The NEFA released during hydrolysis are adsorbed onto feed particles and can be exposed to further metabolism, in a process generally referred to as biohydrogenation, or directly incorporated into bacterial lipids ( Demeyer and Doreau, 1999).
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Irrespective of diet, the proportion of C6:0 to C12:0 is lower, and that of C18:0 and cis-9 C18:1 are higher in milk produced from cows in early lactation (85%), being higher for diets rich in protein, but decreased when high concentrate diets or mature forages are fed ( Harfoot and Hazlewood, 1988 Doreau and Ferlay, 1994 Palmquist et al., 2005). Milk fatty acid composition (g/100g total fatty acids)ī Total mixed ration containing (g/kg dry matter) maize silage (300), Lucerne hay (250) and concentrates (450) supplemented with 750 g/day of calcium salts of palm oil distillate.ĭ Total mixed ration containing (g/kg dry matter) maize silage (293), Lucerne silage (297) and concentrates (410).Į Total mixed ration containing (g/kg dry matter) Lucerne hay (369), steam flaked maize (282) and concentrates (349).ĭuring the onset of lactation, the energy requirements for milk production exceed nutrient intake, and cows experience a period of negative energy balance, causing the mobilisation of long-chained fatty acids from adipose tissue and incorporation into milk fat. Genetic selection for increased milk fat content also results in altered milk fatty acid composition, causing an increase in the proportion of short-chain fatty acids and a concomitant reduction in the amount of long-chained fatty acids ( Palmquist et al., 1993). (1999) concluded that while there were differences between these breeds in the concentrations of C16:0, C18:0 and C18:1, it is questionable if these are sufficiently large to be of practical importance. In a comparison of the Irish Holstein-Friesian, Dutch Holstein-Friesian, Montbeliardes and Normandes cows grazing the same pasture, Lawless et al. Comparisons of milk fatty acid composition of milk from cows of different breeds ( Table 11.5) are consistent with the activity of Δ 9-desaturase being lower and the proportion of fatty acids in milk synthesised de novo being greater for the Channel Island breeds than the Holstein ( Beaulieu and Palmquist, 1995). Milk from Jersey cows contains more fat than that from Holsteins ( Drackley et al., 2001 White et al., 2001) and the proportion of C6:0 to C14:0 of total fatty acids, has, irrespective of diet, been reported to be lower in milk from Holstein than Jersey cows ( Beaulieu and Palmquist, 1995). Even though changes in milk fatty acid composition have typically been realised by inclusion of lipid supplements in the diet, genotype is also an important factor.
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Milk fatty acid composition can be manipulated by nutritional means or through exploitation of naturally occurring genetic variation.