Is the temperature-humidity index the best indicator of heat stress in lactating dairy cows in a subtropical environment?

Abstract

Several temperature-humidity indexes (THI) have been used to estimate the degree of thermal stress experienced by dairy cows. The present objectives were to develop equations using meteorological variables that predicted rectal temperature of lactating cows in a subtropical environment and compare the goodness of fit of these equations to those using 8 different THI. Rectal temperature was measured between 1500 and 1700 h in 1,280 lactating Holstein cows in north central Florida between August and December. Meteorological data recorded in the barn where cows were located included dry bulb temperature (T(db)), relative humidity (RH), dew point temperature, and wind speed. Wet bulb temperature was calculated. In the first series of analyses, regression analysis was used to model rectal temperature using the meteorological variables as well as THI. The r(2) using T(db) (0.41) was slightly less than for models using all but one THI (r(2) between 0.42 and 0.43). The r(2) for equations using T(db) could be improved by adding RH (r(2) = 0.43) or RH and RH(2) (r(2) = 0.44) to the model. In the second analysis, regression analysis was performed using forward selection, backward elimination, and stepwise selection procedures with the meteorological variables. All models gave a similar goodness of fit (r(2) = 0.44). An analysis of variance with rectal temperature as a class variable was performed to determine the least squares means of meteorological measurements associated with hyperthermia. A T(db) of 29.7 degrees C was associated with rectal temperature of 39 degrees C, and a T(db) of 31.4 degrees C was associated with rectal temperature of 39.5 degrees C. In conclusion, T(db) is nearly as good a predictor of rectal temperatures of lactating Holsteins in a subtropical environment as THI. Estimates of values of meteorological variables associated with specific rectal temperatures should prove valuable in relating environmental conditions to the magnitude of hyperthermia experienced by heat-stressed cows.

Adaptation to climate change – exploring the potential of locally adapted breeds

“…A THI of higher than 70 is commonly assumed to be the onset of physiological heat stress (Bohmanova et al, 2007;Dikmen and Hansen, 2009). Consequently, among the 1058 national breed populations with information provided on their general adaptation, all populations described as heat sensitive and drought susceptible, and 86% of poorly adapted occur in countries where the average THI of the hottest month is higher than 70 (Table 8).…”

“…High thermal resistance has a physiological cost in terms of lower performance in non-stressed environments (Williams et al, 2008). The capacity to tolerate elevated temperatures has declined with increasing metabolic heat production related to high milk yield in dairy cattle, and growth rates and leanness in pigs or poultry (Zumbach et al, 2008;Dikmen and Hansen, 2009;Gauly et al, 2013).…”

“…High thermal resistance has a physiological cost in terms of lower performance in non-stressed environments (Williams et al, 2008). The capacity to tolerate elevated temperatures has declined with increasing metabolic heat production related to high milk yield in dairy cattle, and growth rates and leanness in pigs or poultry (Zumbach et al, 2008;Dikmen and Hansen, 2009;Gauly et al, 2013).Beyond thermoregulation, rumen physiology, the ability to walk and reach scarce feed resources, to intake water and rehydrate, to compensate for low feed quality by selecting high-quality diets from different plant components or species, or to respond with increased night-time grazing to high afternoon temperatures, or even genetic aspects of diet selection play a role in adaptation to climate change (Hall, 2004;Gauly et al, 2013). Breeds are known to differ with regard to their ability to survive, grow and reproduce in the presence of poor seasonal nutrition as well as parasites and diseases; they vary in their ability to mobilize body resources to cope with periodic underfeeding and cease reproduction at different levels of BW loss.…”

“…Furthermore, 27% of goats, 28% of cattle, 28% of chicken and 34% of sheep populations live in countries with a range of 08C to 448C. Tropical countries with their more uniformly warm temperatures require adaptation to a lower temperature range between 208C and 348C (Figure 3).A THI of higher than 70 is commonly assumed to be the onset of physiological heat stress (Bohmanova et al, 2007;Dikmen and Hansen, 2009). Consequently, among the 1058 national breed populations with information provided on their general adaptation, all populations described as heat sensitive and drought susceptible, and 86% of poorly adapted occur in countries where the average THI of the hottest month is higher than 70 (Table 8).…”

Dairy cattle in a temperate climate: the effects of weather on milk yield and composition depend on management

“…Cattle generate metabolic heat as a by-product of milk synthesis and so higher yielding animals experience heat stress at lower THIs than lower yielders (Kadzere et al, 2002). An animal's thermal tolerance is also affected by solar radiation and the velocity of ambient air (Dikmen and Hansen, 2009;Graunke et al, 2011;Hammami et al, 2013), while increasing precipitation (ppt) is associated with declining milk production (Stull et al, 2008). Weather-related stressors could potentially affect performance immediately or have a delayed impact, and yet few studies have explored the time interval between weather events occurring and impacting milk traits (St-Pierre et al, 2003).…”