Environmental Fate and Effects of Dichloroacetamide Herbicide Safeners: “Inert” yet Biologically Active Agrochemical Ingredients

Abstract

Safeners are included in many commercial herbicide formulations to selectively protect crops from injury induced by active ingredients. Despite their bioactivity, safeners are classified as inert from a regulatory perspective, and as such, safeners have received minimal attention in the peer-reviewed literature regarding their environmental fate and effects. Herein, we review what is known about the uses, physicochemical properties, environmental transformations, and (eco)toxicological effects of dichloroacetamide safeners, which represent one of the most commonly used safener classes (estimated use of >2 × 10 6 kg/year in the United States). We particularly highlight transformation pathways that may enhance biological activity and/or persistence; for example, limited studies suggest dichloroacetamides can transform via dechlorination into products with increased bioactivity. We also identify several research needs to improve our understanding of the environmental fate and potential risks of this overlooked agrochemical class, which in turn will enhance the efficacy and safety of future herbicide safener formulations.

Assessing the toxicity of the “inert” safener benoxacor toward Chironomus riparius: Effects of agrochemical mixtures

“…Given such a wide distribution in natural waters, we postulate that benoxacor, as a component of several commercial metolachlor formulations, will have a comparably extensive occurrence. Nevertheless, to our knowledge, environmental occurrence data for benoxacor have not been reported for any environmental compartment (including surface waters or sediments) .…”

“…Under anaerobic conditions in the presence of ferrous iron and iron (hydr)oxides, benoxacor and other dichloroacetamide safeners can undergo reductive dechlorination, yielding monochloro products with structures similar to their active herbicidal coformulants (Figure ) . Accordingly, environmental transformation reactions of dichloroacetamide safeners could conceivably result in increased environmental hazard as a result of the generally greater toxicity of (mono)chloroacetamides relative to dichloroacetamides .…”

“…However, it appears that aquatic organisms may be more sensitive to benoxacor, given an acute median lethal concentration (LC50) of 0.63 mg/L for freshwater algae, and an LC50 of 2.9 mg/L for trout, Onchorhynchus mykiss . Benoxacor can be classified as highly and moderately toxic to aquatic plants and animals, respectively . Chronic experiments with benoxacor demonstrated a no‐observed‐effect concentration (NOEC) of 0.354 mg/L in carapace length of D. magna compared with a NOEC of 0.1 mg/L for chronic exposure of D. magna to S ‐metolachlor .…”

The Effect of Salicylic Acid and 20 Substituted Molecules on Alleviating Metolachlor Herbicide Injury in Rice (Oryza sativa)

“…Synthetic herbicide safeners such as fenclorim, dichlormid, flurazole, fenchlorazole, and dymrone have since been commercialized for crop protection [17][18][19][20]. However, some commercially available herbicide safeners (e.g., dichlormid, benoxacor, and furilazole) are toxic to both aquatic organisms and mammals [21]. The environmental toxicity of these compounds has led to a greater search for highly efficient, highly active, and environmentally friendlier herbicide safeners, representing a new direction for the development of pesticides.…”

Development of chemically active metal oxide composite nanofiber filters for water treatment

“…Neonicotinoids, implicated in declining populations of pollinating insects [43], have only recently been explored as pollutants present in aquatic ecosystems across the United States [44,45]. Likewise, herbicide safeners, used to protect desirable crops from herbicides, are bioactive and can impact ecosystems despite being largely termed "inert" for decades [46].…”