Milk Fat Synthesis
As a general rule of thumb, the fatty acid digestibility decreases as the fats get more saturated. For example, unsaturated fatty acids (USFA) will have higher digestibility as compared to saturated fatty acids (SFA) as shown in Figure 1. Individual fatty acids such as the oleic acid can aid the formation of micelles and help improve the digestibility of fats.
One of the reason why highly saturated fatty triglycerides have poor digestibility is because of their high melting point and low solubility. This will prevent the digestive enzymes from breaking them down.
Figure 1. Fatty acid digestibility decreases as the fats get more saturated
One of the major sites of fat production in the dairy would be the mammary gland. The synthesis of the milk fat accounts for approximately half of the energy requirements for milk production. There are many factors affecting the milk fat % such as different type of breeds, stage of lactation and the diet of dairy cow.
Milk Fat Synthesis
Milk fat is produced within the mammary gland of the cows. It originates from 3 main sources:
The direct (de novo) synthesis in the mammary gland from acetate and butyrate produced by fermentation in the rumen. This contributes roughly 40 - 50% of the total milk fat.
Fats from the cow's diet.
The mobilisation of body fat/loss of body condition
The directy (de novo) synthesis results in the formation of all the short to medium chain fatty acids (C4:0 - C14:0) and roughly half of the palmitic acid (C16:0) fatty acids found in milk. The rest of the C16:0 and longer chain fatty acids (C18 and higher) are from lipids circulating in the blood plasma which arise in turn from dietary fatty acids and from fat mobilised from body reserves (body fat).
Milk Fat Manipulation
Altering Milk Fat Percentage (%)
It is very easy to manipulate and alter the milk fat %. There are many ways to do it. Just by changing the ingredients in the cow's diet or the physical form of the diet, this will be enough to alter the milk fat %. A summary in the table below shows how some dietary factors can affect the milk fat percentage.
As a summary for the table above, high starch, low fibre rations will cause low milk fat %. Adding rumen-active oils can also lead to the production of particular trans fatty acids which are a major cause of the decrease in milk fat %.
Milk Fat Depression
What Causes Milk Fat Depression?
There are many factors that will affect the milk fat percentage. When the cow's diet contains rumen-active oils comprising of unsaturated fatty acids, this will cause milk depression. Alteration in rumen fermentation could also cause milk depression. For example, a high cereal/low fibre diets will drive down the rumen pH (more acidic), causing growth of particular strains of bacteria in the rumen. These bacteria will modify the unsaturated fatty acids in the diet (biohydrogenation process) to produce specific trans fatty acids (for example, trans-10, cis-12 conjugated linoleic acid (CLA)) which are very potent at reducing the milk fat %. The diagram below will illustrate this process.
Figure 2. Biohydrogenation pathway leading to milk fat depression