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Animal Production Research, MTT Agrifood Research Finland, Jokioinen, FIN 31600, Finland
* To whom correspondence should be addressed. E-mail: kevin.shingfield{at}mtt.fi.
Cis-9, trans-11, the major isomer of conjugated linoleic acid (CLA) in bovine milk fat, is derived from ruminal biohydrogenation of 18:2 (n-6) and endogenous conversion of trans-11 18:1 (vaccenic acid; VA) in the mammary gland. Most evidence to date suggests that endogenous synthesis is the major source of cis-9, trans-11 CLA, but the extent of VA desaturation is less well defined. Four lactating cows were used in consecutive 4 x 4 Latin squares to examine changes in milk fatty acid composition and secretion in response to abomasal infusions of lipid supplements enriched with cis-9, trans-11 CLA (88.8%) or VA (29.4%). Treatments were infused over 4-d, followed by a 3-d washout, during 7 d experimental periods and administered to deliver 0, 3, 6, and 12 g cis-9, trans-11 CLA/d (Expt. 1) or 0, 7.5, 15 and 30 g VA/d (Expt. 2). Infusions of cis-9, trans-11 CLA increased linearly milk cis-9, trans-11 CLA concentrations from 0.68 to 1.46 g/100 g fatty acids. Abomasal infusions of VA increased linearly milk VA and cis-9, trans-11 CLA content from 1.22 to 2.72 and 0.61 to 1.24 g/100 g fatty acids, respectively. Changes in milk fatty acid secretion indicated that 28.9% of VA was converted to cis-9, trans-11 CLA. Results provide evidence that conversion by 
9-desaturase to cis-9, trans-11 CLA in the lactating cow is independent of postruminal VA supply. In conclusion, endogenous synthesis via VA was equivalent to 
21% of the response to increases in cis-9, trans-11 CLA available for absorption.
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