The green tractor trundling across a Manitoba field with an empty cab looks like it’s on a collision course with Matt Reimer’s combine—until it neatly turns to pull alongside so he can pour freshly harvested wheat into its trailer. The robot tractor isn’t a prototype or top-of-the-line showpiece. It’s an eight-year-old John Deere that the 30-year-old Mr. Reimer modified with drone parts, open-source software and a Microsoft Corp. tablet. All told, those items cost him around $8,000. He said that’s about how much he saved on wages for drivers helping with last year’s harvest.