Dairy cows are homeothermic animals and need to maintain a constant body temperature of around 38.6֯C (+/- 0.5֯C) for efficient metabolism. To maintain this temperature, the heat generated from normal rumen digestion and metabolism must be lost to the environment. When that environmental temperature is high, this heat is lost more slowly, causing the cow’s body temperature to rise, resulting in heat stress. To dissipate body heat, dairy cows react by reducing feed intake (typically by 10-15%, but as much as 30%) and rumination time, are selective in what they eat (forages increase rumen activity and therefore heat production so cows will seek out concentrates or feeds that produce less heat), increasing respiration rate (>80 breaths per minute), standing time and water intake (by up to 20% and can drink up to 20L per minute!), excessive salivation, drooling and panting. This leads to a suboptimal rumen environment and function: lower rumen pH, lower volatile fatty acid and microbial protein production and nutrient digestibility. Requirement for maintenance increases as the cow attempts to lose body heat.
Heat stress is a result of a complex relationship between temperature and humidity. The Temperature Humidity Index (THI) is used to determine occurrence and severity of heat stress. Do you know the ambient temperature and relative humidity in your sheds? Figure 1 shows the amount of time we spent above the heat stress threshold in the South West last summer.