deciding which hay!

Alfalfa and other legume hays(also includes clover) are too high in nutrients. Most importantly calcium. Adults that are not pregnant cannot use all those nutrients so they buildup either in the body or in the urine when they try to get rid of the excess. Eventually it causes problems in alot of pigs. Alfalfa also tends to be stemmier and browner than grass hays.

All grass hay like the orchard grass and timothy but there is also bluegrass, brome, and bermuda are good for guinea pigs. The first 3 are generally the most preferred by animals but that's definitely not a complete list of all grass hays. They should have unlimited amounts of grass hay and it doesn't really matter what you feed so long as they'll eat it and it's of good quality.

A 3rd group of hay is grain hay. Those are hays made from oat, wheat, barley and other grains. These make good treats but too much grain can cause bloat in guinea pigs and most grain hays are usually referred to as straw instead because they come out a golden color instead of green. They also don't have as good of nutritional values as grass hays. Grain hays (especially oat) do make good bedding for injured or pregnant horses and the guinea pigs enjoy the leftover bits that fall off the bales. The earlier the hay is cut the better it is for the pigs because it will have less grain and no mature grains.