Peripartal changes in diurnal rhythms in blood metabolites and ruminal fermentation profiles in dairy cows.

This study aimed to characterize circadian rhythms in blood metabolites and ruminal fermentation parameters in dairy cows during the transition period. Forty Holstein cows (BW = 779 ± 100 kg; parity = 2 ± 1.2; 305ECM = 13,863 ± 2,490 kg) were monitored from -35 to 60 DIM. Cows were fed once daily at 1011 (±23 min) and 0925 h (±26 min) during close-up and early lactation, respectively. Blood and rumen samples were collected at 9 h intervals over 3 consecutive days to assess 24-h rhythmicity at -12 ± 5 d prepartum (PRE) and 14 ± 2 DIM postpartum (POST). The fit, amplitude, and acrophase of the daily rhythms were evaluated by fitting the data to a linear form of sine and cosine functions using a linear mixed model in R with the fixed effect of stage of lactation (PRE vs POST), collection day (1, 2, and 3), parity, and random effect of cow. During PRE, plasma glucose, insulin, and NEFA exhibited 24-h rhythmicity, with a trend for rhythmicity in BHB. During POST, rhythmicity was observed for NEFA and BHB. Mesor glucose and insulin decreased from PRE to POST, whereas mesor NEFA and BHB increased. Acrophase shifted from PRE to POST for and BHB (1954 to 1632 h), while NEFA acrophase remained unchanged. Ruminal fermentation parameters, including total VFA and molar proportion of individual VFA, exhibited daily rhythms during PRE and POST. In contrast, ruminal NH3 rhythmicity was present in PRE but not POST. Acrophase shifted from PRE to POST periods for total VFA (2045 to 1952 h) and butyrate (2055 to 1958 h). Changes in glucose, BHB, and NEFA rhythmicity suggest that while diurnal glucose turnover and incomplete fatty acid oxidation shift around calving, lipid mobilization exhibited rhythmicity around calving, with greater amplitude postpartum, indicating more pronounced diurnal variation after calving. The lack of rhythmicity in NH3 after calving could reflect a reorganization of N metabolism, driven by changes in diet, feeding behavior, and rumen microbial activity. Overall, these results demonstrate that circadian rhythms in blood metabolites and ruminal fermentation in a 1 × /d feeding program are dynamic across the transition period, with calving representing a key inflection point in the temporal regulation of dairy cow metabolism.