Feral cats can physically eat dog food and they no doubt will eat dog food if they are starving but it is not nutritionally adequate for a feral cat and therefore over the long term you would harm a feral cat. I guess people are asking the question in the title because they are concerned about feral cats starving and they have some leftover dog food. They think, let’s put it to good use.
If I am correct in my assessment then it’s a nice thought but cats and dogs have different nutritional requirements so its impractical. Although cats are, as I’m sure you know, strict carnivores and dogs are omnivores despite being classified as being in the order Carnivora, they need different petfood formulations.
Cats need a higher protein content so although some brands of dog food contain higher protein levels, normally it is not high enough for cats. Therefore a feral cat eating dog food would not have enough protein.
Taurine is an essential amino acid that cats cannot synthesise (create themselves) but they need it and therefore it has to be in their diet. It is added to commercially prepared cat foods. It is rarely added into dog food therefore to feed a feral cat with dog food for a long time may cause dilated cardiomyopathy, loss of vision and digestion problems.
Vitamin A is also a dietary element that feral and domestic cats can’t synthesise. Therefore it must be added to their diet. Dog food contains vitamin A but not to a high enough level for optimum cat food. Cats will develop possible night blindness, muscle weakness and deterioration and poor quality coats if their diet lacks sufficient vitamin A.
Arichidonic acid is a fatty acid that cats cannot manufacture themselves and therefore it must be in their diet. Dogs can create this fatty acids therefore commercially prepared dog food does not normally contain this fatty acid. If a cat ate dog food all the time they may suffer from abnormal kidney and liver values and occasionally increased skin issues.
Niacin is vitamin B3 which is also important to a cat’s diet as they can’t make it themselves. Most modern cat foods are fortified with niacin. Dogs can synthesise sufficient levels of niacin from tryptophan therefore they do not need it in their diet.
The bottom line is that you could feed feral cats for a short time with leftover commercially prepared dog food but it would not be right to do this for any length of time as it would leave feral cats deficient in essential nutrients as described above.
Note: I have referred to petmd.com for some of this article.