Comparative Toxicity of Herbicide Active Ingredients, Safener Additives, and Commercial Formulations to the Nontarget Alga Raphidocelis Subcapitata

Abstract

Chloroacetanilide herbicides are used worldwide to control weeds that affect crops such as corn, soybeans, and cotton. These herbicides are frequently paired with a "safener," which prevents herbicidal damage to the crop without diminishing weed control. Formulated herbicide products that include safeners and other ingredients are infrequently assessed for toxicity. Our goal was to understand the potential toxicity of safeners and herbicide + safener formulations relative to the toxicity of associated active ingredients. We quantified the concentration of safeners in commercially available formulations and tested effects on nontarget algae, Raphidocelis subcapitata, when exposed to individual herbicide active ingredients, safeners, and commercial formulations. The median effective concentrations (EC50s) causing 50% reduction in population growth for the herbicide active ingredients S-metolachlor and acetochlor were 0.046 and 0.003 ppm, respectively. The safeners benoxacor, AD-67, furilazole, and dichlormid were all substantially less toxic than the herbicides and were not toxic at environmentally relevant concentrations. The commercial formulations Dual II Magnum ® , Me-Too-Lachlor II ® , Harness ® , and Surpass EC ® all resulted in EC50 values that fell within the 95% confidence interval of the associated active ingredient herbicide. Interestingly, a significant increase in cell size was observed when algae were exposed to all the formulations, herbicides (acetochlor and S-metolachlor), and safener (dichlormid). The safener furilazole caused a significant decrease in cell size, whereas benoxacor and AD-67 had no observed effect on algae cell size. Significant algae cell size effects all occurred at or above the EC50 concentrations for each chemical, suggesting that other morphological effects may be occurring. Importantly, safeners in commercial formulations appeared not to impact toxicity to R. subcapitata compared with the active ingredient alone.

Review on the Discovery of Novel Natural Herbicide Safeners

“…Certain reports have revealed that even commercial safeners are detrimental to aquatic and mammalian organisms. Therefore, it is urgent to develop novel herbicide safeners with low toxicity, high efficiency, and environmental friendliness. − …”

Research Progress in the Design and Synthesis of Herbicide Safeners: A Review

“…There are still few safeners for dicotyledonous plants. Some new safeners have failed to enter the market as a result of various problems. , The environmental impact of safeners as chemicals has not yet been determined. , The above problems have been deeply explored, and it is particularly important to explore more innovative new safener structures.…”

Microbial Biotransformation Products and Pathways of Dichloroacetamide Herbicide Safeners

“…Dichloroacetamide safeners are coapplied with chloroacetamide herbicides to selectively protect crops from unintended herbicide toxicity. − Due to their extensive use (>8 × 10 6 kg/year globally) and hydrophilic nature, the four most common dichloroacetamides (AD-67, benoxacor, dichlormid, and furilazole) have been detected in surface waters throughout the midwestern U.S., yet their environmental fates remain largely underinvestigated. ,− Existing research, including studies by our groups, indicates that safeners can transform in the environment to yield products with increased biological activity and, in some cases, increased toxicity. ,, For example, dichloroacetamides in iron-rich anaerobic environments can undergo reductive dechlorination to yield more toxic products, including formation of CDAA (known as allidochlor or 2-chloro-N,N-bis­(prop-2-enyl)­acetamide; an herbicide banned in the United States due in part to human health concerns) from dichlormid, as well as monochloro-benoxacor (toxic toward insect larvae; LOEC = 0.1 mg kg –1 ) from benoxacor. ,, Recently, we probed dichloroacetamides' environmental fate, focusing on photolysis and hydrolysis. , Only benoxacor transformed by direct photolysis, and hydrolysis rates were slow and only environmentally relevant under basic (pH 10–11) conditions. , Thus, there are potentially significant environmental fate processes relevant to dichloroacetamide safeners, notably microbial biotransformation, that remain uncharacterized and may yield transformation products of concern.…”

“… 3 , 5 − 11 Existing research, including studies by our groups, indicates that safeners can transform in the environment to yield products with increased biological activity and, in some cases, increased toxicity. 7 , 11 , 12 For example, dichloroacetamides in iron-rich anaerobic environments can undergo reductive dechlorination to yield more toxic products, including formation of CDAA (known as allidochlor or 2-chloro-N,N-bis(prop-2-enyl)acetamide; an herbicide banned in the United States due in part to human health concerns) from dichlormid, as well as monochloro-benoxacor (toxic toward insect larvae; LOEC = 0.1 mg kg –1 ) from benoxacor. 7 , 11 , 12 Recently, we probed dichloroacetamides' environmental fate, focusing on photolysis and hydrolysis.…”

“… 7 , 11 , 12 For example, dichloroacetamides in iron-rich anaerobic environments can undergo reductive dechlorination to yield more toxic products, including formation of CDAA (known as allidochlor or 2-chloro-N,N-bis(prop-2-enyl)acetamide; an herbicide banned in the United States due in part to human health concerns) from dichlormid, as well as monochloro-benoxacor (toxic toward insect larvae; LOEC = 0.1 mg kg –1 ) from benoxacor. 7 , 11 , 12 Recently, we probed dichloroacetamides' environmental fate, focusing on photolysis and hydrolysis. 13 , 14 Only benoxacor transformed by direct photolysis, and hydrolysis rates were slow and only environmentally relevant under basic (pH 10–11) conditions.…”