Modern dairy breeds like the Holstein Friesian breed are developed in northern countries, meaning they are tolerant towards cold weather conditions but susceptible to heat. In many areas around the world, summers are hot and long. This is not only a challenge to humans but even more so for dairy cows, as they are already exposed to heat stress when temperatures rise above 22°C, especially when accompanied by high humidity. Table 1 illustrates the various levels of heat stress in dairy cows according to the temperature-humidity index (THI).
Table 1. Heat stress and temperature humidity index (THI).
Effects of heat stress
With rising outdoor temperatures, a cow’s body temperature also rises. This comes along with an increase in respiratory rate, salivation and water consumption, indicating noticeable discomfort. What follows is usually a drop in dry matter intake (DMI), milk yield and milk fat production, and impaired reproductive performance. High producing and fresh cows are particularly affected; however, heat stress also has an underestimated negative effect on dry cows and heifers. Several physiological changes are connected to heat stress in dairy cows. For example, the alteration of blood ow distribution towards the peripheral tissues helps the cow cope with the heat by increasing heat loss. Panting is also a way of cooling the body, which is important for cattle as their ability to sweat is limited. However, with increased panting, the acid-base balance and pH of the blood might be altered, possibly leading to respiratory alkalosis. On the other hand, the shift towards higher proportions of grain in summer diets might influence the rumen and result in rumen acidosis. As such, the subsequent drop in DMI and milk yield and failures in reproductive performance are not surprising.