The primary conjugated linoleic acid (CLA) isomer in milkfat, cis-9, trans-11 CLA, is a proven anticarcinogen in rodents and can be derived directly from ruminal biohydrogenation of linoleic acid. The major fatty acid in pasture, however, is alpha-linolenic acid; the biohydrogenation of which does not produce cis-9, trans-11 CLA as an intermediate. Therefore, in grazing cows, cis-9, trans-11 CLA must be produced by other sources, possibly via D9-desaturation of trans-11 vaccenic acid (TVA), a biohydrogenation product of both alpha-linolenic acid and linoleic acid. Studies were conducted to i) determine the importance of endogenous synthesis of cis-9, trans-11 CLA in pasture-fed cows and ii) increase the level of cis-9, trans-11 CLA. The first study involved abomasal infusion of sterculic oil (SO) to inhibit the activity of the D9-desaturase enzyme. Abomasal infusion of SO decreased the concentration of cis-9, trans-11 CLA in milk fat by 71% (P<0.01; 3.6 c.f. 12.1 ± 0.15 mg/g fatty acid). The presence of cis-9, C 1 0:1, cis-9, C12:1 and cis-9, C14:1 in the milk fat of cows following SO treatment indicated D 9-desaturase was not completely inhibited. Using the changes in these fatty acids to evaluate the extent of D9-desaturase inhibition with SO treatment, an estimated 87-100% of cis-9, trans-11 CLA in milk fat was of endogenous origin. In the second study, sunflower oil at 4% DMI/cow/day invoked a 38% increase (P<0.01) in milk fat TVA (61.3 c.f. 44.4 ± 0.51 mg/g fatty acid) and a subsequent 28% increase (P<0.01) in cis-9, trans-11 CLA concentrations compared to control (28.5 c.f. 22.3 ± 0.19 mg/g fatty acid). Results demonstrate that endogenous synthesis is the major source of cis-9, trans-11 CLA in pasture-fed dairy cows and levels can be enhanced by increasing the supply of the ruminally-derived precursor, TVA.

JK, Kay, TR Mackle, MJ Auldist, NA Thomson, and DE Bauman

Proceedings of the New Zealand Society of Animal Production, Volume 62, Palmerston North, 12-15, 2002
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