After All, Tomorrow Is Another Day for the Transition Cow: Depending on Liver and Reproductive Health, of Course.

Infertility in lactating dairy cows has been attributed to metabolic stress during the transition period. Potential metabolic alterations that dysregulate ovarian functions have not been completely catalogued. Our objective was to characterize metabolic parameters of dairy cows during the transition period. First, we examined the metabolic profiles in circulation to pinpoint time points of major changes. We collected weekly blood samples from Holstein cows (N=15) from three weeks before to 12 weeks after calving. Glucose levels reduced during pre-calving weeks to reach a nadir at 3 weeks post-calving (P<0.05) and the first increase in glucose concentration occurred at 10 weeks post-calving (P<0.05). Also, β-hydroxybutyric acid levels increased from calving until week 3 of lactation (P<0.05) and subsequently, returned to baseline. Levels of triglycerides decreased during pre-calving weeks, while significant increase occurred at 5 weeks post-calving (P<0.05). Total cholesterol concentrations increased from the 3^rd to 7^th week post-calving (P<0.05). Oxidative stress indicator, glutathione, decreased to reach a nadir by 7 weeks in lactation (P<0.05). Thus, post-calving weeks 3 to 7 are associated with major alterations in metabolic indicators in circulation. In the second experiment, we evaluated changes in hepatic and granulosa cell (GC) mRNA levels and circulating metabolic indicators during the periods of major metabolic changes listed above. We collected blood and liver biopsies from Holstein cows at 3 weeks pre-partum, during calving week and 7 weeks post-partum, this last liver sample accompanied collection of GCs from the dominant follicle by follicular aspiration. Cows were separated into two groups, where Group 1 (N=4) consisted of cows that lost less than 0.75 body condition score (BCS) during the sampling trial and Group 2 (N=4) consisted of cows that lost equal to or greater than 0.75 BCS. Lipid metabolism and oxidative stress were evaluated in hepatic tissue by qPCR. Transcript abundance revealed a decrease in CYP7A1 (P<0.05) and tendency for decrease of LDLR (P<0.1) in cows from Group 2 at 3 weeks pre-partum. Transcript abundance of SRB1, ACAT1, PON1 and APOB100 remained unaltered between groups at all time points (P>0.05). Assessing reproductive competence through mRNA abundance of genes required for growth and ovulation (SRB1, FSHR and LHR) in GCs remained unchanged (P>0.05). Taken together, transition period features increasing cholesterol, triglycerides and decreasing glucose and glutathione compared to pre-calving period. Further studies, including liver function assays, are required to thoroughly investigate the relationship between liver health, reproductive health and alterations in circulating metabolic indicators.