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