Associations between health status and the probability of approaching a novel object or stationary human in preweaned group-housed dairy calves.

Neonatal calf diarrhea (NCD) and bovine respiratory disease (BRD) are the 2 most prevalent diseases affecting the welfare and productivity of preweaned dairy calves in the United States. Early detection of these diseases improves both the probability of recovery and animal welfare. Group housing of preweaned calves is increasing in popularity and disease detection in a socially competitive environment can be challenging. One method of detecting disease is through the observation of key behaviors that occur during illness. Novel object and stationary human approach tests can be used to measure exploratory behavior, which is decreased during the expression of sickness behavior. The objectives of this study were (1) to determine associations between 4 categories of health status and the probability of calves approaching a novel object or stationary human, and (2) to determine the associations between rectal temperature and the probability of calves approaching. Holstein heifer calves (n = 75) in group housing were tested weekly for the first 6 wk of life for their willingness to approach a novel object (OBJ) or stationary human (SH). After the approach tests, calves were evaluated by research staff using a standardized health-scoring system. Treatment records were obtained from the farm to determine detection of BRD, NCD, and umbilical infections. Associations between probability of approach and BRD status were analyzed using a linear mixed model with a logit-transform (PROC GLIMMIX), controlling for calf as a random effect. All models controlled for week, pen, pen order, and test order. Calves with clinical signs of BRD on test day were 0.5 [95% confidence interval (CI) = 0.3-0.9] and 0.5 (95% CI = 0.3-0.9) times as likely to approach the OBJ and SH compared with healthy calves, respectively. Calves with a fever (rectal temperature ≥ 39.4°C) were 0.4 (95% CI = 0.2-0.8) and 0.4 (95% CI = 0.1-0.7) times as likely to approach the OBJ and SH compared with calves without fever, respectively. Calves that were recovering from NCD on test day, but were not showing clinical signs of NCD, were 0.4 (95% CI = 0.2-0.9) times as likely to approach SH, compared with healthy calves. These results suggest that approach tests may be useful for the identification of calves with clinical signs of BRD and calves with a fever in group housing.