ABSTRACT: Yields of protein and fat are positively correlated with yield of milk but increased milk yield can dilute the percentages of protein and fat in milk. Milk components can be altered through ration formulation. Fat is easier to change than protein which is easier to change than lactose. Substrates for mammary synthesis of milk components are provided by fermentation in the rumen and by digestion in the small intestine. Substrates like trans octadecenoic acids can inhibit mammary synthesis of fat. Imbalances of amino acids can lower mammary synthesis of protein. Carbohydrates affect milk yield through the supply of glucose to the mammary gland and milk protein through growth of ruminal bacteria. Fibre is needed to maintain normal rumen function. Through altered carbohydrate fermentation and decreased bacterial growth, subclinical rumen acidosis can decrease yields of milk, protein and fat. Buffers affect milk fat by increasing acetate:propionate and by decreasing ruminal synthesis and mammary uptake of trans octadecenoic acids. Rumen bacteria need degradable protein. Escape protein should contain amino acids that promote synthesis of milk protein. Balancing rations for amino acids increases mammary synthesis of protein and milk yield is increased in early lactation cows. Rations with added fat need to contain more rumen escape protein. Ionophores provide a means of increasing the ratio of protein:fat in milk
