Table 2 Advantages and disadvantages of different methods used to measure body temperature and activity in free-ranging sheep
From: Technologies for the automated collection of heat stress data in sheep
Method of deployment | Invasiveness | Difference from CT | Disadvantages | Advantages |
|---|---|---|---|---|
Indwelling rectal logger | Moderate | 0 ℃ | Expulsion risk, constrains biological function, affected by faecal temperature [46], deployment difficulties [47], duration limited by battery power, not real-time | Core temperature [51], high collection frequency, remote measurement, detailed and accurate temperature profile, indwelling without surgery, secure data storage |
Indwelling vaginal logger | Minimal | Infection risk, affected by changes in blood flow during oestrus [52, 53], female animals only, data logger size limitations, duration limited by battery power, not real-time | Close correlation to core [51], high collection frequency, detailed and accurate temperature profile, quick response time ~ 8 s, simple to implant, indwelling without surgery, no biological function constraints | |
Tympanic (ear canal) probe | Minimal | Infection risk, prone to displacement/dislodgement, influenced by environmental factors, high activation threshold [19] | Simple to implant, devices commercially available, visual signal to identify pre-determined temperature breach [19], real-time wireless transmission | |
Reticulorumen bolus (rumen) | Minimal | + 0.45–0.75 ℃ [66] | Irretrievable before slaughter, temperature influenced by; water volume [60, 61], feed intake levels and rumen microorganisms [65] | Sensitive to reading body temperature [58, 59], no infection risk, suitable for long-term use, real-time wireless transmission [56], simple administration [55], |
Infrared thermography; ocular region, udder, muzzle, vulva, forehead, flank | Non-invasive | −1.7–2.1 ℃ [130] | Influenced by; ambient radiations [130], dust, temperature, humidity and obstacles [46], affected by wool coverage [15] and age, stress, gender and reproductive state [86] | Contactless/non-obstructive, > 3000 m range, quick response time 1 ms to 250 ms [30], measure changes in blood flow related to heat load [25], potential to measure respiration |
Subcutaneous microchips: abdomen, flank, tail-base, shoulders | High (surgical intervention) | Possibly irretrievable, risks safety of product for human consumption, requires surgery, accuracy is site-dependent, data collected by handheld receiver [86], not real-time | Highly secure, allows permanent placement, accurate, high resolution of data | |
GPS collars | Non-invasive | N/A | Large data output, extensive analysis required, not real-time | Measures spatial movement, more effective than direct flock observation [115], non-obstructive [116] |
Accelerometers: neck, leg, chest, ear | Non-invasive | N/A | Large data output, difficult to determine new behavioural algorithms, data can be misinterpreted | Potential to determine respiration associated with heat stress [29, 112], can measure mutually exclusive behaviours and activity |