Cargill Inc. is developing a software service that guides farmers on how to plant crops, a foray that pits the agricultural conglomerate against a host of rivals seeking to harvest reams of data to sell "prescriptive planting" technology in North America. Wayzata-based Cargill began selling the service in two U.S. states, Minnesota and South Dakota, this summer and plans a broader push over the next several years as it seeks to help farmers produce larger crops and navigate weather shifts, officials said.