Many people feed their cows some hay in winter and it is important to be able to figure out on average how much hay is required per cow.  There are certainly a lot of assumptions we have to make but I think the general concept is important and then you can figure out how to use this for your specific herd.  I will be talking about some percentages but will try to keep them as uncomplicated as possible so I will use a 1,000-pound cow as the example although most beef cows today weigh more than 1,000 pounds. 

Dr. Glen Selk with Oklahoma State indicates that high-quality forages are fermented more rapidly in the rumen, leaving a void that the animal can refill with additional forage, and that’s why forage intake increases.  For example, low-quality forage that is less than 6% crude protein will be consumed at about 1.5% body weight or about 15 pounds per day while high-quality grass hay above 8% crude protein will be consumed at 2% of body weight or 20 pounds per day.  Cows will even consume more alfalfa and green pasture at about 25 pounds per day. 

So if we look at high-quality grass hay and a 1,000-pound cow, they will consume about 20 pounds per day at 100% dry matter. If we assume the hay is about 10% moisture, they will consume 22 pounds on an as fed basis.  Assuming 15% hay wastage, this cow would need about 30 pounds of grass hay to be hauled to the pasture and if she is in late pregnancy or nursing a calf, this would increase to about 35 pounds of hay per day.  Hopefully, this gives you some idea on calculating the amount of hay you need per cow.