 20 or 40 g/ha primisulfuron suppressed Canada thistle (59 to 75%). Tank mixtures of nicosulfuron or primisulfuron with 2,4-D or dicamba generally increased Canada thistle control (75 to 87%) compared with control by these herbicides applied alone. Treatments that controlled Canada thistle generally increased corn yields compared with the weedy controls. Yield increases were most dramatic in 1991 when precipitation was low.

Flower production and pollen dispersal patterns of diclofop-methyl-susceptible and -resistant Italian ryegrass were examined to determine whether evolution of herbicide resistance could be controlled by cross-fertilization between biotypes. The susceptible and resistant ryegrass biotypes differed in timing and abundance of both ovule production and pollen release such that pollen from the susceptible plants had a much greater chance of fertilizing the resistant plant population than vice versa. Susceptible ryegrass, growing with or without wheat competition, produced more than 60% of its seeds before any pollen from resistant plants was released. In contrast, throughout the course of resistant plant seed generation, pollen from susceptible plants composed at least 30% of the total pollen load. These phenological differences, in conjunction with herbicide treatment results for third-generation ryegrass seedlings, suggest that gene flow can be used to reduce development of diclofop-methyl resistance in ryegrass populations within wheat cropping systems. In a wheat field infested with resistant ryegrass, cessation of herbicide application and sowing of a susceptible ryegrass biotype can be expected to reduce the evolution of diclofop-methyl resistance by at least 6% per year.

Seed response to grinding in a hammer mill or extrusion through a pellet die was determined. Less than 1% of spotted knapweed, sulfur cinquefoil, timothy, and alfalfa seeds were intact after grinding to pass through a 1-mm screen. Intact seeds of all tested species except sulfur cinquefoil were able to germinate after grinding. Extruding unground alfalfa seed through a die to make hay pellets decreased viability 60 to 63%. After grinding and extrusion through a grain pellet die, 53% of intact alfalfa seeds in the pellets were viable. These data demonstrate that feed pellets can contain viable weed seeds.

The growth, development, and reproductive potential of several populations of organic arsenical-susceptible (S) and -resistant (R) common cocklebur biotypes were compared under noncompetitive field conditions. Plant height, leaf area, aboveground dry weights, and relative growth rate (RGR) were measured periodically during the growing season. Days to flowering, bur dry weight, and number of burs per plant were also recorded. Arsenical S- and R-biotypes were similar in all measured parameters of growth, development, and reproductive potential. Populations within each biotype varied occasionally in plant height, leaf area, aboveground dry weights, and reproductive potential.

Barnyardgrass and morningglory control POST with glufosinate at 840 g a.i./ha 28 d after treatment was 79 to 85% and 83 to 90%, respectively, when no more than 35 d elapsed between initial spring soil tillage and herbicide application. For the same rate of glufosinate, prickly sida and hemp sesbania were controlled 68 and 92%, respectively. Comparable barnyardgrass control was obtained with glufosinate at 560 and 840 g/ha, which was greater than at 420 g/ha. Hemp sesbania control was similar for all rates of glufosinate. In comparison, paraquat at 1050 g a.i./ha controlled 40 to 65% barnyardgrass, 44 to 75% morningglory, 41% prickly sida, and 92% hemp sesbania. With 840 g a.i./ha glyphosate and SC-0224, barnyardgrass, morningglory, prickly sida, and hemp sesbania were controlled 55 to 89%, 55 to 81%, 45 to 61%, and 56 to 68%, respectively. Soybean yield was 5.8, 7.6, 6.0, and 5.9 times greater than the nontreated check for 1050 g/ha paraquat and 840 g/ha glufosinate, glyphosate, and SC-0224, respectively.

Ten herbicide treatments were evaluated for early season control of three broadleaf weeds and effects on marketable potato yields in low-organic, coarse-textured soil. All treatments controlled prostrate pigweed 100%. Trifluralin with metolachlor or with EPTC did not control kochia well. Pendimethalin alone or with EPTC controlled Russian thistle poorly, and produced the lowest marketable tuber yields. Fluorochloridone at 0.6 kg ai/ha caused chlorosis and reduced potato yield 11%. Treatments with metribuzin tended to have high potato yields.

Sequential extraction and chromatographic procedures were used to isolate inhibitors of yellow nutsedge growth from sweet potato periderm tissue. Most of the inhibitory activity was found in a single high pressure liquid chromatography peak that contained 1.2% of the periderm dry weight. Concentration-response data indicated that yellow nutsedge is highly sensitive to this fraction. Several other fractions inhibited yellow nutsedge root growth, but they did not compose a major portion of the inhibitory capacity of the periderm extracts.

The genus Bidens L. is in the Asteraceae (Compositae), the largest family of flowering plants, and one which contains many of the world's weed species. Beggarticks' phenomenal spread and colonization is due partly to their effective pollination mechanisms and their distinctive dispersal adaptations, which allow seed distribution by humans, animals, wind, and water.