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What factors impact the level of exposure of plants and animals to glyphosate?

Category: Environment and Wildlife

The amount of area treated, frequency of applications and rate of product applied are some of the many factors that determine the exposure of plants and animals to glyphosate. Only plants and animals living directly in treatment sites, or that visit the sites within a few weeks of the treatment, are likely to be exposed to significant levels of glyphosate or surfactants in the formulated products.

Across Canada glyphosate-based herbicides are applied to approximately 150,000 ha annually, equivalent to approximately 1/3rd of of the area harvested by clearcutting. The most common use scenario is for release of conifer seedlings from competing vegetation and involves aerial applications made to specifically targeted regeneration sites within a few years (1-4 years) post-harvest. Treated sites are distributed throughout the forest landscape and will typically receive a treatment, at a rate of ~ 2 kg acid equivalent (a.e.) per hectare, once or twice in a 40-80 year rotation cycle. Plants within the targeted competing vegetation canopy (deciduous brushy species or tall herbaceous species such as trembling aspen, raspberry or calamagrostis grass) are likely to intercept a relatively higher proportion of the spray than plants closer to ground level. Animals resident within the targeted spray blocks at the time of application (Aug-mid September) and which have limited forage range or mobility are likely to experience relatively higher exposures. Resident animals or those which visit the site shortly after treatment may also be exposed to residues via ingestion of contaminated foods or in the much shorter-term via desorption from treated surfaces. Since glyphosate is relatively non-persistent, such environmental exposures will be short in duration (a few days to a few weeks) and will involve concentrations that diminish rapidly through those time frames. In forest-use scenarios, major water bodies such as streams, rivers, ponds and lakes are usually protected by buffer zones, thus most aquatic organisms are unlikely to be exposed to toxicologically significant levels. Aquatic and terrestrial life-stages of amphibians which frequently are associated with small, shallow and commonly ephemeral wetlands, and which may occur within treated areas, represent a unique case of relatively higher potential for exposure

For more info on aquatic and terrestrial life stages of amphibians, see also

Recent risk analyses (PMRA 2015) conclude that typical uses of glyphosate-based herbicides, including forest use and even those involving multiple applications as typical of various agricultural crops, pose low risk to birds, mammals, soil organisms, fish and amphibians. In the more specific context of glyphosate-based herbicide use in forest management, where single exposures at lower rates approximating 2 kg a.e/ha are involved, Durkin (2003) concluded that none of the hazard quotients for acute or chronic scenarios reach a level of concern and that such analyses support the conclusions previously reached by the USEPA that potential effects on birds, mammals, fish and invertebrates are minimal.

In forest use scenarios, exposure of plants or animals is a function of the frequency, method and rate of application which control the initial levels in various environmental compartments and the probability of any particular organism actually being exposed. Persistence and fate of residues within the various compartments (e.g. soils, vegetation, water) determine the potential duration of exposure and whether such residues may be biologically available. Finally the characteristic growth pattern of plants or natural behavior of animals including how it forages and interacts with its environment, may influence the potential route and degree of exposure. Given that glyphosate-based herbicides are typically applied only once within the first 1-4 years in a forest successional cycle that may vary from 40-80 years, the probability of exposure for most organisms is quite low. The potential for significant exposures are further restricted to only those plants or animals that occur or forage within these regenerating blocks at the time of application, or given known herbicide residue dissipation rates, within say 8 weeks post-treatment. In Canada, the area of forest land area treated with glyphosate-based herbicides is estimated to be approximately 150,000 ha annually or approximately 19% of the area that is clearcut harvested, while the majority of the area is regenerated naturally or using non-chemical alternatives. This is a particularly important aspect as it governs the probability that wildlife will actually be directly exposed as well as the proportion of habitat that may be influenced by the herbicide treatment in the context of potential indirect effects. Glyphosate-based herbicides are typically applied by air at an average application rate of 1.9 kg a.e./ha (Thompson 2011). Much of the aerially-applied spray is intercepted by the target woody/shrub layer, with proportionally lower amounts of chemical being vertically distributed to ground herbaceous vegetation and soil layers (Thompson et al. 1997). As such, plants that occur in the upper layers of the typically tiered plant community (e.g. alder, trembling aspen) and organisms that may be foraging in this layer (e.g. birds, moose) at the time of spray or shortly thereafter are likely to experience relatively higher exposures than those which are restricted to the ground layer (e.g. earthworms, small mammals). As no direct application to streams, rivers, ponds and lakes are permitted and given that these systems are protected by buffer zones, most aquatic organisms will not be directly exposed. As an exception, there is potential for direct exposure to aquatic organisms (e.g. amphibians) associated with small, shallow, typically ephemeral wetlands that may occur within or immediately adjacent to spray blocks but not mapped or visible from the air and thus not excluded from the over-sprayed area (Thompson et al. 2004). Several ecotoxicological field studies have now been conducted in Canada to explicitly address this issue with results indicating that risk to either aquatic larval or terrestrial adult amphibians are minimal.

In April 2015, the PMRA released their latest review of glyphosate and declared that the weight of evidence indicates that glyphosate does not present unacceptable risk to human health. The full PMRA glyphosate review can be found here or please visit here for a summary version of the full PMRA review.

PMRA (Pest Management Regulatory Agency). Proposed re-evaluation decision – Glyphosate. PRCD2015-01. 13 April 2015. Available electronically at: pmra.publications@hc-sc.gc.ca

Durkin PR. Glyphosate - Human health and ecological risk assessment report. Syracuse Environmental Research Associates Inc, Fayetteville NY 2003.

Thompson DG. Ecological impacts of major forest use pesticides. Chapter 5, In: Sanchez-Bayo, F., P. van den Brink and R.M. Mann (Eds.). Ecological impacts of toxic chemicals. Bentham Science Publishers Ltd. 2011; pp 88-110.

Thompson DG, Pitt DG, Staznik B, Payne NJ, Jaipersaid D, Lautenschlager RA, et al. On-target deposit and verticle distribution of aerially released herbicides. For. Chron.. 1997; 73(1):47-59.

Thompson DG, Wojtaszek BF, Staznik B, Chartrand DT, Stephenson GR. Chemical and Biomonitoring to Assess Potential Acute Effects of Vision® Herbicide on Native Amphibian Larvae in Forest Wetlands. Environmental Toxicology and Chemistry. 2004; 23(4):843-9.