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Milk Composition & Synthesis
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Milk Synthesis
Milk Fat

Alterations in Milk Fat Content

Milk fat may be depressed in cows as a result of insufficient dietary energy intake, especially during early lactation. Cows in good flesh during the early lactation have higher milk fat percent than cows that enter lactation in thin condition. Most high-producing cows lose weight during this interval; therefore, energy intake should be maintained as high as possible without causing the cows to go off feed. When the loss in body fat is too rapid, ketosis can occur.

Cows fed high grain throughout the dry period will produce milk which is richer in fat and SNF for a few months postpartum than cows on a more modest feeding regimen. However, this predisposes cows to metabolic disorders and is not economical. The recommended practice is to challenge feed the dry cows only during the last 2 or 3 weeks before parturition and then increase grain during early lactation.

Low Fat % can occur because of -

Energy Shortage - Occurs between 90 and 150 days of lactation. The cow is usually thin and has a fat test at 2.5 - 3.0 %; milk protein is also reduced. Generally the ration is unbalanced

Milk Fat Depression - Occurs at any stage of lactation. Cow probably is in good body condition. Fat test is at 0.9 - 2.5 %. Cow is off-feed. Milk protein % is higher than milk fat %.

Some dietary components will markedly reduce fat percentage when fed to dairy cows without alteration of milk production. These include when feeding high grain, low roughage rations which will stimulate milk production but depresses milk fat percent. This occurs when roughage is restricted to 30% or less of the dry matter fed. Other dietary factors which contribute to milk fat depression include: feeding finely ground ( less than 1/8 inch) or pelleted diets; feeding certain oils or polyunsaturated fatty acids (cod liver oil and extracted seed oils); feeding flaked corn, heated starches or expanded grains; or grazing certain pasture plants (lush spring pastures, young oats, pearl millet).

To the right is a corn silage and grain Total Mixed Ration for dairy cows that has been separated though a series of wire meshes of different sizes. This is used to estimate the relative proportion of finely ground up to coursely gound feed material.

When fat percentage is reduced to 2% or less it is the true milk fat depression syndrome. The milk fat depression syndrome seems to be related to an imbalance of production of volatile fatty acids (VFA) in the rumen.

Diets which cause increases in milk fat include feeding protected lipids. Diets with less than 5% fat in diet inhibits rumen bacteria. Dietary unsaturated FA's are saturated by the microbes. Sodium salts of FA's can act as a detergent and kill microorganisms. Diets which can increase milk fat also include: feeding certain saturated fatty acids; feeding high roughage diets; and frequent feeding.

[For reviews on altering milk fat see Grummer, 1991, J. Dairy Sci. 74:3244; Gibson, 1991, J. Dairy Sci. 74:3258; Ney, 1991, J. Dairy Sci. 74:4002.]

Supplemental Dietary Fat

(also see Schneider et al., 1988, J. Dairy Sci. 71:2143; Middaugh et al., 1988, J. Dairy Sci. 17:3179; Sutton, 1989, J. Dairy Sci. 72:2801)

High producing cows, even when fed grain at recommended maximal levels, can remain in energy deficit, particularly during early lactation. The energy demands of high milk production combined with dry matter intake limited by rumen capacity, can compromise efforts to maximize milk yield. Increasing grain concentrate in the diet (above 60% of total diet dry matter) only leads to other problems such as acidosis, reduced fiber digestion, lowered feed intake, more frequent off-feed problems, and lowered milk fat percent. The answer would seem to be increasing the energy density of the diet. Supplementation of diets with fat (replacing a portion of the grain concentrate to maintain forage:concentrate ratios) is an acceptable approach. Added fat is only effective during early lactation, or at least in those cows producing in excess of 60 lbs. per day. The normal diet contains ~3-4 % fat, but this can be increased to 7-8 % of total diet dry matter. However, large quantities of supplemental fat can depress ruminal fermentation of fiber.

Milk fat percent should be maintained or perhaps increased if it was originally depressed because of high grain feeding. The milk fat response to supplemental fat can be highly variable, depending on the amount of supplement, physical form of the fat, and the fatty acid composition of the supplement. Supplemental fat generally does result in a slight decrease in milk protein percent. The magnitude of this depressed milk protein percent varies, but is usually up to about 0.3 % units. Fatty acid composition of milk fat is altered by supplementation of the diets with fat. Changes in milk fat fatty acid composition reflect the fatty acids supplemented to the diet. Generally there is an increase in the proportion of long chain fatty acids (saturated or unsaturated depending on the dietary source) and a decreased proportion of short and medium chain fatty acids. Most fat supplements contain long chain fatty acids.

Several other nutrient changes need to be considered when supplementing diets with fat. Adequate fiber needs to be available in the diet to stimulate ruminal fermentation. Calcium complexes with the fatty acids to form soaps, resulting in lowered availability of Ca for absorption. Calcium level in the diet should be adjusted when fats are added. And, the decrease in proportion of grain in the diet that occurs when adding fats results in lowered available energy to the rumen microorganisms, and therefore less microbial protein synthesized and made available to the cow. A good source of dietary protein that is of low rumen degradability will make up for this deficit.

The type of fat used in the diet can substantially affect the result. Seed oils extracted from plants (such as cottonseed oil, sunflower seed oil, soybean oil and cod-liver oil) have a negative effect on ruminal fermentation. An "inert" fat source will not alter ruminal fermentation. Acceptable supplemental fats include oil seeds (such as whole cottonseeds, whole soybeans, and whole sunflower seeds), tallow, hydrolyzed animal-vegetable blends, calcium salts of fatty acids, and prilled fats.

Milk Fat
Milk Composition & Synthesis