Association of diarrhea severity with activity and milk feeding behaviors in group-housed dairy calves

The objective of this study was to assess associations between diarrhea and activity (step count, activity index, lying time, lying bouts) and milk feeding behaviors (milk intake, drinking speed, rewarded visits, time spent in the feeder) in the days leading up to and during a diarrhea bout in group-housed dairy calves. Holstein calves were fed ad libitum from automated milk feeders and monitored daily for health from 3 to 28 d of age. Activity was measured daily using accelerometers placed at 3 d of age. Diarrhea was diagnosed based on fecal scoring (loose feces for ≥2 d or watery feces for ≥1 d) and further classified as mild or severe based on clinical signs of dehydration (skin tent >2 s or eye recession >2 mm for ≥2 d) or depression (≥2 d). Diarrheic calves (n = 22; 11 mild, 11 severe) were pair-matched by age and pen to healthy controls (n = 22). Mixed linear regression models assessed effects of health status, day relative to diagnosis (d -3 to d 7), and their interaction on activity and milk feeding behaviors, and relative changes from baseline (d -3). Mildly diarrheic calves were diagnosed at 12.5 ± 3.8 d of age, whereas severely diarrheic calves were diagnosed at 9.4 ± 1.5 d of age. Severely diarrheic calves showed dehydration or depression on the day of diagnosis or 1 d afterward (10 ± 1.5 d of age). From 0 to 5 d after diagnosis, severely diarrheic calves took 510 ± 121 fewer steps per day than healthy calves and 371 ± 143 fewer steps per day on d 0, 1, 3, and 4 than mildly diarrheic calves. Mildly diarrheic calves only showed reductions on d 2 and d 5. Activity index followed a similar pattern. Relative to baseline, severely diarrheic calves had a 27% reduction in step count on d -1. On d 0, step count decreased by 47% in severely diarrheic calves and 28% in mildly diarrheic calves, whereas healthy calves remained near baseline. Relative changes in activity index mirrored these patterns. For milk feeding behaviors, severely diarrheic calves consumed 2 ± 0.85 L/d less milk than healthy calves on d 0 and tended to consume 1.5 ± 0.85 L/d less on d 1, 3, and 4, whereas mildly diarrheic calves tended to consume 1.5 ± 0.85 L/d less on d 0 and d 1. Relative to baseline, severely diarrheic calves had a 10% reduction in milk intake on d -1. On d 0, milk intake decreased by 20% in severely diarrheic and 10% in mildly diarrheic calves, whereas healthy calves increased by 24%. Severe diarrhea in group-housed calves was associated with a decline in activity and milk intake, with changes occurring before clinical signs. Early deviations in these behaviors may help identify disease severity and support automated health monitoring systems for timely detection.