Feed & Additive Magazine Issue 17 June 2022

ISSUE FOCUS 38 FEED & ADDITIVE MAGAZINE June 2022 UNDERSTANDING HEAT STRESS IN DAIRY TO ALLEVIATE IT "Heat stress (HS) is caused by several weather factors: temperature, relative humidity, solar radiation, air movement, and precipitation. Due to climate change and the selection of high-producing animals, dairy cows are more confronted with high temperatures while getting more sensitive to HS, compromising their performance. Here is an insight on HS origin, its consequences, and mitigation strategies." ORIGIN AND DETERMINATION OF HEAT STRESS Parameters received by cows such as thermal radiation, wind speed, and precipitation are complex to be recorded, while temperature and relative humidity are simple measures. Thus, the most common indicator to estimate the magnitude of HS is based on these parameters and is called Temperature Humidity Index (THI). Johnson proposed the following equation in 1962 (in °F, then adapted to °C), and different thresholds have since been identified (cf. Figure 1): THI = (1.8 x T⁰Cdb + 32) – [(0.55 - 0.55 x RH/100) x (1.8 x T⁰Cdb – 26)] where; T⁰Cdb = dry bulb temperature (°C), and RH = relative humidity Cattle maintain a relatively constant core body temperature (CBT) over a wide range of weather. The CBT is a result of heat production and losses. In normal conditions, there are several sources of heat production by cows: basal metabolism, physical activity, rumination and digestion, and production metabolism. HS is observed when cows cannot maintain their CBT, leading to the rising of CBT and the impairment of metabolic functions. High-producing cows are more sensitive to HS than Delphine Lacombe Global Technical Manager Ruminant Delacon Biotechnik GmbH Figure 1. Calculated THI from air temperature and relative humidity (based on Habeeb et al., 2018) - THI < 68: no heat stress - THI > 72: milk yield impaired, calf rate is affected - THI > 78: milk production is strongly impaired, dry matter intake is reduced - THI > 82: signs of severe heat stress

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