Fatty acids in the rumen – Not everything stays the same
Fats are present in almost all feedstuffs for dairy cows but vary significantly in concentration and fatty acid composition. Particularly high levels of unsaturated fatty acids are found in grass silage, legumes, and linseed, which are rich in linoleic acid (C18:2) and linolenic acid (C18:3). In contrast, maize silage and rapeseed mainly contain linoleic acid (C18:2) and oleic acid (C18:1).
Most of these fatty acids are available in the rumen and are altered by rumen microbes. A study by Boerman et al. (2015) highlights significant changes in the fatty acid profile from ingested feed to the outflow into the small intestine (see figure). The majority of the ingested unsaturated fatty acids (oleic acid, linoleic acid, linolenic acid) do not “survive” microbial digestion in the rumen.
Fatty acid flow through the rumen
Fig. 1: Fatty acid flow through the rumen, adapted from Boerman et al. (2015)
Calcium soaps – No protection for unsaturated fatty acids
Numerous studies have examined the effects of different feed rations with and without the addition of vegetable oils, oilseeds, and calcium soaps based on palm or rapeseed. Calcium soaps do not protect unsaturated fatty acids from modification or hydrogenation in the rumen. Due to the prevailing pH conditions, they break down and release the fatty acids.
Rumen microbes convert polyunsaturated fatty acids from the feed into stearic acid – a process known as biohydrogenation. This transformation serves as a natural protective mechanism, as high concentrations of unsaturated fatty acids are toxic to many rumen bacteria. Excessive amounts of unsaturated fatty acids in the diet can impair rumen digestion and negatively affect milk fat content.
Boerman et al. (2015) found that approximately 90% of linolenic acid, 80% of linoleic acid and over 70% of oleic acid are converted into stearic acid in the rumen. In contrast, saturated fatty acids such as palmitic acid and stearic acid pass through the rumen unchanged. Compared to the fatty acid profile of the feed ration, the fatty acid composition available for digestion in the small intestine consists predominantly of stearic acid.
Conclusion:
Unprotected, unsaturated fatty acids are converted into stearic acid in the rumen through biohydrogenation. Calcium soaps also do not provide effective protection for unsaturated fatty acids. Dairy cows can efficiently digest high amounts of stearic acid and utilise it as an energy source. The use of rumen-protected fats (BEWI-SPRAY® fat powders) with sufficient levels of stearic acid provides a readily available energy source without burdening the rumen.
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