ISSUE FOCUS FEED & ADDITIVE MAGAZINE June 2024 31 stress at lower THI levels, around 62. THI does not account for radiation, making it more suitable for housed dairy cattle rather than those grazing in direct sunlight. The Dairy Heat Load Index (DHLI) is a relatively new index used to assess the risk of heat stress in dairy cows. It's designed to be a more suitable predicter for grazing cattle. The DHLI uses humidity and temperature measurements from a "black globe" placed where the cattle graze, providing a more accurate assessment of the conditions cows face. WHY DOES HEAT STRESS IMPACT SUSTAINABLE DAIRY FARMING? Increasing the Lifetime Daily Yield (LDY), (the average production per day from birth to culling), is crucial for sustainable dairy farming. LDY is influenced by milk production per lactation and the number of lactations per cow. Heat stress can lead to systemic immune activation and inflammation, which negatively impacts both. While systemic immune activation often occurs around the transition to lactation, heat stress can happen at any time, exacerbating the problem. When cows experience heat stress, they try to cool themselves by reducing their dry matter intake (DMI), redirecting blood flow to their skin, and increasing evaporation through sweating and panting. This cooling response leads to several issues: • Reduced Nutrient Intake: Lower DMI means fewer nutrients for milk production. • Gut Integrity: Reduced blood flow to the intestines can cause "leaky gut", leading to systemic immune activation and inflammation. • Respiratory Alkalosis: Panting decreases blood CO2 levels, increasing blood pH and causing rumen acidosis, further exacerbated by saliva loss. Heat stress also negatively affects cow fertility. Elevated body temperatures can impair oocyte quality, embryonic growth, and overall reproductive function. Additionally, heat-stressed cows may show reduced signs of estrus due to lethargy. MANAGING HEAT STRESS EFFECTIVELY: MITIGATING ACTIONS Cows lose heat through radiation, convection, conduction, and evaporation. If these methods are insufficient, cows will start panting and sweating. However, cows have a low sweating rate, and high Mild Moderate Severe Heat stress level THI Effect 72-78 78-82 82 or Higher mild heat stress and reduced fertility reduced milk production significant milk production losses and severe heat stress symptoms Table 1. Heat Stress level of cows related to the Thermal Heat Index (THI) level and the effect on cows 20 30 40 50 60 70 80 25 24 23 22 21 20 I/day Temperature Humidity Index (THI) Figure 1. Changes in milk yield in grazing first lactation cows related to THI. Adjusted from Hammami et al 20131.
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