Low Milk fat syndrome/ Milk fat depression in dairy cows
Diet-induced Milk fat depression (MFD) syndrome is a prevalent problem in many dairy herds feeding high-yielding dairy cows. Milk fat depression (MFD) is characterized by a decrease in milk fat yield of up to 50% with no change in milk yield or yield of other milk components. Milk fat depression is classically observed in ruminants fed highly fermentable diets or diets high in plant oils.
Normally, about half of milk fatty acids are produced in the mammary gland from acetate and butyrate generated from rumen fermentation (Palmquist and Jenkins, 1980). Acetate is produced mainly from forage/fiber fermentation in the rumen, hence the importance of fiber digestion on supplying acetate for milk fat synthesis. About 40 to 50% of milk fat comes from dietary sources of fat and less than 10% of the cow’s mobilized fat stores (Palmquist and Jenkins, 1980). However, early lactation cows in negative energy can have a higher proportion of milk fat coming from their fat stores. It is generally believed that milk fat depression occurs when mammary milk fatty acid production (de novo synthesis) is inhibited or reduced.
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| How to correct Low milk fat in Dairy Cows |
The fat content in milk can be altered positively or negatively by dietary changes. Diets formulated to maximize milk production utilize a high proportion of concentrates and/or a high content of specific fatty acids. Sometimes, these energy-rich diets can exert negative effects on milk fat, causing low milk fat syndrome, widely known as milk fat depression (MFD).
Unsaturated fatty acids are processed into saturated fat in the rumen. MFD occurs when pathways change during the ruminal biohydrogenisation of fatty acids and produce trans-10, cis-12 CLA.
Common causes for the production of this compound include excessive intake of unsaturated fatty acids, reduced rumen pH, or the cow’s diet may be too quickly fermentable (not enough forage and too high in rapidly fermentable grains) which causes faster rumen throughput.
Milk fat is present primarily as triacylglycerols containing fatty acids with different lengths of carbon chains and different degrees of saturation. Fatty acids with a carbon chain length from 2 to 8 carbons are called short-chain, from 10 to 12 carbons are called medium-chain, from 14 to 18 carbons are called long-chain, and more than 18 carbons are called very-long-chain. These same fatty acids can be classified according to the number of double bonds in the carbon chain as saturated (no double bonds), monounsaturated (a single double bond), and polyunsaturated (more than 1 double bond). Finally, the configuration of the double bond in the carbon chain can vary. Double bonds can be present in the native cis configuration in which the hydrogen atoms are on the same side of the carbon chain or in a trans configuration in which the hydrogen atoms are on opposite sides of the carbon chain. Unsaturated fatty acids with a trans double bond, also called trans fats, are normally present in milk because they are synthesized by bacteria living in the rumen of cows.
Fat appears in milk through two major routes:
1) Fat consumed by the cow or stored in the adipose tissue is absorbed into the bloodstream. The mammary gland then moves these fats from the blood directly into milk. About half of the 16-carbon fatty acids and all longer-chain fatty acids in milk come from the diet or fat reserves.
2) The mammary gland can synthesize short- and medium chain fatty acids by pulling compounds from the blood that were produced in the rumen during fermentation of feed by resident microorganisms. When dairy cows develop MFD, the synthesis of the shorter-chain fatty acids is compromised. As a result, the proportion of fatty acids having up to 16 carbons in the milk fat decreases more dramatically than those fatty acids with 18 or more carbons (. Therefore, MFD is characterized primarily by a reduction in fat synthesis by the mammary gland. Several theories have been proposed to explain the physiology behind this reduction in fat synthesis.
The Cause of Milk Fat Depression:
The depression of milk fat synthesis in the mammary gland starts in the rumen. Most feed sources of fat for dairy cows in both forages and concentrate ingredients contain unsaturated fat and fatty acids. Because unsaturated fatty acids contain one or more double bonds along their carbon chain, they are liquid at room temperature (70° F). These liquid unsaturated fatty acids are “active” and can be toxic to some rumen bacteria. The most abundant dietary source of unsaturated fatty acid is linoleic acid (C18:2) from corn, soy, and other oil seeds. To detoxify linoleic acid and other unsaturated fatty acids they undergo biohydrogenation by rumen bacteria to mainly stearic acid (C18:0) which is a saturated fatty acid that is inactive and inert in the rumen. When the rumen microbial population is normal and stable, most of the linoleic acid is converted to stearic acid (C18:0) which results in normal milk fat . However, when the normal rumen microflora is altered or disrupted, an alternative biohydrogenation pathway can occur resulting in fatty acid intermediates being formed in the rumen such as trans-10, cis 12 conjugated linoleic acids (CLA) . The ruminal production of as little as 1 to 2 grams of the aforementioned CLA isomer can directly inhibit milk fat production in the mammary gland . Consequently, the direct cause for milk fat depression is straightforward, however, the indirect cause(s) of a disrupted/altered rumen environment is more complicated.
Milk Fat Depression Theories
Milk fat depression is generally observed when dairy cows are fed diets containing a high proportion of concentrates (particularly readily fermented sources of starch), a low proportion of forages or forages that are finely chopped, and a moderate to high proportion of unsaturated fatty acids. A more current theory is that the combination of high grain and high unsaturated fatty acids in the diet causes the microorganisms in the rumen to produce more trans fatty acids. Some of these trans fats have suppressive effects on fat synthesis in the mammary gland. This theory was confirmed after scientists induced MFD by infusing these trans fats directly into the abomasum of dairy cows. Low forage, high concentrate diets cause the rumen fluid to be more acidic, which alters the microbial population because some bacteria are sensitive to acidic conditions. The shift in rumen microflora favors accumulation of trans fatty acids that can depress milk fat synthesis after absorption into the blood.
Avoiding Milk Fat Depression:
Within a given herd, environment and nutrition are likely to explain most of the variation in bulk tank milk fat content from day to day. In hot climates, the summer months typically result in depression in milk fat concentration. Although the exact mechanisms are not entirely clear, it is thought that reductions in milk fat during hot months are the result of changes in eating patterns of dairy cows and reduced buffering capacity of saliva because of panting. It is also possible that increased body temperature during heat stress might have a direct effect on fat synthesis by the mammary gland. Therefore, proper cooling of cows is critical for producing milk in hot environments. This requires shade, forced ventilation, and evaporative cooling.
Because the two basic conditions for LMFs are the consumption of polyunsaturated fatty acids and an acidic ruminal environment, measures to minimize milk fat depression should focus on identifying the nutritional components (diet formulation or dietary management) that favor those two conditions.
Buffer and Alkalinizing Agents
Sodium bicarbonate and sesquicarbonate are rumen buffers commonly used in diets of dairy cattle to improve milk fat synthesis. Sodium bicarbonate is typically fed at 0.7%–1% of the diet dry matter to neutralize the organic acids produced in the rumen. This makes the rumen fluid less acidic, thereby minimizing the risk of MFD. Concurrent with buffers, feeding of alkalinizing agents such as magnesium oxide to increase dietary magnesium content to 0.35%–0.40% of the diet dry matter also tends to favor milk fat synthesis.
Ionophores
Diets for lactating dairy cows in many countries can be supplemented with ionophores to improve feed efficiency and minimize the risk of ketosis. In the India , the ionophore monensin is typically fed to lactating cows at 5.5–11 mg per lb of feed (12–24 ppm) on a dry basis. Because ionophores kill some of the populations of rumen microorganisms, it shifts the rumen microflora favoring populations that produce more trans fatty acids. When ionophores are combined with diets high in starch and in unsaturated fatty acids, the risk for MFD substantially increases. Therefore, it is prudent to evaluate ionophore feeding practices when dietary circumstances favor greater use of starch and fat sources.
Dietary Unsaturated Fatty Acids
Polyunsaturated fatty acids are commonly present in dairy cattle diets. Vegetable oils present in grain byproducts and oilseeds as well as fish oil from fishmeal are common sources of unsaturated fatty acids implicated with LMF. The contribution of high-fat byproducts can be more problematic given the variability in composition of some of them. Increased use of distiller’s grains from either corn or sorghum, which have between 8% and 14% crude fat, has been the culprit causing LMF on many farms. In order to minimize the risk of LMF, the amount of polyunsaturated fatty acids present in the diet should be controlled. In many cases, the total dietary fat in lactating cow rations should be less than 6% of the dry matter, and less than 3% is suggested to be unsaturated fatty acids.
Balancing Dietary Carbohydrates
Excessive amounts of fermentable carbohydrates, particularly starch, can depress rumen pH and favor LMF. Most lactating cow diets contain approximately 35%–40% non-fiber carbohydrates of which 70%–75% is from starch, with the remainder provided by sugars and soluble fiber. Excessive starch feeding (i.e., diets with > 28% starch) predisposes cows to MFD. This is particularly important when the starch source is rapidly fermentable in the rumen, such as that from extensively processed grains (steam flaking or finely ground) or grains harvested with high moisture (high-moisture corn).
On the other hand, diets that provide sufficient fiber, particularly from forages with long particle size that stimulate cud chewing and saliva production, maintain a more stable rumen environment and favor milk fat synthesis. Typical dairy cattle diets contain 40%–55% forage, and the dietary fiber (neutral detergent fiber) from forage sources should make up at least 20% of the dietary dry matter, in most cases. When high-fiber byproducts are used in the diet to replace forage fiber, it is prudent to increase the total amount of dietary fiber at the same time that dietary starch is reduced. This is because the fiber in byproducts does not have the same ability to stimulate cud chewing and salivation as forage fiber.
Feeding Management:
Although diet composition is key in preventing LMFs, feeding management and diet presentation to cows cannot be neglected. Preventing slug feeding by minimizing competition in the feed bunk is critical, which allows less dominant cows to consume feed that has not been sorted through by more dominant cows. As space in the bunk is reduced, cows increase their eating rate to compensate for less time available for eating. Also, proper presentation of diet minimizes sorting against longer particles. Although mixing of diets should preserve long particles, particularly forages, the length of long particles should not be excessive. Excessive long forage particles (usually longer than the muzzle of the cow) favor sorting against them. Use of wet ingredients, such as silages and wet byproducts (as well as feeding of molasses), tends to favor agglomeration of feed particles and prevent sorting.
Supplemental Dietary Fat:
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.