Estimates of heat stress relief needs for Holstein dairy cows1

Abstract

Estimates of environmental heat stress are required for heat stress relief measures in cattle. Heat stress is commonly assessed by the temperature-humidity index (THI), the sum of dry and wet bulb temperatures. The THI does not include an interaction between temperature and humidity, although evaporative heat loss increases with rising air temperature. Coat, air velocity, and radiation effects also are not accounted for in the THI. The Holstein dairy cow is the primary target of heat stress relief, followed by feedlot cattle. Heat stress may be estimated for a variety of conditions by thermal balance models. The models consist of animal-specific data (BW, metabolic heat production, tissue and coat insulation, skin water loss, coat depth, and minimal and maximal tidal volumes) and of general heat exchange equations. A thermal balance simulation model was modified to adapt it for Holstein cows by using Holstein data for the animal characteristics in the model, and was validated by comparing its outputs to experimental data. Model outputs include radiant, convective, skin evaporative, respiratory heat loss and rate of change of body temperature. Effects of milk production (35 and 45 kg/d), hair coat depth (3 and 6 mm), air temperature (20 to 45 degrees C), air velocity (0.2 to 2.0 m/s), air humidity (0.8 to 3.9 kPa), and exposed body surface (100, 75, and 50%) on thermal balance outputs were examined. Environmental conditions at which respiratory heat loss attained approximately 50% of its maximal value were defined as thresholds for intermediate heat stress. Air velocity increased and humidity significantly decreased threshold temperatures, particularly at higher coat depth. The effect of air velocity was amplified at high humidity. Increasing milk production from 35 to 45 kg/d decreased threshold temperature by 5 degrees C. In the lying cow, the lower air velocity in the proximity of body surface and the smaller exposed surface markedly decrease threshold temperature. The large variation in thresholds due to environmental and animal factors justifies the use of thermal balance-based indices for estimating heat stress. Such an approach may make possible estimates of threshold temperatures at which heat stress relief is required for widely different cattle types and environmental situations.

Livestock production system management responses to thermal challenges

“…Above a threshold environmental condition, body temperature (BT) begins to increase as a result of the animal's inability to adequately dissipate the excess heat load by increased respiratory vaporization. There is a concomitant decrease in FI as BT increases, which ultimately results in reduced performance (production, reproduction), health, and well-being if adverse conditions persist (Hahn and Mader 1997;Berman 2005). Thresholds, such as noted in Fig.…”

“…2, equivalent THI thresholds shown in Table 2 were developed for RRs, assuming a solar radiation (Hahn and Osburn 1969) of 800 Wm −2 , a wind speed of 0 ms −1 , dry-bulb temperature range of 25 to 40°C, and RH between 30 and 50%. Berman (2005) determined environmental threshold temperatures for dairy cows based on environmental factors and characteristics of the cows. Environmental factors included temperature, humidity, wind speed, and direction, while cow characteristics included body weight, milk production level, and hair coat depth.…”

Heat production and retained energy in lactating cows held under hot summer conditions with evaporative cooling and fed two rations differing in roughage content and in vitro digestibility

“…It is well accepted in the literature that the main factors that might affect the partitioning of energy in lactating cow are level of dry matter intake (DMI), environmental conditions that might affect both DMI, energy expenditure as heat production (HP), the production level of milk and body tissue gain and individual variability between cows with respect to the efficiency of energy utilization for maintenance and production (Berman, 2005;Brosh, 2007).…”

“…Voluntary DMI can decrease by as much as 6% when ambient temperatures rise above 308C (Eastridge et al, 1998). Therefore, under hot weather conditions during the summer, most of the dairy farms in Israel cool the lactating cows by 'evaporative cooling', namely an arrangement whereby first the cows are made wet using sprinklers and then exposed to fans to promote evaporation (Flamenbaum et al, 1986;Berman, 2005). At present, in Israel only minor changes are made to the total mixed ration (TMR) composition in order to adapt it to the hot weather conditions.…”

Climatic effects in Central Europe on the frequency of medical treatments of dairy cows

“…Berman (2005) stated that heat stress relief is influenced by environmental factors, including temperature, humidity, wind speed and direction, and by animal characteristics such as body weight, exposed body surface, milk production and hair coat. As in regions of intensive dairy production in general, heat stress is of increasing concern in Central Europe.…”