> Honest question: what would be the rationale to prevent farmers from repairing their machinery themselves instead of through the manufacturer (in this case, Deere)?

Simple: an old tractor or truck - give me a decent shop with tools, lifts and a crane, and I can repair almost everything that is broken. Some light doesn't work, engine doesn't turn on? Hand me a multimeter and I can troubleshoot it.

Modern cars, trucks and tractors are essentially computers on wheels. Good luck for average Joe Farmer troubleshooting why the hydraulic valve doesn't work? Can be everything from a faulty valve or cabin switch over broken shielding somewhere along the CAN (or other) high speed bus leading to garbled transmissions to a software bug somewhere in the control unit. The farmer can easily fix a broken component - but good luck finding the issue in the latter two failure modes, I doubt it's even possible for a small freelance shop as it requires HF communications or IT knowledge combined with specialized tooling (signal analyzer, some kind of debugging tool for the CU).

The problem there is that forcing manufacturers to open up the documentation is that this equals giving everyone and their dog including their competitors access to the "secret sauce" that is software. This is the unfortunate reality and I have no idea how to balance the right to repair for farmers willing to dive deep into complex technology with the interest of JD (or other manufacturers) that China doesn't simply dump the software and clones it outright.

Probably in a world without capitalism this might work out in favor of right to repair, but in the current economy... I'd settle for a middle groumd, giving the right for people to obtain and install spare parts on module scale, but not forcing opening up the software.

Edit: oh and that doesn't even touch the issue that old cars/trucks/tractors were designed to have ample space to work in, and that components tolerated abuse by idiots screwing around. Forget to properly clean the fuel line of gunk? No issue for an old truck, it might spew some smoke and that's it, but on the modern diesel engines with thousands of bars of pressure on the fuel rail? The tiniest amount of contaminants will brick the entire injector system if you're in bad luck...