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Lipids or fats, found in plant and animal products, are a key energy source in dairy cows' diet. They are typically low, between 2-6% of dry matter in pasture-based diets, as excess can harm rumen function and milk production. Fats come as triglycerides which have saturated or unsaturated fatty acids. Unsaturated ones, toxic to some rumen microbes, undergo biohydrogenation in the rumen to become non-toxic saturated fatty acids. When cows digest fats, they convert them into energy, milk fat, or store them as body fat. If cows have a negative energy balance, they break down body fat to maintain energy. Supplementing fats needs careful consideration due to cost and potential negative impacts.
A lipid or fat is a substance found in plant and animal products that is not soluble in water. On a weight basis, fat contains more than twice the energy content of carbohydrates.
Dietary fats are found in feedstuffs such as oilseeds (e.g. cottonseed or whole soybean) or by-products (e.g. palm kernel extract) and are present in relatively small amounts in many forages (e.g. 2 – 3 % in pasture and maize silage).
Thus, the dietary fat content for dairy cows is typically low, between 2-6% DM for most pasture-based diets. This is because dietary fat contents in excess of 8% can negatively impact on rumen function, fibre digestion, dry matter intake (DMI) and milk production.
If supplementing with fat - as with any other supplement - it is important to look at the gain in milksolids and milk revenue in return for the cost of the fat supplement. Fat supplements are generally expensive and the gains in milk revenue are rarely profitable.
Dietary fats are generally in the form of a triglyceride. A triglyceride consists of a glycerol backbone joined to 3 fatty acids (Figure 1). Theses fatty acids can be saturated (no double bonds between the carbons) or unsaturated (one or more double bonds between two carbons).
Figure 1. Triglyceride structure containing a glycerol backbone and 3 fatty acids.
The free fatty acids that are released into the rumen are either saturated (no double bonds) or unsaturated (at least one double bond). The number and position of double bonds in a fatty acid can alter the way in which the fatty acid is digested and can also alter its biological activity.
Unsaturated free fatty acids are toxic to many rumen microbes, in particular those that are involved in fibre digestion. In an attempt to detoxify these fatty acids, they undergo biohydrogenation in the rumen. Biohydrogenation is a process in which hydrogen ions are added to the carbons that contain a double bond. The additional hydrogen ions change the double bond to a single bond. When unsaturated free fatty acids are completely biohydrogenated they become saturated fatty acids.
If the lipid content of the diet is less than 6%, the biohydrogenation process can generally keep up with hydrolysis of triglycerides, and detoxify the majority of unsaturated fatty acids; however, if the cow’s diet contains too much fat, hydrolysis still occurs and releases free fatty acids, but the biohydrogenation process is overwhelmed. Many of the fatty acids will not be completely saturated and rumen function is reduced.
This results in:
Saturated fatty acids are generally found in animal fats and in some by-product feeds, such as palm kernel expeller. Because these fatty acids are already saturated, when they are hydrolysed and released into the rumen, they are not toxic to the rumen microbes and pass through the rumen unaltered, providing an energy source for the dairy cow.
In lactating cows, fats are directed to the mammary gland, where they are broken down into free fatty acids and a glycerol backbone. The fatty acids are taken up into the mammary gland, repackaged into triglycerides and secreted in the milk.
About 40% of the fat in milk comes from these circulating free fatty acids, while the remaining 60% of milk fat is synthesised within the mammary cells (de novo synthesis) from acetate and butyrate (created during carbohydrate metabolism).
If a cow is in a positive energy balance and is gaining body condition, some of the fats from dietary lipids are partitioned towards adipose tissue. As in the mammary gland, the fats are broken apart and the resulting glycerol and free fatty acids are transported into the adipose tissue. Once in the adipose tissue, they are reconnected to a glycerol backbone to form a triglyceride and stored as body fat. This process is known as lipogenesis.
The triglycerides stored in the adipose tissue are broken down to form glycerol and free fatty acids, a process known as lipolysis. The glycerol and free fatty acids are released into the blood stream and then taken up by the liver. The free fatty acids in the blood stream are termed non-esterified fatty acids or NEFAs and blood NEFA content is often used as an indicator of the energy status of the cow. Generally, the higher the blood NEFA content, the more negative the energy status of the cow.
In the liver, the glycerol contributes to glucose production, while the free fatty acids undergo a process known as ß-oxidation to provide energy for the cow. If the cow is in a severe state of negative energy balance these free fatty acids can be converted to ketone bodies, such as ß-hydroxybutyrate, which are used as an alternative energy source by a variety of tissues. Excess production of ketone bodies is indicative of the metabolic disorder ketosis.
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