Funny you should mention that! I just got blessed with a pickup truck load of old out of date large bags of dog and pet feeds from a local farm feed supply that I do business with, that has agreed to sell my homemade bagged compost. I got about $500.00 worth of old dog/cat foods! It is so cool!
My plans is first to feed my cat and dog, the best bags. Then I plan on using it as a soil amendment/fertilizer for my lawn, a protein and bacterial food ingredient for my aerated compost tea recipes, and lastly in my hot compost piles as a nitrogen activator for hotter piles.
Most cat feeds contain fish meals. Most dog feeds contain chicken or beef meals. They all are high in protein/nitrogen, calcium, phophorus, and potassium. All meat products contain calcium phosphates.
There is a little salt in these feeds. However, I don't care, because they is about the same amount of NaCl salt in animal maures, and I use horse manure all the time in my composting schemes. It hasn't harmed my no-till garden soils yet. Compost buffers soil pH, and it buffers many mild toxins and salts in soil also.
The entire Kingdom of God can be totally explained as an Organic Garden (Mark 4:26)