Milk fat is present in milk in the form of small, coated droplets of triglycerides. It forms an oil-in-water emulsion and is produced in the udder. With over 400 different fatty acids identified, milk fat is one of the most complex natural fats. Almost all of these fatty acids occur only in trace amounts; only about 15 different fatty acids occur in significant quantities of over 1%. The proportion of saturated fatty acids (without double bonds) is around 65–70%, while the proportion of unsaturated fatty acids (C18:1, C18:2, C18:3) is around 30–35%. Fatty acids with a length of up to 16 carbon atoms (C16:0) can be synthesized by the cow itself. In this so-called de novo synthesis, the even-numbered fatty acids C4:0 to C14:0 are produced in their entirety, as well as about half of the C16:0 (palmitic acid). This formation takes place mainly in the udder from the precursors acetate (acetic acid) and beta-hydroxybutyrate/butyric acid, which are produced in the rumen during the microbial digestion of feed (see Figure 1). The propionic acid formed in the rumen serves as a precursor for glucose formation and thus forms the basis for the synthesis of milk sugar (lactose). A functioning rumen flora is therefore essential for high milk yield. The other half of the C16:0 (palmitic acid) and the C18 fatty acids do not originate from the dairy cow’s own metabolism, but come directly from the feed ration or the mobilization of body fat.

Figure 1: Milk fat – origin of the fatty acids

The relationship between excessive feeding of unsaturated fatty acids and milk fat reduction (Milk Fat Depression, MFD) is described by the RUFAL value (Jenkins et al., 2009). The total amount or content of unprotected unsaturated fatty acids in the feed (“Rumen Unsaturated Fatty Acid Load,” RUFAL) provides information about the influence on rumen fermentation and thus about the potential for unwanted milk fat reduction.
The RUFAL value includes unsaturated fatty acids and is calculated as the sum of oleic acid (C18:1), linoleic acid (C18:2), and linolenic acid (C18:3). High levels of unsaturated fatty acids are often initially attributed to concentrate feed or fat powders. However, due to the amount fed daily and their high proportion in the ration, the basic feedstuffs contribute significantly to the total amount of unsaturated fatty acids in the rumen (RUFAL) (see Table 1).

Table 1: Fatty acid composition of individual feedstuffs and RUFAL values

Just 8 kg DM per day of corn or grass silage provides more than 200 g RUFAL per day. Due to their insufficient rumen protection, calcium soaps also have high RUFAL values – with almost 50% in fat or around 400 g per kg DM. Studies by Chilliard et al. (2009) showed that extruded linseed, with its high alpha-linolenic acid content, does not provide sufficient protection against biohydrogenation. At the same time, negative effects on dry matter intake and a significant, undesirable reduction in milk fat content were observed.

The intake of unprotected unsaturated fatty acids (RUFAL) should be limited to 600 to 650 g per day or 3% of dry matter. However, recent studies by Mannai et al. (2016) show that the effect of increasing RUFAL values increases continuously. There is therefore no real limit for the content or quantity of unsaturated fatty acids. It is recommended to keep the RUFAL value as low as possible to avoid negative effects on the milk fat content. Many laboratories offer the determination of fatty acids and the calculation of the RUFAL value when analyzing basic feedstuffs. This allows the risk of an unwanted reduction in milk fat to be assessed when calculating rations.