We 'limit-fed' hay and DDG for 12 winters; starting in 2007, when there was virtually NO hay available locally.... I saw folks payin $100 for a roll of cornstalks (after grain harvest) or CRP 'residue', released for haying in late Oct... some rolls had a pile of 1" diameter honeylocust stems left over after the cows picked through and ate what overmature grass was present. After that first year, we decided to continue.... feeding 25# hay/cow/day + whatever amount of DDG that our ration calculator indicated was needed to meet TDN and CP requirements. We had a split herd - Spring and Fall calving - which had very different nutritional requirements.
You need a reasonable nutritional analysis on hay and supplement, but there are several good ration-balancing programs (U. of MN Beef Cow Ration Calculator, and CowCulator are two we've used) that can help you tailor feeding, with consideration of breed, cow size, stage of pregnancy/lactation, desired body condition score, etc.
Mark Reynolds said, "Some think that ANY hay is better than snowballs and don't have a clue about hay analysis. (Saying any hay is better than snowballs is a real pet peeve of mine)."
I agree, wholeheartedly... I've seen way too many cattle, over the years, starving to death with a rumen full of poorly-digestible hay - and nothing else. If Crude Protein is less than 7%, the rumen microbes can't function to break down the cellulose in the plant materials... it just sits there, barely moving through the GI tract, causing decreased intake of more indigestible materials, so the cattle begin catabolizing body fat and muscle stores.