Balance between herbicidal activity and toxicity effect: A case study of the joint effects of triazine and phenylurea herbicides on Selenastrum capricornutum and Photobacterium phosphoreum

• Individual and joint effects of triazines and phenylureas were determined.
• The outlier of the individual herbicides was explained by molecular docking.
• Mechanistic hypotheses on herbicide mixture of two species were proposed.
• Prometon-phenylureas were recommended, whereas simetryn–phenylureas were not.

The use of herbicide mixtures has become a cost-effective strategy against the evolution of herbicide resistance to protect global food production. Much research has focused on investigating either the herbicidal activities or the toxicity effects of herbicides; however, few of them have investigated both factors. This study investigates the balance between herbicidal activity for Selenastrum capricornutum and toxicity effect toward Photobacterium phosphoreum by determining the joint effects of triazine (simetryn, atrazine, prometon and prometryn) and phenylurea (fenuron, monuron, monolinuron and diuron) herbicides. The results showed that among the four triazines, only simetryn exhibited a unique effect (formation of a pi–sigma bond with the D1 microalga protein and an H-bond with the Luc photobacterial protein); and among 16 triazine–phenylurea binary mixtures, only the mixtures containing simetryn resulted in TU1 values (herbicidal activities of mixtures on S. capricornutum) >TU2 values (toxicity effects of mixtures on P. phosphoreum). However, the other 12 mixtures, which did not contain simetryn, showed the opposite result (TU1 < TU2). A comparison of TU1 with TU2 showed that additive effects occurred more frequently for TU1, whereas antagonism effects occurred more frequently for TU2. Based on these results, a preliminary mechanistic hypothesis of herbicide mixtures was proposed. Meanwhile, some suggestions are provided firstly for herbicide combinations based on the balance between herbicidal activity and toxicity effect, which will encourage thoughtful efforts for how to best combine herbicides in a sustainable way.