Validation of serum gamma-glutamyl transferase activity and body weight information for identifying dairy calves that are too young to be transported to auction markets in Canada.

Dairy calves are at risk of being stressed when transported during the first week of life. A new Canadian federal rule will forbid transportation of calves younger than 9 d old to auction market. However, in the absence of reliable information to determine birth date, other indirect methods would be of interest. This study aimed to determine the prediction accuracy of body weight, Brix refractometry, and serum gamma-glutamyl transferase (GGT) activity for determining if a calf was not fit to be transported (i.e., <9 d old). For this purpose, we used 284 calves with a known birth date from a cross-sectional and a prospective cohort study. A logistic regression model was built based on multivariable analysis as well as a misclassification cost term analysis. Because of the collinearity between GGT activity and Brix value and lower discrimination of Brix value, the GGT activity was retained for the main model. The final logistic regression model contained body weight and log-transformed GGT activity value. The misclassifications of the logistic model was minimized using a model probability threshold ≥0.55 with a sensitivity of 70.4% and a specificity of 77.3%. This probability threshold was relatively robust for various prevalence and false negative to false positive cost ratios. The prediction accuracy of this model was moderate at the individual level, but is helpful in calves with a reasonable suspicion of being less than 9 d old.