Long-term effects of abomasal infusion of linoleic and linolenic acids on the enrichment of n-6 and n-3 fatty acids into milk fat of lactating cows

Abstract

Our objective was to compare abomasal infusions of linoleic (18:2n-6) and α-linolenic (18:3n-3) acid on the incorporation of n-6 and n-3 fatty acids (FA) into the milk fat of dairy cows and to evaluate their potential carryover effects. Six rumen-cannulated multiparous Holstein cows (252 ± 33 DIM) were fed the same diet and assigned to 1 of 2 treatments in a completely randomized design with repeated measures. Treatments were abomasal infusions (67 g/d total FA) of (1) n-6 FA blend (N6) to provide ∼43 g/d 18:2n-6 and 8 g/d of 18:3n-3; or (2) n-3 FA blend (N3) providing 43 g/d 18:3n-3 and 8 g/d 18:2n-6. The treatment period lasted from d 1 to 20, and the carryover period lasted from d 21 to 36. Compared with N6, the N3 treatment increased the yields of total n-3 FA, 18:3n-3, and eicosapentaenoic acid (20:5n-3) from d 4 to 20, decreased total n-6 FA from d 8 to 20 and 18:2n-6 from d 8 to 16, and tended to decrease arachidonic acid (20:4n-6) from d 12 to 16. During the treatment period, n-3 FA transfer efficiency to milk for N3 was 47%, whereas n-6 FA transfer efficiency to milk for N6 was 39%. A similar pattern was observed for milk FA content, with N3 increasing the contents of total n-3 FA, 18:3n-3, and 20:5n-3 from d 4 to 20; and decreasing total n-6 FA from d 4 to 20, 18:2n-6 from d 4 to 16, and 20:4n-6 from d 12 to 16 compared with N6. We only observed positive carryover effects for N3, with the treatment increasing or tending to increase the yield of n-3 FA until d 28 and increasing the content of total n-3 FA until d 26. We observed no carryover effects for N6. In addition, we did not detect 22:6n-3 either during the treatment or carryover periods. In conclusion, abomasally infusing N3 and N6 for 20 d increased the yields and contents of n-3 and n-6 FA in milk fat, respectively. Interestingly, the increases were more pronounced in n-3 than in n-6 milk FA, with a transfer efficiency of 47% and 39% during the treatment period, respectively. Furthermore, a positive carryover effect was observed only for n-3 FA.

Summary: Mammals do not synthesize essential fatty acids (FA) vital for proper physiological functioning; thus, these FA must be provided in the diet. The essential FA linoleic (18:2n-6) and a-linolenic acids (18:3n-3) are partially converted to longer-chain FA: 18:2n-6 to arachidonic acid (20:4n-6), and 18:3n-3 to eicosapentaenoic acid (20:5n-3) and docosahexaenoic acid (22:6n-3). We compared abomasal infusions of 18:2n-6 (N6) and 18:3n-3 (N3) on the incorporation of essential FA into the milk fat of cows. Infusions lasted 20 days, with subsequent carryover evaluation until day 36. During the treatment period, n-3 FA transfer efficiency to milk for N3 was 47%, whereas n-6 FA transfer efficiency to milk for N6 was 39%. In addition, only n-3 FA had a positive carryover effect. However, the absolute quantity of n-3 FA in milk fat was still very low, especially considering 20:5n-3 and the lack of 22:6n-3.

Figure 1

Yields (g/d) of total n-3 and n-6 FA, 18:2n-6, 18:3n-3, 20:4n-6, and 20:5n-3 in milk fat of cows during treatment (from d 1 to 20) and carryover periods (from d 21 to 36; treatment and carryover periods separated by red dashed line). Treatments were abomasal infusions of ∼43 g/d of 18:2n-6 (N6) or 18:3n-3 (N3). Trt × Day = interaction between treatments and day. When the interaction tended to be significant (P ≤ 0.15), comparisons were performed between treatments within each day with tendencies at *P ≤ 0.10; and significances at **P ≤ 0.05 and ***P ≤ 0.05. Error bars represent SEM. Bar plots represent milk FA yield response to additional FA (FAYR, %) of cows during treatment (from d 1 to 20) and carryover periods (from d 21 to 36). Intercepts for each day were compared against zero with significance at †††P ≤ 0.01. Error bars represent SEM.

Figure 2

Contents (g/100 g FA) of total n-3 and n-6 FA, 18:2n-6, 18:3n-3, 20:4n-6, and 20:5n-3 in milk fat of cows during treatment (from d 1 to 20) and carryover periods (from d 21 to 36; treatment and carryover periods separated by red dashed line). Treatments were abomasal infusions of approximately 43 g/d of 18:2n-6 (N6) or 18:3n-3 (N3). Trt × Day = interaction between treatments and day. When the interaction tended to be significant (P ≤ 0.15), comparisons were performed between treatments within each day with significances at **P ≤ 0.05 and ***P ≤ 0.01. Error bars represent SEM.