Cereals refer to edible grain parts. Botanically, they are considered as fruits that grow from various kinds of cultivated grasses and have bran, germ, and endosperm. However, there are other plants with edible grains including quinoa, buckwheat, and chia.
The most commonly cultivated ones include corn, wheat, rice, barley, oats, rye, millet, soybeans, and sorghum. Although considered as grains, legumes such as soybeans, beans, peas, are not classified as cereals.
Can rabbits have cereals?
No. Do not give your rabbits cereals whether they are in their whole form or have been processed. No Honey Nut Cheerios, frosted flakes, honey bunches of oats, Froot Loops, Lucky Charms, Frosted Mini Wheats, Raisin Bran, and so on.
Not everyone is going to agree with our advice on these foods. Of course, we understand that in their whole form, these grains have vitamins, proteins, minerals, carbohydrates, and fats. Bunnies definitely require all these nutrients.
According to PETA, “feeding your rabbit cereal—like muesli, processed food made up of flaked maize, peas, pellets, grains, and seeds—can lead to tooth and tummy problems.” In their natural habitats, they hardly eat any kind of grains including legumes.
RSPCA Victoria shares similar sentiments, they say “do not feed you pet cereals, grains, nuts, seeds, corn, beans, peas, bread, biscuits, sweets, sugar, breakfast cereals, chocolate or any garden plants that are toxic to rabbits”
Although a source  states that “unsugared whole-grain cereals such as Corn Chex, Cheerios, and bite-sized Shredded Wheat” as safe for rabbits. Let us look into why were are not why are advising you to avoid them avoid them?
Why they are not good
First, they are starchy (they are high in carbohydrates and sugars) while their fiber quantity is low. On the other hand, the rabbit’s lagomorphic digestion system is designed to handle low-energy, high-fiber foods. Therefore, excessive amounts of starch can disrupt the normal cecal flora leading to cecal dysbiosis, enteritis and so on.
Secondly, fiber especially the indigestible one is very important in promoting gut motility as well as removal of any ingested fur. Remember while grooming, bunnies may ingest fur and they cannot vomit it out. Therefore, foods with little or no fiber increase the chances of furballs as well as GI stasis.
Thirdly, since they have high amounts of fats and carbs, cereals are highly calorific. Therefore, their excessive consumption may cause weight gain and obesity in your rabbits.
Technically speaking, most of these grains are not poisonous or toxic. Occasional very small amounts may be harmless if fed with other high fiber foods. However, you have seen some of the possible effects this highly calorific grains might have.
Therefore, you are advised to avoid any starchy foods including all grains and their products as well as sugar. Therefore, avoid cookies, pieces of bread, oats, breakfast cereals, corn, pasta, nuts, beans, and other seeds.