I PRINCIPLES OF WEED CONTROL IN /CALIFORNIA

I I A

Sponsored By CALIFORNIA WEED CONFERENCE Box 2454 ElMa:cero, CA 95618

Thomson Publications P.O. Box 9335 Fresno, CA93791 -'7,...... __,~------~------6 CHEMICAL CONTROL METHODS

INTRODUCTION

by Floyd M. Ashton 1, Alden S. Crafts 2 and Harry S. Agamalian 3

In the last half-century, weed control has become one of the principal tech­ nologies responsible for the increased agricultural production characteristic of this period. The USDA reported that U.S. farmers were expected to have used 3.5-to-4-billion dollars worth of pesticides in 1984. This usage represents -,~ 550 million pounds of active ingredients, 450 million pounds of which are herbicides. Given that every arable acre of land in the world has a potential infestation of weeds that may interfere with crops, it is not surprising that growers are becoming increasingly aware of the role of weeds in limiting production. Herbicides are also used on noncrop land to control vegetation. It is of paramount importance to understand that chemical weed con­ trol is only one method of controlling weeds which often must be inte­ grated with other methods for optimum results. Such combinations of methods result in the various systems approaches to vegetation manage­ ment discussed in chapter 9.

COMPONENTS

A successful chemical weed-control program depends on the appropriate interaction of the plant, the herbicide, and the environment. When one considers the complexity of the many diverse species of crops and weeds, the great array of herbicides, and the infinitely variable environment, it becomes apparent that any discussion of chemical weed control must be developed from an initial understanding of these three components.

1, 2. University of California, Davis, CA. 3. University of California Cooperative Extension, Monterey County, CA. I 98/Principles of Weed Control in California

The Plant

Both crops and weeds have specific characteristics that must be considered in developing a successful chemical weed-control procedure. These include form, growth, and function. Since these aspects are discussed in chapters 1 and 2, only those most relevant to chemical. weed control will be men­ tioned briefly here. In order to select the proper herbicide and the method of application, one needs to know: (1) whether the crop and/or weed is an annual, biennial, or perennial; (2) the stage of growth of each (e.g., germi­ nating seed, seedling, or established plant); and (3) the growth form (e.g., upright or horizontal leaves, deep or shallow root system, etc.). In general, annual weeds are easier to control than established peren­ nial weeds. Germinating seeds and young seedlings of perennial weeds are as easy to control as annual weeds up to the time they develop their peren­ nial characteristics. In general, young weeds are easier to control than older weeds. Germinating seeds can usually be controlled with an appropriate soil-applied herbicide. Emerged seedlings can usually be controlled with an appropriate foliar-applied herbicide; these may be translocatable (systemic) or have only contact action. Although established annual weeds may be controlled by a contact herbicide, better control is usually obtained with a translocatable herbicide. With a contact herbicide, dormant lateral buds of broadleaf weeds or the growing points of grasses may develop to reestab­ lish the plant. Established perennial weeds are the most difficult to control. Their control requires a translocatable herbicide, usually with repeat treatments. Sometimes, when herbicides are used to control annual weeds in a perennial crop (e.g., in orchards), perennial weeds may subsequently invade the area and present a much more difficult weed-control problem. . .. ;·, ··_.",.·

Herbicides .i

In a recent issue of Weed Science over 150 herbicides were listed, many of which are formulated in more. than one way, providing a great array of commercial products. Every herbicide has its specific chemical, physical, and biological properties. Of primary concern here is their movement and degradation in plants and soils. Information regarding such properties allows one to pre­ dict how they may be used in the field. The translocation pattern of a given herbicide often determines how it is used, whether it is applied to leaves or roots via the soil. Herbicides translocated in the symplastic system (phloem transport) are usually applied to the leaves, and those translocated in the · apoplastic system (xylem transport) are usually applied to the soil. A few herbicides (amitrole, for example) are readily translocated in both systems. The general translocation pattern is usually specific for a given herbicide; however, the extent to which it is translocated may vary in different species. Translocation patterns of many herbicides are provided in table 1. •

Chemical Control Methods/99

TABLE 1. RELATIVE MOBILITY AND PRIMARY TRANSLOC,ATION PATHWAY(S) OF HERBIC./D$S 1 (ASHTON AND CRAFTS, 1981)

Free Mobility Limited Mobility Little or No Mobility In apoplast In symplast In both In apoplast Insymplast In both 3 3 cacodylic acid glyphosate amitrole barban phenoxys chloramben · bensulide chlorpropham maleic hydrazide asulam bipyridyliums endotha113 DCPA chloroacetamides dalapon chloroxuron fenac3 dinoseb desmedipham dicamba fluridone3 naptalam dinitroanilines diphenamid3 DSMA perfluidone nitriles diphenylethers methazole MAA phenoxys1 napropamide3 MSMA propanil norflurazon3 picloram phenmedipham 2,3,6-TBA r thio<;arbamates triazines uracils ureas2

1Herbicides may also move from the symplast to the apoplast and vice versa. The rate of translocation niay vary between plant species and under different environmental conditions. 2Exi:ept chloroxuron, limited apoplast. · 3Translocation rate varies widely between species. Reprinted by permission ofJohn Wiley & Sons, New York. Copyright ©1981,

A herbicide that is not translocated and/or that is degraded rapidly in plants will not control perennial weeds. The degradation of a herbicide usually makes it essentially nonphytotoxic. Paraquat and trifluralin are not translocated, whereas amitrole and glyphosate are. Glyphosate is not read­ ily degraded in plants, whereas chlorpropham is degraded rapidly. In soils, a herbicide that is rapidly degraded (e.g., propham) is not effective for perennial weeds or industrial sites but may be quite suitable in short-season crops. A herbicide that is degraded slowly in soils (e.g., prometon) is ideal for industrial sites:' Herbicides that are bound tightly to soils and are not subject to downward movement with rainfall are not effective for the con­ trol of deep-rooted species.

The Environment

Various aspects of the environment can have profound effects on the suc­ cess of a given herbicide application. These aspects include soil type, soil microflora, water (rainfall or type of irrigation), temperature, and sunlight. A given herbicide may or may not be bound significantly to soil particles. The major components of the soil responsible for this binding are organic matter and clay. In addition to its effect on the movement of herbicides in soil, this binding may also reduce their phytotoxicity. Many product labels • indicate that higher application rates of the herbicide should be used on soils containing considerable clay and/or organic matter to be effective. 100/Principles of Weed Control in California

Some herbicides (e.g., paraquat, glyphosate) are bound so tightly thatthey are essentially nonphytotoxic in most soils. Most herbicides are mainly degraded by soil microorganisms; therefore, environmental conditions fa­ vorable for their growth (warm temperatures and adequate moisture) accel­ erate herbicide decomposition. Excessive rainfall shortly after a foliar application may remove the herbicide before it is taken up by the plant or leach it below the appropriate horizon after a soil application. The influ­ ence of the type of irrigation will be discussed later. In general, tempera­ ture influences the effectiveness of foliar-applied herbicides more than soil-applied herbicides. For example, a postemergence application of dinoseb in peas should be made when the temperatures after application are ex­ pected to be between 70 and 85°F. Lower temperatures may give inade­ quate weed control, and higher temperatures may result in crop injury. Several herbicides (e.g., paraquat) require sunlight to be effective; under low light or cloudy conditions they may be less effective and/or the appear­ ance of the phytotoxic symptoms may be delayed.

SELECTIVITY

Selectivity is a term used to indicate that one plant species (the weed) is injured by a given herbicide while another species (the crop) is not. The uninjured species is considered to be tolerant and the injured species susceptible. This is an extremely important concept, in that it means that weeds can be controlled by a herbicide without injuring the crop. Selectivity is· relative, not absolute, because· excessive rates of any herbicide or extreme environmental conditions can obliterate the differ­ ence between tolerant and susceptible species and both may be injured. It is desirable to have herbicides recommended for use in crops with at least a "2X" safety factor-that is, applications· at twice the recommended rate will not injure the crop. However, in some cases this degree of selectivity is not achieved and extreme care should be taken to make certain the proper rate is applied. Occasionally a tolerant weed not controlled by the herbicides used in the crop will become a problem. This usually occurs when crop rotation .. is. not practiced and/or there are a limited number of herbicides available -for the crop. Sometimes a combination or sequential·application of two or more herbicides is required to control all the weeds. Selectivity is more difficult to obtain for weeds that are closely related to the crop (e.g., night­ shades in tomatoes). The mode of selectivity for an individual herbicide between a given weed and the crop is usually very specific. The basis of selectivity may be a differential growth pattern, depth of rooting, location of growing point, leaf cuticle, etc., or a physiological, biochemical, biophysical difference. Perhaps the most common basis of selectivity between two species is their Chemical Control Methods/101

.A. A I \ i \ ; \ ; \ ; \ ; \ ; \ . \ i . \ '/ \

Figure 1. Selective placement of herbicides applied after crop emergence accomplished with a shield protecting the crop (left), and a directed application (right) in which spray is directed towards the base of crop plant, favoring minimum crop coverage and maximum . weed coverage. ·

relative ability to change the herbicide molecule to a nonphytotoxic form. For example, atrazine is rapidly degraded in corn but not in most weeds. Other reported differences in selectivity include absorption and translocation. Selectivity can also be obtained with relatively nonselective herbicides by using application methods that maximize weed exposure and minimize crop exposure to the herbicide. These include directed and shielded sprays (fig. 1), recirculating sprayers, roller applicators, and wick/wiper applicators.

Resistance

Resistance may be considered a unique type of selectivity in which the two plants involved are diffl;jrent biotypes of the same species. The resistant biotype survives and grows normally at the usual effective dose of the herbicide. The resistant and susceptible biotypes are identical in appearance. The resistant biotype occurs in very low numbers in the native population. It is only with the continuous use of the same herbicide for several years that the resistance becomes apparent. Triazine resistance is the most exten­ sively studied example. The triazine molecule binds to a protein of the photosynthetic electron transport system and blocks photosynthesis in the susceptible biotype but not in the resistant biotype. By 1982, triazine resis­ tance bad been confirmed in at least 18 genera and 30 species located in at least 23 states (USA), four provinces of Canada, and seven countries of Europe. Additional information on resistance is available in the monograph on this subject edited by LeBaron and Gressel (1982).

OIL APPLICATIONS

Most soil-applied herbicides are used to control annual weeds. They inter­ fere primarily with weed growth at the stage of seed germination or seed­ ling establishment. They usually have little, if any, effect on mature weeds. 102/Principles of Weed Control in California

...

····· •.• ·.·.· ·=· ......

~-::::::j Herbicide

Figure 2. The position of the herbicide in the soil profile influences control of shallow- or deep-rooted crops and/or weeds. When herbicide remains near the surface, deep-rooted crop is not affected, while weed is controlled (left). When the her~icide is leached deeper in the soil Profile, shallow-rooted crop is not affected, while deep-rooted weed is controlled (right).

Often the seedlings never emerge from the soil; and if they do emerge, they are usually stunte.d and misshapen. Therefore, these herbicides must be present in the soil horizon occupied by the germinating weed seeds. This placement is accomplished by soil-incorporation or preemergence application followed by rainfall or overhead irrigation. Normal furrow irriga­ tion following a preemergence treatment does not place the herbicide in the soil horizon where weed seeds germinate. A few soil-applied herbicides control established annual weeds, as well as herbaceous and woody perennial weeds. They are usually nonselective and used on noncrop land. They require sufficient rainfall or irrigation to leach them into the absorbing root zone of the weed. This is necessary because they are mainly photosynthetic inhibitors and must be absorbed by the roots and translocated ·to the leaves via the apoplastic transport system (xylem) to be effective. Herbicides are applied to the soil as preemergence- or preplant­ incorporated treatments, and they may be selective or nonselective. Selec­ tivity is essential in crops but undesirable in noncrop situations where all vegetation must be controlled (fig. 2).

Preemergence Herbicides

These herbicides are applied to the soil surface after the crop is planted but before the crop or weeds emerge. Rainfall has to occur or overhead irrigation must be applied within several days after application for them to Chemical Control Methods/103

be effective. In areas of trequent rainfall, this is probably the most common method used for the control of annual weeds in crops. Occasionally 'preemergence' is used to refer to a situation where the

TABLE 2. PERSISTENCE OF BIOLOGICAL ACTIVITY AT THE USUAL RATE OF HERBICIDE APPLICATION IN MOIST-FERTILE SOILS, SUMMER TEMPERATURES, IN A TEMPERATE CLIMATE1 (KLINGMAN AND ASHTON, 1982)

1 Month or Less 1-3Months 3-12 Months3 Over 12 Months 4 (Temporary (Early-season (Full-season (Total vegetation effects) . control) control) control)

acrolein acifluorfen alachlor isopropalin arsenic amitrole bentazon ametryn linuron borate AMS bifenox atrazine metobromuron bromacil benzadox butylate bensulide metolachlor chlorate cacodylic acid CDAA bromoxynil mitribuzin fenac chloroxuron CDEC buthiadazole monolinuron fluridone (in soil) dalapon chloramben chlorobromuron monuron hexaflurate 2,4-D chloropropham cyanazine napropamid karbutilate 2,4-DB cycloate cyprazine norflurazone picloram dinoseb (DNBP) desmedipham DCPA norea prometon 0 diquat2 diallate dicamba oryzalin tebuthiuron DSMA diphenamid dichlobenil oxyfluorfen terbacil endothall dipropetryn difenzoquat , pendimethalin 2,3,6-TBA fluorodifen EPTC dinitramine pronamide glyphosate mecoprop diuron propazine ioxynil methazole ethalfl uralin prosulalin metham naptalam fenuron sechumeton methyl bromide norea fluchloralin simazine MCPA pebtlate fluometuron terbutol MCPB prometryn fluridone (in trifluralin MH propac~lor water) molinate propham MSMA pyrazon nitrofen siduron paraquat2 silvex phenmedipham TCA propanil terbutryn propham thiobencarb triallate 2;4,5-T vernolate 1 These are approximate values atJd will vary as discussed in the text. 2 Although diquat and paraquat molecules may remain unchanged in soils, they are adsorbed so tightly that they become biologically inactive. 3 · At higher rates of application, some of these chemicals may persist at biologically active levels for more than 12 months. · · 4 At lower rates of application, some of these chemicals may persist ~t biologically active levels fodess than 12 months. Reprinted by permission ofJohn Wiiey & Sons, New York. Copyright ©1982. A 104/Principles of Weed Control in California

crop has not emerged but the weeds have. In this case, one should be more specific and indicate that the herbicide is applied preemergence to the crop and postemergence to the weeds. Contact-type herbicides can frequently be used this way in crops that emerge later than most of the weeds, e.g., paraquat in peppers.

Preplant Soil-Incorporated Herbicides

These herbicides are incorporated into the soil before planting the crop. Methods of incorporation are discussed later in this chapter. This tech­ nique was initially developed to minimize the loss of volatile herbicides (e.g., EPTC) from the soil surface. However, shortly thereafter it was found that many herbicides that proved ineffective in the arid southwest as preemergence treatments with furrow irrigation were effective when incor­ porated into the soil.

Herbicide Persistence in Soils

Persistence of herbicides in the soil is a critical factor. On an industrial site, long persistence is desirable; however, on crop land, it should last just long enough to give season-long weed control but not long enough to in­ jure the following crop. Atrazine is interesting in this regard, because it is relatively persistent. It is used at high rates on noncrop land for total vegetation control, and allow rates in com and sorghum for the control of most annual grasses and broadleaf weeds. It is selective because it is rap­ idly degraded in these crops but not in most weeds. However, even at low rates it can persist long enough to injure other crops planted the following year. The average persistence of many herbicides is given in table i Herbicides are inactivated in soils by· biological, chemical, and physical means. Inactivation by biological and chemical means usually involves a degradation of the molecule, whereas physical means involve its tight binding to other substances. For most herbicides, biological inactivation is probably the most important, and this primarily involves microorganisms. ; Therefore, conditions favorable to the growth of microorganisms acceler­ t ( I ate herbicide degradation in soils. These factors include warm tempera­ ' ' ~ tures, adequate moisture, and sufficient aeration. Excessively high or low temperatures, high or low soil moisture, and compacted soils decrease their l ' biological inactivation. i: FOLIAR APPLICATIONS L r I Herbicides applied to the leaves are considered to be of two types, contact r or translocated. In general, the contacHype herbicides are nonselective, ,,:i.'.. 01 but the translocated herbicides may be either selective or noriselective. : lI i,, -l I ' ; ~ I I Chemical Control Methods/105

-Contact Herbicides

Contact herbicides have their action on that part of the plant to which they are applied, usually the leaf. Since they do not move to untreated parts of . the plant, they are relatively ineffective on perennial weeds with regenera­ tive rhizomes and stolons. Such perennials rapidly recover from a contact­ herbicide treatment. However, these contact herbicides are very effective on a broad spectrum of young annual weeds because they are relatively. nonselective. Paraquat is an example of this type of herbicide.

Translocated Herbicides

Translocated herbicides applied to the leaves move from the treated leaves to other parts of the plant and may act primarily at these distant sites. They tend to accumulate in such areas of rapid growth as growing points, root tips, and areas of rapidly elongating shoots and roots. They are effec­ tive on both annual and perennial weeds. The degree of movement of the translocatable herbicides varies considerably. Amitrole and glyphosate are very mobile, whereas 2,4-D has limited mobility. The relative mobility and primary translocation pathway of many herbicides are provided in table 1. This table includes both foliar- and soil-applied herbicides.

FATE IN THE ENVIRONMENT

Scientists and the general public are becoming increasingly concerned about the fate of pesticides in the environment. Most herbicides are relatively nontoxic to man; they are designed to injure plants, not animals. The toxi­ cological properties of herbicides are given in the Herbicide Handbook (1983) of the Weed Science Society of America. Figure 3 illustrates the great diversity of processes that lead to the detoxification, degradation, and disappearance of herbicides from the site of application. Some may be lost by drift during the application, but this can be minimized by not spraying when the wind exceeds 5 mph and avoid­ ing small droplets .in the spray. Some may be subject to· photodecomposition in the air and on plant or soil surfaces. Herbicides with a high vapor pres­ sure (e.g., EPTC) are lost by volatilization unless incorporated into the soil. Some may be lost by the runoff of surface water. Such losses are minimized under good agricultural practices. Herbicides that enter the plant via leaves or roots are subject to degradation within the plant. Herbicides that enter the soil from a direct application or plant residues undergo nu­ merous fates, as shown in figure 3. Some can be leached downward and ultimately reach the water table; however, this is not common and usually only occurs in relatively light, sandy soils with excessive rainfall or irrigation. Most herbicides remain in the upper soil profile where they are subject to 106/Principles of Weed Control in California !

Drift t Photodecom position Volatilization

Soi 1 Surface

0 t 0 0 .. Diffusion ----1.,~ 0 MovementO in Water Absorp~on t O 0 0 0 ~ Microbial Absorption Soil o_.----. Metabolism O Solution 0 0 0 0 0 Adsorption 0 Desorption 0 Chemical 0 Leaching Reactions O in Water 0 0 Water Table· o~.

0 Herbicide Molecule Figure 3. Diagrammatical sketch of the interrelations of processes that lead to detoxification, degradation, and disappearance of herbicides. (Courtesy Sheets and Kaufman, 1970.) ·

biological and nonbiological degradation. Such processes usually involve a relatively rapid breakdown :of the herbicide molecule; however, a few herbi­ cides (e.g., glyphosate and paraquat) are inactivated by being strongly bound to clay and organic matter.

ADDITIONAL READING

Anderson, WP. 1983. Weed Science: Principles, 2nd ed. West Publishing Co., St. Paul. 655 pp. Ashton, F.M. and A.S. Crafts. 1981. Mode of Action of Herbicides, 2nd ed. John Wiley & Sons, Inc. New York. 525 pp. Ashton, F.M. and WA. Haroey. 1971. Selective Chemical Weed Control Univ. of Calif Gire. 558. 17 pp. Beste, E.E., ed. 1983. Herbicide Handbook of the Weed Science Society of America,· Urbana, Ill. Brown, A. WA. 1978. Ecology of Pesticides. John Wiley & Sons, Inc., New York. 525 pp. Crafts, A.S. 1975. Modern Weed Control. Univ. of Calif Press, Berkeley. 440 pp. Klingman, G.C. and F.M. Ashton. 1982. Weed Science: Principle and Practices, 2nd ed. John Wiley & Sons, Inc., New York. 449 pp. LeBaron, H.M. and]. Gressel, e