Can you give guinea pig hay?

no

Absolutely. Hay is a Guinea pig's prime source of food. You need to supplement fresh grass or herbs and give them vitamin A and C (for example, with vitamin-enriched guinea pig food from the pet store), but there should always be plenty of hay. They sit on it, they borrow in it, and they eat it.

A Guinea pig's teeth never stop growing, so it is essential to feed them tough matter such as hay to help them wear their teeth out. A diet based on soft fresh food such as salads will lead to overgrowing teeth, which will cause pain and injury (and require a trip to the vet).