Is this a dairy cow or a beef cow? Cows dont need a certain amount of feed. Instead we like to balance rations based on pounds of protein, fiber carbohydrates and non-fiber carbohydrates then add in appropriate levels of vitamins and minerals. Grass hay provides a small amount of protein (10-14%) and legume hay (alfalfa) provides larger amounts of protein (20-24%). Both types of hay have fiber carbohydrates and this is balanced in a ration as NDF (neutral detergent fiber).
Dairy cows need protein for milk production and fetal growth. Beef cows need it primarily for fetal growth and smaller amounts of milk production. The fiber carbohydrates provide the rumen microbes with fuel to manufacture amino acids for growth and milk production.
As a general rule, cows can eat about 1.5 to 2% of their body weight in NDF and that is the way we balance the ration with a computer.
Dairy cows usually eat about 100 lbs of feed a day, beef cows about 30-50% of this amount depending on the type of feed. Dairy cows eat dry hay and haylage (fermented hay) and this account for about 30 lbs of their intake if quality is good. Beef cows would eat more hay and less starch so they could eat 15-20 lbs.
The main factor with hay is the quality. Good quality alfalfa hay is 20% protein, 30% ADF and 40% NDF when you test it at a lab (always recommended).
As a general rule they need to eat within these guidelines but it depends on the levels of the other things in their diet. I dont balance any of our cattle rations for sweet feed. The other components would be grain for energy and corn silage for energy and fiber. Hope that helps