A direct comparative study of dairy cows and goats was performed to characterize the animal performance and milk fatty acid (FA) responses to 2 types of diets that induce milk fat depression in cows as well as a diet that increases milk fat content in cows but for which the effects in goats are either absent or unknown. Twelve Holstein cows and 12 Alpine goats, all multiparous, nonpregnant, and at 86 ± 24.9 and 61 ± 1.8 DIM, respectively, were allocated to 1 of 4 groups and fed diets containing no additional lipid (CTL) or diets supplemented with corn oil [5% dry matter intake (DMI)] and wheat starch (COS), marine algae powder (MAP; 1.5% DMI), or hydrogenated palm oil (HPO; 3% DMI), according to a 4 × 4 Latin square design with 28-d experimental periods. Dietary treatments had no significant effects on milk yield and DMI in both species, except for COS in cows, which decreased DMI by 17%. In cows, milk fat content was lowered by COS (-45%) and MAP (-22%) and increased by HPO (13%) compared with CTL, and in goats only MAP had an effect compared with CTL by decreasing milk fat content by 15%. In both species, COS and MAP lowered the yields (mmol/d per kg of BW) of C16 FA in goats, but not in cows, and the >C16 FA yield decreased with MAP in both species. HPO supplementation increased the milk yield of C16 FA in cows. Compared with CTL, COS induced an increase of trans-10,cis-12 conjugated linoleic acid by 18 fold in cows and 7 fold in goats and of trans-10 18:1 by 13 fold in cows and 3 fold in goats. Moreover, other conjugated linoleic acid isomers, such as trans-10,trans-12 and trans-7,cis-9, were increased to a greater extent in cows (8 and 4 fold, respectively) compared with goats (4 and 2 fold, respectively) on the COS treatment. In both species, the responses to MAP were characterized by a decrease in the milk concentration of 18:0 (3 fold, on average) and cis-9 18:1 (2 fold, on average) combined with a 3-fold increase in the total trans 18:1, with an increase in trans-10 18:1 only observed in cows. Compared with CTL, the response to HPO was distinguished by an increase in 16:0 (10%) in cows. This comparative study clearly demonstrated that each ruminant species responds differently to COS and HPO treatments, whereas MAP caused similar effects, and that goats are less sensitive than cows to diets that induce a shift from the trans-11 toward the trans-10 ruminal pathways.

Recent knowledge is presented on the composition of ruminant milk fat fractions and the nutritional strategies known to alter their amount. The development of lipidomic and proteomic analyses has allowed for the characterization of minor components, such as proteins, liposoluble vitamins, and phospholipids of the milk fat globule (MFG), in addition to the triacylglycerols (TAG), which are the major constituents of the MFG core. Few differences in these components among ruminant species exist, and they have been outlined mainly on the fatty acids (FA) profile of the TAG, whereas comparative data are still lacking on vitamins and proteins. The effects of dietary treatments enriched in n‐3 polyunsaturated FA (PUFA) on the composition of the milk fat fraction are explored. In particular, pasture and plant oilseeds increase milk n‐3 PUFA and cis‐9,trans‐11 CLA and decrease saturated FA, whereas data with new feed resources, such as algae, are still rare. The peculiarities of the response of the milk fat to diets that induce a milk fat depression in cows but in lesser extent in small ruminants are described. The potential effects of polar lipids, proteins, and liposoluble vitamins of the MFG on human health are reviewed, highlighting the nutraceutical properties of milk. Practical Applications: This review provides an overview of the different components of the milk fat fraction in ruminant species and on nutritional strategies to alter their amounts to improve the nutritional quality of milk. Furthermore, this review presents recent data on species peculiarities of the milk fat fraction composition and of its response to nutritional factors, which offers a promising model to identify news levers of regulation of this fraction and foster the identification of new feeding strategies to better control milk fat composition and feed efficiency. This review synthesizes data on the composition of the milk fat fraction in ruminant species, reports advances in nutritional strategies to alter their amounts and species‐specific responses to nutrition, as well as the potential effects on human health of the major and minor components of this fraction.

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