Metabolic and energy status during the dry period is crucial for the resumption of ovarian activity postpartum in dairy cows

It is well known that the degree of negative energy balance in high-producing dairy cows is the major cause of delayed resumption of the ovarian cyclicity that closely relates to fertility. Recent evidence suggests that the energetic situation during early lactation critically affects nutrient partitioning, metabolism, and the reproductive axis, whereas the effect of energy status during the dry period is widely unknown. The aim of the present study was to investigate the effect of energy status throughout the entire dry period until early lactation on the onset of the ovarian cyclicity. Blood samples were taken in 23 cows from dry off at 8 wk before expected parturition to 8 wk postpartum for the analyses of metabolites and hormones, and milk samples were obtained 3 times weekly from d 7 of lactation onward to confirm luteal activity and pregnancy by milk progesterone analysis. Energy balance (EB) was measured weekly during the last 6 wk of the dry period and every other week after parturition. Liver biopsies were obtained at 8 wk before expected calving, within 1 d after calving, and at 4 wk postpartum to measure the mRNA abundance of various gluconeogenic enzymes and metabolic hormone receptors. Cows showing luteal activity within 3 wk postpartum were defined as ovulatory during the first follicular wave postpartum (OC), whereas cows without luteal activity within 3 wk postpartum were defined as anovulatory (AC). Energy balance and, concomitantly, plasma concentrations of glucose, insulin, insulin-like growth factor-1, 3,5,3'-triiodothyronine, and thyroxine were higher in OC than in AC during the dry period. Plasma thyroxine concentrations and body condition score during the postpartum period were higher in OC than in AC. At the mRNA level (19 cows), hepatic insulin receptor decreased from dry off to early lactation, and mRNA of pyruvate carboxylase was highest at parturition and decreased in early lactation in AC only, whereas both parameters remained unchanged in OC. The mRNA abundance of phosphoenolpyruvate carboxykinase-mitochondrial increased from dry off to parturition in both groups, remained high in OC, and decreased again in early lactation in AC. However, none of the investigated gene transcripts differed between OC and AC cows. Thus, ovarian function postpartum appears to be crucially influenced by the energy status during the dry period, which is reflected by timely changes in hepatic mRNA abundance of only a few key metabolic factors in the liver.