The European Union faces a consequential decision over glyphosate, one of the world’s most widely used weedkillers. The debate over the chemical pits the farm industry against campaigners who say it poses environmental and health risks. EU nations are poised to vote this week on whether farmers can keep using it for another decade. A ban could cost the French and U.K. agriculture sectors about $1 billion each. The herbicide is used in popular products made by firms such as Monsanto Co., and the EU accounts for about one-sixth of glyphosate use.