EPIDEMIOLOGY OF SUBCLINICAL HYPOCALCEMIA IN EARLY-LACTATION DAIRY CATTLE AND A POTENTIAL DIAGNOSTIC TOOL FOR MEASUREMENT OF BOVINE BLOOD IONIZED CALCIUM

One of the challenges for dairy cows transitioning to a new lactation is the maintenance of optimum blood calcium concentrations that can successfully support the demands for milk production and the active immune system. Failure to do so can increase the risk of the animal to suffer from early-lactation diseases along with impaired performance. Subclinical hypocalcemia (SCH) is now being researched by many groups, though knowledge in this area is still in its infancy. The objectives of this research were to: 1) evaluate potential associations of cow-level variables in the prepartum period (and particularly plasma Mg concentration) that could be risk factors for SCH at calving, and at 2 days-in-milk (DIM) using a previously established cut-point for disease classification; 2) determine the associations of plasma calcium at calving with the most common early-postpartum health disorders, culling risk, reproductive and milk production performance, and 3) evaluate the analytical performance of a calcium ion-selective electrode technology as a potential cow-side instrument for measurement of bovine blood ionized calcium. Prepartum plasma magnesium concentration was not associated with SCH in the immediate postpartum. Risk factors associated with SCH if classified at calving or at 2 DIM varied, which highlighted the potential differences of lactational outcomes depending on when blood is tested for SCH relative to parturition. In multiparous cows, lower plasma calcium concentration collected within 12 hours of parturition was associated with an increased risk of displaced abomasum, and higher milk production. Calcium concentration at calving was not predictive of any outcome in primiparous animals. Lastly, evaluation of ionized calcium in bovine blood cow-side demonstrated to be possible using an ion-selective electrode prototype. Instrument short-term imprecision, as measured by coefficient of variation, demonstrated to be within acceptable limits of performance according to general guidelines for point-of-care devices, and were within 5% . Higher sensitivity and specificities were found for the prototype at 3 different cut-points. Therefore, as a qualitative method, the prototype can represent a significant advance to the veterinary field due to its potential application as a cow-side test with lower cost