Adult guinea pigs need timothy hay, not alfalfa. The difference comes down to calcium: alfalfa contains 1.2–1.8% calcium compared to timothy’s 0.4–0.6%, and that gap matters for long-term urinary health. Alfalfa is appropriate only for guinea pigs under six months old, pregnant females, or nursing mothers. For everyone else, timothy or orchard grass should make up 70–80% of daily food intake.
Quick verdict:
- Timothy hay is the best choice for adult guinea pigs (6+ months, not pregnant/nursing)
- Alfalfa hay is the best choice for growing pigs under 6 months and pregnant or nursing females
- Orchard grass hay is the best choice for adults who refuse timothy or need a softer texture
At a glance
| Feature | Timothy Hay | Alfalfa Hay | Orchard Grass |
|---|---|---|---|
| Crude Protein | 7–12% | 15–20% | 7–11% |
| Calcium | 0.4–0.6% | 1.2–1.8% | 0.3–0.5% |
| Ca:P Ratio | ~2:1 | ~6:1 | ~2:1 |
| Best for | Adult guinea pigs (6+ months) | Growing pigs (<6 months), pregnant/nursing | Adults who refuse timothy |
| Biggest weakness | Some pigs find it less palatable | Excess calcium causes urinary stones in adults | Less widely available |
Timothy hay — best for adult guinea pigs
Timothy hay is a grass hay with the fiber guinea pigs need for gut function and the coarse texture that naturally files their continuously growing teeth. Adult guinea pigs require 0.8–1.2% dietary calcium for maintenance, and timothy delivers exactly that range without excess.
The dental benefit is non-negotiable. Guinea pig teeth grow throughout their lives, and constant chewing on coarse hay is what keeps them worn to a functional length. I’ve fostered pigs with overgrown incisors from soft-diet histories—those require regular vet trims under sedation.
Strengths:
- Calcium level (0.4–0.6%) matches adult maintenance needs without creating urinary mineral buildup
- Coarse fiber supports both gut motility and natural dental wear
- Widely available and affordable
Weaknesses:
- Some guinea pigs find it less tasty than alfalfa and may initially refuse it
- Quality varies by batch; reject any hay with visible mold or excessive dust
Best for: Any guinea pig over six months old that is not pregnant or nursing. This is the default hay for life.
Alfalfa hay — best for young and reproductive guinea pigs
Alfalfa is a legume hay with significantly higher protein (15–20%) and calcium (1.2–1.8%) than grass hays. That profile is appropriate for the rapid growth phase in pigs under six months and for the increased nutritional demands of pregnancy and lactation. Outside those windows, it’s a liability.
The calcium number is the issue. Adult guinea pigs eating primarily alfalfa can develop hypercalciuria — excess calcium in the urine. This doesn’t always cause immediate symptoms, but over months or years it increases the risk of bladder sludge and urinary stones, especially in males (their narrower urethra makes blockages more dangerous). According to research published in the Journal of Exotic Pet Medicine, dietary calcium above maintenance levels is a documented risk factor for urolithiasis in guinea pigs.
Strengths:
- Higher protein and calcium support rapid growth in young pigs
- Palatable — most guinea pigs prefer the taste
Weaknesses:
- Calcium content (1.2–1.8%) exceeds adult maintenance needs and contributes to urinary stone formation
- Inappropriate for long-term feeding in adults
Best for: Guinea pigs under six months old, pregnant females, and nursing mothers. Transition to timothy by six months unless there’s a specific reproductive reason to continue.
Timothy hay versus alfalfa for guinea pigs: the calcium breakdown
The AVMA exotic pet guidelines recommend grass hays for adult guinea pigs specifically because of the calcium-to-phosphorus ratio. Alfalfa’s 6:1 ratio is roughly three times higher than timothy’s 2:1. Guinea pigs cannot simply excrete excess calcium at will — it concentrates in urine, and that’s how sludge starts.
I’ve fostered pigs who came in with chronic bladder issues traced back to years on an alfalfa-heavy diet. The condition is often asymptomatic until it’s advanced. By the time you see blood in the urine or your pig straining to pee, there may already be stones that require surgical removal. Switching a young pig to timothy at the six-month mark is preventive care, not dietary fussiness.
Life-stage framework:
- 0–6 months: Alfalfa is appropriate; growing pigs need the extra calcium and protein
- 6+ months (non-reproductive adults): Switch to timothy or orchard grass
- Pregnant/nursing females: Alfalfa during pregnancy and lactation only; return to timothy afterward
- Senior pigs (4+ years): Continue timothy unless a vet advises otherwise for a specific health issue
Orchard grass hay — best for picky or senior adults
Orchard grass is another grass hay with a nutritional profile nearly identical to timothy (0.3–0.5% calcium, 7–11% protein). It’s slightly softer and often more palatable to pigs who refuse timothy. If your adult guinea pig won’t eat timothy but will eat orchard grass, that’s a fine substitution.
The ASPCA guinea pig care guide lists orchard grass as a safe primary hay for adults. The only practical downside is availability — it’s less common in pet stores than timothy, so you may need to order it online.
Best for: Adult guinea pigs who refuse timothy hay, or older pigs with minor dental sensitivity (though if dental disease is present, that’s a vet issue, not a hay-texture issue).
Common mistakes
Feeding alfalfa to adults because “my pig has always eaten it and is fine.” Asymptomatic hypercalciuria can persist for years. Bladder sludge and stones develop slowly. The absence of visible symptoms does not mean the diet isn’t creating metabolic stress. I’ve seen this pattern repeatedly in foster intakes — pigs on alfalfa their whole lives who develop urinary issues at three or four years old. Switching to timothy earlier would have been easier than managing stones later.
Assuming softer hay is better for older guinea pigs. If an older pig genuinely cannot chew hay, the problem is usually advanced dental disease — and that requires a vet, not a hay swap. Switching to alfalfa because it’s “softer” addresses a symptom while introducing a new risk (calcium overload). Orchard grass is the softer alternative if texture is the issue.
Treating all hay as nutritionally equivalent. Grass hays (timothy, orchard, meadow grass) and legume hays (alfalfa, clover) have fundamentally different mineral profiles. Choosing randomly sets up mineral imbalances. Always check what type of hay you’re buying.
When to see a vet
Any guinea pig should see an exotic-animal veterinarian (not just general practice) annually. If you’re switching hay types or your pig shows any of these signs, seek care immediately:
- Straining, crying out, or difficulty urinating
- Blood in urine (visible red or pink tinge)
- Inability to urinate (emergency — can be fatal within hours)
- Hunched posture, lethargy, or loss of appetite
- Overgrown front teeth visible when the mouth is closed
Dietary changes alone cannot reverse advanced urinary stones. Your vet may recommend ultrasound imaging and potentially surgical intervention. According to UC Davis School of Veterinary Medicine, early intervention for urolithiasis significantly improves outcomes.
The decision is life-stage, not preference
Choose hay based on where your guinea pig is in life. Growing pigs and reproductive females need alfalfa’s extra calcium. Everyone else — adult males, spayed females, neutered males, senior pigs — should eat timothy or orchard grass as their primary hay, offered unlimited and refreshed daily. Preference matters for getting your pig to actually eat the hay, but nutritional appropriateness comes first. If your adult pig refuses timothy, try orchard grass before defaulting back to alfalfa.