Both dogs and cats are known to eat grass on occasion, though it's more common in dogs. When they suffer from gas or other mild gastrointestinal issues, they may seek out grass to remedy the situation. When grass is consumed, the blades tickle the throat and stomach lining, which usually results in vomiting. This can actually be beneficial, as the grass absorbs any impurities that may be causing the dog discomfort.
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For more about why companion animals eat grass, please see our blog post on this topic.