A comprehensive evaluation of microchips to measure temperature in dairy calves

Elevated temperature is often an indicator of an immune response and used in the diagnosis of illness in dairy calves; however, measuring rectal temperature is labor intensive and often not measured daily on the farm. The objective of this study was to measure body temperature using a microchip and determine an appropriate implant site that would passively read body temperature in dairy calves. First, the precision of the temperature microchips and the rectal thermometer were tested ex vivo. Then, Holstein bull calves (n = 12) at 14 ± 12 d (mean ± SD) of age were implanted with microchips subcutaneously by the scutulum of the ear, subcutaneously in the upper scapula (SCAP), and intramuscularly in the trapezius muscle of the neck. One week after implantation, a temperature reading was taken for every microchip implant site using a radiofrequency ID (RFID) reader, as well as rectally and in the tympanic membrane using a digital thermometer every 60 min for 24 h in each calf (hereafter, the hourly study). Additionally, microchip readings and rectal temperatures were taken daily at 0800 h from 8 wk of age (n = 9; 57 ± 12 d of age) until 2 wk postweaning for a subset of the bull calves used in the hourly study (hereafter, the daily study). In the ex vivo trial, the microchip readings were very highly correlated with the rectal thermometer (r = 0.96), and the average coefficient of variation between microchip readings was very low (0.12 ± 0.03%). The relationships between the microchip readings within ear, SCAP, and neck and rectal and tympanic temperatures were analyzed using Pearson correlations and Bland-Altman plots. The ear and neck readings were strongly correlated for the hourly study [individual animal correlation; median (Q1, Q3), r = 0.78 (0.73, 0.84)] and for the daily study [r = 0.79 (0.73, 0.89)] across calves. However, rectal temperature was not significantly correlated with ear, SCAP, neck, or tympanic temperature for the daily and hourly studies. Results suggest that temperature microchips measure temperature appropriately, but temperature is dependent upon the implant site in calves, and temperature measured at ear, SCAP, and neck implant sites cannot be used to estimate rectal temperature. Future research should determine thresholds for fever that are specific to implant sites in calves.