Sorry, I was in a hurry when I made my last post.
For comparison:
All straws have the same DM (dry matter) content.
Barley straw has 5.8 MJ/Kg (Megajoules per kilogram) DE (digestable energy), and is 3.4% cp (crude protein).
Oat straw has 6.2 MJ/Kg DE, and is 3.4% c p.
Rye straw has 5.8 MJ/Kg DE, and is3.7% c p.
Wheat straw has5.8 MJ/Kg DE, and is 3.7% cp.
Early cut grass hay is around 8.5 MJ/Kg DE, and 13% cp; while
Late cut grass hay is around 8.0 MJ/Kg DE, and 8.5% cp.
These figures are all dependant on the quality of the fodder, ie the maturity of the crop, and time of harvesting. They are to be considered as guides or general indicators. Source: Veterinary Notes For Horse Owners: Captain M. Horace Hayes FRCVS.
Incidentally, the book has an excellent chapter on the feeding of horses, and shows how to work out the amount of feed required in relation to the workload on the horse. As well as those outlined above, there are values given for a large array of feedstuffs.