Many factors influence the composition of milk, the major components of which are water, fat, protein, lactose and minerals.
Nutrition or dietary influences readily alter fat concentration and milk protein concentration. Fat concentration is the most sensitive to dietary changes and can vary over a range of nearly 3.0 percentage units. Dietary manipulation results in milk protein concentration changing approximately 0.60 percentage units. The concentrations of lactose and minerals, the other solids constituents of milk, do not respond predictably to adjustments in diet.
Milk composition and component yields also can be affected by genetics and environment, level of milk production, stage of lactation, disease (mastitis), season and age of cow.
There are various feeding management practices that can enhance levels of milk fat and protein concentration in milk. Feeding strategies that optimize rumen function also maximize milk production and milk component percentages and yield.
Normal Sources of Variation in Composition
Genetics and Environment
Table 1 contains the breed averages for percentage of milk fat, total protein, true protein and total solids. A change in milk composition using traditional breeding techniques occurs slowly, although new techniques of genetic manipulation may allow faster progress in the future. Yields of milk, fat, protein and total solids are not easily impacted by genetics; heritability estimates for yield are relatively low at about 0.25.
Table 1. Breed averages for percentages of milk fat, total protein,true protein and total solids
Meanwhile, heritability estimates for milk composition are fairly high at 0.50. Conversely, environmental factors such as nutrition and feeding management will impact yield more than the actual percent composition of the major milk constituents.
The priority placed on each genetic trait depends upon its economic or profit impact. Milk yield per cow tends to receive the most attention by producers. However, component yields should not be overlooked. Genetic selection should be directed toward increasing fat, protein and nonfat solids yields. But, because component percentages tend to have negative genetic associations with yield traits, a change in these percentages is not likely to be achieved through genetic selection alone.
Level of Production
Yields of fat, protein, nonfat solids and total solids are highly and positively correlated with milk yield. Under selection programs that emphasize milk yield, fat and protein yields also increase. However, the percentages of fat and protein in the total composition decrease.
The concept of milk component yield versus milk composition can be illustrated by comparing different bulk tank production averages with similar protein composition. If the tank average increases from 65 to 70 pounds while protein composition remains constant at 3.1 percent, an additional 0.16 pound of protein is produced per cow per day. However, if the percentage of protein increases from 3.1 to 3.2 percent while the bulk tank average production remains at 65 pounds, protein production (yield) increases by only 0.07 pound per cow per day.
Table 2. Change in milk constituents associated with elevated somatic cell counts
Stage of Lactation
The concentration of milk fat and protein is highest in early and late lactation and lowest during peak milk production through midlactation. Normally, an increase in milk yield is followed by a decrease in the percentages of milk fat and protein, while the yields of these constituents remain unchanged or increase.
Although other diseases can affect milk component content and distribution, mastitis has been the predominant disease studied. Table 2 shows the compositional changes in milk constituents associated with elevated somatic cell counts (a measure of severity of the disease). Mastitis results in a reduction in fat and casein content and an increase in whey content of milk. These changes in the milk proteins, in conjunction with alterations in lactose, mineral content and milk pH, result in lower cheese yields and altered manufacturing properties. Milk from cows with elevated somatic cell counts (greater than 500,000 somatic cells/ml) has longer coagulation time and forms weaker curds than milk from cows with lower somatic cell counts.
Milk fat and protein percentages are highest during the fall and winter and lowest during the spring and summer. This variation is related to changes in both the types of feed available and climatic conditions. Lush spring pastures low in fiber depress milk fat. Hot weather and high humidity decrease dry matter intake and increase feed sorting, resulting in lower forage and fiber intake.
While milk fat content remains relatively constant, milk protein content gradually decreases with advancing age. A survey of Holstein Dairy Herd Improvement Association (DHIA) lactation records indicates that milk protein content typically decreases 0.10 to 0.15 unit over a period of five or more lactations or approximately 0.02 to 0.05 unit per lactation.
Maximizing Rumen Function Can Increase Milk Components
There are several strategies that producers can use to enhance rumen function and the resulting milk components. Producers who use records, such as those provided by DHIA, can critically evaluate their nutrition and feeding management programs.
Feed provides the nutrients that are the precursors, either directly or indirectly, of the principal milk solids. Thus, an increase in feed intake usually results in the production of a greater volume of milk. In general, the proportional increases in fat, protein and lactose yields are approximately the same as the proportional increase in milk volume. Milk composition changes little.
It is critical to maximize feed intake of cattle so that negative energy balance is minimized during early lactation. As cows consume more energy than they use, body weight is regained, losses in body condition are minimized and cows produce milk of normal fat and protein content. Increasing feed intake, and the resulting overall increase in energy, can increase milk protein content by 0.2 to 0.3 percent.
High-producing cows should eat 3.5 to 4.0 percent of their body weight daily as dry matter. If a herd is consuming less than this, production of solidscorrected milk may be limited. Major factors that can affect feed intake include:
• Feed bunk management (keep feed bunks clean, not empty)
• Feeding frequency
• Feed sequencing
• Ration moisture between 25 and 50 percent (to optimize dry matter intake)
• Social interactions and grouping strategy of the herd
• Abrupt ration changes
• Physical facilities
• Environmental temperature
Increased feeding frequency of lowfiber, highgrain diets increases milk fat levels. The greatest increase occurs in diets of less than 45 percent forage and when grain is fed separately as in parlor feeding. When diets are fed as a total mixed ration, feeding frequency becomes less important as long as the feed remains palatable and is fed and mixed a minimum of once a day. During hot weather, more frequent feeding helps keep feed fresh and palatable.
On a dry matter (DM) basis, the minimum ratio of forage to concentrate required to maintain normal milk fat percentage is approximately 40 to 60. This ratio should serve only as a guide; other dietary factors influence the general effects that a decreased ratio has upon rumen fermentation. These effects include decreased rumen pH, increased propionic acid production and reduced fiber digestion. Obviously, type and physical form of ingredients that contribute to the forage or concentrate portion of this ratio must be considered.
The proper feeding of concentrates involves maintaining proper foragetoconcentrate ratios and nonfiber carbohydrate levels. Feeding appropriate nonfiber carbohydrate levels can improve both milk fat and protein levels, while overfeeding leads to milk fat depression of one unit or more and often increases milk protein percent by 0.2 to 0.3 unit.
Nonfiber carbohydrates (NFC) include starch, sugars and pectin. The percentage of nonfiber carbohydrate is calculated as NFC = 100 – (% Protein + % NDF + % Fat + % Ash). Depending on the digestibility of the neutral detergent fiber (NDF) present, nonfiber carbohydrates should range from 34 to 40 percent of the total ration dry matter. In most instances, a nonfiber carbohydrate level between 36 to 38 percent is considered ideal. This level is typical of diets with less than 60 percent forage. Diets with greater than 60 percent forage may be deficient in nonfiber carbohydrates.
Table 3. Grain feeding guidelines
When feeding for component changes, limit the amount of grain consumed during one feeding to 5 to 7 pounds to avoid rumen acidosis and offfeed problems that result in reduced fat content of milk. Grain feeding guidelines to maximize milk fat and protein production are provided in Table 3. Limit grain consumption to a maximum of 30 to 35 pounds per cow daily.
Manure containing large amounts of undigested corn or with a pH less than 6.0 can indicate too much grain or an imbalance of nonfiber carbohydrates in the diet. Fibrous byproducts such as soybean hulls can replace starchy grain and reduce the severity of milk fat depression in rations high in nonfiber carbohydrate.
The type of grain and processing method can have a significant impact on the site and extent of starch digestion of a particular diet and resulting milk component composition and yield (Table 4). Generally, ground, rolled, heated, steamflaked or pelletized grain increases starch digestibility and propionic acid production in the rumen. Steamflaked corn or sorghum compared to steamrolled corn or dryrolled corn or sorghum consistently improves milk production and milk protein yield. In six comparisons, steamflaked corn increased milk protein percentage and yield and decreased milk fat percentage compared to steamrolled corn. Milk fat yield remained unchanged in these trials. Twentyfour comparisons of dryrolled and steamflaked sorghum have produced similar results. These results are attributed to increased total tract starch digestibility, increased recycling of urea to the intestinal tract and increased microbial protein flow to the small intestine.
Extensive use of grains, such as wheat, that consist of a rapidly fermentable carbohydrate and overprocessing of grains can result in severe milk fat depression, offfeed problems and reduced milk yield. It is important to match carbohydrate and protein sources and to carefully monitor nonfiber carbohydrate levels in the diet to ensure proper fermentation patterns and to maximize milk component content and yield.
Table 4. Rate of rumen starch digestion asimpacted by grain type and processing method
Ration Fiber Levels
The level of fiber feeding and the physical size of fiber particles contribute to the effectiveness of a fiber source for stimulating rumination (cud chewing), buffer production (salivation) and maintenance of normal milk fat and protein composition. Feeding of finely ground forages inadequately stimulates rumination and lowers saliva production. This results in a rumen fermentation pattern that produces a higher proportion of propionic acid and, in turn, reduces milk fat percentage. In most situations, forage comprises no less than 40 to 50 percent of the total ration dry matter or should be included in the diet at no less than 1.40 percent of body weight. Cows should receive a minimum of 5 pounds of roughage (fiber) that is at least 1.5 inches long per day.
Cows require a minimum acid detergent fiber (ADF) level of 19 to 21 percent in the ration dry matter. Maintain total neutral detergent fiber (NDF) intake above 26 percent of the total ration dry matter. Provide 75 percent of the NDF as forage. Below these levels, cows are at an increased risk for acidosis, feed intake fluctuations, laminitis and rapid and extensive body condition loss especially in early lactation. Suggested guidelines for NDF intakes from forages are presented in Table 5.
Table 5. Forage and total neutral detergent fiber(NDF) intake guidelines
Table 6. Summary of feeding management practices and their potential impact on milk fat andprotein concentration
Protein Feeding Guidelines
Generally, dietary crude protein level affects milk yield but not milk protein percent, unless the diet is deficient in crude protein. Normal changes in dietary protein ranges do not consistently affect milk fat percentage. Theoretically, insufficient amounts of rumendegradable protein might result in decreased milk fat percentage if the concentration of ammonia in the rumen does not support the optimal digestion of fiber and microbial growth.
The crude protein requirement for a 1,350pound cow producing 3.6 percent milk fat ranges from 14.0 percent of total dry matter (TDM) for 50 pounds of milk to 18.0 percent TDM for 100 pounds of milk. Depending on the stage and level of production, the recommended level of undegradable intake protein (UIP) ranges from 32 to 38 percent of crude protein. Keep soluble protein between 30 to 32 percent of crude protein or about half of the degradable protein intake level.
It is essential to meet the cow's requirement for both crude protein and rumenundegradable protein to avoid a negative impact on dry matter intake and fiber digestibility. Studies of diets containing no supplemental fat show that each 1 percent increase in dietary protein, within the range of 9 to 17 percent, results in a 0.02 percentage unit increase in milk protein. The additional synthesis of protein by mammary tissue likely is linked to limiting amino acids. Table 6 (page 5) summarizes the various feeding management practices and their potential impact on milk fat and protein concentration.
Many factors can influence milk composition. This is an important point to remember when evaluating the potential to improve a herd's milk composition and component yields. Certainly, genetics plays an important role, but changes here are slow. Producers who pay attention to detail, keep disease to a minimum and adjust their management program as the seasons dictate will be in the best position to take advantage of nutrition management changes that maximize rumen function. The resulting increase in milk components should help improve their bottom line.