A 1,200-pound cow needs approximately 24 pounds of hay per day during the winter. When temperatures drop below 32 degrees Fahrenheit, a cow burns more fuel in order to obtain the calories necessary to maintain body heat. At that point the cow requires more hay. If a rancher's goal is to maintain the cow's weight, then the amount of hay is 2 percent of the cow's weight.
There are other factors besides weather to take into consideration when determining how much hay a cow should receive: weight of the animal, objectives of the task, quality of the hay and available forage.
A cow may be a little underweight after giving milk to a calf all summer. If she is thin more hay should be added. If the cow is with a calf she needs to add more weight, thus more hay.
The quality of the hay is important because lower-quality hays contain less protein and less energy for the cow. The availability of grass in the fields determines how much hay is required. Keep in mind that the dry, brown grass in the winter does not have the nutrition necessary to maintain a cow. A supplement must be considered in those circumstances in order to bring the protein and energy levels up.