The study objective was to compare the ability of Brix values and the D2Dx immunity test to predict the mortality risk of calves under 30 days of age. Heifer calves at 2-3 days of age were enrolled from two farms: Farm A, which has a low calf mortality rate (n = 849, 2.9% mortality), and Farm B, which has a high calf mortality rate (n = 698, 11.7% mortality). Serum samples were collected from the calves when they were 2-3 days of age. The D2Dx scores and Brix values of serum samples were compared between calves that died or survived between 3-30 days of age. For both farms, calves that survived their first 30 days of life had greater serum Brix values (9.05 vs. 8.48, p = 0.002 for Farm A; 9.26 vs. 8.99, p = 0.05 for Farm B) and D2Dx scores (0.0281 vs. 0.0036, p < 0.001 for Farm A; 0.0342 vs. 0.0286, p = 0.05 for Farm B) at 2-3 days of age compared with calves that died. On Farm A, the D2Dx score shows significantly better sensitivity (96% vs. 48%) and Area Under the Curve (0.87 vs. 0.69) than the Brix value in predicting calf mortality. However, similar results were not observed for the high-mortality farm (Farm B) due to other factors in addition to calf immune status that influenced calf survival. This study concludes that the D2Dx immunity test can be used to identify newborn dairy calves with low immunity and increased mortality risk; however, the prediction is most effective if no other risk factors are involved.
Keywords: D2Dx immunity test; calf mortality risk; humoral immunity; transfer of passive immunity.