Prepare a concrete or compacted gravel pad slightly larger than the diameter of the tire you will be using, near the water source for your trough.
Use a reciprocating saw or a very sharp, heavy knife to remove the "bead" or sidewall on one side of the tire.
Position the tire, cut side up, on the prepared pad and center it.
Soak the pad thoroughly with water before mixing the concrete, so that it will not absorb any of the water from the concrete when poured and dry the mixture prematurely.
Mix concrete according to the directions on the bag.
Pour the concrete mix into the center hole of the tire where it sits on the pad and carefully work it under and around the opening to make good contact with both the pad and the underside of the tire around the bead. Fill about halfway to the rim of the bead.
Position the piece of scrap metal fencing--as concrete reinforcement--on top of the freshly poured concrete and continue filling the opening until it is flush or slightly above the bead.
Allow the concrete to firm up slightly, then fill the lower portion of the tire with water until a layer spills over the top of the poured concrete about ½ inch to 1 inch deep. This will protect the fresh concrete from drying out and allow a harder cure.
Keep the concrete wet for three to five days minimum. The longer it sets, the better and harder it will become when cured. Allow at least a week before using the trough to water livestock.
Fill the trough with water and allow it to overflow until the excess cement washes out of the trough; or pump out the water, rinse thoroughly, and refill.