From what I gather, the common expression is "imprenditori a titolo principale", which refers to FARMERS who earn more than 50% of their income through farming and spend more than 50% of their working time in that farming activity / agricultural holding. This definition is used in EU regulations when defining people that may be entitled to financial aid: "farmerS practising farming as his main occupation". If it is not clear whether the text refers specifically to farmers, I would therefore talk about self-employed individuals but leave the sense of “main occupation”. Hope this helps.