Between September and December, soybean volumes shipped through Cargill’s river terminals in Louisiana are down 40 percent year on year. After US President Donald Trump imposed new tariffs on Chinese goods, Beijing punched back with duties on US exports including most of its $20bn in agricultural commodity sales. Soybean exports, worth $12bn in 2017, were hit hardest. The oilseed’s conquest of farm fields in the past 20 years has largely been down to US farmers gambling on China’s demand for…