The term “lipid” refers to a broad class of substances that are insoluble in water or other aqueous solvents, but are soluble in organic solvents such as ether, chloroform, hexane, acetone, and certain alcohols. Most normal forages, grains, and byproducts contain some amount of lipid. This discussion will focus only on those lipids that are found as part of feedstuffs or as commercial fat supplements and which provide a source of fatty acids as energy-yielding nutrients to cows. The major lipid constituents in dairy cow nutrition are:

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Triglycerides: Major lipid type found in cereal grains, oilseeds, animal fats, and byproduct feeds. Also the type of lipid making up milk fat.

 

Glycolipids: Major lipid type found in forages.

 

Phospholipids: Minor component of most feeds. Form the cell membrane of all animal cells, and the surface of milk fat globules. Also important in fat digestion in the small intestine of cows.

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Free fatty acids: Minor component of dairy feeds, but major component of certain fat supplements. Major form of transport of fatty acids in the cow’s blood (same as nonesterified fatty acids or NEFA).

 

Within each of these classes there is considerable variability due to chemical makeup that results in different physical characteristics of the fat. These are discussed in more detail below.

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Triglycerides

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The basic unit of triglycerides is a molecule of glycerol (a three-carbon alcohol) combined with three fatty acid molecules. The term "fat " usually means a triglyceride in a form of solid or semi solid at room temperature whereas the "oil" is a triglyceride in a form of liquid at room temperature. Whether something is a fat or oil will very much depend on the fatty acids that are attached to the glycerol backbone. For example, beef fat trimmed from a cut of meat is a semi-solid at room temperature, but corn oil is liquid. They both are triglycerides but the beef fat has a higher melting point because of the greater content of saturated fatty acids while the corn oil is in liquid form due to its high content of unsaturated fatty acids. Chemically, unsaturated fatty acids contain one or more double bonds linking the carbon atoms of their chain. The double bonds in the otherwise straight-chain structure of the fatty acid molecule causes a "kink". This "kink" will cause the fatty acids to be packed less tightly with one another, making the oil liquid at colder temperatures. The well balanced mix of saturated and unsaturated fatty acids allows the triglyceride the correct physical properties for a particular organism. For example, at body temperature, the triglycerides of beef muscle are mostly fluid; seed oils must remain fluid at extremely low environmental temperatures and so contain mostly unsaturated fatty acids.

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The main function of triglycerides in plants or animals is to store energy. As components of dairy cow rations, triglycerides are mostly broken down in the rumen to release the fatty acids and the glycerol with the exception of triglyceride being extremely saturated. Examples of this would be a highly hydrogenated tallow, grease or vegetable oil.

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Glycolipids

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The lipids of forage stems and leaves are primarily glycolipids, which are similar to triglycerides except they have two or more sugars linked to one position of the glycerol backbone instead of the third fatty acid. The most common are galactolipids, which have galactose (a component sugar of milk lactose) linked to the glycerol. The two fatty acids that make up the glycolipids are generally unsaturated. Glycolipids are structural components of plant tissues. In most forages, whether fresh, dry, or ensiled, glycolipids are fairly completely
broken down in the rumen of the cow.

 

Phospholipids

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Phospholipids make up the cell membranes of plants and animals. They consist of a glycerol backbone with two fatty acids attached, with the third position of glycerol attached to a phosphate group that links an organic base such as choline, ethanolamine, serine, or inositol to the molecule. In the rumen, bacteria largely remove the base group and fatty acids from the phospholipids in dietary ingredients. However, the protozoa and bacteria in the rumen also make their own phospholipids for their cell membranes. Usually, therefore, the amount of phospholipid leaving the rumen is more than the amount consumed by the cow in the diet.

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Free fatty acids

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Free fatty acids are not attached to a glycerol molecule. Fatty acids consist of a hydrocarbon chain ranging in length from 14 - 18 carbons in feeds. Marine oils such as fish oil contains longer-chain fatty acids with 20 - 24 carbon atoms in their chains. Saturated fatty acids will have all the carbon atoms linked together in a single bond whereas unsaturated fatty acids will have one or more double bonds. The first double bond is found linking the ninth and tenth carbons, while additional double bonds are located farther down the chain away from the acid head group. The most abundant fatty acid in forages is linolenic acid (18:3), which contains 18 carbon atoms with three double bonds in the chain. In cereals and oilseeds, the most abundant fatty acid is linoleic acid (18:2). Both these fatty acids are called "polysaturated" fatty acids, or PUFA, because they contain two or more double bonds. In animal fats, oleic acid (18:1) is the most common fatty acid. Oleic acid is a "monosaturated" fatty acid. In cows' milk fat, palmitic acid (C16:0) is the most common fatty acid. This saturated fatty acid is also the most abundant fatty acid in palm oil, which is where it gets its name. In ruminant milk and body fat, stearic acid (18:0) is also fairly abundant because it results from bacterial modification of dietary unsaturated fatty acids in the rumen.

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For unsaturated fatty acids, double bonds are either in "cis" or "trans" configurations. The presence of cis-configurations makes fatty acids more fluid, thus having a lower melting point than the presence of trans-configured bonds. All unsaturated fatty acids in naturally occurring plant lipids have only cis-double bonds. Trans-double bonds result from microbial modification of cis double bonds, or also result from chemical hydrogenation. As hydrogens are added to the unsaturated double bonds, they are converted to single bonds, converting unsaturated fatty acids to saturated fatty acids. The first step in these hydrogenation processes, whether microbial or chemical, is conversion from cis- to trans-orientation.

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Most natural lipids in feedstuffs are not free fatty acids. In oils, increasing free fatty acid content is a sign of rancidity. In dairy cow nutrition, nearly all long-chain fatty acids found as part of forage-derived glycolipids or the triglycerides in cereals or oilseeds will ultimately be converted to free fatty acids before they can be absorbed. In addition, fats supplemented to the diet as triglycerides, calcium soaps or salts of fatty acids, and free saturated fatty acids all will be absorbed as free fatty acids. Like triglycerides, the degree of saturation affects the physical characteristics. Saturated fatty acids are solids at environmental and body temperature, while
unsaturated fatty acids are fluid. Non-ruminant animals cannot digest saturated free fatty acids very well, while ruminants like dairy cows have evolved elaborate mechanisms to be able to digest saturated fatty acids with high efficiency. This is essential because saturated free fatty acids constitute the main type of lipid reaching the intestine for absorption from “normal” or
“natural” ruminant diets.