What is it with celebrated US investors buying into South American farm operators? A month after Florida-based Kopernik Global Investors, run by renowned stockpicker David Iben, took a 5% holding in SLC Agricola, which controls 470,000 hectares of Brazil, BrasilAgro has lured a US heavyweight investor too. Ruane, Cunniff and Goldfarb has bought a 5.1% stake in BrasilAgro, which has a total landbank of more than 250,000 hectares – an area about the size of Luxembourg – in Brazil and Paraguay. Ruane,…