Donna, as long as it's dry food, I don't think you have to throw it away and I know Hills S/D is not the cheapest kind either, so try giving them the amount similar to two table spoonfulls at a time, and if there's still some left, pour it on TOP of the new ration, but use clean bowls. (I bet you do already) They'll eat it if they're hungry!

I buy bags of 2 kg at a time and pour some in a plastic container which contains the amount similar to two large mugs. The bag I keep in a dark, cool storage room.