What Do Guinea Pigs Eat? Hay, Greens, Pellets, and Vitamin C

Guinea pigs eat grass hay, and they eat a lot of it. Unlimited good quality grass hay in front of them all day, about half a cup to a cup of fresh vegetables per pig, a small measured portion of plain guinea pig pellets, and clean water is the entire diet, and the Merck Veterinary Manual sets it out in exactly that order.

The reason a guinea pig cannot simply eat what a rabbit eats is vitamin C. Guinea pigs cannot make their own, so they need a dietary source every day or they develop scurvy, and Merck’s nutrition chapter for rodents says plainly that they should be offered vitamin C fortified guinea pig pellets rather than rabbit pellets for that reason. Weight is the earliest signal that a feeding routine has drifted, so weigh your pigs on a kitchen scale once a week and write the number down. You can log a weekly weight on a Creatures profile in about ten seconds, and the guinea pig species page collects the rest of the care reading in one place.

Below is what goes in the bowl and the hay rack every day, how much of each, which foods to leave on the shelf, and the point at which a feeding problem stops being a feeding problem and becomes a call to an exotics veterinarian.

Hay is most of the diet, not a bedding material

If you get one thing right, get this one right. The main food, in Merck’s owner guidance, is unlimited good quality grass hay such as timothy or orchard grass. Humane World for Animals puts a number on it and says an unlimited quantity of fresh grass hay should make up around 80 percent of the diet, with continuous access to aid digestion. The RSPCA says good quality hay should make up the majority of the diet and should be available at all times. Merck’s professional chapter gives the most usable mental picture of the quantity involved: a guinea pig should get through roughly its own body size in hay.

Two things depend on that volume. A guinea pig’s teeth grow continuously, and Merck notes that hay is what wears them down. The long fiber also keeps the gut moving, and Merck is blunt about the alternative: too little fiber alongside too many pellets and sugars leads to gas, bloating, pain, loss of appetite, and potentially rapid decline and death.

Which hay, and the alfalfa question

For an adult, grass hay is the answer: timothy, orchard, meadow or oat. Alfalfa is a legume rather than a grass, and it carries more protein and calcium, which is where the guidance splits. Merck’s professional chapter says growing guinea pigs under six months old, along with pregnant and lactating sows, require alfalfa hay and pellets. Purdue Veterinary Medicine takes the more cautious line, treating alfalfa as acceptable only when grass hays are unavailable because the protein and calcium load can contribute to obesity and bladder stones.

Those are not really in conflict once you separate the life stages. Alfalfa is a tool for animals that are building bone or feeding babies, and grass hay is the everyday food for everyone else. If your guinea pig has a history of bladder stones or is otherwise on a restricted diet, that call belongs to your veterinarian rather than to a general guide.

One hay rule has no exceptions. The RSPCA states that you should never feed lawnmower clippings, which will upset a guinea pig’s digestive system and make it ill. Hand picked fresh grass is a different matter, and the RSPCA recommends offering it daily where you can.

Vitamin C is the fact that makes guinea pigs different

Guinea pigs share an unusual trait with humans: they cannot manufacture vitamin C, so every day’s supply has to arrive in the food. Merck’s owner guidance states it directly, and its professional chapter notes that the requirement rises for growing, pregnant, lactating and sick animals.

When the supply falls short, the result is scurvy. The picture Merck sets out is broad: loss of appetite, dental problems, swollen joints, bleeding under the skin or into organs, weakness, a rough coat, lameness, weight loss, diarrhea, sudden illness, and possibly death. Those signs are a veterinary emergency, not a cue to buy a supplement. Merck’s own treatment line is daily vitamin C by mouth or by injection as directed by a veterinarian, which is exactly where that decision belongs. This guide deliberately gives no dosing figures, because the right amount and the right form depend on the animal in front of the vet.

Prevention is the part you own, and it has two halves. The first is a guinea pig specific pellet fortified with vitamin C. The second is fresh vegetables that carry it naturally, with bell pepper doing most of the work.

Vitamin C fades in the bag

Fortified pellets lose their vitamin C as they sit. A bag can lose a substantial amount in just a few months, Merck warns, which is why it tells owners to buy fresh pellets regularly. Purdue is firmer still, recommending that you buy pellets in small quantities, check that they are freshly milled, and treat older pellets as not containing adequate vitamin C. Buy small bags, store them cool, dark and dry, and finish them promptly. The RSPCA makes a related point about the bowl itself: put out a fresh portion every day rather than topping up yesterday’s, because the vitamin C degrades over time.

Putting vitamin C in the drinking water is a popular shortcut and a poor one. Merck says supplementation in the water is not recommended, both because it demands frequent changes and because it can put a guinea pig off drinking. Purdue notes the same instability, that vitamin C breaks down in sunlight and water. Feed the vitamin, do not dissolve it, and take any supplement question to an exotics veterinarian.

Pellets: plain, guinea pig specific, and measured

Pellets are a supplement to hay, never the main course. For adults, NC State’s veterinary hospital recommends a timothy based pellet with no added fruits, nuts or seeds, and Merck says timothy based pellets with less calcium are preferable for adults while alfalfa based pellets suit young or pregnant animals.

The daily amount is one of the places published guidance genuinely varies. About two tablespoons, an eighth of a cup, is the figure on Merck’s owner page. One to two tablespoons is NC State’s number for most adults. Merck’s professional chapter is stingier at roughly one to two teaspoons, and Purdue is more generous at one to two ounces. What the four figures agree on is that this is a small measured portion rather than a full bowl. One to two tablespoons per adult per day sits in the middle of the published range, and the exact figure matters far less than whether the hay rack is always full.

Rabbit pellets are not a substitute. Merck’s rodent and lagomorph nutrition chapter says guinea pigs should be offered commercial guinea pig pellets fortified with vitamin C instead of rabbit pellets, and the reason is precisely that rabbits make their own vitamin C and their food is not formulated to supply it.

Why a plain pellet beats a colorful mix

Muesli style foods, the mixes with flakes, colored pieces, corn and seeds, invite selective feeding: the guinea pig eats the sweet, starchy bits and leaves the fibrous ones, so the ration that actually goes in is not the balanced ration on the label. A single uniform pellet removes that possibility, which is why veterinary teaching hospitals and welfare organizations consistently recommend one.

It is worth being straight about the strength of the evidence in this species. A 2016 survey of 150 owners covering 344 guinea pigs, published by Norman and Wills in the journal Animals, found that around a fifth of the animals received muesli either alone or alongside pellets, and reported no significant relationship between any individual dietary component and the presence of dental disease. The dental case against muesli is strongest in rabbits, and in guinea pigs the argument rests on the selective feeding mechanism and on the recommendations of veterinary and welfare bodies rather than on a demonstrated link. That is still a good enough reason to buy the plain pellet, and it is a better reason than a statistic that does not exist.

Fresh vegetables, every single day

Fresh produce is the natural source of vitamin C in the diet, so it is a daily job rather than a treat. Humane World for Animals recommends half a cup to one cup total per guinea pig per day, mostly leafy greens. NC State scales it to the animal at roughly half a cup per two pounds of body weight. One to two cups a day is the higher figure in Merck’s professional chapter. Half a cup to a cup per pig is a defensible everyday target, and the vet who knows your animal can move it.

The daily backbone. Leafy greens make up the bulk: Humane World lists romaine, red and green leaf lettuces, mustard or turnip greens, cabbage, kale, cilantro and parsley, and recommends adding red or green pepper and broccoli daily for vitamin C. The NC State recommendation goes further and routes most of a guinea pig’s vitamin C through bell peppers specifically, which are high in vitamin C and low in sugar, at one to two tablespoons of pepper per day. The produce list in Merck’s professional chapter is similar in shape, favoring leafy greens plus low sugar fruits such as bell peppers, cucumbers and tomatoes.

Variety is doing real work. Merck’s professional chapter cites research finding that guinea pigs offered seven to ten, or more than ten, types of produce were significantly less likely to develop uroliths, the bladder stones Purdue flags as a risk of high calcium feeding. Rotating what goes on the plate is therefore a health measure, not a garnish.

Go easy on the high calcium and high oxalate greens. Humane World notes that reducing or eliminating foods high in oxalates, naming spinach, parsley and strawberries, can help guinea pigs prone to bladder stones. The RSPCA sorts broccoli, spinach, kale, carrot pieces and apple slices into the few times a week column for their sugar or calcium content, rather than the daily one.

Change the menu slowly. Merck’s instruction is that all changes to the diet should be made gradually, and NC State adds that a guinea pig unaccustomed to greens should be introduced to them slowly to prevent diarrhea. Uneaten fresh food should come out after a few hours, Merck adds, so it does not spoil in the pen.

Fruit and treats

Fruit is sugar, and this is the other place the sources part company. Merck’s owner page allows small amounts of carrot or high fiber fruit such as apple or pear strictly as treats. Merck’s professional chapter is stricter and says other fruits, including berries and oranges, along with carrots and sugary treats, should not be fed at all.

The safe reading that satisfies both is simple. Nothing sugary is required, because bell pepper and leafy greens cover the vitamin C. If you give fruit or carrot, treat it as an occasional small piece and never as a daily item. NC State draws the same line, allowing carrots and fruits only in very small quantities, roughly a half inch piece a few times a week, and specifically advises against yogurt drops, other high sugar commercial treats, breads and cereals.

Water

Fresh water must be available at all times. Guinea pigs drink more from a bottle than from a bowl, Merck notes, though the same page warns that they frequently contaminate the nozzle, so the bottle and its spout need checking and cleaning daily. NC State recommends a bottle changed daily, and Merck suggests a small heavy ceramic dish for food that cannot be tipped. The RSPCA advises checking water twice a day and, for guinea pigs housed outdoors, preventing it from freezing.

Foods to leave on the shelf

• Iceberg lettuce. Both the RSPCA and Purdue list it among foods to avoid. Dark loose leaf lettuces are the better choice.
• Lawnmower clippings. The RSPCA says never, because they upset the digestive system.
• Seeds, nuts, dairy products and candy. Purdue names all four.
• Yogurt drops, sugary commercial treats, bread and cereal. NC State advises against them.
• Rabbit pellets as the everyday pellet. No vitamin C fortification, per Merck.
• Muesli style mixes. Selective feeding makes the eaten ration different from the labeled one.
• Potatoes, garlic, avocado, coconut and citrus fruits. The RSPCA groups these among foods that can be harmful.

Feeding through the life stages

Growing guinea pigs under six months, and sows that are pregnant or nursing, need more of everything: Merck’s professional chapter specifies alfalfa hay and alfalfa based pellets for them, and notes that the vitamin C requirement rises during growth, pregnancy, lactation and illness. Adults move to grass hay and a timothy based pellet. Older animals, and any guinea pig with a dental, urinary or weight history, should be fed to a plan their veterinarian sets rather than to a general article, and that includes this one.

Housing shapes appetite more than people expect. Guinea pigs are social animals that should not be kept alone, and hay needs to be somewhere clean, dry and always reachable rather than trodden into soiled bedding. The companion guinea pig cage size and setup guide covers floor space, hay placement and pairs. Coat type changes the grooming job rather than the menu: a long coated Sheltie, Texel or Alpaca eats exactly the same diet and simply needs hay combed out of the coat more often.

When feeding becomes a veterinary matter

A guinea pig that stops eating is an emergency, not a wait and see. The low fiber pathway, in Merck’s description, runs from gas and bloating to pain, loss of appetite, and potentially rapid decline and death, and rapid is the operative word. Call an exotics experienced veterinarian the same day if you see any of the following:

• Refusing food, or eating noticeably less than usual
• Weight loss on your weekly weigh in, or a sudden drop
• Drooling, a wet chin, or dropping food while chewing
• Droppings that are small, misshapen, or missing altogether
• Any of Merck’s scurvy signs: rough coat, lameness, swollen joints, weakness, bleeding, or sudden illness

Do not start a home treatment while you wait. Vitamin C given by mouth or injection is prescribed and dosed by a veterinarian for a reason, and an appetite loss that looks dietary can be dental, urinary or something else entirely.

Keeping the record that makes the diet visible

Diet problems in guinea pigs show up as trends before they show up as symptoms, and a weekly weight is the cheapest early warning there is. Keep three things: the weekly weight, what greens actually got eaten, and the date you opened the current bag of pellets, since vitamin C fades from the day it is milled.

That is exactly the job a Creatures profile does. Add each guinea pig once, then add a record for weights and diet notes, keep vet visits in health and medical records, and set reminders and upcoming care for the pellet reorder and the annual check. Everything lands on the animal’s own profile, and the profile tabs guide explains where each entry appears.

Frequently asked questions

What do guinea pigs eat every day?
Unlimited grass hay, about half a cup to a cup of fresh vegetables led by leafy greens and bell pepper, a small measured portion of plain guinea pig pellets, and clean water. Hay is the bulk of it, at around 80 percent of the diet by Humane World for Animals’ guidance.

Do guinea pigs need a vitamin C supplement?
They need vitamin C every day, and for most healthy guinea pigs a fortified guinea pig pellet plus daily fresh vegetables supplies it. Whether an individual animal needs a supplement, and in what form and amount, is a decision for an exotics veterinarian. Putting it in the drinking water is the one route Merck advises against, since it needs changing frequently and can reduce how much a guinea pig drinks.

Can guinea pigs eat carrots and fruit?
Only as occasional small treats, and neither is needed. Small amounts of carrot or high fiber fruit are allowed as a treat in Merck’s owner guidance, while its professional chapter advises against feeding fruits and carrots at all. NC State’s limit of a half inch piece a few times a week is a practical middle ground.

Can guinea pigs eat rabbit food?
Not as their regular pellet. Rabbits produce their own vitamin C, so rabbit food is not formulated to supply it, and Merck specifically directs guinea pig keepers to vitamin C fortified guinea pig pellets instead.

Is iceberg lettuce bad for guinea pigs?
Both the RSPCA and Purdue list iceberg lettuce among the foods to avoid. Romaine, red and green leaf lettuces and other dark leafy greens do the same job with far more nutritional value.

Can guinea pigs eat grass from the garden?
Fresh hand picked grass is good for them and the RSPCA recommends offering it daily where possible. Lawnmower clippings are not the same thing: the RSPCA says never feed them, because they upset the digestive system.

How much hay should a guinea pig get through?
Merck’s professional guidance is that a guinea pig should ingest roughly its own body size in hay, which is a useful check on whether the hay rack is genuinely full rather than topped up.

Do this next on Creatures

Feeding guinea pigs well is mostly a routine, and a routine is much easier to hold when the weights, the vet visits and the reminders live in one place instead of a notebook.

Still deciding on a name for the new arrival? The guinea pig name generator is a good place to start, and the guinea pig species page links the rest of the care library.