If you add salt to water the freezing temperature will decrease. At 23 % by weight of salt the freezing temperature reaches -21 degrees C, about -6 F. Adding more salt has no more effect, it's saturated. The salt/water ratio versus temperature is linear in between.
Now sea water has 3.5% by weight of salt which would lower the freezing point to -3 degrees C or about 26 degrees F. I am sure you would not want your hogs to drink sea water. They would die. Ethylene glycol antifreeze is also deadly. Don't use it.
Sugar, which would probably not be harmful to the hogs, would also lower the feezing point.
But the amount of sugar you need is 10 times the amount of salt. Too expensive.
I recommend installing a heater unit in your hog trough.
Ethanol also lowers the freezing point, but you might not want to deal with drunken hogs.
(published on 12/05/2007)