Investigating the presence of crop applied herbicide mixtures in aquatic systems and its possible risks

PhD (Environmental Sciences), North-West University, Potchefstroom Campus, 2019.Crop production is important to feed the growing global population. Over the last few decades, measures to improve crop production have been adopted around the world. This include the use of genetically modified maize to kill insect pests and resist glyphosate effects, (Bt and Roundup Ready) and also using various pesticides. The chemicals used in agricultural activities are mostly water soluble. These compounds therefore end up in the environment as complex mixtures. Their effects are unpredictable and they may act or interact differently when present in mixtures, than in their single capacity. The research question of this study was whether the herbicides, glyphosate and 2,4-D, and Cry proteins, from Bt maize, have endocrine disruptive effects when present in mixtures. Previous work did not address mixtures containing this specific combination of compounds that was tested in the current study. In order to obtain a mixture of the specific target compounds, the idea was to conduct a field trial in which different cultivars were planted and sprayed with different combinations of the above-mentioned herbicides. This gave a better idea of the agricultural chemicals that were introduced into the field. After the field trial, soil was collected and extracted with rainwater to target the bioavailable fraction. The endocrine potential of the mixtures containing the target compound was determined by using in vitro reporter-gene assays. The MDA-kb2 cells have both androgen (AR) and glucocorticoid (GR) receptors and were used to measure (anti)androgenic effects. H4IIE-luc cells determined the xenobiotic potential and an indirect endocrine disruptive potential of the compounds. The cells were exposed to: the single active ingredients (pure compounds) of glyphosate, 2,4-D and Cry1Ab; formulations Roundup®, and 2,4-D amine SL; environmentally relevant concentrations of the active ingredient, and formulations; as well as the rainwater extracts. The effect on cell proliferation of the same suite of compounds was assessed using the 3-(4,5-dimethythiazol-2-yl)-2,5-diphenyl tetrazolium bromide (MTT) viability assay. The results of the current study revealed that the environmental extracts that received a pre-and post-emergent Roundup® application facilitated androgen receptor binding. The testosterone equivalents (TTEQs) derived from AR activation and the dexamethasone equivalents (DexaEQs) derived from GR binding exceeded the drinking water trigger values specifically derived for bio-assays. Exceeding the trigger values act as a warning signal prompting further investigation. The environmental extracts containing 2,4-D suppressed AR activation, but only slightly, as these responses were not detected in the AR inhibition assay. The following compounds caused inhibition of the AR from strongest to weakest inhibitor: Roundup® > Cry1Ab > 2,4-D > D:RR. Most of the compounds tested were responsible for increased cell proliferation presenting evidence that they could stimulate cancer cell growth. Use of contaminated water sources can lead to chronic exposure to these mixtures and cause endocrine disrupting effects in humans or aquatic life. The findings of this study highlight the need for additional monitoring of water resources due to the effects the target compounds might pose to non-target organismsNational Research Foundation (NRF)Doctora