Arterial blood ionised calcium activity in periparturient Holstein cows fed an alkaline low-energy density or acidifying high-energy density close-up prepartum rations

Periparturient dairy cows often experience metabolic and health challenges due to impaired Ca homeostasis. Therefore, improving Ca metabolism and monitoring functional Ca status are essential during this critical transition phase. The objective was to test the effects of different feeding strategies in the close-up dry period on arterial blood ionised Ca concentration (iCa) of periparturient Holstein cows. A total of 28 Holstein dry cows were fed a common far-off dry cow ration (FAR; grass silage and barley straw-based, dietary cation-anion difference (DCAD) of +300 mEq/kg DM) and randomly allocated to four experimental treatments at the beginning of the close-up period (-21 days relative to expected parturition). The treatments were (1) continuation of FAR ration (FAR; n = 7), (2) high-energy acidifying ration with MgCl₂ (MGC-70; maize silage and mechanically expelled rapeseed cake based, DCAD of -70 mEq/kg DM, n = 6), (3) high-energy acidifying ration with MgCl₂ and NH₄Cl (MGC-100; maize silage and mechanically expelled rapeseed cake based, DCAD of -100 mEq/kg DM, n = 7), (4) a 7 days transition diet (grass silage:MGC-70 ratio of 20:80 DM basis, DCAD of 0 mEq/kg DM), followed by 14 days of MGC-100 feeding (OVE; n = 8). During the close-up period, urine samples were collected weekly. Arterial blood samples were collected on -12, +0.5, +1.5, and +2.5 days relative to parturition by puncturing the arteria auricularis caudalis. On -7 day relative to parturition, urine pH in MGC-70, MGC-100 and OVE was lower than in FAR, indicating metabolic acidosis. Furthermore, MGC-100 and OVE had lower blood pH than FAR on -12 day relative to parturition. Although having higher postpartum arterial blood pH, cows fed acidifying close-up diets had a higher postpartum iCa than FAR. Tendency for an increased arterial iCa was detected in MGC-100 already on +0.5 day postpartum and MGC-70 tended to have an increased iCa on +2.5 day relative to parturition. Plasma total Ca concentration (tCa) was greater in cows fed acidifying close-up diets compared with FAR on +2.5 day, but tCa was not affected by treatments on +0.5 day and +1.5 day relative to parturition. Therefore, the present results indicate that the determination of physiologically active Ca status in periparturient dairy cows can differ depending on the chosen biological indicator (iCa vs tCa). Overall, feeding maize silage-based acidifying close-up rations improved iCa status in periparturient cows. Sampling of arterial blood from the arteria auricularis caudalis is a method to be considered in future studies evaluating functional Ca status.