I myself have been very interested on the hay matter with guinea pigs. The only things I have heard of is Orchard Grass, Timothy, and Alfalfa. Alfalfa is good for pregnant or young piggies, but others shouldn't have it... something to do with the content (if anyone knows why this is so please reply) Timothy is good for all guineas, and should (if this is what you use) be provided at all times. Guinea's can eat as much hay as they want! It helps to wear down their teeth so they won't have to be grinded down!!!! (yuck yuck) Please investigate before you just buy hay, and don't just read the back of the product you pick up-everyone wants you to think their product is the best!!!!!
hope this helps
guineagal!