Guinea pig hay: choosing it, judging a batch, storing it, and how much they actually eat

Quick answer

Green, springy, sweet-smelling grass hay with long stems. This is the single item that decides whether a guinea pig's teeth and gut work, and it should never run out.

Hay is not bedding with a nutritional bonus. It is the main food, and the quality of the hay you buy has a bigger effect on a guinea pig's health than almost any other purchase you make.

Two separate systems depend on it. Guinea pig teeth grow continuously for life and are worn down only by the long, sideways grinding stroke that long-stem forage forces. Their hindgut fermentation only keeps moving if large fibre particles keep arriving. Pellets and vegetables satisfy neither requirement, which is why a guinea pig can be fat, full and heading for dental surgery at the same time.

• Grass hay should be available at all times and make up the great majority of what a guinea pig eats by volume.
• Judge hay with your eyes, nose and hands before it goes in the cage. Green, sweet, springy, dust-free.
• Alfalfa, also sold as lucerne, is a legume, not a grass hay. It suits growing, pregnant and nursing animals and is the wrong staple for a healthy adult.
• Mould and dust are the two things that turn hay from a health food into a cause of respiratory disease.
• Weekly weighing catches a hay problem weeks before you can see one.

:::danger
Never feed hay that smells musty, sour, or like a damp cellar, and never feed hay with grey, white or black patches, warm spots inside the bale, or visible clumping.

Mould in forage does two things to a guinea pig. The spores are inhaled into a small, easily overwhelmed respiratory tract, and the mycotoxins produced by the fungus damage the liver and gut. Guinea pigs are already among the pets most prone to fatal respiratory infection, and mouldy hay is the version of that risk you control completely.

If you have to think about whether a batch is acceptable, it is not. Throw it out. Do not shake a suspect bale near your own face either, because those spores affect people too.
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Decide in 60 seconds

Why hay and not something more convenient

The teeth never stop growing.

All twenty of a guinea pig's teeth are open-rooted and grow continuously through life. This is an adaptation to a diet of abrasive, silica-rich grasses. In a wild diet, growth and wear stay balanced automatically.

The wear only happens with the right jaw movement. Chewing long, tough stems requires a wide lateral grinding stroke that drags the upper and lower cheek teeth across each other. Chewing a pellet or a piece of cucumber uses a shorter, more vertical motion, and does not produce that grinding.

So a guinea pig fed mostly on pellets and salad still wears its incisors, which is why owners see normal-looking front teeth right up until the animal stops eating. The cheek teeth, which you cannot see without an instrument, overgrow into sharp spurs. In guinea pigs the lower cheek teeth characteristically grow inwards over the tongue and can bridge across it, physically trapping it.

The gut only moves if fibre keeps arriving.

Guinea pigs are hindgut fermenters. Bacteria in a large caecum break down cellulose that the animal cannot digest itself, and the products are packaged into caecotrophs, which the guinea pig eats directly.

Gut motility in this system depends on a steady supply of coarse, long-particle fibre stretching the gut wall. Reduce it and motility slows, gas builds, the caecal flora shifts, and appetite falls. That is the road to bloat and to gut stasis, which in a guinea pig is an hours-not-days emergency.

Particle size, not fibre percentage on a label, is what matters here. A product can be high in fibre and still fail this job entirely if the fibre has been ground and pressed.

It occupies the day.

A guinea pig in a grassland spends most of its waking hours foraging and chewing. Hay is the only food that lets a caged animal do that, and it is why hay-fed pigs are calmer and chew their housing less.

Which hay, and which is not hay

Grass hays: the staple.

Timothy, meadow, orchard grass, ryegrass, oaten, brome and cocksfoot all work. Which of these you can actually buy depends entirely on where you live. Timothy and orchard dominate North America, meadow and timothy dominate the UK and much of Europe, and oaten hay is the common option across Australia. In hot, humid regions imported hay is expensive and locally sold hay has more often been baled or stored damp, so the quality check below matters more, not less.

A mixed meadow hay with several grass species and a few edible herbs is usually more interesting to eat than a single-species hay, which helps intake.

First cut and second cut.

First cut is coarser and stemmier with more seed heads. Second cut is softer, leafier and richer. Most guinea pigs prefer second cut, and most guinea pigs need the tooth wear that first cut provides. A practical compromise is to offer a coarser hay as the main supply and use a softer one to tempt a fussy or recovering animal.

Alfalfa, also sold as lucerne.

This is a legume, not a grass. It is higher in protein and much higher in calcium.

It is genuinely useful for a growing youngster, a pregnant or nursing sow, or an underweight animal working back to condition, and in those cases it should be offered alongside grass hay rather than instead of it.

It is the wrong staple for a healthy adult. The extra calcium adds to a urinary system already prone to sludge and stones, and the extra energy adds weight the joints and heart do not need.

Dried grass products.

Products sold as dried grass or high-temperature dried forage are richer and more aromatic than hay, and useful as an appetite prompt or a treat. They are not a replacement for the physical work of chewing long hay.

Hay cubes, compressed blocks and hay-based pellets.

The fibre is ground and pressed. The chewing stroke is wrong and the particle size is wrong. Useful for transport and boarding, not as the daily supply.

Straw.

Bedding, not food. Barley straw is fine to bed on if you want it, but it is nutritionally close to empty and the stiff, hollow stems are one of the commoner causes of a corneal scratch in a guinea pig.

How to judge a batch before your pigs eat it

Do this every time, including with brands you have bought for years. Hay is an agricultural product and batches vary far more than packaging suggests.

Look.

Green through to pale green-gold is what you want. A bleached, whitish-yellow bale has been rained on or left in the sun and has lost much of its vitamin content. Brown, and especially brown with a slightly caramelised smell, means it heated in storage. Grey, white or black patches mean mould.

Smell.

Fresh hay smells sweet and grassy, a little like a summer field. Musty, sour, cellar-like or sharply ammoniacal all mean reject. Do not press your face into a suspect bale to check.

Feel.

It should be dry, cool and slightly springy, with stems that bend before they snap. Damp or warm inside a bale means microbial activity is still going on. Powdery and crumbling means it is old and will be dusty.

Shake a handful in a shaft of light.

A visible cloud is a respiratory hazard, and for a guinea pig with any history of snuffles, sneezing or eye discharge it is disqualifying.

Look for what should not be there.

Baler twine, plastic, wire and thistles are the common contaminants. Ragwort, which stays toxic after drying, is the one to learn to recognise in your own region. Barley awns and foxtail seed heads migrate into eyes, gums and skin and are worth picking out.

Pellets are the part you measure. Hay is the part you never measure and never let run out. If the pellet bowl empties fast and the hay pile does not shrink, the ratio has gone the wrong way.

How much, and how to present it

The volume rule most rescues use is a pile roughly the size of the guinea pig itself, per pig, per day. That looks like far too much until you watch how much is trampled, soiled and rejected.

Guinea pigs are fussy about hay they have sat on. Refreshing the pile at least twice a day usually raises intake more than changing brand does.

Where to put it.

The most reliable arrangement is a large, shallow tray or box filled with hay that they can sit in and eat from. Guinea pigs eat and toilet in the same place, and giving them a hay area they can occupy increases intake substantially compared with a rack on the wall.

A warning about racks and balls.

Wire hay racks, hanging hay balls and mesh feeders are a genuine injury risk. Guinea pigs get a leg, a head or a jaw through a gap and cannot free themselves, and unattended entrapment can be fatal. If you use a rack, use a solid-sided or closely woven one with no gap a limb can pass through, and check it every day.

Multiple stations.

In a bonded pair or group, one hay pile is one resource to guard. A dominant pig can quietly restrict a subordinate's intake without any visible fighting. Two or three separate hay areas remove the problem.

Never let it run out overnight.

Guinea pigs feed heavily at night. An empty rack at bedtime means many hours without fibre, and the gut notices.

Staging: what a hay problem looks like over time

Early, and easy to miss.

The guinea pig eats readily but leaves the stemmy parts and takes only the soft leaf. Pellets and vegetables go first and fast. Droppings become slightly smaller or slightly fewer. Weight on the scale drifts down while the animal still looks the same.

Established.

Food is picked up and dropped again. There is dampness under the chin or on the chest fur, which is drooling rather than spilled water. The guinea pig approaches food, tries, and turns away. Weight loss becomes visible as the shoulder blades and spine become easy to feel.

At this point the cause is usually cheek teeth that have already overgrown, and it needs an exotics vet with a proper oral speculum, not a look with a torch.

Late.

Refusing food altogether, hunched posture, no droppings, and a tongue that may be trapped under bridging molars. This is gut stasis on top of dental disease and both need treating at once.

The value of catching it early is enormous, because a dental correction on an animal that is still eating is a far smaller undertaking than one on an animal that is already in stasis.

Storage, and how good hay goes bad

Hay that arrives perfect can be ruined in a fortnight.

Never store it sealed in plastic.

Hay continues to release moisture. Sealed in a bag or a plastic box with a tight lid, that moisture condenses and mould follows. Bags the hay arrived in are for transport, not storage.

Cool, dark, dry, and off the floor.

A pallet, a slatted shelf or a crate lets air move underneath. Concrete floors and garage corners wick moisture upwards. Direct sun bleaches out vitamins.

Breathable containers.

A woven basket, a hessian sack, a slatted box, or a lidded container with drilled ventilation all work. The aim is airflow with protection from rain and rodents.

Buy for your climate, not for the discount.

A large bale is cheaper per kilogram and worthless if you cannot store it dry. In humid or monsoon regions, small quantities bought often is the better economy.

Rodents and wild birds.

Hay stored where wild rodents can reach it can be contaminated with urine. That is a real disease route, and it is a reason to keep hay in a sealed room rather than an open shed even if the container itself is breathable.

Do not try to rescue a bad batch.

Freezing, drying, sunning or picking through mouldy hay does not remove mycotoxins. Compost it.

Variation that changes the answer

Growing, pregnant and nursing animals.

Alfalfa alongside grass hay is appropriate here, and the extra calcium and protein are being used rather than stored.

Seniors and animals with existing dental disease.

They often shift towards softer second-cut hay because chewing hurts. Follow the preference to keep intake up, but treat the preference itself as a symptom and get the mouth examined, because moving permanently to soft food accelerates the overgrowth that caused it.

Guinea pigs with a stone or sludge history.

Grass hay only, no alfalfa, and pay attention to water intake as well.

Long-coated breeds.

Peruvians, Shelties and Coronets collect hay in the coat and, more importantly, around the rear end where it mats with urine and faeces. Trim the rear and check it daily in warm weather, because that combination is how flystrike starts.

Seasonal fresh grass.

Growing grass is excellent and guinea pigs love it. Introduce it in small amounts over a week or two, pick it rather than using lawnmower clippings, and never feed clippings at all, because they begin to ferment within minutes of cutting and cause serious bloat.

Hot, humid climates.

Buy smaller amounts more often, check every batch, and accept that a bag which was fine when opened may not be fine two weeks later.

An empty hay area is the thing to notice. Hay left untouched for a day, or eaten far more slowly than usual, is often the first sign of a mouth problem.

The routine that catches problems early

The daily salad matters for vitamin C, but it is a small part of the volume. If the salad is the highlight of the day and the hay is barely touched, the balance needs correcting before the teeth do.

What never to do

Do not treat hay as optional bedding that some guinea pigs happen to eat.

Do not sort usable hay out of a mouldy bale. Contamination is not confined to the visible patches.

Do not use straw, hay cubes or dried grass as the main fibre source.

Do not hang a wire hay ball or open mesh rack in the cage and leave it unsupervised.

Do not feed lawnmower clippings, ever, and do not feed grass from verges treated with herbicide or heavily used by dogs.

Do not decide that a guinea pig has simply gone off hay. Guinea pigs do not lose interest in hay for no reason, and reduced hay intake in a previously keen animal is a clinical sign until proven otherwise.

When to get help

Contact an exotics or small-mammal veterinarian the same day for any of these:

• No droppings, or a marked drop in droppings, with reduced eating
• Refusing hay while still taking soft food
• Drooling, a wet chin or chest, or food picked up and dropped
• Weight falling for two or more consecutive weeks
• Sneezing, eye or nose discharge, or noisy breathing after a dusty batch of hay
• A guinea pig found caught in a hay rack, even if it appears unhurt afterwards

Bring these to the appointment, because they shorten the diagnosis considerably:

• Your weekly weight record, ideally as a list of dates and numbers
• What hay you use, how it is presented, and roughly how much disappears per day
• A photograph of the droppings next to a coin for scale, and of the hay itself
• What is eaten first and what is consistently left behind
• Any vitamin C supplement, pellet brand and daily vegetable list
• The date of the last dental examination, and whether it was conscious or under anaesthesia

Ask specifically for the cheek teeth to be examined with a speculum. A torch and a look at the incisors misses the problem that hay is there to prevent, and normal front teeth are not evidence of a healthy mouth.