The main difference you want to be aware of is legume hays and grass hays. Legume hays - in particular alfalfa - are very rich in nutrients like calcium and protein and are to be limited in pigs over the age of around 6 months.
Grass hays are the good all-rounder hays, good for pigs of all ages. Different types of grass hays include:
- Meadow
- Timothy
- Orchard Grass
- Oat
- Brome
- Rye
And of course there are dried grasses such as Readigrass/Just Grass, which are usually heat-treated (dried) so lock in all the nutrients - unlike hays which are left ot dry naturally and tend to lose most of their nutrients.