The short answer for an adult guinea pig
Timothy hay is the staple to build the bowl around for a healthy adult guinea pig. Its stems are long and fairly coarse, so a pig has to work its jaws side to side to chew them, and that grinding is exactly what keeps continuously growing teeth at a workable length. The fibre also keeps food moving steadily through the gut. A guinea pig should be able to reach clean hay at any hour, not just at set meals, because grazing little and often matches how its digestion is built to run.
When a different hay fits better
Not every guinea pig is in the same life stage, and that changes what belongs in the rack. Young, growing guinea pigs and pregnant or nursing sows have higher energy and calcium needs, so a richer legume hay such as alfalfa is often mixed in or offered alongside grass hay for that period, then eased back once the pig is a settled adult. Orchard grass and meadow hay are useful grass-hay alternatives for an adult who turns away from timothy or for an owner who finds timothy dusty to handle. They are softer and sweeter smelling, and a picky pig will sometimes eat noticeably more of them, which matters more than chasing a single perfect variety.
What does not change across those choices is the role of hay itself. Pellets and vegetables sit on top of a hay-based diet; they do not replace it. If your guinea pig fills up on pellets and only nibbles hay, tooth wear and gut movement both suffer even though the bowl looked full.
How to serve it so your pig actually eats it
Put hay in more than one place. A rack keeps some strands clean, but many guinea pigs eat more from a loose pile on the floor of the enclosure where they can burrow in, choose stems, and graze while they rest. Offer at least a body-sized pile per pig and top it up rather than waiting for it to vanish, because the last dusty remnants at the bottom are the part pigs leave behind. Remove soiled, damp hay when you refresh, since wet hay quickly smells stale and a pig will avoid the whole area around it.
Store the bale or bag somewhere dry and airy, off a cold floor, and shake out each handful before serving to loosen dust. Hay that smells sweet and looks green to faded gold is inviting; hay that smells musty, feels damp, or shows dark mould patches should be thrown away rather than picked through, because a guinea pig will eat around bad stems only if it has a clean alternative.
Signs the hay routine needs a rethink
Watch how much disappears overnight. A sudden drop in hay eating, droppings that shrink or slow, selective eating of only the soft leafy bits, or food being taken into the mouth and dropped can all point to tooth discomfort or a gut that is not moving well. Weight loss in a guinea pig who still rushes to the vegetable bowl but ignores hay is another pattern worth taking seriously. Those changes call for a prompt call to a veterinarian familiar with guinea pigs rather than a wait-and-see week, because guinea pigs hide illness well and go downhill quietly. Meanwhile, note when the change started and what hay you were feeding, so you can describe the pattern clearly at the appointment.
Keeping the staple this steady is easier when the refresh is a named job instead of a vague intention, and KinsLe Care Teams lets you set the twice-daily hay top-up as a shared duty with a timestamp when it is done.