Do you need cdl for dump truck for farm use?

Couple different variables here, one being state law, and one being gross vehicle weight. If it's under 26,000 lbs. (for example, something like an F650), then no, because it would be under CDL weight.

If it's over 26,000 lbs. GVWR, then it'll depend on a number of things. If it's registered as a farm vehicle, or is unregistered and will not leave the property the farm is on, you can get by without a CDL. If it will be traveling on public roads, then it depends on what your state permits. Your state may or may not permit the driver to have a non-commercial Class B licence.

If you intend to be able to hire out the dump truck to third parties, then it needs to be registered as a commercial vehicle, have a state fuel tax or IFTA sticker, have a DOT registration number or state exemption number, and will require the driver to possess a Class B CDL. I know a lot of farmers who have dump trucks for use on their farm, but also hire them out in order to generate extra revenue for themselves.

Ultimately, you would need to contact your state's Division of Motor Vehicles and explain the full extent of what the truck's usage would be in order to get their determination.