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Milk is an easily digestible source of nutrients and bioactive factors, its composition reflects the neonate's needs, and changes from colostrum to transitional and mature milk. Our objective was to measure milk fat, lactose, total carbohydrate, and protein content in parallel with global proteome of homogenate milk samples to characterize changes across the three phases of swine lactation. Milk samples were collected from multiparous sows (n=9) on postnatal day 0 (D0; colostrum), 3 (D3; early transitional), 7 (D7; late transitional) and 14 (D14; mature). On D3, percent fat (16 ± 2.1) and lactose (3.8 ± 0.3) were higher (P<0.05) than on D0 (10 ± 3.9, and 1.5 ± 0.3; respectively). Levels of fat and lactose were not different between D3 and D14. Percent total protein decreased (P<0.05) between D0 (11 ± 2.1) and D3 (5 ± 0.7), but there was no significant change in percent protein between D3 and D14. Total carbohydrates increased (P<0.05) between D3 (944 ± 353 µg/ml) and D14 (1150 ± 462 µg/ml). Quantitative proteomic analysis using liquid chromatography tandem mass spectrometry (LC-MS/MS) of homogenate D0, D3, and D14 milk samples (n=6) identified 772 protein groups which corresponded to 501 individual protein-coding genes. A total of 207 high confidence proteins were detected in n=3 sows/day. Of the high confidence proteins, 81 proteins were common amongst all three days of lactation. Among the proteins that decreased between the days (FDR < 0.05) were multiple apolipoproteins and XDH which decreased between D0 to D3. Proteins that increased across the days (FDR < 0.05) were complement factors and14-3-3 proteins (YWHAQ, YWHAE). Our data provide a good characterization of milk proteome changes that likely reflect mammary function as well as the neonate's phase-specific developmental needs. This data may be useful in developing approaches to enhance the health and welfare of swine.

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