Chemical Brush Control: Assessing the Hazard

Reprinted from the JOURNAL OF FORESTRY, Vol. 69, No. 10, October 1971 Reproduced by USDA Forest Service for official use. PLEASE DO NOT REMOVE FROM FILES CHEMICAL BRUSH CONTROL: ASSESSING THE HAZARD hazard from the use of any chemical requires consider- ABSTRACT—An adequate evaluation of the hazard asso- ation of both the likelihood of exposure and the toxicity ciated with the use of any chemical agent requires consid- of the chemical (15). eration of both the toxicity of the material and the potential for exposure of nontarget organisms. The hazard can be high only if both the toxicity of the chemical and the Likelihood of Exposure to Herbicides potential for exposure to a significant dose are high. The The likelihood that a nontarget organism will be relatively large doses of 2,4-D, amitrole, 2,4,5-T, and exposed to a significant dose is determined by the picloram required to produce acutely toxic responses in behavior of the chemical. Behavior is the initial dis- most nontarget organisms are not likely to occur from nor- tribution, subsequent movement, persistence, and fate mal chemical brush control operations on forest lands. The short persistence, lack of biomagnification in food chains, of chemicals in the environment. Chemical behavior and the rapid excretion of these herbicides by animals dictates the magnitude and duration of exposure and preclude chronic exposure and, therefore, chronic toxicity. thus the nature of a toxic response. A long history of field use and research shows our com- Herbicides applied aerially are distributed initially mon brush control chemicals can be used with minimum among four components of the forest environment—air, hazard to the quality of our environment. vegetation, forest floor, and surface waters. The amount of chemical entering each portion of the environment is determined by the chemical, equipment used, condi- P reoccupation with the projected needs of the nation tions of application and environmental factors. for wood fiber has obscured similarly increasing de- mands for forage, water, wildlife, and areas for purely Distribution in Air recreational purposes. If we accept the premise that all these needs must be satisfied, we must compensate for a Appreciable amounts of herbicide may be dispersed decreasing production base for wood products by in- I A toxic dose is one that causes an adverse effect; it is not creasing unit productivity of land devoted primarily to restricted to a lethal dose. timber production. Fertilizers, insecticides, and herbicides are important tools in increasing forest productivity, but the use of these chemicals has risks as well as benefits. Therefore, we must know in advance the consequence of each practice involving their use. The purpose of this paper is to assess the hazard to nontarget organisms from the routine use in forest brush control of four common herbicides: 2,4-dichlorophe- noxyacetic acid (2,4D); 2,4,5-trichlorophenoxyacetic acid (2,4-5-T); 3-amino-1,2,4-triazole (amitrole); and 4-amino-3,5,6-trichloropicolinic acid (picloram). Logan A. Norris The hazard of using a herbicide is the risk of adverse effects on nontarget organisms. Two factors determine the degree of hazard: (1) the toxicity of the chemical and (2) the likelihood that nontarget organisms will be exposed to toxic doses. Toxicity alone does not make a chemical hazardous. The hazard comes from exposure THE AUTHOR is principal chemist, Forestry Sci. Lab., Pac. North- to toxic doses of that chemical. Even the most toxic west Forest and Range Exp. Sta., U.S. Forest Serv., Corvallis, chemicals pose no hazard if organisms are not exposed Ore. Adapted from a paper presented at the 1970 SAF National to them. Therefore, an adequate assessment of the Convention, October 13. OCTOBER 1971 715 Fig. 1. Recovery of 2,4-D, amitrole, 2,4,5-T, and picloram from red alder forest floor material. Amitrole, 2,4-D, and 2,4,5-T applied at 2 pounds per acre and picloram at 0.5 pounds per acre in water (19). 120 100 90 80 1c"-- 70 a) U cr) 60 LEGEND cc>- 50 w 0 2,4-D CD 40 U A Amitrole w CC El 2,4,5-T 30 Picloram 20 10 20 40 60 80 100 • 120 140 160 180 TIME (days) in the air in the vapor phase or as fine droplets called land. Air dispersion of herbicides can be minimized by "drift." Studies in western Oregon revealed that from taking full advantage of recent advances in spray 20 to 75 percent of some aerially applied herbicides did technology, equipment and chemicals. not reach the first intercepting surfaces.2 The hazard from this loss is probably minimized by atmospheric Distribution in Vegetation dilution and the fact that most chemical brush control The amount of spray material intercepted by vegeta- operations do not involve large, contiguous blocks of tion depends on the nature and density of the vegetation, the physical characteristics of the spray material, 2 Michael Newton, Logan A. Norris, and Jaroslav Zavitkov- and the rate of application. A single-level community, ski. 1966. Unpublished data. Sch. of Forestry, Oregon State like grass, may offer maximum concentrations of herbi- University, Corvallis, Ore. • cide to organisms which use this vegetation as a food source. Table 1. Residues of Herbicide 1 in Forage Gross. In tests with 2,4-D, picloram, and 2,4,5-T, highest Herbicide residue concentrations in forage grasses were found immediate- Weeks after treatment 2,4-D1 2,4,5-P Picloram3 ly after spraying (Table 1). The herbicide levels de- clined markedly with time due to growth dilution, parts per million 0 100 100 1 35 60 60 2 50 30 32 Selective control of competing species with herbicides can 4 30 15 increase the growth rate of suppressed conifers. 8 6 6 24 16 2 16 52 3 Rate of application-1 pound per acre. s Data from Fig. 4 of Morton et al. (16). _ _ ‘,, Data from Table 5 of Getzendaner (9). -- HERB IC ME \ APPLICATION Table 2. Disappearance of Herbicides from Mammals. \ ,. Herbicide Mammal Excretion Reference • — — — — percent ofdose in days 2,4-D Rat 95 1 (14) Sheep 96 3 (3) 2,4,5-T Mouse 76 (23) Cow 89 4 (21) Amitrole Rat 95 I ( 7) Picloram Cow 98 4 ( 8) 716 J OURNAL OF FORESTRY Table 3. Acute Toxicity of Herbicides.1 Organism 2,4-D 2,4,5-T Amitrole Picloram Birds: LD,,o, mg/kg 360-2000 300 2000+ 2000+ No effect, ppm2 7203 600 2000+3 1000 Rodents: LD,,o, mg/kg 375-800 400-950 5000+ 2000 No effect, ppm2 1500 8003 2000+3 3000 Ruminants: LD50, mg/kg 400-800 500-1000 2000 No effect, ppm2 24003 1200 20003 Other mammals: LD5o, mg/kg 100 100 1200+ No effect, ppm2 500 2003 2000+3 Fish: TL m, ppm, 1-60 1-30 325 13-90 No effect, pprn5 0.16 0.16 326 1.06 Other aquatics: TLm, ppm 1-5 0.5-50 20 1+ No effect, ppm, 0.1 6 0.056 26 0.16 A list of references for specific values in this table is available from the author. Concentration in diet for a limited exposure period which causes no acute effect. Assumes daily food consumption is 10 percent of body weight and that 20 percent of LD:,o in daily diet has no acute effect. 48-hour median tolerance limit, i.e., the concentration of herbicide in the water which will kill 50 percent of an exposed population of aquatic organisms in 48 hours. Concentration in water which has no acute effect following 48 hours exposure. 6 Assumes 10 percent of TL m has no effect. metabolism, excretion of herbicide from the plant roots, Distribution in Forest Floor (9, 16). The half- and weathering of surface residues The forest floor is a major receptor of aerially life of amitrole was about one day in sugar beet, corn, applied herbicides. The behavior of herbicides in the and bean leaves (16). Amitrole residues completely forest floor will determine the magnitude of the dose disappeared from sugar cane in eight weeks, even after and the duration of exposure to soil microorganisms, applications of up to 40 pounds per acre (10). These the amount of Chemical available for uptake by plants, reports and the resprouting of brush which commonly and the potential for movement of herbicide residues occurs a year following spraying on forest lands suggest into water. that high residues of 2,4-D, amitrole, 2,4,5-T, and picloram do not persist for long periods in vegetation. Chemicals in the forest floor may be either volatilized and reenter the atmosphere, adsorbed by soil colloids Intense competition for light, water and nutrients can and organic matter, leached through the soil profile, markedly reduce regeneration success. Herbicides can re- absorbed by plants, or degraded by chemical or biologi- duce vegetation density and permit conifer establishment. cal processes. The physical properties of amitrole and picloram prevent their evaporation from the forest floor. The phenoxy herbicide esters could volatilize, but the rapid hydrolysis of these esters to nonvolatile forms may restrict this process (2). Degradation is the only means by which the total load of an environmental pollutant can be reduced. Ami- trole, 2,4-D, picloram, and 2,4,5-T are degraded in forest floor material (19). In one study with red alder (Alma rubra Bong.) forest floor material, 80 percent of the amitrole, and 94 percent of the 2,4-D were degraded in 35 days (Fig. 1). One hundred and twenty days were required to degrade 87 percent of the 2,4,5-T.

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