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A rumen bolus is a useful tool to monitor core body temperature in lactating dairy cows in a sub-tropical summer
The ability to vary body temperature is a common thermoregulatory response in mammals and rumen boluses may allow frequent monitoring of such variations during heat stress episodes. Twenty four Holstein Friesian lactating dairy cows were fed either a total mixed ration plus wheat (TMRW), TMR plus Bioprotect, a starch binding agent (TMRB), or TMR plus Corn (TMRC). The only difference between diets was the type of grain contained in the TMR. Cows had ad libitum access to water and feed in shaded pens at the University of Queensland, Gatton Campus, Dairy research facilities, 27.4986 °S, 153.0155 °E, 89 m elevation during 29 d in summer 2015. Rumen temperature (RuT) was recorded over 15 d at 20 min interval using transponder rumen boluses (RFID transmitters; Smartstock, USA) placed in the ventral sac of the rumen. Rectal temperature (RT) was measured once every four days in the morning (0700 -1000 h) and the temperature humidity index (THI) was calculated from an on-site weather station. Data were analysed using the restricted maximum likelihood (REML) and Pearson correlation analysis procedure for GenStat V15. Treatment groups were considered statistically different at P ≤ 0.05. Average THI during the experimental days was 72.4 ± 2.0 (mean ± SD) with 76 % of the days having THI ≥ 72 which is the critical THI threshold for dairy cows. TMRC fed cows had lower RT than TMRW and TMRB (38.8 vs 39.1 and 39.1 ⁰C respectively; P <0.001) and RT was directly correlated to THI and RuT in all diets (P <0.001). Diet had no significant effect on RuT (39.6, 39.7, 39.5 ⁰C for cows fed TMRB, TMRC and TMRW respectively; P >0.05). Cows had large variations in RuT during the day that weren’t associated with THI. For example, RuT was higher overnight (2000 to 0500 h) than during the day with the maximum occurring between 2100 and 0300 h. The minimum RuT occurred around the AM feeding (0600 to 1000 h) after which RuT gradually increased during the day. There was a positive association between RuT and mean daily THI during the bolus data collection period (P <0.001). The data obtained from our study demonstrated that the variation in RuT follows daily THI and that the association between RT and RuT is positive. Therefore, RuT enables a sensitive prediction of RT when cows are exposed to high ambient temperatures.
Keywords: dairy cows, heat stress, rumen temperature