Grass Herbicides Research Articles

Evolution of Herbicide Programs in Sugarbeet

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.

Effects of herbicides on wallaby grass (Danthonia spp.). 2. Established plants

Three experiments were conducted at Tamworth, New South Wales, in 1990-92 to assess the effects of a range of broadleaf and grass herbicides, 6 rates of glyphosate and paraquat + diquat, and 4 rates of 2,4-D amine and diuron on the phytotoxicity, dry matter yield, inflorescence number, and seed yield of established Danthonia (wallaby grass) plants. Phytotoxicity was scored visually on a 0-5 scale (0, no phytotoxicity; 5, 100%). Danthonia dry matter yields were not affected (P<0.05) by any herbicide tested, except glyphosate applied at 2360 g a.i./ha. Phytotoxicity was highest for the glyphosate treatment (1440 g a.i./ha) in that study. All rates of glyphosate, and paraquat + diquat rates >250 + 150 g a.i./ha reduced (P<0.05) inflorescence number compared with the unsprayed control. Paraquat + diquat had no significant effect on seed yield, but glyphosate rates >360 g a.i./ha reduced (P<0.05) seed yield compared with the control. In 1992, no damage was observed on 2,4-D amine treatments and phytotoxicity scores were low (<2) for all diuron treatments. Danthonia inflorescence numbers and seed yields were higher (P<0.05) on plots mown in early spring than on unmown treatments. The lack of effect of herbicides such as diclofopmethyl, simazine, fenoxaprop-ethyl, and diuron on Danthonia dry matter yield, and the low phytotoxicity, indicated that a range of chemicals may be used on established Danthonia swards to control perennial and annual grass weeds. Seed yield was more affected than dry matter yield by herbicide, and paraquat + diquat should be used in preference to glyphosate on seed crops.

The Effect of Giant Foxtail (Setaria faberi) Plant Height on Control with Six Postemergence Herbicides

Field experiments were conducted from 1986 to 1988 to evaluate giant foxtail control at four plant heights by six POST grass herbicides. Clethodim, fenoxaprop, fluazifop-P, haloxyfop, quizalofop, and sethoxydim controlled 95% or more of 60-cm giant foxtail all three years at the labeled herbicide application rates. Control of 7-, 15-, and 30-cm giant foxtail with fenoxaprop, fluazifop-P, quizalofop, and sethoxydim was dependent upon soil moisture conditions at application. Clethodim and haloxyfop provided the greatest control (89 to 99%) regardless of plant height or soil moisture conditions. No herbicide injured soybean. Soybean seed yield with POST grass herbicides was equal to or greater than yield in the hand-weeded plots.

Occurrence of a herbicide-resistant acetyl-coenzyme A carboxylase mutant in annual ryegrass (Lolium rigidum) selected by sethoxydim

The spectrum of herbicide resistance was determined in an annual ryegrass (Lolium rigidum Gaud.) biotype (SLR 3) that had been exposed to the grass herbicide sethoxydim, an inhibitor of the plastidic enzyme acetylcoenzyme A carboxylase (ACCase, EC 6.4.1.2), for three consecutive years. This biotype has an 18-fold resistance to sethoxydim and enhanced resistance to other cyclohexanedione herbicides compared with a susceptible biotype (VLR 1). The resistant biotype also has a 47- to >300-fold cross-resistance to the aryloxyphenoxypropanoate herbicides which share ACCase as a target site. No resistance is evident to herbicide with a target site different from ACCase. The absorption of [4-14C]sethoxydim, the rate of metabolic degradation and the nature of the herbicide metabolites are similar in the resistant and susceptible biotypes. While the total activity of the herbicide target enzyme ACCase is similar in extracts from the two biotypes, the kinetics of herbicide inhibition differ. The concentrations of sethoxydim and tralkoxydim required to inhibit the activity of ACCase by 50% are 7.8 and >9.5 times higher, respectively, in the resistant biotype. The activity of ACCase from the resistant biotype was also less sensitive to aryloxyphenoxypropanode herbicides than the susceptible biotype. The spectrum of resistance at the whole-plant level is correlated with resistance at the ACCase level and confirms that a less sensitive form of the target enzyme endows resistance in biotype SLR 3.

Dos graminicidas para piñales

En fincas de la Autoridad de Tierras en Vega Baja y Barceloneta se realizaron dos experimentos con herbicidas contra gramíneas en plantilla y retoño de piña de 1990 a 1991. Se evaluó el fluazifop-p-butyl a razón de 0.28 y 0.56 kg ia/ha y el quizalofop a razón de 0.224 y 0.448 kg p/ha en aspersiones sobre el follaje y dirigidas. Se realizaron dos aplicaciones posemergentes a una plantilla de 12 meses y dos aplicaciones a un retoño de 30 meses. Ambos herbicidas controlaron excelentemente las gramíneas sin que se observaran síntomas de fitotoxicidad. El rendimiento más alto de piñas en el experimento de plantilla se obtuvo con el quizalofop aplicado sobre el follaje a razón de 0.224 kg ia/ha (69,289 kg/ha). Hubo diferencias significativas (P = 0.05) con tres de los otros tratamientos. En el experimento de retoño, el rendimiento mayor se obtuvo con el fluazifop-p- butyl en aplicación dirigida a razón de 0.56 pa/ha (51,585 kg/ha). No hubo diferencias significativas (P = 0.05) con los demás tratamientos.

Investigation of an emulsifiable oil adjuvant and its components on the activity of a new grass herbicide by factorial experimentation

AbstractThe herbicidal activity of a new compound [(±)‐(EZ}‐2‐[1‐allyloxy‐imino)propyr]‐3‐hydroxy‐6‐(4‐cyanophenyl)cyclohex‐2‐ene‐1‐one] (I), selective to broad‐leaf plants, and partially selective in some cereal crops was enhanced by an emulsifiable paraffinic oil (HVI 60E).In order to determine the amount of HVI 60E to be applied with an emulsifiable concentrate of I (IE) a factorial herbicidal assay involving six concentrations of I and five concentrations of HVI 60E (both sets including zero concentration) was conducted to obtain response surfaces of herbicidal activity. The total selectivity of IE in broad‐leaf species remained for all HVI 60E concentrations. The activity against grass weeds, except Poa annua L., increased markedly with increasing HVI 60E concentration and enhancement of performance of IE by HVI 60E in broad‐leaf crops could be pursued without fear of crop phytotoxicity.However, the increases in activity against grass weeds were accompanied by increases in phytotoxicity against cereal crops. The response surfaces of Echinochloa crus‐galli(L.) Beauv. and Zea mays L. were coincident and this result terminated further testing in this outlet. However, the response surfaces of Avena fatua L. and Triticum aestivum L. were reasonably well separated. Further tests in this outlet and in broad‐leaf crops are planned around optimum cost‐performance ratios derived from combinations with cost‐response surfaces.The results of a further lest showed that both components (oil and emulsifier) of HVI 60E were approximately equivalent in adjuvant enhancement activity and so, on cost grounds, a crop oil (low emulsifier content) would be preferred over a crop oil concentrate (high emulsifier content).

Postemergence Grass Herbicide Activity Changes with Adjuvant and pH and Sodium Level in Spray Solutions

Abstract Water quality is known to affect herbicide efficacy. Nursery well water containing naturally high pH and sodium levels was evaluated as spray solution for the postemergence grass herbicides Poast (sethoxydim), Fusilade (fluazifop P) and Acclaim (fenoxapropethyl). Using crabgrass as a test plant, high pH & sodium solution water reduced the effectiveness of all herbicides in one study. Other studies revealed that Poast (sethoxydim) was influenced the most with high pH & sodium water while the activity of Fusilade (fluazifop-P) and Acclaim (fenoxaprop-ethyl) was less affected. The solution water inhibition of Poast (sethoxydim) was overcome when Dash (a proprietary blend of surfactants used as a replacement for oil concentrate) was used as an adjuvant.

New Selective Grass Herbicides and Their Hydrolytic Properties as Pro-herbicides.

New derivatives of pyridinyloxyphenoxypropionic acid were synthesizd, which have a keto group in their ester-alkyl structures. Some acylmethyl esters, e.g., acetylmethyl ester, hydroxyacetylmethyl ester and diazoacetylmethyl ester, were found to show outstanding herbicidal activity against grass weeds. A study of the hydrolysis rate of esters with regard to their pro-herbicide action is also discussed.

Economics of Johnsongrass (Sorghum halepense) Control in Soybeans (Glycine max)

Field trials were conducted from 1986 to 1988 at Clarkedale, AR, to evaluate selective POST grass herbicides for johnsongrass control in soybeans. Herbicide treatments were trifluralin PPI followed by single and sequential POST applications of fenoxaprop, fluazifop, quizalofop, or sethoxydim. In 1986, sequential applications of sethoxydim and fenoxaprop controlled more johnsongrass 4 WAT than single treatments. In 1987 and 1988, all herbicides except fenoxaprop controlled more than 79% of johnsongrass. The number of johnsongrass stems was reduced by cultural practices and herbicides after 2 yr. The lowest cost of control and the highest average net return over the 3-yr period was obtained with quizalofop.

Carpetgrass Response to Postemergence Herbicides

Carpetgrass is a warm season grass used in the south as low maintenance turf, especially in wet areas where other turf species do not persist. The response of carpetgrass to postemergence (POST) herbicides currently available for turf producers has not been thoroughly investigated and was the objective of four experiments conducted over a 2-yr period. Slight reduction in turf quality followed application of atrazine (2.2 kg ai ha-1), bentazon (2.2 kg ai ha-1), imazaquin (0.4 and 0.6 kg ai ha-1), mecoprop (1.1 kg ai ha-1), triclopyr (0.6 kg ai ha-1), metsulfuron (0.2 kg ai ha-1), 2,4-D (1.1 kg ai ha-1), 2,4-D plus dicamba (0.8 + 0.3 kg ai ha-1), and 2,4-D plus dicamba plus mecoprop (0.9 + 0.1 + 0.5 kg ai ha-1). Marginally acceptable turf quality followed sulfometuron (0.2 kg ai ha-1), sethoxydim (0.3 and 0.6 kg ai ha-1), and diclofop (2.2 kg ai ha-1) treatments. Unacceptable turf quality resulted from asulam (2.2 kg ai ha-1) and MSMA (1.1 and 2.2 kg ai ha-1) treatments. Carpetgrass tolerated most POST broadleaf and sedge herbicides but was sensitive to most POST grass herbicides used in other turf species, with the exception of sethoxydim.

Postemergence Control of Large Crabgrass (Digitaria sanguinalis) with Herbicides

Four field locations were used to compare environmental factors on efficacy of four postemergence grass herbicides in Kentucky bluegrass and bermudagrass turf. Imazaquin, BAS 514, and tridiphane were compared to fenoxaprop-ethyl, a registered herbicide, for postemergence control of large crabgrass. Best large crabgrass control was provided by BAS 514 and fenoxaprop-ethyl. Use of BAS 514 gave equal turf quality, phytotoxicity to the turf, large crabgrass control, and stand compared to fenoxaprop-ethyl. Imazaquin and tridiphane reduced turf quality and were phytotoxic to both turf types, and did not control large crabgrass adequately.

Development of a Selective Herbicide, Quizalofop-ethyl

Quizalofop-ethyl, ethyl 2-[4-(6-chloro-2-quinoxalinyloxy)phenoxy] propionate is a selective grass herbicide developed by Nissan Chemical Industries, Ltd. Quizalofop-ethyl possesses a wide herbicidal spectrum for grass weeds with a quick action at low application rates by foliage applications. It shows good safety for non-gramineous crops. A research for a grass killer herbicide, focussed on heterocyclicoxy phenoxy proionic acid, began in 1978. During investigations on the various condensed heterocyclic moieties, the quinoxalinyloxy phenoxy propanoic acid derivatives were found to have a high potential for grass herbicides. Through the optimization of derivertives, the title compound was selected as a candidate for development. In field trials conducted in major producing areas of soybeans, cotton, sugar beet and other broad leaf crops, quizaolfop-ethyl demonstrated sufficient control at 0.5-1.5g a.i./a for annual grass species and at 1.25-2.5g a.i./a for perennial ones. Quizalofop-ethyl was translocated from treated leaves to meristem tissues of grass weeds within a day and attacks these parts, followed by causing necrosis. Quizalofop, which is the metabolite of quizalofop-ethyl, was proved to be a potent inhibitor of fatty acids synthesis through acetyl-CoA carboxylase inhibition. A main metabolic pathway of quizalofop-ethyl was the hydrolysis to quizalofop in plants, soil and mammals. Following hydrolysis, cleavages of ester bonds, hydroxylations and conjugation of quizalofop occurred. Toxicity of quizalofop-ethyl in mammals, fish and birds was very low. Its potential hazard to the environment was minimal.

Tolerances of canola, field pea, lupin and faba bean cultivars to herbicides

The grain yield responses of cultivars of canola, field pea, lupin and faba bean to the recommended rates and twice the recommended rates of pre- and post-emergence herbicides were examined in field trials conducted at Wagga Wagga, New South Wales, from 1986 to 1989. Significant (P<0.05) reductions in grain yield of some field pea cultivars resulted from the recommended rate of registered post-emergence herbicides: Cressy Blue (22%), Derrimut (20%) and Dundale (13%) with metribuzin (0.23 kg a.i./ha); Wirrega (26%) with cyanazine (1.0 kg a.i./ha); Wirrega (15%) and Maitland (13%) with methabenzthiazuron (0.6 kg a.i./ha). The canola lines Hyola 30 and BLC 198-82 also had significant reductions in yield (16-19%) from the recommended rate of clopyralid (0.09 kg a.i./ha). There were differences in cultivar (or advanced line) tolerance to other herbicides at twice the recommended rate: clopyralid (0.18 kg a.i./ha) applied post-emergence in canola; diflufenican (0.2 kg a.i./ha) and MCPA (0.75 kg a.i./ha) applied post-emergence in field pea; preemergence treatments of metribuzin (0.46 kg a.i./ha), cyanazine (3.0 kg a.i./ha) and simazine (3.0 kg a,i./ha), and post-emergence treatments of simazine (2.0 kg a.i./ha) and diflufenican (0.2 kg a.i./ha), in lupin. Faba bean cv. Fiord tolerated pre-emergence treatments of terbutryne (2.0 kg a.i./ha), prometryne (3.0 kg a.i./ha) and triallate (1.6 kg a.i./ha), and there was seasonal variation in faba bean tolerance to pre-emergence treatment with cyanazine (3 kg a.i./ha), metribuzin (0.42 kg a.i./ha) and simazine (2.0 kg a.i./ha). The crops tolerated the early post-emergence grass herbicides: clethodim (0.48 kg a.i./ha), diclofop-methyl (1.5 kg a.i./ha), fluazifop-P (0.21 kg a.i./ha), haloxyfop (0.156 kg a.i./ha), quizalofop (0.28 kg a.i./ha), sethoxydim (0.372 kg a.i./ha). Herbicides that were tested in field pea and found to have only marginal selectivity at the recommended rate (even though some cultivars were tolerant) were terbutryne + MCPA (0.4 + 0.15 kg a.i./ha), diuron (0.4 kg a.i./ha) and pyridate (1.35 kg a.i./ha). Pyridate was non-selective in canola, lupin and faba bean. Faba bean tolerance to glyphosate (0.18 kg a.i./ha) was achieved in 1989 if application was delayed from 11 to 15 weeks after sowing.

Differential Effects of UAN on Antagonism with Bentazon

The effect of urea ammonium nitrate (UAN) on the antagonism of sethoxydim, haloxyfop, or the methyl ester of haloxyfop activity by bentazon was evaluated in greenhouse and field trials on yellow and giant foxtail. Including UAN in the spray solution in the absence of bentazon did not enhance the activity of any of the three grass herbicides. However, adding UAN to sethoxydim or haloxyfop in the presence of bentazon decreased the bentazon antagonism of grass activity. Conversely, UAN increased bentazon antagonism of the activity of haloxyfop methyl ester. The differential effect of UAN was not linked to effects on spray solution pH.

The influence of the application of grass herbicides on the production of dryland medic and lucerne pastures in the Rûens area of the southern Cape

Reports on a trial which was conducted over a period of three seasons in which the advantages of the removal of grass weeds from dryland lucerne and medic pastures determined. Two herbicide treatments, an unsprayed control and a grass herbicide treatment were compared for three seasons. Illustrates with graphs and a table; A trial was conducted over a period of three seasons in which the advantages of the removal of grass weeds from dryland lucerne and medic pastures were determined. The pastures were grazed at four stocking rates, i.e. 6, 8, 10 & 12 non-lactating merino ewes and wethers/ha. Two herbicide treatments, an unsprayed control and a grass herbicide treatment were compared for three seasons. The herbicide treatment consisted of two applications, one each of fluazifop-butyl and propyzamide, respectively, during the first two seasons. On the lucerne pastures the removal of the grass weeds resulted in an increased wool production and, to a lesser extent, ADG, which also resulted in an economic advantage. On medic pastures, however, the removal of the grass weeds did not result in any animal production or economic advantage. Herbicide treatment, however, did result in a higher medic pod yield, which should increase the productive lifespan of these pastures.Language: English Keywords: Economic analysis; Fluazifop-butyl; Merino sheep; Pastures; Propyzamide; Wool production; adg; animal production; dryland; fluzifop-butyl; grass; lucerne; treatments; weeds; yield; production; grasses; herbicides; southern cape; south africa; ruens; stocking rates; medicago sativa; medicago truncatula

TOLERANCE OF ORNAMENTAL GRASSES TO POSTEMERGENCE GRASS HERBICIDES

Ornamental grasses are popular landscape plants and often encounter turf encroachment or other grass weed problems. Several postemergence grass herbicides are available for use in turf and ornamentals and herbicide tolerance information is needed for ornamental grass species. Fifteen ornamental grasses including species from the genera Calamagrostis, Cortaderia, Eragrostis, Erianthus, Miscanthus, Sorghastrum, Spartina, Panicum and Pennisetum were field planted in Clemson, SC in May 1989 and Festuca species in November, 1989. Herbicide treatments were fenoxaprop-ethyl, fluazifop-P and sethoxydim at 0.4 kg a.i.·ha-1 applied 4 weeks after planting and an untreated control. Height and injury evaluations were taken at weekly intervals and plants were harvested 10 weeks after treatment. Fenoxaprop-ethyl treated Calamagrostis and Festuca species treated with all herbicides were the only treatments that were the same as untreated controls in terms of % injury, height and dry weight. Three ornamental Calamagrostis species were evaluated in a greenhouse study to determine the level of tolerance to fenoxaprop-ethyl at 0.4, 0.8, 1.6 and 3.2 kg a.i.·ha-1. No visual injury symptoms were seen on any treatments as compared to untreated controls but growth rates of the youngest leaves did vary among species shortly after treatment.

TOLERANCE OF ORNAMENTAL GRASSES TO POSTEMERGENCE GRASS HERBICIDES

Ornamental grasses are popular landscape plants and often encounter turf encroachment or other grass weed problems. Several postemergence grass herbicides are available for use in turf and ornamentals and herbicide tolerance information is needed for ornamental grass species. Fifteen ornamental grasses including species from the genera Calamagrostis, Cortaderia, Eragrostis, Erianthus, Miscanthus, Sorghastrum, Spartina, Panicum and Pennisetum were field planted in Clemson, SC in May 1989 and Festuca species in November, 1989. Herbicide treatments were fenoxaprop-ethyl, fluazifop-P and sethoxydim at 0.4 kg a.i.·ha-1 applied 4 weeks after planting and an untreated control. Height and injury evaluations were taken at weekly intervals and plants were harvested 10 weeks after treatment. Fenoxaprop-ethyl treated Calamagrostis and Festuca species treated with all herbicides were the only treatments that were the same as untreated controls in terms of % injury, height and dry weight. Three ornamental Calamagrostis species were evaluated in a greenhouse study to determine the level of tolerance to fenoxaprop-ethyl at 0.4, 0.8, 1.6 and 3.2 kg a.i.·ha-1. No visual injury symptoms were seen on any treatments as compared to untreated controls but growth rates of the youngest leaves did vary among species shortly after treatment.

Inhibition of acetyl-coenzyme A carboxylase by two classes of grass-selective herbicides

The selective grass herbicides diclofop, haloxyfop, and trifop (((aryloxy)phenoxy)propionic acids) and alloxydim, sethoxydim, and clethodim (cyclohexanediones) are potent, reversible inhibitors of acetyl-coenzyme A carboxylase (ACC) partially purified from barley, corn, and wheat. Although inhibition of the wheat enzyme by clethodim and diclofop is noncompetitive versus each of the substrates adenosine triphosphate (ATP), HCO{sub 3}{sup {minus}}, and acetyl-coenzyme A (acetyl-CoA), diclofop and clethodim are nearly competitive versus acetyl-CoA since the level of inhibition is most sensitive to the concentration of acetyl-CoA (K{sub is} < K{sub ii}). To conclusively show whether the herbicides interact at the biotin carboxylation site or the carboxyl transfer site, the inhibition of isotope exchange and partial reactions catalyzed at each site was studied with the wheat enzyme. Only the ({sup 14}C)acetyl-CoA-malonyl-CoA exchange and decarboxylation of ({sup 14}C)malonyl-CoA reactions are strongly inhibited by clethodim and diclofop, suggesting that the herbicides interfere with the carboxyl transfer site rather than the biotin carboxylation site of the enzyme. Double-inhibition studies with diclofop and clethodim suggest that the ((aryloxy)phenoxy)propionic acid and cyclohexanedione herbicides may bind to the same region of the enzyme.

Photosynthesis as an Index of Turfgrass Growth Following Application of Herbicides

The effects of sethoxydim, cloproxydim, and fluazifop on photosynthesis and growth of St. Augustinegrass [Stenotaphrum secundatum (Walt.) Kuntze `Floralawn'], bahiagrass (Paspalum notatum var. saurae Parodi `Pensacola'), and centipedegrass [Eremochloa ophiuroides (Munro.) Hack.] were evaluated to determine if photosynthesis could be used as a rapid, nondestructive measure of relative susceptibility. Field and greenhouse studies were conducted using infrared CO2 analysis to estimate photosynthesis. Under field conditions, St. Augustinegrass was susceptible to sethoxydim and fluazifop applications, as indicated by a 40% and 38% reduction in apparent photosynthesis, respectively. Bahiagrass incurred a respective 62% and 51% reduction in apparent photosynthesis from sethoxydim and fluazifop application. Growth of these species, as measured by foliage dry weight, was also inhibited by both herbicides. Centipedegrass growth was unaffected by sethoxydim, but was reduced 48% by fluazifop. Under greenhouse conditions, centipedegrass apparent photosynthesis was reduced by sethoxydim and cloproxydim (41% and 51%, respectively), while fluazifop caused a 71% reduction. Growth of centipedegrass was significantly reduced only by fluazifop (83%). These studies indicated that in vivo photosynthetic measurements may provide a sensitive, rapid, and nondestructive method for determining the susceptibility of turfgrasses to postemergence grass herbicides. chemical names used: 2-[1-(ethoxyimino)butyl]-5-[2-(ethylthio) propyl]-3-hydroxy-2-cyclohexen-l-one (sethoxydim); (E,E) -2-[1-[[(3-chloro-2-propenyl) oxy]imino]butyl] -5-[2-(ethylthio) propyl]-3-hydroxy-2-cyclohexen-l-one (cloproxydim); and butyl ester of 2-[4-[[5-(trifluoromethyl)-2-pyridinyl]oxy]phenoxy]-propanoate(fluazifop).

Natural products phytotoxicity A bioassay suitable for small quantities of slightly water-soluble compounds

A large variety of secondary metabolites that can inhibit germination and/or seedling growth are produced by plants in low quantities. The objective of this study was to develop a bioassay capable of reliably assessing reductions in germination percentage and seedling length of small-seeded plant species caused by exposure to minute quantities of these compounds. The germination and growth of alfalfa (Medicago saliva), annual ryegrass (Lolium multiflorum), and velvetleaf (Abutilon theophrasti) were evaluated against six known phytotoxins from five chemical classes; cinmethylin (a herbicidal cineole derivative) was selected as a comparison standard. Each phytotoxin, dissolved in a suitable organic solvent, was placed on water-agar in small tissue culture wells. After the solvent evaporated, imbibed seeds were placed on the agar; after three days, germination percentages and seedling lengths were measured. Compared to a commonly used filter paper procedure, this modified agar bioassay required smaller quantities of compound per seed for comparable bioassay results. This bioassay also readily permitted the measurement of seedling length, a more sensitive indicator of phytotoxicity than germination. Seedling length decreased sigmoidally as the toxin concentration increased logarithmically. Phytotoxicity was a function of both compound and plant species. Cinmethylin, a grass herbicide, reduced the length of annual ryegrass seedlings by 90-100%, whereas that of alfalfa and velvetleaf was inhibited slightly. The agar bioassay facilitated the rapid and reliable testing of slightly water-soluble compounds, requiring only minute quantities of each compound to give reproducible results.