Ban on triazine herbicides likely to reduce but not negate relative benefits of GMHT maize cropping

The UK Farm-Scale Evaluations (FSE) compared the effects on biodiversity of management of genetically modified herbicide-tolerant (GMHT) spring-sown crops with conventional crop management1. The FSE reported larger weed abundance under GMHT management for fodder maize2, one of three crops studied. Increased seed production may be important for the long-term persistence of these arable weeds and may benefit invertebrates, small mammals and seed-eating birds1. In three-quarters of FSE maize fields, growers used atrazine on the conventionally managed half, reflecting contemporary commercial practice3. Withdrawal of the triazine herbicides atrazine, simazine and cyanazine from approved lists of EU chemicals4 could therefore reduce or even reverse the reported benefits of GMHT maize1,2,5. Here we analyse effects of applications of triazine herbicides in conventional maize regimes on key indicators6, using FSE data. Weed abundances were decreased greatly relative to all other regimes whenever atrazine was applied before weeds emerged. Here, we forecast weed abundances in post-triazine herbicide regimes7,8. We predict weed abundances under future conventional herbicide management to be considerably larger than that for atrazine used before weeds emerged, but still smaller than for the four FSE sites analysed that used only non-triazine herbicides. Our overall conclusion is that the comparative benefits for arable biodiversity of GMHT maize cropping would be reduced, but not eliminated, by the withdrawal of triazines from conventional maize cropping.