MSU Extension

Many dietary and non-dietary factors influence milk components. In this article, we will focus on added fat, dietary protein and forage to concentrate ratio. Any variable that changes rumen fermentation can affect milk components.

Sources of milk components

Volatile fatty acids (VFAs) (acetate, propionate, and butyrate) are produced in the rumen by digestion. Digestion of concentrates yields propionate that is converted to glucose to fuel the body and leads to increased milk volume. Digestion of fiber produces acetate and butyrate. Much of the butyrate produced is converted to beta-hydroxybutyrate and used to provide energy for the rumen wall. Acetate and beta-hydroxybutyrate are used in the udder to synthesize about half of the fat in milk. The other half of milk fat is provided by the pool of fatty acids circulating in the blood that originate from dietary sources and, in early lactation, from body fat mobilization. Fat concentration is sensitive to dietary changes and can be easily altered 1.0 percentage point within 7-14 days.

Milk protein is synthesized from amino acids in the blood originating from rumen microbes or dietary derived protein. Microbial protein provides 50-75% of amino acids in the blood. Milk protein production is usually limited by the amino acid that is in shortest supply in relation to requirement. Because the cow uses many amino acids for the functioning of bodily tissues in addition to producing milk protein, milk protein is more difficult to manipulate via nutrition. Protein concentration is seldom altered more than 0.1 to 0.4 percentage points. Changes may take up to 3-6 weeks to occur.

Added fat

Extensive research has been done in the area of fat feeding to dairy cows in the last 25 years. The development of commercially available rumen by-pass (or inert) fats has given dairy producers options for additional energy sources and has also allowed a higher amount of fat in the diet. Milk composition response to added dietary fat is variable and dependent on the source of fat. Rumen available unsaturated fats primarily come from plant sources. These fats may leave the rumen as specific trans-fatty acids, which may lead to milk fat depression. Replicated studies have shown an inverse relationship between specific trans-fatty acids in milk and milk fat content and yield. A study at Cornell University proved that milk fat depression is linked to trans-10, cis-12 conjugated linoleic acid (CLA) produced in the rumen. Under some circumstances, grain feeding has been shown to increase the production of this fatty acid, which is why milk fat depression often occurs when feeding supplemental fats in combination with a high grain diet.

Added fat in the diet sometimes decreases milk protein percent. Often, higher milk yield results when feeding additional fat, so milk protein yield may not differ. A significant amount of research has tried to determine the mechanism by which dietary fat causes a dilution of milk protein. No conclusions have yet been determined.

Dietary protein

Dramatic changes in the amount and source of protein in the diet exhibit relatively little change in milk protein content, unless the diet is deficient in protein. A summary of studies done by Jenkins and McGuire (2005) showed that diets containing 15-19% crude protein, including a wide variety of sources, resulted in milk protein ranging from 2.85 to 3.27%. In 1978, Michigan State University’s Dr. Roy Emery established that protein content of milk increases only about 0.02% for each 1% increase in dietary protein. Dietary protein, when fed within a normal range, has little effect on milk fat.

Protein type and quality can affect milk protein concentration and yield. Using non-protein nitrogen (NPN) sources, such as urea, as a main source of protein can reduce milk protein by 0.1-0.3 percentage units. Rations should be analyzed for crude protein, rumen degradable protein, rumen undegradable protein, soluble protein, and possibly for amino acids.

Lysine and methionine are recognized as the two most limiting amino acids to milk protein production. Supplemental rumen-protected methionine and lysine are available commercially. Level of production responses have varied in research where methionine and lysine have been supplemented in the diet. Several research studies have demonstrated increases in milk production, feed intake, and content and yield of milk protein have been documented. Response is dependent upon many factors including parity, stage of lactation, production level, and basal levels of lysine and methionine. The NRC, 2001 recommends a balance of 3:1 of methionine to lysine. Many ration balancing programs calculate the ratio and grams of metabolizable methionine and lysine to help determine if additional supplementation of rumen-protected amino acids may have a fit in your herd’s nutrition.

Forage to concentrate ratio

In most cases, increasing the concentrate in the diet increases protein percent and yield. Limited research indicates that energy intake plays a greater role in milk protein yield rather than fiber or protein content of the diet. Energy shortage may reduce milk protein by 0.1-0.4 percentage units.  Rapidly fermentable concentrate is associated with increased milk protein content due to production increases of propionate and microbial protein. Because a minimum concentration of forage is needed to maintain rumen health, increasing concentrates above 60% of ration dry matter is not practical. Overfeeding concentrate may increase milk protein, but reduce milk fat yield and impair animal health.

Typically, milk fat percentage responds negatively to increasing concentrates in the diet. Decreasing the amount of forage in the diet will result in less acetate available to synthesize milk fat. In addition, grain feeding has been shown to enhance the microbial production of trans-10 fatty acids that cause milk fat depression.