Division of Agricultural Sciences UNIVERSITY OF CALIFORNIA GENERAL-CONTACT WEED KILLERS Alden S. Crafts CALIFORNIA AGRICULTURAL Experiment Station Extension Service CIRCULAR 447 GENERAL-CON TA Chemicals that destroy all growth — weeds and crops. Anywhere you want no vegetation — along high- ways, railroads, fence lines, around farm and industrial buildings. Weeds harbor crop pests and diseases, are a fire hazard, prevent access to buildings. Some pointers how to use general-contact weed killers on There are many conditions under which general-contact sprays are effective, and a variety of suitable spray materials are on the market. This circular gives an over-all picture of the different sprays and some of the principles underlying their successful applica- tion. Each user will have different problems. You will have to decide which type of spray is best for your particular situation. The following pointers will serve as a guide to the selection of the best method. On tender succulent weeds use the contact herbicides mixed with water. Dow general and Sinox general herbicides are examples. For average mixed weed populations on roadsides, ditchbanks, and similar places, use fortified oil emulsions. WEED KILLERS T HOW DO THEY WORK? ^ WHAT TYPES ARE THERE? ^ HOW ARE THEY APPLIED They kill plants by con- tact — must be sprayed on plants to wet them thoroughly. In grasses sprays must creep down the stems to kill plant crowns. There are three types — water-soluble chemicals; oils and fortified oils; emul- sions of water and oil with or without fortifying agents. As sprays — by hand, ma- chine, sometimes by air- plane. For mixed weed populations that are predominantly grass, use fortified oil emulsions of higher oil content (formulas, page 15). For tough or heavy grasses and for oil-tolerant weeds use weed oils. For preplanting or preemergence sprays use oils or oil emul- sions containing only oil-soluble toxicants. Water-soluble chemi- cals may injure the crop. For spot treatment of shallow-rooted perennials and against weeds that come up through cracks in pavement, ditch lining, or gravel drives use fortified oils or weed oils of high toxicity. To dry out seed crops prior to harvest use oil extracts or light weed oils such as Annalos 7 and Union 4060 distillate. THE AUTHOR: Alden S. Crafts is Professor of Botany and Botanist in the Experiment Station, Davis. MAY 1955 Replaces Extension Circular 137 GENERAL-CONTACT WEED KILLERS ALDEN S. CRAFTS Where vegetation is unwanted As preplanting, preemergence, and postemergence sprays What they are It is important to remember that the weed killers dis- cussed in this circular destroy all vegetation, wanted and unwanted. Of course, many chemicals, if applied in suf- ficient quantity, will kill all growth. The materials dis- cussed here are those which are most practical (because they are cheaper, more available, more effective) in killing vegetation. Information on selective weed killers (those that kill weeds but do not injure crop plants) is contained in Circular 157. Soil sterilization for weed control is de- scribed in Circular 446. Where they are used General-contact weed killers are often used in and around areas such as these: Roadsides Ditchbanks Railroad rights-of-way Fence lines Dry yards Parking areas Landing fields Firebreaks Highways Pole lines Billboards Farm buildings Warehouses Lumber yards Irrigation structures Industrial plants Ordinarily, general-contact sprays are not used on crop plants. However, they may be applied as preplanting or preemergence sprays if they do not carry readily available toxicants that will leach into the soil, persist there, and injure the crop. Preemergence treatments must be made on very young weeds. Recently, general-contact weed killers are also being used as directed postemergence sprays in corn, onions, bulbs, sugar cane, milo, and other crops that are robust and resistant to injury from sprays applied at the base [4] of the plant. Postemergence sprays are more effective and economical on young weed growth. Older weeds require more volume, more toxicant, and more careful spraying. Why they are used In many cases, mowing or disking of unwanted vegeta- tion is not satisfactory. It may not remove the weeds thoroughly enough, and may have to be repeated too often. General-contact weed killers if used repeatedly can maintain an area in a weed-free condition. Farmers may want weed growth removed from road- sides and ditchbanks because it encourages crop pests and diseases which threaten their fields and orchards. Exam- ples are curly top virus on sugar beet and bean thrips. Weed growth can be dangerous on highways where it cuts off the motorist's view. Or it may block the way to pump houses, headgates, toolsheds, and other structures. Chemicals are effective protect crops remove obstacles How they work General-contact weed killers destroy by contact of the entire plant surface with toxicants contained in the sprays. To kill nongrassy weeds by sprays you must: 1. Apply sufficient toxicant to kill the bulk of vegeta- tion and allow considerable excess for unevenness of ap- plication and differences in density of plant growth. 2. Distribute spray solution so that the growing parts of the weed are completely covered. These requirements can be met fairly readily by water solutions. Where vigorous, weedy grasses are present, you need a third requirement for the weed killer to do its job: The solution must creep into the crowns to prevent re- newal of growth by tillers. This can be done only by oil or by solutions having the properties of oils. Wetting agents will greatly increase the spreading and wetting properties of sprays mixed only with water, or of emul- sions, but the high spreading and creeping qualities of oils can never be completely duplicated. One way to increase the killing power of such sprays and emulsions is to increase concentration or volume, or both, until the necessary killing power is obtained. In some cases, this may be done rather easily; in others, the increase in dosage and hence in cost is such that use of oil alone is more economical. Only experiments under your own conditions can help you decide which of these two types of contact herbicide is more useful to you. The toxic effect of an oil on the plant depends in part on how volatile it is, and how much unsaturated com- On nongrassy weeds: by contact On grasses: by preventing renewal of growth [5] Oils cause rapid leaf burning or yellowing of leaves pounds it contains. Oils vary in their toxicity. Some kill all plants; some are selective, and kill only weeds, leaving certain crop plants undamaged. Light unsaturated com- pounds cause a rapid burning of leaves called acute tox- icity. Heavy unsaturated compounds injure the growing parts and cause a chlorosis (yellowing of leaves). This injury comes on much more slowly and is called chronic toxicity. Very light unsaturated compounds, such as those from gasoline stock, cause burning of the leaves. Injury is not complete, however, if the spray incompletely satu- rates the plant, because these oils may evaporate before all tissues are killed. Unsaturated compounds of medium weight are very toxic to grasses and most weeds. They do not kill plants of the carrot family, except at high concentrations. They are usually found in unrefined petroleum distillates, such as stove oil, at concentrations between 20 and 30 per cent. Heavy unsaturated compounds, such as those in diesel and other heavy fuels and in aromatic weed oils kill plants slowly by chronic toxicity. Crop plants as well as weeds are killed by such oils. Jeep equipped for spraying of weeds along fence lines and roadsides. How they are applied General-contact weed killers can be applied by hand or machine equipment; oil and oil emulsions also by air- plane. For detailed information on booms, nozzles, and spray equipment, see Extension Circular 389, "Chemical Weed-Control Equipment." Here are some details about water-soluble chemicals and emulsions, since these mixtures involve special prob- lems that do not apply to straight oil sprays. For satisfactory application of emulsions and sprays mixed with water alone, pressure at the nozzles should be 100 pounds or more. On booms the nozzles should be spaced so that the spray fans meet at approximately the height of the vegetation. The boom should not be more than 18 inches above the vegetation; the full driving force of the spray is needed to wet the plants thoroughly. Most satisfactory results are obtained by using double coverage from different angles. Where application is made by hand, spraying should be slow and methodical so that all vege- tation is thoroughly wet. If these precautions are observed, weeds should be controlled almost as effectively as where straight oil sprays are used. As sprays, wetting plants thoroughly Insects and plant diseases growing on weeds may be killed by general-contact sprays. [7] What types there are Common chemicals Advantage: low cost Disadvantage: dangerous in use Phenol compounds 1. Water-Soluble Chemicals Two types of chemicals used as general-contact weed killers dissolve readily in water and may be used in water solution as sprays: the well-known common chemicals, and the new phenol compounds. Unless they are highly selective, water-soluble chemi- cals are not ordinarily used for preplanting, preemer- gence, or postemergence treatments in crops because they may be leached into the crop roots by rainfall or irrigation. All water-soluble herbicides require thorough applica- tion. Follow labels for formulas for mixing. Which water- soluble chemical you will choose will depend on its availability; ease of use; toxicity, as it relates to cost, dosage, and effectiveness; and hazards (poison, fire, and corrosion of equipment). Common chemicals. These include: sodium chloride, calcium chloride, sodium chlorate, sodium arsenite, so- dium hydroxide, sodium borate, sulfuric acid, and many others. These chemicals have been widely used as general- contact sprays but no longer are generally recommended because some may be extremely dangerous to humans and animals. Some may have injurious effects on the soil or leave toxic residues on crops or in the soil. Some of these chemicals are manufacturers' by-products. If you are near the place of production, you may buy them cheaply. For example, one such material is bittern. But there are some dangers involved in their use. So- dium arsenite, for instance, attracts livestock and has killed many animals. It is also dangerous to the spray operator, to pets, and to humans who happen to come into contact with it. Use it only when you can avoid these risks. Another common chemical, sodium chlorate, presents a different hazard: if spray containing this chemical dries on clothing, dead plants, wood, or other surfaces, they become highly inflammable, ignite by friction and burn like gunpowder. Strong acids and alkalies are corrosive to machinery and dangerous to use. Phenol compounds. Among them are pentachlorophe- nol, dinitrocresol, dinitro secondary butyl phenol, dinitro secondary amyl phenol, and their sodium, ammonium, or amine salts. Most of these compounds are sold under trade names. Their content in such commercial products is shown on [8] the labels. Examples of these products are: Contax, Sinox General, Dow General, Du Pont Dinitro weed killer, Knoxweed 55, and Endothal. Sodium pentachlorophenate or sodium dinitro cresylate may be used as general-contact sprays in many situations if their toxicity is increased by activating them with an acid salt, such as ammonium sulfate, aluminum sulfate, or sodium bisulfate. Sodium pentachlorophenate and sodium dinitro cresylate, at concentrations of 1 to 2 per cent (or even less when no grasses are present) will kill weeds that have been protected from full effects of sun and wind, and consequently, do not have thick leaf sur- faces. Such protected areas are found, for example, in deep drainage ditches, shady orchards, and lath-houses. Higher concentrations up to 5 per cent may be used on hardier weeds. If grasses are present, include a wetting agent in the spray solution. The ammonium salt of dinitro secondary butyl phenol is so toxic that it will kill even grasses in their seedling stages. When used at concentrations of 0.5 per cent and over, with a good wetting agent, this herbicide has proved effective in killing mixed weeds on roadsides, ditchbanks, etc. On highways in central California, fire strips sprayed with this material in April could be burned sufficiently within two to three weeks to provide adequate fire pro- tection for adjoining crops and pastures. Phenol compounds are highly toxic; small amounts are required so that hauling is reduced to a minimum. If used in large quantities, they can be bought at prices that compare favorably with those of fuel oils. They have no serious poison hazard to livestock. They form true solutions and require no agitation once they are dissolved. They kill certain oil-resistant weeds, such as sweet fen- nel, yellow star thistle, mayweed, and pineapple weed. They are not economical for use on coarse, vigorous grasses, such as wild oats, foxtail, ripgut, and the like. On these weeds, an oil spray is necessary, because it will creep down the grasses and penetrate the crowns, where a water spray, unless applied in very large amounts, will run off. They are not useful on perennial grasses. Advantages: reduced labor; low cost; safe for livestock Disadvantage: not practical on coarse grasses, perennial grasses CAUTION Fumes or spray drift should not be inhaled. A died as a result of undue from din number ol exposure itrocresol compounds spray operators have to spray drift. [9] 2. Oil sprays Diesel oil Advantages: useful against mixed weeds growing densely; cheap; safe Disadvan tage: large amounts are needed Smudge-pot oil: similar to diesel Stove oil Advantage: useful as pre- emergence spray In contrast to sprays in water solution, oils wet plant surfaces readily and tend to spread as thin films and run down the stems. They penetrate the crowns of grasses, to the growing tissues that form new shoots. If an oil spray wets the tops of grasses thoroughly, the film may creep from 4 to 6 inches down the stems and kill all tissue from which new shoots might grow. This accounts for the satis- factory results usually obtained with diesel and smudge- pot oil sprays, both of which are standard materials for weed killing. The content of aromatic compounds in general-contact weed oils should be above 25 per cent and may be as high as 50 per cent or more. Oils are often used as pre- planting, preemergence, and, occasionally, as postemer- gence sprays because they are not readily leached into soils. Diesel oil for weed killing is readily available from oil dealers, and should be bought under the name of diesel fuel oil. Automotive diesel oil, or Diesel No. 1, is less desirable as a weed oil because it is more highly refined and less toxic to plants. Diesel combines a low acute toxicity with a medium chronic toxicity. This means, it kills plants slowly. Be- cause it clings to the waxy plant surface, it wets plants easily and covers them well. It is very effective against mixed weeds, including grasses, and will kill completely even where growth is so dense that spray cannot be forced down into the bases of the plants. Being low in cost (in many areas), noncorrosive, and safe to handle, diesel oil is an excellent herbicide for general weed control. Because diesel is relatively low in toxicity, a large vol- ume per acre is required. This makes transportation costs relatively high. It also makes diesel unfit for low-volume application by airplane unless fortified. Plants that are resistant to oil require too large an application to make use of diesel economical. Because of its value as a fuel, widespread use of diesel as a weed killer seems unwise. Smudge-pot oil. This may be used in place of diesel be- cause its properties are similar. It is often less refined and hence may be more toxic. Stove oil. Although this oil may be used as a contact herbicide, it is largely limited to preemergence spraying in cropped areas. It may be used as a preemergence spray in row crops even after crop plants have started breaking through the soil if the crops are tolerant of the oil. Some tolerant plants are carrots, celery, parsnips, and parsley. (For use of stove oil on carrot crops, see Extension Circular 136.) [10] It is lighter and less concentrated in chronic toxicants than diesel oil, and is therefore less effective for general weed spraying, especially where there are large grasses. Some refined stove oils are so low in toxicity that they are almost worthless as herbicides. Kerosene distillate. This is useful, if obtainable, as are other petroleum distillates in the same general boiling range. The gravity of such oils should range between 27° and 38° A.P.I, (gravity of oils is determined by the Ameri- can Petroleum Institute and expressed in degrees A. P. I.). Below 27° A.P.I, oils are heavy and may be difficult to apply in cold weather; above 38° they lack the persistent action of the chronic toxicants of heavier oil fractions. They are somewhat selective and are therefore ineffective on many oil- tolerant plants. Low-grade oils. In certain oil-producing regions, low- quality oils that are perfectly satisfactory for killing weeds may be bought from local refineries. Usually such oils are high in sulfur or aromatic compounds and may be un- suitable for use as lubricants or fuels. The properties that make them unfit for these uses may make them especially good weed killers from the standpoint of toxicity. Where such oils are available, they prove very useful in weed control. Oil extracts. In the refining of medicinal oil, kerosenes, lubricating and other finer oils, solvent extraction is used to remove the more reactive unsaturates. Lighter grades of these by-products are recovered from the extract and used as paint thinners, cleaning fluids, and solvents. When these materials have been removed there may still remain a dark, foul-smelling substance of little commercial value. This residue is high in aromatic compounds and is ex- tremely toxic to all plants. It is sold in limited quantities as a weed killer. (Annalos No. 7 Weed Killer, of the Tide- water Associated Oil Company, and 4060 distillate, of the Union Oil Company, are examples.) Such oils are very useful as herbicides, but only limited amounts are avail- able. They may be used to fortify diesel oil when it has been so highly refined that it does not have enough killing action. They are used for preharvest desiccation of seed crops. Weed oils. As demands for gasoline increase, stove, diesel, and smudge pot oils are used for its manufacture, and new types of oils are being made available in their place. These new oils are largely fractions resulting from the cracking operations used in gasoline manufacture and, if unrefined, may be higher in unsaturates than the pres- ent types. This increases their toxicity. Nontillage pro- grams in orchards, and an increased use of oil in weed Disadvantage: less effective Kerosene distillate: effective at gravity of 27° to 38° A.P.I. Low-grade oils: useful where available Oil extracts: useful as fortifiers Weed oils: control Johnson, Bermuda grasses [ii] Fence line treated with a general-contact herbicide— 30 gallons diesel oil, 1 quart dinitro, and 70 gallons of water. This quantity will cover about an acre. control are creating a great demand for weed-killing oils. To meet this need the large refiners are offering special products combining maximum spreading and wetting properties with a balanced toxicity sufficient to kill all weeds without wasting toxic compounds. Many oils of this type are now available. Examples are Annalos No. 11, Richfield A, Pentox No. 2, etc. These weed oils are par- ticularly effective and economical for controlling Johnson and Bermuda grasses. Fortified oils Fortified oils. Oils of low toxicity may have their killing power increased by adding fortifying agents such as phenol compounds or sulfur. It usually costs less, per unit of toxicity, to use fortifying agents than to use an amount of unfortified oil of equal toxicity. The oil itself acts mainly as a carrier for the toxic fortifying agent. In addition to increasing the killing power, fortifica- tion provides extra toxicity where an oil is being used in emulsion or in low-volume application by airplane. It also makes oils more toxic to certain oil-tolerant weeds, such as sweet fennel, wild carrot, yellow star thistle, and mayweed. Most of these compounds are sold under trade names, with their content in the product shown on the labels. [12] Pentachlorophenol and dinitrocresol are about equally toxic in oil solution. For most purposes in the field, a 0.5 per cent solution — roughly, 4 pounds per 100 gallons of oil — is enough; the same dosage rate applies to sulfur. Dinitro secondary butyl phenol (active ingredient in Dow General) is about three times as toxic, and only about 1% pounds per 100 gallons are required. Dinitro second- ary amyl phenol (one active ingredient in Sinox General) has similar properties. Where oil tolerant weeds, such as sweet fennel, yellow star thistle, and mayweed predomi- nate, the fortifying agent may be increased. These special weed types, while resistant to oils may be susceptible to 2,4-D, and dusts or sprays of this chemical may be used as separate treatments, preferably before the oil spray. In addition to the fuel oils, several less toxic oils are useful in weed control if they are fortified. Examples are wash and slop oils, reclaimed solvent and lubricating oils, old crankcase oil, and gas-drip oil. In fact, the wetting and creeping properties of any oil may be combined with the high toxicity of a fortifying agent to produce a weed killer. Fortified oils are more toxic than straight fuel oils, and the volume used may be cut to the smallest amount that will give thorough coverage. In one case, where 150 gal- lons of straight diesel oil per mile were required to spray ditches in the Sacramento River delta, 75 gallons of diesel containing 3 pounds of dinitrocresol or 1 pound of dinitro secondary butyl phenol produced better results. In an- other instance, 14 gallons of fortified oil per acre were applied by airplane to kill small weeds in a field where onion bulbs had been planted for seed production. The field was too wet for application by ground sprayer, or for cultivation or hoeing. A similar application has proved effective for eliminating small annual weeds in alfalfa. (For information on weed control in alfalfa, apply to the Botany Division, Davis, for the circular Weed Control in Perennial Legumes.) Fortified oils are more rapid in their action than straight diesel or heavier fuel oils. When grass along the water line of irrigation ditches is sprayed to control the breeding of mosquitoes, ditches can be restored to normal use after only 24 hours. By killing quickly, fortified oils provide a longer period for burning. This makes them especially useful on highways where it is necessary to burn while the surrounding vegetation is still green. Fortified oils will kill oil- tolerant weeds. They are use- ful for spot treatments on clumps of sweet fennel, poison Advantages: low volume; rapid action; useful in airplane application; and against oil-tolerant weeds [13] Disadvantage: misuse is expensive Emulsions: Needed where high-volume applications are necessary (for example, on tall weeds) hemlock, artichoke thistle, St. John'swort (Klamath weed), or other shallow-rooted perennials. They kill all weeds, including grasses, that come up through cracks in sidewalks, pavement, and concrete ditch lining. They are useful for treating Bermuda grass and similar pests in gravel walks and railroad ballast, and for treating indi- vidual weeds, such as dandelions, plantains, and chicory in lawns and permanent pastures. Fortifying agents are expensive (around $12 or more per gallon) and if used where not required they are wasted. This often happens when inexperienced men are put in charge of weed spraying. All oils have some toxicity and only where this is not sufficient to kill all the weeds are fortifying chemicals required. 3. Emulsions Emulsions are mixtures of oils and water. Most emul- sion have a water base in which droplets of oil are sus- pended. Invert emulsions have an oil base in which droplets of water are suspended. The extreme toxicity of the recommended fortifying agents makes it possible to prepare fortified oils with much greater killing power than spreading ability. Such solu- tions may be applied in low volume from airplanes. Similar applications can be made by ground rigs if high- pressure, small-orifice nozzles are used. Although low-volume application of fortified oil is ef- fective on small weed growth, more volume is required when plants have grown to the stage where they cover the soil and protect their own crowns. When this point has been reached, the fortified oil must be applied in emulsion. Weed-killing emulsions may have the fortify- ing chemical dissolved in either the water or the oil or both. In fortified oil emulsions it is usually necessary to use wetting agents. Agitation in the spray tank is necessary because fuel-oil emulsions are not stable enough to hold throughout the period required for application. Emulsion formulas. The formulas given below should be mixed on a weight basis. One hundred gallons of emul- sion weighs roughly 800 pounds; 1 per cent would repre- sent 8 pounds per 100 gallons; Vi per cent, 4 pounds; ^4 per cent, 2 pounds; and % per cent, 1% pounds. Since diesel oil weighs about 8 pounds per gallon, a satisfactory approximation can be made by adding the oil on a volume basis — 2 per cent would be 2 gallons of oil and 98 gallons of water; 6 per cent, 6 gallons of oil and 94 gallons of water. [14] A satisfactory emulsion may be made using: 4 pounds of pentachlorophenol 8 gallons of aromatic oil 4 pounds of oil-soluble wetting agent 92 gallons of water. In preparing this formula, add the pentachlorophenol to the aromatic oil and agitate until it is all dissolved, than add the oil-soluble wetting agent and agitate until a uniform solution is obtained. This makes a stable con- centrate that may be stored. To prepare the spray solu- tion, mix equal volumes of concentrate and water, agitate vigorously until a stable emulsion is formed, then add the remaining water, with agitation. A similar emulsion can be made by using % per cent dinitro secondary butyl phenol in place of the V2 per cent pentachlorophenol. If an emulsion of great stability is required, the above formula should be homogenized. This breaks up the oil droplets to such an extent that they will remain in sus- pension. Another popular formulation includes: 1 quart dinitro concentrate 15 gallons of diesel oil 85 gallons of water. For old, tough weeds use: 1 quart dinitro concentrate 40 gallons of diesel oil 60 gallons of water. These formulas are satisfactory for broad-leaved weeds and grasses grown in sheltered locations. Where more vigorous grasses are present, the percentage of oil may be increased or the volume of water decreased. It usually does not pay to use over 4 pounds of pentachlorophenol (or IVs pounds dinitro secondary amyl or butyl phenols) per 100 gallons of spray. If more oil is required to handle grasses, it may be mixed with the fortified oil of the above formulation, and the whole mixture may be emulsified. For this supplementary oil, the least expensive oil avail- able may be used provided it has the proper viscosity (not over 50 sec). Mixtures up to 50 per cent oil have been used against large, rapidly growing wild oats, barley, and foxtail in dormant alfalfa. The grower will learn by ex- perience how much oil is required and how to adjust for the height and vigor of the vegetation. Density of growth and plant maturity, with respect to possible tillering and late germination of additional seed, must also be con- sidered. Formula 1 Formula 2 Formula 3 (for tough weeds) [15] Emulsions Advantages: low cost; little hauling; useful against oil- tolerant weeds; and as preemergence, postemergence sprays Disadvan tage: agitator required Uses for emulsions. Emulsions kill oil-resistant weeds at a minimum cost. Chickweed, pineapple weed, and yel- low star thistle have been killed in experimental tests in an onion crop where selective herbicides failed and where straight oil would have seriously harmed the crop. Although the emulsion injured the onions, they recovered rapidly. Yellow star thistle and mallow, both of which are resistant to diesel, have been killed on roadsides by applications of emulsions. The composition of emulsions may be adjusted to meet almost any condition. The oil content may be held to 10 per cent on tender weeds in orchards or in ditches, or may be increased to 25, 40, or 50 per cent in fighting heavy, vigorous infestations of grasses. Within this wide range there may be an appreciable saving of oil. And in all cases, oil-tolerant weeds are killed. As the oil content is raised above a level of 50 per cent, a point of diminish- ing return is reached where the relative costs of oil, forti- fying agent, and wetting agent must be balanced with the inconvenience of mixing and effectiveness of treatment. Under most conditions, emulsions containing fortified oil are cheaper than straight oil because the toxicants are so much more effective than those present in oil alone. For example, formula 1 on page 15 can be prepared for about one-half the cost of a straight oil spray. Use of emulsions cuts the cost of long-distance hauling of fuel oils. This becomes a real factor in regions far removed from oil refineries. For preemergence or postemergence use of general- contact weed killers, those having toxicants insoluble in water are least likely to cause injury to the crop. Forti- fied oil emulsion sprays may be used as preplanting or as post-planting preemergence treatments to reduce com- petition by weeds during the early stages of crop growth. In using emulsions, the spray tank must be equipped with a mechanical agitator. Also, where large, vigorous grasses are present, results may never be quite so good as with oil because of the difference in creeping ability. In order that the information in our publications may be more intelligible it is sometimes necessary to use trade names of products or equipment rather than complicated descriptive or chemical iden- tifications. In so doing it is unavoidable in some cases that similar products which are on the market under other trade names may not be cited. No endorsement of named products is intended nor is criticism implied of similar products which are not mentioned. Co-operative Extension work in Agriculture and Home Economics, College of Agriculture, University of California, and United States Department of Agriculture co-operating. Distributed in furtherance of the Acts of Congress of May 8. and June 30, 1914 J Earl Coke, Director. California Agricultural Extension Service. 30m-5,'55(B861)JF [16]