To avoid completely full, frozen water troughs and ensure that there is always access to open water (which is hugely important for horses any time of the year) simply fill a jug half full of salt water. It will always float and never freeze. Horses quickly catch on to pushing down this "bobber" to access open water. How simple is that?! We love it!
Just then a ginger man, dripping wet and covered head to toe in swamp mire, BURST through the door of the inn with a flopping mermaid over his shoulder. "WATER!" One of the gnarly men handed her a glass. "NOT ENOUGH!" She RAN back outside and saw the horse's water trough. "THAT'LL DO!" And she THREW the Mermaid into the through with a mighty SPLASH!