A dairy cow has specific feed requirements if the cow is to produce milk to her full potential. Most dairy cattle are fed grain and forages (hay, pasture, or silage) and various supplements mixed together to form a balanced food or ration. The main requirements of a dairy ration are energy and protein sources, vitamins, and minerals along with water.
Carbohydrates are the main source of energy in dairy cattle diets. Forages (hay, silage, pasture) and grains are the main sources of carbohydrates. These include simple sugars, starch, and cellulose. Because of their rumens (stomachs) cattle are able to use cellulose as a carbohydrate unlike simple stomached animals such as pigs. Fats are another source of energy. Common sources of fat in the diet are soybeans, tallow, and whole cottonseeds. Dairy cattle only require about 5 % of fat in the diet. Any more will decrease appetite and may cause diarrhea.
Protein is needed to produce milk and to maintain body condition. Proteins are built from amino acids. Proteins are broken down by the microbes in the cow’s rumen into amino acids to turn it into forms that the dairy cow can use. Proteins are found in grains and grain by-products. Sometimes specific amino acids are added to dairy rations in order to boost the amounts available for the cow's body to use.
Vitamins are needed in small amounts to support various bodily functions. They are categorized as water soluble vitamins and fat soluble vitamins. The water soluble vitamins are the B-vitamins and vitamin C. The B vitamins are usually made by the rumen. Vitamin C is not needed by the cow as it is able to make this vitamin in its body. Fat soluble vitamins--or their precursors--need to be fed to dairy cattle. These include Vitamin A, Vitamin D, Vitamin E, and Vitamin K.
Minerals are essential to the diet and play many roles in the body. They are divided into macro minerals and micro minerals. Macro minerals are needed in larger amounts than micro minerals. Macro minerals needed by dairy cattle are: calcium, phosphorus, sodium, chlorine, magnesium, sulfur, and potassium. Micro (trace) minerals needed by dairy cattle are: cobalt, copper, iodine, iron, manganese, selenium, zinc. Usually macro minerals are given as a supplement that is mixed into the ration. Micro minerals and salt (sodium and chlorine) are given to cattle through loose granules or solid, 40 to 50 pound blocks that the cattle can lick.
Water is absolutely necessary for proper digestion and hydration. Fresh, uncontaminated water should be available to cattle at all times. Dairy cattle that are producing large amounts of milk can drink up to 25 gallons of water a day.