ISSUE FOCUS FEED & ADDITIVE MAGAZINE July 2025 51 Exposure to temperatures just above TNZ causes an adjustment in heat transfer by increasing blood flow to the skin. This increases the removal of metabolic heat to the periphery where it is lost. However, when the ambient temperature exceeds the skin surface temperature (~ 36° C), heat is transferred from the warmer air to the cooler skin, adding to body heat. To maintain thermal balance, evaporative cooling—through sweating and panting—must increase to match the additional heat load. Unlike more heat tolerant animals like horses, cows only have about 800 sweat glands per cm². As a result, cows sweat inefficiently and rely more on increased respiration for evaporative cooling, increasing from 40 – 60 breaths per minute to over 100 breaths per minute. The onset of panting impacts the cow’s acid/base balance and can lead to respiratory alkalosis. At the same time, water requirements increase dramatically—heat stressed cows have increased water requirements of up to 200 liters per day. CONSEQUENCES OF HEAT STRESS When heat dissipation is no longer sufficient, physiological adaptation mechanisms kick in. The cow reduces feed intake as microbial fermentation in the rumen generates additional heat leading to a cascade of physiological responses (Figure 5). As blood distribution is diverted to the skin to dissipate heat, blood flow to organs such as the rumen and liver is reduced, further compromising digestive efficiency and metabolic processes. This can lead to reduced energy supply, increased oxidative stress, mobilization of body reserves, hormonal imbalances and reduced milk yield. Immune defenses are also compromised, making animals more susceptible to infection and inflammation. The negative effects of heat stress on dairy cattle are well documented. In addition to reduced health and performance, the reproduction and fertility of dairy cows are severely affected, affecting the next generation. The fertility of a dairy cow is influenced by many factors, but environmental factors appear to be the most important. ↑Body temperature ↑Blood to skin ↓Blood to intestine ↓Microbial efficiency ↓Rumen function ↓Feed utilization ↑Rumen acidosis ↓Nutrient absorption ↓Intestinal damage ↑Leaky gut ↓Immunity ↑SCC ↑Oxidative stress ↓Available energy ↑Morbidity/mortality ↓Milk production ↓Conception rates ↑Maintenance energy ↓Rumen buffering ↓Saliva production ↑Drooling ↑Panting ↓Feed intake Figure 5. Heat stress is a combination of internal and external stimuli that works to increase body temperature and produce a physiological reaction
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