My little Maltese gets a 2/3c cooked meal a day. He eats his cooked spinach better than my kids did. I cook for him, batches of cubed chicken with grated carrots, some peas, grn beans, brown & wild rice. Other times I switch veggies to broccoli and califlower and switch to 98% fat free grnd beef. Most of the times I use chicken. I Always put brown & wild rice in each batch too. I portion his food into 2/3c bags and freeze them. Been doing it for years too. Of course my little guy is only 8 pounds. For bigger dogs just mix the portions with their dry food. They'll love it and it's really healthy. Once a week I scramble 2 eggs using one yoke only with spinach He scarfs that up. He eats better than I do.
No salts, no spices, no grapes Thumper approves this message.. LOL
The Logic of Nature: Dogs  Horses and rabbits are herbivores, or vegetarians. They have large front incisors to grasp plant matter. Their jaws shift side-to-side to grind the plant matter on their large, flat molars. The long GI tracts of herbivores are designed to ferment these foods into usable nutrition. Omnivores such as humans and Bearded Dragons, are designed to chew & process both meat and plant matter. Dogs, cats, and snakes are carnivores, with sharp interlocking teeth designed to hold onto prey. Snakes and cats are obligate carnivores, meaning they must consume a meat diet to maintain health; whereas dogs are scavenging carnivores in addition to a meat-based diet, can consume other foods with no problems.
By nature's design, rabbits & horses were never designed to consume meat. Dogs, cats, and snakes are not designed to consume grains. So as you would expect, when you feed your pets foods they weren't designed to eat, you're asking for trouble. BUT, this does not mean they cannot consume veggies or fruits in moderation. Some dogs actually enjoy the taste of fruit.
The person who wrote the previous answer (found below) is incorrect. Dogs are not carnivores, they are omnivores. They eat meat, fruits, vegetables, and grains. You should check with your vet to see what the exact proportions might be if you are planning on replacing store bought dog food, but it is definitely ok for them to eat fruits and veges, and is, in fact, good for them. Most dog foods contain fruits, grains, and vegetables in them because of this. Only toxic fruits and veges I have come across in my research are grapes/raisins, garlic, onion, and mushrooms, but there may be more. Carrots, apples, bananas are great for dogs and in most foods/treats bought at pet stores. Be advised not to give them too much fruit or veges, as just like humans, too much can cause diarrhea or upset stomach. Here are some links to check out about foods toxic to pets from people working in the animal fields:
In actuality, spinach is one of the best vegetables you can give a dog. It is high in iron, and is especially recommended for anemic or nursing dogs.
"Previous incorrect answer: Depending on how much they eat, it probably won't hurt them if they get a small bit, but it's definitely not good for their diet. Dogs are carnivores and as such, their digestive system isn't designed to digest plant material like spinach. It can make them sick, and it's not recommended. It's best to stick to a balanced diet with only a very small amount of vegetable matter."