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Equipment doesn’t break when it’s sitting in the shed; it breaks in the field, often when there’s not enough time to get equipment back to the shop, run to town for a part, make the repair, and still finish the task before bad weather moves in or a marketing deadline passes.
For this reason, it’s handy to have not only commonly needed spare parts on hand but also a mobile tool kit, either in a box bolted to the tractor frame or in the cargo area of a pickup truck or ATV. This allows you to get basic tools to the equipment and make minor repairs quickly. The minimum field tool kit is a hammer, large pair of pliers, flat head and Phillips head screwdrivers, and a couple of sizes of adjustable wrenches.
If you have more room, add a grease gun, penetrating oil, pry bar, shop rags, and whatever else seems to be needed regularly; we routinely carry a set of socket wrenches. Also include some zerks, cotter pins, hitch pins, nails, nuts and bolts, and pieces of wire for jury-rigging emergency repairs; once the equipment is back in the shed you can remove the jury-rig (such as a nail for a cotter pin) and put in the proper part.