Altered innate immune system functions exert a pivotal role in impairing the adaptation of dairy cows to new lactation (1–3). A dysfunctional immune system detrimentally affects performance and predisposes dairy cows to develop infectious and metabolic disorders (4–6). It has been hypothesized that the changes in the fatty acids (FAs) profile of plasma and cellular lipid fractions [i.e., decreased polyunsaturated FAs (PUFAs)/saturated FAs (SFAs) ratio] that are induced by the massive mobilization of fat depots at the onset of lactation, alter the leukocyte functions (7–10), and induce uncontrolled inflammatory responses in early lactating dairy cows (11–14). Among PUFAs, Omega-6 FAs (n6-FA) and Omega-3 FAs (n3-FA) are named “essential FAs” because they must be provided through the diet (15). Dairy cows commonly develop severe deficiencies for n3-FAs due to the massive inclusion of n6-FAs sources in the diets, leading to unbalanced PUFA ratios in all the body compartments, including immune cells phospholipid membranes (PLM) (16).

Supplementing n3-PUFAs to lower the plasma n6/n3 ratio represents a fascinating strategy to improve dairy cows' adaptation to the new lactation challenges through coping with the immune dysfunctions (14, 17–22). In fact, n3-FA are well known for their anti-inflammatory and immune-modulatory actions (i.e., shifted oxylipids profile in favor of anti-inflammatory mediators, reduced series 1 and 2 prostaglandins production, and improved leukocytes functions) (23–27). That said, supplementing the diet with feed sources containing n3-PUFAs is hindered by several limitations in dairy cows (i.e., feed intake depression, modifications in the profile of dietary FAs induced by rumen biohydrogenation, milk fat depression) (16, 28, 29), limiting dietary-administrable FAs to the rumen-protected forms. A promising strategy to cope with these limitations could be administering n3-PUFAs through venal infusion, but scarce are the studies performing a similar treatment on transition dairy cows. A major concern in supplementing huge amounts of fats by venal infusion during the early lactation could be aggravating the liver lipids load, sorting negative effects on animals' health (30, 31). Eicosapentaenoic acid (C20:5n-3 -cis–EPA) and docosahexaenoic acid (C22:6n-3 -cis – DHA) have the highest rate of inclusion in immune cell PLM among all the n3-PUFAs, representing the most valuable choice to effectively modulate immune cells functions through supplementing a small amount of lipids (15, 32, 33).

This experiment aimed to investigate the effect of infusing small amounts of EPA and DHA through the jugular vein of postpartum dairy cows on their performance and plasma profiles reflecting FA composition, inflammation, and metabolic conditions. The hypothesis was that venal infusion could alter the plasma FAs profile and that the anti-inflammatory properties of EPA and DHA could mitigate the inflammatory condition physiologically affecting dairy cows at the onset of lactation, positively affecting their performance.

Excluding the FA plasma profile, the time effect was highly significant (P < 0.01) for most of the other parameters in the study. Thus, the time effect will be presented only in the Results section for the plasma FA profile.

At the plasma total lipids profile level, n3FA cows had higher plasma proportions of EPA and especially of DHA at 3 DFC compared to CTR cows, and they maintained them until 10 DFC (at 24 h and 8 days after the last administration). Previous studies reported no trace of EPA and a low and stable proportion of DHA in the plasma total lipids FAs profile of dairy cows at their calving transition (11), and the outcomes obtained in this study are thus probative that fish oil infusion effectively elevates the availability of these long-chain n-3 PUFAs in plasma of postpartum cows. Such an elevation of EPA and DHA abundance detected in plasma aligns with the improved rheological parameters (i.e., reduced rCT and increased a30) of the milk from n3FA cows at 3 DFC. Changes in the FAs profile of plasma are reflected by those of the milk fat (16), and higher inclusion of long-chained PUFAs in the milk fat positively affects the cheesemaking features, improving the cheese yield (48). This also suggests that long-chained n3-PUFAs administered through venal infusion are partially diverted to the mammary gland, which could reduce their plasma availability over time.

The occurrence of a systemic inflammatory state is a recurring condition in dairy cows at the onset of lactation (3, 49). The lowered ROMt concentration found in the plasma of n3FA cows at 1 DFC and the lowered haptoglobin concentration at 2 and 3 DFC suggest that the fish oil infusion is effective at mitigating their systemic inflammatory state as compared to CTR cows. In fact, ROMt elevation in plasma of early lactating cows could be driven by leukocytes activation during inflammation (50), while haptoglobin is the main acute-phase protein in cows, and it serves as a systemic inflammation marker by reflecting an acute-phase response occurring at the liver level (51, 52).

The mitigated inflammatory conditions of n3FA cows could likely be driven by a greater EPA and DHA inclusion in the PLMs of their leukocytes as compared to CTR cows. In fact, the enrichment of leukocytes PLMs with EPA and DHA is known for playing an anti-inflammatory action through inhibiting the release of proinflammatory mediators (i.e., prostaglandins) and altering the formation of lipid rafts involved in NF-kB activation (19). Despite the plasma total lipids FAs profile assessed in the present study only reflects the changes in the neutral lipids and NEFA fractions, it could be hypothesized that EPA and DHA elevation detected at the plasma total lipids level could have come with an even greater elevation of these n3PUFAs in the leukocytes PLMs. In fact, plasma FAs are differentially spared among the body compartments depending on the length of their carbon chains (11, 53). Long-chained PUFAs are preferentially included in the PLMs (54, 55). Among these, EPA and DHA are known for their high inclusion rate in the PLM of leukocytes at the expense of ARA (an n6-FA representing the main PUFA composing leukocytes PLMs in normal conditions) (19, 32).

Results obtained in this study contradict those obtained when administering n3 PUFAs through diet and that report no effect on any acute-phase protein biomarker (21, 56). Thus, greater effectiveness of the venal infusion in favoring the inclusion of n3 PUFA in the leukocytes PLMs could be hypothesized, suggesting that it is a potential strategy to cope with limitations hindering lipid nutrition in dairy ruminants. The short time frame occurred between calving and the administration of the first lipid infusion could also account for the effectiveness of n3 PUFA in mitigating plasma haptoglobin elevation in this study, as acute inflammation driven by calving is reflected by a massive haptoglobin peak in healthy dairy cows during the first few days after calving (57). Despite that, haptoglobin was the only inflammatory marker affected by the infusion in the present study, and no effect was found on the other acute-phase proteins (i.e., ceruloplasmin and negative acute-phase proteins). We can thus speculate that the amount of n3 PUFA provided by the 3 intravenous infusions is not sufficient to fully overcome the inflammatory condition faced by dairy cows at the onset of lactation.

The effectiveness of the fish oil infusion in mitigating the systemic inflammatory state of n3FA cows could account for the higher DMI they had during the first week of lactation as compared to CTR cows because cytokines tuning the inflammatory processes reduce feed intake through exerting an anorexic power on the hypothalamus in periparturient dairy cows (58). We can speculate that the greater DMI of the n3FA cows at the onset of lactation could have modified the relative abundances of dietary- and lipomobilization-related FAs in their plasma profile as compared to those of CTR cows (i.e., increased abundance of MUFAs at 3 DFC and increased abundance of cis-11,14-eicosadienoic acid counterbalanced by a reduced abundance of myristic acid and cis-10-pentadecanoic acid at 10 DFC). Yet a detailed discussion of these differences from a metabolic standpoint could be hard to assess because little is known about the metabolic pathways regulating minor FA abundance in the plasma of dairy cows (11). Besides inducing changes in the FA profile of n3FAcows, greater feed intake of these animals could account for their greater milk yield recording and lower plasma urea at 1 and 2 DFC, suggesting they have greater energy availability to sustain lactation requirements and a lower amount of endogenous amino acids deserved to gluconeogenetic processes at the beginning of lactation (59, 60) as compared to CTR cows. Although only numerical trends were detected afterward, greater milk yield was maintained by the n3FA cows during the whole experimental period, suggesting that the lower lactose and urea nitrogen content of their milk is driven by a dilution effect. While the reason behind the BHB elevations detected in n3FA cows at 14 DFC and CTR cows at 21 DFC remains unclear, but it seems to be independent of the lipid infusion and likely consequential to the adaptative response of the animals against other minor stressors that occurred along with the lactation.

The elevation of long-chained n3 PUFAs observed at the plasma total lipid profile level until 10 DFC demonstrated lipid infusion to have a long-lasting effect on the FAs' availability. This could account for an anti-inflammatory action exerted on leukocytes and resulting in mitigating oxidant species release and in the lowered inflammatory status observed in n3FA cows after calving. These alterations in inflammation-related plasma analytes accompanied a higher feed intake and milk yield, confirming that the performances of dairy cows at the onset of lactation largely depend on the severity of the inflammatory conditions they experienced in this phase (61).

Despite these promising outcomes, haptoglobin was the only inflammatory marker affected by the infusion, and no effect was found on the other acute-phase proteins, suggesting that the transient effects of n3PUFAs infusions on plasma analytes are not long lasted and are not very impactful to improve overall inflammatory status within the period assessed by the study. Furthermore, the small sample size suggests the present research to be considered a pilot study, and further experiments testing the effect of a similar lipid infusion performed for a longer time frame and on a wider experimental sample are required to confirm the effectiveness of n3PUFAs infusion at modulating the inflammatory conditions experienced by peripartum dairy cows.

The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation.

The animal study was reviewed and approved by Authorization of Italian Health Ministry N 1047/2015-PR.

FP-C, AM, and ET: conceptualization, methodology, writing—review and editing, and supervision. MM: formal analysis, investigation, and writing—original draft preparation. ET: resources and funding acquisition. MM and FP-C: data curation. FP-C and ET: project administration. All authors have read and agreed to the published version of the manuscript.

Research supported by the Università Cattolica del Sacro Cuore Line D.1 – 2011 on Studio sulla utilità della somministrazione di acidi grassi omega-3 nelle bovine da latte.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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