Herbicides: Non-Target Species Effects

Introduction

Pesticides such as inorganic chemicals (e.g., sulfur, arsenic, or other metal compounds) have been used in agriculture for centuries to protect crops. In more recent times, especially after World War II, organic pesticides have been discovered and increasingly used to suppress unwanted plants, insects, and other organisms that interfere with crop production. The main pesticides used are categorized by their target organisms: insecticides used to kill or suppress insects, fungicides used for pathogens, and herbicides for weeds. In this entry, we are mostly concerned with herbicides because of their considerable use, especially in North America, and their phytotoxic effects. The definition accepted by the Weed Science Society of America is that an herbicide is “a chemical substance or cultured organism used to kill or suppress the growth of plants.”111

Phytotoxicity refers to the capability of herbicides or other pesticides to exert toxic effect on plant growth, reproduction, and survival. By extension and in addition to terrestrial and aquatic vascular plants, it usually includes other primary producers such as algae and cyanobacteria, which will not be considered in this entry.

This entry will first consider the history, types, and main uses of herbicides. There are advantages and disadvantages of using herbicides in agriculture and forest management. Benefits have long been established; however, the limitations and undesirable effects attributed to herbicide use are still debated. The environmental impact will be examined, including environmental exposure, phytotoxicity, and toxicity to different trophic levels, as well as the multitudinous factors to take into account in risk assessment. Lastly, the techniques and limitations of the phytotoxicity assessments currently used to determine environmental risk evaluations will be discussed. Mitigation measures and alternatives to herbicide use will be discussed in conclusion.

History, Types of Herbicides, and Their Use

DNOC or 4,6-dinitro-o-cresoi, developed in 1935, was the first organic herbicide used to control weeds. This herbicide was also used as an insecticide, fungicide, and a defoliant and was shown to be toxic to animals.121 It is no longer used in many countries. In the 1940s, several phenoxy herbicides were discovered and many are still used today, including 2,4-D and MCPA [(4-chloro-2-methylphenoxy)acetic acid]. In the 1950s, other herbicides appeared on the market, including diuron, diquat, paraquat, and triallate. In the 1960s, the tri- azine herbicides were developed and are still considerably used, especially atrazine. Glyphosate, which is still the most widely used herbicide worldwide, was developed in the 1970s. In the 1980s, low-dose high-efficacy herbicides such as sulfonylureas and imidazolinones were found. Since the late 1990s, a new method for controlling weeds has emerged with the development of herbicide tolerant crops, i.e., genetically engineered crops resistant to glyphosate and, to a lesser extent, glufosinate ammonium. More genetically modified herbicide tolerant crops will no doubt be engineered or bred in the future.

Herbicides exhibit different mechanisms of action.|!J1 They include disruption of photosynthesis (ura- cils, substituted ureas, and triazines), inhibition of lipid biosynthesis (carbamothioates such as EPTC [s-ethyl dipropylthiocarbamate], triallate, clethodim, fluazifop, and metolachlor), inhibition of cell division (dinitroanilines such as trifluralin and pendimethalin), plant hormone mimics (the phenoxy herbicide 2,4-D, MCPA, the benzoic acid dicamba, and the picolinic acid picloram), inhibition of amino acid biosynthesis (glyphosate, sulfonyl ureas including metsulfuron methyl and chlorsulfuron, imidazolinones including imazethapyr), blockage of carotenoid biosynthesis (clomazone), and disruption of cell membranes (acifluorfen and bipyridylium compounds such as diquat and paraquat). Other herbicides such as glufosinate ammonium act by inhibiting glutamine synthetase, thus leading to a complete breakdown of ammonia metabolism in affected plants. Uncouplers of oxidation phosphorylation, such as the widely used bromoxynil, interfere with plant respiration.

Many herbicides act primarily on systems unique to plants, e.g., photosynthesis, but some herbicides act at more than one site of action. Undoubtedly, the secondary mode of action of some herbicides could explain their relatively high toxicity to animals (see below). In the case of some herbicides, the precise mode of action is unknown and exact molecular sites of action remain to be determined.141

The number of herbicides listed in the Weed Science Society of America reached 374 in 2010.151 In Canada, there are 500 pesticide active ingredients (the ingredient to which the pesticide is attributed) and 7000 pesticide formulated products (mixture containing one or several active ingredients and for- mulants) available since many formulated products contain a mixture of active ingredients.161 The number of herbicide active ingredients registered in Canada amounts to approximately 125.161 The majority of herbicide use occurs in agriculture where in modern practices they dominate weed control practice. Approximately 90% of areas planted with corn, cotton, potato, wheat, and soybean were sprayed with herbicides in the United States in 2004.|!|

While the benefits of using herbicides from an agronomic perspective are well known, the undesirable effects of herbicides on the environment have not always been considered carefully. Regardless of the method of application, it is generally accepted that misplacement will take place through drift at the time of application or through runoff, leaching, and volatilization from soil or plants or from particles moving with contaminated soil after application has occurred.

Benefits to Agriculture and Forest Management

Benefits of herbicide to agriculture, forest management, and other agronomic applications leave no doubt as to their utility, although some applications for cosmetic reasons (e.g., domestic use, horticulture, golf courses) are more questionable. The negative relationship between crop yield and weed density is well established.111 Weeds are well-adapted species that compete with crops in disturbed environments and will reduce crop yield depending on their germination timing and densities, growth patterns, and growth rates. However, there may be cases where weeds can be beneficial to crops. As an example, field experiments were conducted to evaluate the effects of nicosulfuron and imazethapyr, a sulfonyl urea and an imidazolinone herbicide, respectively, for the control of johnsongrass (Sorghum halepense L.), a weed difficult to control in corn fields.1"1 It was noted that corn vigor was greatly reduced in plots where these two herbicides were applied and where johnsongrass was reduced. In order to verify if there was an unwanted effect of the two herbicides on corn, small plots were sprayed with the two herbicides separately. Two other treatments were also included in the experimental design: control of johnsongrass via mechanical means and no treatment. It was noticed that in plots where johnsongrass was removed, corn was more prone to being attacked by the maize dwarf mosaic and maize chlorotic dwarf viruses, which are transmitted by aphids or leafhoppers. Results revealed that in treated plots, virus disease was increased because the preferred host johnsongrass was suppressed. Furthermore, there was no change in yield between treatments. Crop yield was reduced in treated plots (whether through mechanical or chemical treatments) due to increased virus severity, while in non- treated plots, reduced corn yield was due to more competition from johnsongrass.

Herbicide Use and Exposure to Primary Producers

to satisfy increasing needs for more cropping areas. Consequently, there are almost no pristine grassland prairies left where native plant and animal communities can survive. The Mixedwood Plains are a smaller area in southern central Canada (1.5% of the land cover) where 50% of the Canadian population lives and where agriculture is also very intense. In this region, a large portion of the Carolinian forest and the mixed-wood forest has disappeared to satisfy human needs. In the United States, agriculture covers 48% of the land and is concentrated in the mid-west regions1201 where original ecosystems have largely vanished. The same pattern is repeated in Europe where mostly seminatural habitats remain in most countries. What remains of these ecosystems interspersed in a sea of intensively cultivated land can be greatly impacted by herbicide use.

Herbicide exposure to non-target environments can occur when application is performed with aircraft, mist-blower, and ground applications through overspray or via spray drift, vapor drift, revolatilization from soil and plants, runoff, or dust particles moved by wind or water.1141 Depending on the equipment and prevailing weather conditions during application, the amount of sprayed herbicide that will deposit in hedgerows and other field edges from multiple consecutive spray tracks can reach 1% to 10% of the application rate within 10 m of a single swath with ground equipment (Boutin and Jobin1211 and references therein), and much more with mist-blower sprayers and aerial applications.1221 Herbicides can travel a considerably longer distance with aerial equipment applications, for example, 500 m downwind from the source.123,241

With ground application, it was found that herbicides could cover long distances. A study was undertaken to assess the protection afforded by buffer zones from herbicide drift (Boutin and Baril, unpublished data). Surveys of nontarget plants situated in two small woodlots adjacent to crop fields were conducted prior to (May) and after (May, June, and July) herbicide application. Vegetation was surveyed for community composition and symptoms of herbicidal impact. The experimental work was conducted in southwestern Ontario, Canada, under normal field operation conditions, for soybean sprayed with imazethapyr in 1993, corn with dicamba in 1994, and wheat with MCPA in 1996, following the usual rotation in southwestern Ontario. The buffer zone was defined as a 12 m wide seeded strip of crop, upwind of a woodlot, where herbicides were not applied. Each treatment consisted of four transects, divided into sampling points, at lm, 2 m, 4 m, 8 m, 16 m, and 32 m distances into the woodlots. Herbicide application occurred in the early morning or evening, when no precipitation was forecasted, when wind speed was at or less than 8 km/hr, and when the direction of the wind was across the soybean field into the woodlot.

Results revealed that herbicides could move up to 32 m into woodlots (Figure 1). Up to 43% of the vegetation of one quadrat (of the 23 species) in the woodlots showed visual effects characteristics of herbicidal injury: discoloration, bleaching, epinasty, yellow or brown spots, etc. Effects were less pronounced in transects abutted to buffer zones. The plants most affected [e.g., raspberry (Rubus idaeus L.), goldenrod (Solidago canadensis L.), and ash tree (Fraxinus spp.)] were species of open areas growing in the first few meters of the woodlots. In some cases, effects lasted for more than 2 mo.

Vapor drift can also migrate a long way (Franzaring et al.1251 and references therein) causing recurrent sublethal effects on native plants not only in bordering seminatural habitats but also in more remote habitats. Presence of airborne herbicides in the atmosphere has been reported in Europe1261 and North America.1271 In the Netherlands, it was found that non-target vegetation was repeatedly exposed to small amounts of herbicides (Franzaring et al.1251 and references therein). Pesticides used in agriculture in southern Canada, including some herbicides, have been reported in the arctic environments.1281 Of the 10 chemicals surveyed by Hoferkamp et al.,1291 9 were detected in the arctic. Traveling distances for these chemicals ranged from 55 km to 12,100 km. Persistence in the environment of these herbicides can explain their presence in the arctic where they are transported via the air or by dust.

Undoubtedly, the large number of herbicides available for use, the geographical extent of their use on different crops, and the quantity applied together with the method of application suggest a high probability of exposure to primary producers and other wildlife.

Number of plants showing herbicide effects in two woodlots

FIGURE 1 Number of plants showing herbicide effects in two woodlots (L and H) surveyed during 3 years prior to and after herbicide spray under normal field operations. Fields were sprayed with the herbicides imazethapyr in 1993, dicamba in 1994, and MCPA in 1996. Plants were surveyed in quadrats situated between 1 and 32 m from crop fields. A12 m buffer zone within fields was used in half of the fields while the other half was sprayed right to the field edge. Four transects were placed for each treatment. In total, 116 species were inventoried and 35 species were found to be sensitive to herbicides during the course of the 3 years the study lasted.

 
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