The secretion of milk fat and the composition of milk fatty acids (FA) are of great interest with regard to human nutrition, and altering them in dairy cows via dietary manipulation has gained considerable attention because of the implications for human health (Parodi, 1999).
For FA in milk, short chain FA (4 to 8 carbons) and medium chain FA (10 to 14 carbons) arise almost exclusively
from de novo synthesis, and long chain FA (LCFA; >16 carbons) are derived from the uptake of circulating lipids, while FA with 16 carbons originate from both sources (Neville and Picciano, 1997). In contrast to short and medium chain FA,
there is very little LCFA synthesized (>18 carbons) de novo by ruminants and therefore most LCFA must be ingested in the feed if these moieties are to be present in the milk (Elgersma et al., 2006; Chilliard et al., 2007).