Response of clones of Canada thistle from North America and leafy spurge from North America and Eurasia to treatment with chlorsulfuron, clopyralid, and glyphosate was determined. Roots of Canada thistle plants from different locations responded differentially to injury from chlorsulfuron and clopyralid but not to glyphosate. Clones of Canada thistle varied significantly in response to all herbicide treatments. Roots of leafy spurge plants from different locations were differentially susceptible to injury from glyphosate. Clones of leafy spurge varied significantly for all responses to each herbicide. The differential effects of chlorsulfuron, glyphosate, and clopyralid on clones of Canada thistle and leafy spurge suggest that genetically-based differences may account for the failure of the herbicides to kill all plants within collection locations or between collection locations. Cross tolerance to the herbicides was not found in leafy spurge or Canada thistle.

The response of sweet white lupin to several herbicides was evaluated over five years at Charlottetown, Prince Edward Island (PEI) and two years at Cornhill, New Brunswick (NB). Excellent crop tolerance was found to the herbicides: chloramben, ethalfluralin, fluazifop-P, linuron, metobromuron, metolachlor, and trifluralin. None of the above herbicides affected crop grain yield or 1000 seed weight. Sweet white lupin was slightly injured by metribuzin at 500 g ai/ha but yields and 1000 seed weight were not affected. Higher rates of metribuzin reduced lupin yield and 1000 seed weight. Imazethapyr applied POST with 0.25% v/v Agral 90 and 1.0% v/v 28% N fertilizer caused severe crop injury and reduced lupin yields.

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.

Research was conducted at two sites during 1991 and 1993 at Brandon, MB to determine if sodium bicarbonate antagonizes clethodim or quizalofop efficacy and if ultraviolet light is a factor in any antagonism. Sodium bicarbonate in the spray solution reduced the efficacy of clethodim. However, removal of ultraviolet light partially reversed sodium bicarbonate antagonism of clethodim activity with all adjuvants except crop oil concentrate with or without ammonium sulfate. Amigo, crop oil concentrate plus ammonium sulfate, and Merge were the most effective adjuvants for clethodim with distilled water. However, only the adjuvant crop oil concentrate plus ammonium sulfate overcame sodium bicarbonate antagonism of clethodim. Enhance increased the activity of clethodim when sodium bicarbonate was present in the spray solution but overall was not as effective an adjuvant as crop oil concentrate plus ammonium sulfate. Sodium bicarbonate in the spray solution did not reduce quizalofop efficacy.

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.

Leafy spurge control and forage production after various fall-applied herbicide treatment rotations were evaluated in a bluegrass pasture and a mixed grass prairie. Dicamba at 2 kg ae/ha or picloram at 0.6 kg ae/ha alone or combined with 2,4-D at 1 kg ae/ha applied annually for 3 yr provided consistent leafy spurge control of 85% or better, but forage production was greater at the bluegrass pasture site than the mixed grass prairie site. Glyphosate plus 2,4-D at 0.4 plus 0.7 kg ae/ha applied annually or rotated with picloram plus 2,4-D or dicamba provided 90% or better leafy spurge control after 3 yr at the bluegrass pasture. Control was 50% or less at the mixed grass prairie site where forage production was severely reduced by glyphosate plus 2,4-D. All treatments on the bluegrass pasture provided a positive economic return averaging $227/ha after 3 yr. Picloram or picloram plus 2,4-D at the mixed grass prairie site provided net returns averaging $285/ha, but treatment rotations that included glyphosate plus 2,4-D one or more times in 3 yr averaged a net loss of $32/ha due to severe grass injury.

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.

Phenmedipham was registered for use at 1.1 to 1.7 kg ai/ha in 1970 and desmedipham at 1.1 to 1.4 kg ai/ha in 1974 for postemergence control of broadleaf weeds in sugarbeet. Phenmedipham was sold from 1970 through 1981 and desmedipham plus phenmedipham (1:1), as a premixed product, has been sold since 1982. Development of multiple applications of reduced rates began in 1972 primarily to reduce sugarbeet injury. The technique was promoted to farmers after 1976. Ahalf-rate of phenmedipham and/or desmedipham applied twice at a 5- to 7-d interval controlled weeds better and caused less sugarbeet injury than a single full-rate application, and allowed application to smaller than four-leaf sugarbeet. Suggested sugarbeet size for the first application changed from 4-leaf in 1970 to cotyledon to two-leaf in 1992. Earlier application to more susceptible small weeds allowed a further rate reduction. The use of soil-applied herbicides prior to phenmedipham and/or desmedipham increased weed control and sugarbeet injury. Application to small weeds and sugarbeet by air or by ground sprayers at spray pressures over 1040 kPa controlled weeds better and injured sugarbeet more than when applied by ground sprayers at 280 kPa. Desmedipham and phenmedipham rates presently are adjusted for sugarbeet size, application method, soil-applied herbicide, and environment. The suggested initial application rate of phenmedipham and/or desmedipham in 1992 was 0.13 to 0.28 kg/ha as compared with 1.1 to 1.7 kg/ha in 1970. A typical rate of desmedipham or desmedipham plus phenmedipham applied in 1992, including multiple application, was approximately 0.7 kg/ha less than typical rates from 1970 through 1977.

Six experiments were conducted in three rice fields near Harrisburg, AR in Poinsett County in 1991 and 1992 to confirm barnyardgrass resistance to propanil and to compare herbicide treatments for control of propanil-resistant barnyardgrass. In all experiments, standard treatments of thiobencarb applied delayed PRE or tank mixtures of propanil + thiobencarb applied POST controlled propanil-resistant barnyardgrass. In five of six experiments, propanil failed to control barnyardgrass at 6 to 8 kg ai/ha (three times use rates) and in three experiments it provided inconsistent control at 11 kg/ha. Emulsifiable propanil at 11 kg/ha frequently controlled resistant barnyardgrass better than the dry flowable formulation. Treatments that consistently controlled propanil-resistant barnyardgrass included propanil tank-mixed with pendimethalin or quinclorac applied POST to two-leaf grass; quinclorac applied alone delayed PRE or POST or applied POST tank-mixed with thiobencarb, pendimethalin, or a formulated mixture of molinate and propanil; a formulated mixture of propanil and molinate applied POST in tank mixtures with thiobencarb or pendimethalin; and fenoxaprop or fenoxaprop-P alone applied POST to four-leaf grass prior to flood establishment. Herbicide treatments that failed to control resistant barnyardgrass included POST applications of a formulated mixture of propanil and molinate alone and propanil in tank mixtures with bromoxynil or triclopyr. These studies confirm that barnyardgrass from several farms in Arkansas have resistance to propanil and identified treatments that control propanil-resistant barnyardgrass.

Field studies were conducted in 1992 and 1993 to evaluate the efficacy of EPTC plus R-29148 (EPTC+) plus cyanazine and alachlor plus cyanazine. Materials were applied PPI and PRE to corn at 25, 50, 75, and 100% of labeled rates in an aqueous solution (sprays) and as herbicide-impregnated fertilizer. EPTC+ plus cyanazine at all rates applied as sprays or as herbicide-impregnated fertilizer controlled 89% or more of giant foxtail. Velvetleaf and common lambsquarters control was less when alachlor plus cyanazine was applied as herbicide-impregnated fertilizer than when applied as sprays. When sprayed, alachlor plus cyanazine applied PPI controlled 95% giant foxtail only at the 100% rate without cultivation, but at all rates with cultivation. Alachlor plus cyanazine applied as impregnated fertilizer at 50, 75, and 100% labeled rates controlled 88% or more of giant foxtail when these treatments received a interrow cultivation. PRE treatments of alachlor plus cyanazine applied as herbicide-impregnated fertilizer were less effective in 1992 than in 1993 because of lack of timely precipitation following application.

Two or three applications of MSMA applied alone were ineffective in controlling smutgrass in common bermudagrass turf. Where atrazine at 2.2 kg ai/ha or simazine at 2.2 kg/ha was followed by two or three MSMA applications at 2.2 or 4.5 kg/ha, smutgrass control was increased. Metribuzin followed by two or three applications of MSMA was only slightly better than MSMA applied alone. Up to 35% injury to bermudagrass was observed 2 wk after the initial MSMA or the MSMA-triazine treatment, but complete recovery was evident 2 to 4 wk later, even with one or two follow-up MSMA applications at weekly intervals. Bermudagrass injury from the application of triazines followed with MSMA did not differ from MSMA applied alone in two of three experiments. In one experiment, atrazine or metribuzin followed with MSMA applications caused 10 to 25% more bermudagrass injury than MSMA applied alone.

The absorption of 14C-bentazon and 14C-acifluorfen by individual leaves of velvetleaf was measured from the cotyledons to the most recently developed leaf in plants that had developed seven leaves beyond the cotyledons. Herbicide absorption 1 min after application was greatly enhanced by the organosilicone adjuvant Sylgard 309 explaining previously observed enhanced efficacy and rainfastness by this adjuvant. 14C-herbicide absorption was greatest by the cotyledons and decreased upward to the most recently developed leaf. Absorption of the 14C-Sylgard 309 was similar to that of the 14C-labelled herbicides and was not related to the number of stomata or leaf area covered by the spray droplets. These results are consistent with the hypothesis that the mode of action of Sylgard 309 is to enhance cuticular absorption and not stomatal infiltration.

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.

During the early years of herbicide use, the total amount of ai applied per ha increased in attempts to obtain season-long weed control, peaking in the decade of the mid-1970's to mid-1980's. Since then, the chemical load applied for broadleaf weed control has shown a consistent, if not dramatic, decline. A much more significant reduction has occurred in grass weed control. Main reasons for the reduction are a move from PPI and PRE treatments to POST, the development of repeat low-dose herbicide techniques, and the introduction of more active postemergence grass herbicides. In general, this change has been achieved with a concomitant improvement in crop safety. These developments occurred as a coincident benefit in pursuing the target objective of giving growers more convenient and flexible weed control and not as a specific attempt to reduce chemical use. This paper discusses the evolution of weed control programs in the U.K., France, Germany, and the U.S.A. All show a similar trend, though the pace of change has been slower in the U.S.A.

In a two-year study involving two sites in western Canada, the dissipation of bromoxynil octanoate alone at 0.35 or 0.42 kg phenol equivalent (pe)/ha and the tank mixture of MCPA iso-octyl ester at 0.28 kg ae/ha plus bromoxynil octanoate at 0.28 kg pe/ha was monitored in postemergence-treated triticale. Initial foliar residues on the day after application (three- to four-leaf stage of the crop) were of the order of 14 and 5 mg/kg for MCPA and bromoxynil, respectively. Residues of both herbicides decreased to less than the limit of quantification (0.025 mg/kg) of the analytical method 21 d after application. At crop maturity, herbicide residues were non-detectable in the straw and seed. There was little or no effect on the dissipation of bromoxynil residues due to tank mixing with MCPA.

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.

Field experiments were conducted at two locations in Virginia to evaluate the following herbicides: alachlor, diphenamid, diuron, metolachlor, napropamide, norflurazon, oryzalin, oxyfluorfen, paraquat, pendimethalin, and simazine. One experiment involved newly-transplanted apple trees; the others, three in apple and one in peach trees, involved one-year-old trees. Treatments were applied in the spring (mid-April to early-May). Control of annual weed species was excellent with several treatments. A broader spectrum of weeds was controlled in several instances when the preemergence herbicides were used in combinations. Perennial species, particularly broadleaf species and johnsongrass, were released when annual species were suppressed by the herbicides. A rye cover crop in nontreated plots suppressed the growth of weeds. New shoot growth of newly-transplanted apple trees was increased with 3 of 20 herbicide treatments and scion circumference was increased with 11 of 20 herbicide treatments compared to the nontreated control. Growth of one-year-old apple trees was not affected. Scion circumference of one-year-old peach trees was increased with 25 of 33 herbicide treatments.

There is distinct regional variation in forestry uses of herbicides. Different land ownership patterns affect policies and practices, and crop tree species characteristics differ within and among regions. Recent decreases in land areas treated have been attributed to budget reductions, changes in operating conditions, and pressures from the general public. Factors affecting future reductions in amounts of herbicides used will include new chemistry, improved delivery technologies, incorporating vegetation management into all aspects of young stand silviculture, and employment of alternative methods.