UNLIMITED Timothy Hay is recommended for guinea pigs. For young Guinea pigs under the age of 4 months and pregnant or lactating mothers, Alfalfa hay is also recommended. For those cavies, you could mix half Timothy and Alfalfa. Since Alfalfa hay is too high in calcium, it should only be given to adult cavies as an occasional treat.
According to Dr. Curt Nakamura, an exotic vet specialist at Adobe Animal Hospital in Los Altos, California, grass hay is an important part of a cavy’s diet:
“One of the most important items in the guinea pig diet is grass hay, which should be fed in unlimited quantities to both adults and baby guinea pigs. It is important to provide an unlimited source of hay because pellets do not provide enough long fiber to keep their intestines in good working order. The long fibers stimulate muscle contraction of the intestines to improve and maintain gut motility (to prevent gastrointestinal obstruction).
Chewing hay is also important. Like rabbits, the molars in guinea pigs are constantly growing and must be ground down by chewing. Constant chewing on hay promotes healthy and normal wear on their molars. Treats and chew sticks are not efficient at wearing the teeth.
Alfalfa hay is rich in protein and calcium, but when combined with pellets it doesn’t have the proper ratio of calcium and phosphorus. This can lead to improper gastrointestinal motility, such as diarrhea. It also may predispose certain guinea pigs to calcium oxalate bladder or kidney stones. Timothy hay is a better choice and is becoming more readily available. It’s important to keep your guinea pig sleek, so cut down on the amount of protein and calorie-rich pellets while feeding timothy hay.”