Quick changes in milk fat composition from cows after transition from fresh grass to a silage diet

Abstract

Two experiments were conducted to evaluate the effects on milk composition of transition from a fresh grass diet on pasture to a winter diet of mixed grass/maize silage. In trial 1, the change in milk fatty acid profile for six dairy cows in mid-lactation was followed via analysis of milk samples collected from individual cows on days 0, 2, 6 and 14 after the transition of diet. In trial 2, milk of 12 cows was analysed on days 0 and 4 after transition. In trial 1, dry matter intake and milk production did not change during the experiment. After transition, the average milk fat content increased from 43.7 g/kg on day 0 to 54.9 g/kg on day 14 and the milk fatty acid composition altered within two days. The largest changes occurred in the proportions of C14:0 (from 89 g/kg milk fat on day 0 to 117 g/kg on day 14), C16:0 (226–348 g/kg), C18:0 (110–88 g/kg) and C18:1 cis-9 (238–170 g/kg). The average content of conjugated linoleic acid (CLA) decreased from 24.3 g/kg milk fat on day 0 to 4.4 g/kg on day 14, while the proportion of the beneficial n-7 fatty acid (FA) rumenic acid declined from 0.95 to 0.85 of total CLA. Vaccenic acid contents decreased linearly with CLA contents. Two days after transition large changes had occurred, whereas the changes between day 6 and day 14 were relatively small. These results were confirmed by the results of trial 2, which also indicated that most changes actually took place within four days after transition. The milk fatty acid profile of grazing cows was more favourable from a consumer health perspective than that of silage-fed cows.

Introduction

There is nowadays much interest in adaptation of the fatty acid (FA) profile of milk fat due to consumer demands for dietary unsaturated fatty acids (UFA). UFA are favourable against coronary heart diseases (Williams, 2000). Some UFA, particularly vaccenic acid (C18:1, trans-11) (Banni et al., 2001, Turpeinen et al., 2002), omega-3 polyunsaturated fatty acids (PUFA) such as linolenic acid (C18:3) and conjugated linoleic acid (CLA), especially rumenic acid (C18:2 cis-9, trans-11) (Martin and Valeille, 2002) have shown additional positive health effects in experimental animals, such as prevention of mammary gland and skin tumours.

CLA is a term to describe a mixture of positional and geometrical isomers of linoleic acid (C18:2) with conjugated double bonds. The beneficial rumenic acid is quantitatively the most important CLA isomer in milk fat. Also vaccenic acid has been claimed to have anticarcinogenic properties, probably due to the in vivo conversion into rumenic acid (Corl et al., 2003, Turpeinen et al., 2002). Because rumenic acid and vaccenic acid are both n-7 trans fatty acids, which are naturally almost only present in milk and meat from ruminants, Ellen and Elgersma (2004) suggested to refer to them as n-7 fatty acids, or use their individual trivial names, in order to avoid confusion with other CLA-isomers or trans fatty acids in general, which are associated with negative health effects.

Reported effects on the body composition changes, notably a reduction in the fat to protein ratio, are due to the C18:2 trans-10, cis-12 (Park et al., 1999), but liver-fattening and liver-enlarging effects of this CLA isomer were reported in mice (Warren et al., 2003).

There are two pathways for the production of CLA in the dairy cow. Firstly, they are formed as intermediates through incomplete biohydrogenation of PUFA from the diet by anaerobic rumen micro organisms. This reaction is catalyzed by linoleic acid isomerase, produced by the ruminal bacterium Butyrivibrio fibrisolvens (Kepler et al., 1966). Thus, the factors that determine the amount of CLA that is available for absorption from the gastrointestinal tract are the dietary intake of C18:2 and C18:3 and rumen conditions that may affect the growth and activity of B. fibrisolvens (Dhiman et al., 1995, French et al., 2000). Secondly, CLA are formed in the intermediary metabolism by desaturation of monounsaturated fatty acids (MUFA) (Griinari et al., 2000), through endogenous synthesis in the mammary gland from vaccenic acid due to delta-9-desaturase activity (Baumgard et al., 2000). Vaccenic acid may thus serve as a precursor for CLA in the mammary gland (Salminen et al., 1998).

Animals grazing fresh grass have higher levels of CLA (Stanton et al., 1997, Kelly et al., 1998, Lawless et al., 1998, Dhiman et al., 1999) in their milk and meat than those consuming conserved forages. Pasture is a richer source of PUFA than silage (Harfoot and Hazlewood, 1997). Besides, Elgersma et al. (2003a) discovered that whereas in fresh grass 0.98 of the fat was present as esterified fatty acids (EFA), in grass silage a large proportion was present as free fatty acids (FFA). During the conservation process, PUFA in herbage that are precursors for the FA in milk may be lost. Dewhurst and King (1998) and Elgersma et al. (2003a) reported a decrease in FA content and in the proportion of α-linolenic acid in ensiled material compared to fresh grass. Therefore, the transition from summer feeding to winter feeding, i.e. from pasture to silage will decrease the intake of UFA. It is hypothesized that this will shift the ratio between the unsaturated and the saturated milk fat fraction in the direction of the saturated fraction, which implies that less CLA will be present in milk after transition.

Kelly et al. (1998) studied changes during transition from a winter diet to fresh grass, but information on the reverse process is lacking and data on the rate of change during transition are scarce. The objectives of this study were, therefore, to examine the effects of transition and the rate of changes in the profile of milk FA in individual cows during transition from fresh grass to a grass/maize silage diet.