I use the large leaves in place of cabbage in my stuffed cabbage recipes. I have eaten swiss chard for years even when it is 5 ft. tall and the leaves are 18" long. I can't eat a lot of it and I eat it cooked rather than raw because I have kidney stones.

I did a search and found this information. Don't feed it to your animals.
Is Swiss Chard Poisonous?

The intake of Swiss chard by animals such as cats, dogs, guinea pigs, and the like, in large amounts are reported to bring about many health hazards. But, the use of Swiss chard by human is not reported to be poisonous, and is noted for having diuretic, antioxidant and laxative properties. It is also known to be crucial for bone health. Several nutritional benefits are associated with Swiss chard as they are rich in nutrients such as vitamin A, E, K, C, B6, and essential elements like calcium, magnesium, iodine, Beta carotene, Chlorophyll, folate, zinc, and many more that offer added health benefits to us.

Like spinach, Swiss chard has a relatively high oxalate content. This decreases when the vegetable is cooked.

Oxalates interfere with calcium absorption, but the effect is relatively minor and not something to be worried about.

However, eating large quantities of foods with a high oxalate content can result in high oxalate concentrations in body fluids, which can result in the formation of oxalate crystals. Eventually, kidney stones and gall stones could form as a result. For this reason, people with kidney and gall bladder problems are recommended to avoid eating large quantities of oxalate-rich foods.

In this respect, eating raw Swiss chard is no different to using raw baby spinach leaves in salads.

Clare