Heat Stress Can Cut Chilean Milk Yield by 8 Litres Daily
Researchers led by Rodrigo Arias of the Austral University of Chile examined five years of weather data to assess heat stress across Chile’s dairy sector. The study used hourly records from 19 meteorological stations covering the country’s eight dairy macrozones, with more than 100 days assessed in each season between November and March.
The share of days affected by heat-stress conditions varied considerably between years, ranging from 5% to 50% of the period studied. Arias said that a particularly warm summer associated with El Niño could produce heat-stress conditions on at least half of summer days. The estimates concern the spring-summer production season.
Weather indicators produce different estimates
The conventional Temperature-Humidity Index, or THI, indicated that cows experienced heat stress for an average of 5.3 hours a day. A modified measure, known as THIa, added solar radiation and wind speed and raised the estimated exposure to 7.7 hours per day.
The researchers said that relying on temperature and humidity alone may understate the conditions faced by animals, particularly in pasture-based systems. In southern Chile, cows are often exposed directly to sunlight, wind, humidity and ambient temperatures. Arias recommended that farms in those areas consider the adjusted THI or the Comprehensive Climate Index when assessing risk.
Central Chile has more confined operations equipped with roofs and fans, although losses can still be substantial when shade or cooling capacity is inadequate. In macrozones I and II, the study estimated that severe heat stress could reduce output by up to 8 litres per animal per day when solar radiation and wind were included in the calculation.
Economic effects and farm measures
The effects extend beyond milk volume. Heat can lower dry-matter intake, impair fertility, increase the number of days before a cow becomes pregnant again and raise culling rates. Depending on the macrozone and the index used, lost milk and culling accounted for between 66% and 92% of the total economic effect.
For a 150-day season, national losses linked to heat stress were estimated at between $29m and $108.4m. In some macrozones, the difference between calculations based on the two indices exceeded 90%. In macrozones I and II, estimated losses ranged from $83 to $184 per cow per month during summer.
The measures cited in the study include shade in waiting areas, better access to water and correctly designed sprinklers. A cow walking 600 metres to 1 kilometre from pasture to the milking parlour can experience an increase of about 1°C in body temperature, according to records cited by Arias, with the risk particularly relevant during afternoon milking.
Arias also cautioned that cooling equipment developed for other countries may not work in the same way under Chilean conditions. Fine droplets used for misting mainly cool the air, whereas larger droplets are needed to wet the animal and support evaporative cooling. Shade structures require adequate height and ventilation; low roofs can trap humidity and create greenhouse-like conditions. The university has developed the Termómetro Lechero application, which uses weather information to display heat-risk indices and support changes in farm management. The research involved the Austral University of Chile and DeLaval Chile.
