ISSUE FOCUS 50 FEED & ADDITIVE MAGAZINE July 2025 This zone describes the cow’s comfort zone. Within the thermoneutral zone, they do not need to expend additional energy to keep their body temperature stable: heat production and heat loss are in a harmonious balance. On either side of the TNZ is the temperature range at which heat balance can be achieved, but the metabolic rate or evaporative heat loss will change to maintain balance. In this range, metabolic rate increases at low temperatures and evaporative cooling increases at high temperatures. As such, a homeotherm is under thermal stress whenever it is outside of its TNZ. A dairy cow’s TNZ is between approximately 5 – 20° C (Figure 3) although when lactating, they can withstand lower temperatures. Conduction, convection, radiation, and evaporation are just some of the ways cattle can dissipate heat. The difference between a cattle’s body temperature and the ambient temperature determines the success of these techniques and can quickly reach its limits (Figure 4). When the outside temperature rises in combination with high humidity, thermal stress in cows often reaches critical levels as evaporation becomes negligible. Cold Stress Heat Stress Optimal temperature for performance and health Lower critical temperature Upper critical temperature 5°C 20°C Ambient temperature ↓Milk yield ↓Fertility ↓Feed intake ↑Body temperature ↑Respiration rate ↓Lying time Thermalneutral Zone (TNZ) Figure 3. On either side of the TNZ is the temperature range at which heat balance can be achieved, but the metabolic rate or evaporative heat loss will change to achieve balance. In this range, metabolic rate increases at low temperatures and evaporative cooling increases at high temperatures, meaning that more energy or more water is required for thermal homeostasis. As such, a homeotherm is under thermal stress whenever it is outside of its TNZ Potential death Danger >45° C* Moderate 31 - 38° C* Mild 25 - 30° C* None <25° C* Severe 39-45° C* * At 50 % relative humidity ↑↑body temperature ↓↓milk production ↓lying time ↓production ↓feed intake ↑water intake ↓reproduction ↑body temperature ↑saliva production ↑respiration rate ↑shade seeking ↑respiration rate ↑blood vessel dilation minimal impact on milk production ↓↓reproduction ↑↑respiration (panting) ↑↑saliva production Figure 4. The severity of heat stress is strongly dependent on the ambient temperature and humidity
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