Effect of feeding on conjugated cis Δ9, trans Δ11‐octadecadienoic acid and other isomers of linoleic acid in bovine milk fats

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Abstract
Almost 2000 bovine milk fats were analysed by gas chromatography to investigate the influence of typical barn and pasture feeding of cows on trans-C18:2 isomers (with at least one trans double bond) including the conjugated linoleic acid cis Δ9, trans Δ11 (c9,t11) as well as the cis Δ9, cis Δ12 linoleic acid. Moreover, small cow herds were used to determine the influence of pasture feeding with young and older grass as well as the impact of an energy deficit or the variation of quantity and technical treatment of fed rape-seed on the content of C18:2 isomers in milk fat. The contents of trans-C18:2 (w/o c9,t11) and c9,t11 in 1756 milk fats on average amounted to 0.63% resp. 0.75%. These contents increased from barn feeding in winter with 0.46% resp. 0.45% over the transiton period in spring and late autumn with 0.66% resp. 0.76% to pasture feeding in summer with 0.87% resp. 1.20%. Milk fat samples from bulk milk obtained weekly during one year from 4 large West German milk collection areas confirmed and completed the data found for the 1756 milk fats. The percentage of linoleic acid with a mean value of 1.24 (n = 1756) varied irregularly during the different feeding periods. The content of the conjugated linoleic acid c9,t11 could be raised considerably up to triple the normal amount by different changes in feeding. However, the content of trans-C18:1 fatty acids was strongly increased as well, respectively.