Can you restrict which employees use certain equipment without it being considered discrimination?

You could create certifications inside the company. For example: employees can use certain tools only after hands-on training, safety testing, and supervisor approval. A good middle ground might be creating ‘authorized operator’ levels inside the company. Anyone can qualify, but they need the training, safety sign-off, and supervised experience first.

I am backing Homeownerships reply.  You can create certification levels for equipment.  This way you are not discriminating.  If you have a specialized piece of equipment that needs a certain level of know-how so there is no damage done, then make sure you create a certification to reflect that level of expertise needed before you let someone run solo on the gear.

For instance, you said backhoe.  Therefore, create an obstacle course on a piece of your property that has certain challenges to show that the person driving can negotiate the terrain and move in restricted (stall like) areas.  you can set up skeletal set ups of PVC pipe frames, that if wrecked only cost a couple dollars to rebuild instead of someone damaging a client's horse stall to the tune of thousands.

Lay out cones that can be run over and crushed.  Set up things to be moved or lifted or scooped with the buckets. Make each level more difficult with more breakable items.

This way it becomes strictly based on the skill of the employee and they get to try and if things get damaged it's cheap to fix and start over instead of learning at the client's expense.

What about starting a task training program. That way only task trained employees can do that. You can also set it up to where only a certain number of employees can be task trIned