Interpretive Summary: Thermoregulatory response of Brangus heifers to naturally occurring heat exposure on pasture
By: Jackie Walling
Naturally occurring heat exposure is unavoidable for Brangus heifers raised in tropical or subtropical climates. To maximize economic gain, ideally heifers would possess high quality production traits while simultaneously regulating their body temperature efficiently. This article published in the August Issue of the Journal of Animal Science investigates Brangus heifers’ thermoregulatory response to heat exposure on pasture. Researchers assessed phenotypic variability of core body temperature (using vaginal temperature) and sweating rate. Additionally, possible influential factors such as coat type, temperament behavior, and weight were analyzed to see how they effected vaginal temperature and sweating rate.
725 Brangus heifers located at Seminole Ranch in Brighton Reservation, Florida were divided into four groups and evaluated over 5d trials starting August 15th and ending September 12th of 2016. Dry bulb temperature and relative humidity were taken every 15 minutes for the duration of the study. Temperature-humidity index (THI) was calculated to evaluate heat-stress potential according to the livestock weather hazard guide. Data was collected over a 4h period.
Sweating rate was measured with a Vapometer on the rump for Groups 2,3, and 4. Vaginal temperature was measured consecutively for five days every 5 minutes. Interclass Correlations (“the proportion of variance due to permanent environmental and genetic differences among individuals”) were calculated with .47 for sweating rate and .44 for vaginal temperature suggesting one measurement taken for each would suffice to describe individual heifers. High levels of variation existed among heifers in vaginal temperatures, but generally numbers increased approximately one hour following an increase in THI.
Coat type and temperament were both quantified for evaluation. Coat type was measured on a scale from 1 (very smooth) to 5 (woolly). Temperament was broken down into Chute Behavior (scale of 1-5, ranging from calm to rearing/struggling) and Exit Behavior (1 meaning slow/calm, 2 meaning jumping/running/trotting). These factors influenced sweating rate and vaginal temperature. Heifers harboring smoother coats and a calm disposition saw lower temperatures and amounts of sweat. Woolly and unruly heifers saw an increase in both factors. BW served more as an indicator of adaptability. Heifers weighing more had lower vaginal temperatures compared to those weighing less. The additional body mass indicated the heifer was able to obtain adequate ADG meaning it was adaptable to its environment. The lesser cows struggled to gain weight and therefore experienced more heat stress and less ADG in hot environments.
From this study, the Brangus heifers evaluated from the Seminole tribe showed high variability in vaginal temperature even though the environment and conditions remained the same for all. Coat type, temperament, and BW all influenced vaginal temperature and sweating rate giving the advantage to heifers possessing a calm disposition, short coat, and heavier BW. Selecting for heifers bearing such characteristics previously listed would be a way to begin improving the thermoregulatory response of heifers. It would also begin to decrease the variability. Increasing the use of Bos Indicus genetics may also result in increasingly better responses. For the full article, go to the Journal of Animal Science.