The threat of heat stress: how to tackle it efficiently - Part 1: Causes, symptoms, and consequences on milk yield and composition

Let’s discover what heat stress is and how to prevent animals from it, with the advice of our experts. 

Rising temperatures all over the world make heat stress a bigger and bigger threat to animals.  

Let’s dive into this issue, outlining what it is, how it affects cows at every stage of life, as well as milk production. Then, with the support of our American Regional Sales Managers, Mitchell Edgmon and David Kerr, both from the Arizona desert, we will provide some helpful advice on how to efficiently protect your herd from this risk.  

As both our experts underlined, if your animals appear to be irritable, spend too much time standing, suffer from lameness, or pant while temperatures and humidity are high, heat stress probability is significant. In fact, excessively hot and wet climate, as well as solar radiation combined with an insufficient air movement lead environmental temperature exceeding cows’ TNZ (Thermoneutral Zone), with the potential to provoke health issues in the animals.  

In detail, TNZ is the temperature span in which a cow can maintain normal body temperature, without either altering metabolic heat production, or employing evaporative heat loss mechanisms. 

When cows are in heat stress risk, measured through the THI (Temperature Humidity Index), their heat load becomes greater than the capability to disperse it through normal metabolism. They thus increase the water intake, while reducing the feed one, in turn undermining milk production and its composition. 

How does thermal stress affect different stages of the cows’ life cycle? 

Although TNZ depends on breed and milk yield, heat stress affects every stage of the cow cycle, representing a real threat to milking efficiency. 

In case of pre-weaned calves, an excessive heat stress could imply a shift of energy from growth to maintaining body temperature. This could then determine a physiological and behavioural impairment, along with poor growth and higher susceptibility to diseases. 

Moving to lactating cows, the relationship between heat stress and mastitis is evident, as the former has negative effects on cattle immune response to pathogens. 

Moreover, since more energy is used to cool down and less energy is directed to milk production, the result will be an increase in SCC and a decrease in milk quality and quantity. 

Among dry cows as well, a peak in intramammary infection during Summer can be seen. More specifically, at this stage of life, animals have depressed lymphocyte proliferation, which causes a depression in response to antigens. 

Let’s focus on heat stress’ impact on milk production and composition 

As the previous lines suggest, not only is milk affected by heat stress in terms of yield, but also quality and composition. Let’s thus understand how this happens to develop some possible solutions to prevent animals from this risk. 

Figure 2 - Effects of heat stress on cows 

Inspired by: Heat Stress and Dairy Cow: Impact on Both Milk Yield and Composition 

When in thermal stress, metabolic activity is altered, and the feed intake is reduced. This decrease in nutrient absorption leads to irregular rumen function, with the ultimate consequence of reduced milk yield and a negative effect on reproductive potential, as highlighted by our experts. 

Intestinal mucosa loses its barrier function at tight conjunctions’ level, which link intestinal epithelial cells, exposing the organism to poor digestion and intake, as well as to toxins which could cause inflammation. The immune system thus requires energy from the cow, which suffers from insufficient feed intake due to heat stress, further decreasing its milking and reproductive efficiency. 

To sustain the milk production, high-yielding cows have to eat more. Thus, a decrease in feed intake might hinder their dietary requirements for milk production. 

Moreover, too high temperatures and humidity reduce their ability to counteract the excess of heat. Accordingly, heat stress would determine physiological changes in milk composition, such as a reduction in fats and proteins. 

A further factor could be thermal stress’ effect on udders, as it triggers mammary gland involution. Not only does this lead to a decrease in epithelial cells and a consequent decline in milk yield, but it also weakens the immune system, facilitating mastitis infection and its spreading within the herd, deeply undermining milking outcome. 

Heat stress also affects hormones controlling milk production. In fact, it causes an imbalance in the endocrine system and changes in hormone profiles. Lower levels of thyroid hormones and oxytocin combined with higher levels of cortisol and progesterone all lead to a reduction in feed intake and milk production. 

To conclude, potential impact of heat stress on milking efficiency could be massive. It is therefore essential to adopt proper measures to face warmer months and increasing temperatures as prepared as possible. How could you do it? Well, don’t miss our next blog post! 

milkrite | InterPuls thank Mitchell Edgmon and David Kerr for their support.