You can create total mixed ration with just hay and silage. Straw is not "required" for mixed feed -- I just like to use it because silage is worth so much $$ to biogas plant, and hay is a bit harder to make with the mowing+ teddering, etc. Straw is basically 'free' when you do wheat/barley, so I try to use as much as I can in my mixed feed.
Loose straw, used as bedding, increases cow productivity by about 10%. i.e. it takes you from 90 to 100% when using mixed feed, from 35 to 44% when feeding grass or silage. I think there's a line missing in what I typed above - I *think* from memory that feeding grass plus silage is 70%, and adding straw to that gives 80%.
That 10% also makes the wheat/barley fields more profitable, because the straw bi-product is so valuable. I harvest 100k wheat, and get 100k free straw - that I then use to increase my milk production. In hard mode, 1000 cows makes you 250k dollars per day, so that 100k straw is worth $25,000 ($250/t) - that's more than the wheat is worth, by giving it to cows to sleep on!
** and of course, since hard mode silage is worth 400/t to the biogas plant, this straw is clearly an outstanding 'filler' in mixed feed.