Why do cats eat grass?
This is something that almost all cat owners ask themselves, and a question which nobody, including vets, have a clear answer to. However, one thing is certain - grazing is something that comes naturally, not just to domestic cats but also to feral and wild cats.
Eating grass isn't for the nutritional value. Grass is pretty indigestible to cats, and low in nourishment anyway. Grass is mainly fibre and the cat's stomach doesn't have the enzyme needed to digest it. However, grass can help a cat's digestive process by inducing regurgitation of undigested matter. For example, feral cats might nibble on grass blades between mouse snacks. From a cat's point of view a dead mouse is a fiddly thing, and it is impossible for the cat to septe fur and bones from the meat. Therefore the cat gulps down the entire mouse. Once the meat has been digested the hair and bones remain in the cat's stomach. Eating grass makes the cat vomit, and this brings the grass back up, now neatly wrapped around undigested mouse parts. This is probably safer for the cat than passing spiky little bones through its intestines, which might get punctured or blocked.
Domestic cats, especially exclusively indoor cats, will seldom have the chance to digest live prey (your ankles don't count). But even these cats still will nibble on grass if it is provided - and it should be. Remember that cats groom themselves extensively, and their tongues are equipped with little hooks which scoop up loose hair. However, the disadvantage of this is that cats end up swallowing a considerable proportion of the hair they lick off their coats. Hair is not digestible and it can bundle up in the stomach to create a furball. Eating grass may aid in removing the furballs through vomiting before they become unmanageable. A furball on the carpet is no-one's favourite sight, but it is infinitely better than a blockage in the intestines which is painful and traumatic for human and cat and often needs surgical intervention to remove.
Those who make a profession of watching wild cats have noted that when prey is killed and ingested, the first bit of the carcass to be eaten is the intestine. The explanation often given for this is that the intestines contain lots of nutrients needed for good health. That may be also partially true for grass. Although mainly an indigestible fibre lacking almost everything the cat needs in its diet, grass does have a lot of moisture, some trace minerals and the vitamins A and D. Grass also contains chlorophyll, which before the discovery of antibiotics, was a remedy for pain, infection, ulcers, skin diseases, and anemia. Cats, like most other animals, are very skilled in controlling their needs for particular nutrients and finding healthy diet supplements, some of which can come from grass.
Finally, cats may nibble on grass blades for the same reason as country yokels do it. It is quite palatable, and they may enjoy the taste.
Some cat owners make the mistake of preventing their cat from eating grass. There are various reasons for this, one being that it makes their cat vomit. Since most cats make sicking up their stomach contents a major dramatic production, it is hard for the distressed human to understand that eating grass is done precisely to achieve this effect. It is actually highly advisable that cats have access to grass. Many florists and pet stores now offer grass in pots or as seeds which you can grow indoors. Outdoor cats will probably choose the garden lawn as their grass snack bar, so make sure that the grass does not have weed killers or other pesticides which might be harmful. Indeed it may be safer to have grass available in the house even if your cats are outdoor cats, so they have a source of healthy grass.
For whatever reason, or combination of reasons, cats like eating grass. If the cat can't get grass she may try your house plants for taste instead. This could be dangerous because some house plants and flowers are toxic to cats, whilst other sources of fibre, including fabric and wiring are also likely to case distress to human and cat alike. At the same time if a cat starts bingeing on grass followed by frequent vomiting, it is definitely time to consult the vet.