nouvelle img enav


Summer is upon us, and it is already time to think and prepare for the stress that our dairy herds will experience over the next few months. Let’s say, the summer of 2018 brought its share of particularly high temperatures and the impacts on productivity were very important. Moreover, July 2018 was the hottest month observed by the Quebec government in nearly 100 years in the south of the province.

Thermal stress in dairy cows
When we talk about heat stress, the term THI is regularly mentioned. In fact, THI is the abbreviation of the temperature and humidity index. It is calculated using the temperature, in degrees Fahrenheit, as well as the relative humidity (RH,%) in the following equation:
THI = T – [0,55 – (0,55 × RH / 100)] × (T – 58)
Dairy cows experience heat stress when the THI reaches 68. This stress is reflected among other things by a rectal temperature exceeding 38.5 oC and a respiratory rate exceeding 60 cycles per minute. At this stage, the impacts are manifold and can first be seen in the state of health of the animals.
When health is impacted, the energy of the animal is not devoted to milk production. Thus, performance declines result in a decrease in voluntary dry matter intake, a decrease in milk production, an increase in metabolic disorders, a decrease in the percentage of fat and protein, a decrease in reproductive performance and a weakening of the immune functions.

What about heat stress in Canada?
Being located further north, we sometimes tend to think that our cows are not subject to it. Studies show that, on the contrary, Canadian dairy cows are also subject to a decline in yield during the summer season caused by heat and humidity. As demonstrated by the study conducted at Laval University, cows subjected to several consecutive days of heat stress had lower milk, fat and protein yields than cows subjected to normal temperature, humidity and environmental conditions. The cows most affected by heat stress were the third and more lactations, with declines of 4.1 and 2.9 percent in the production of fat and protein in the milk. Not counting the longer-term impact on reproduction and overall health, thermal stress causes a decrease in direct income for the producer.

Effect on immunity
To understand the impact of heat stress on immunity, let’s start by defining what is normal immune function. In times of stress, the functions of the immune system fluctuate. When the burden of pathogens in the environment is not too high, together with a certain decrease in the functions of the immune system, the animal manages to handle the situation effectively and the negative impacts are limited.


As shown by Figure 1, when the burden of pathogens in the environment is higher and the stress is, at the same time, more intense, the animal will develop different diseases. Age, level of production, genetics, nutrition, transport, toxins, calving, THI, etc. are examples of the stresses and factors that affect immunity.Now, what is happening to immune function? In times of stress, it is first possible to observe an increase in blood cortisol. Cortisol, a steroid hormone secreted by the adrenal gland, is important in many situations such as farrowing. The objective is not to inhibit this increase. However, this rise in cortisol in turn causes a decrease of the protein L-Selectin in the blood. Neutrophils are considered to be the dairy cow’s first line of defense against pathogenic invaders and L-Selectin is a marker of innate immunity. Decreased L-selectin expression prevents neutrophils from functioning normally and penetrating pathogen-invading tissues.
This situation has dramatic consequences for the immediate response of the immune system and the clinical outcome of exposure to an infectious organism. Thus, in times of stress, such as heat and extreme humidity, the defenses of the immune system are less able to play their role as they should.


On the left of Figure 2, we find pictorially a blood vessel and immune cells in a normal situation. Yellow beads around neutrophils represent L-selectin cells. However, on the right side, there is a less optimal situation where the neutrophils do not manage to cross the blood vessel to get to the site of infection.
The weakening of immune function is therefore associated with the inability of neutrophils to migrate to tissues attacked by pathogens. These immune cells are less effective at destroying invading organisms, including phagocytosis, making the dairy cow highly susceptible to infectious diseases.
Recent research suggests that acute exposure to high THI damages cells in the digestive tract of the cow, allowing pathogens and endotoxins to enter the bloodstream. Following the introduction of these pathogens, there is activation of a strong immune response that requires a large amount of glucose, which would otherwise be used for milk production.
Cows exposed to high THI during the dry period also respond poorly to vaccination during this period. Thus, the secretion of antibodies is weaker and therefore the innate immune functions (phagocytosis of neutrophils and oxidative burst after calving) are altered. If the innate immune functions are negatively affected, it is now shown that this stress will have an impact on the health of the calf to be born as well as on its future development.

The addition of fans and water nozzles in the barn is a way to defeat the summer heat.

How to prepare for heat stress
Strategies to improve neutrophil function during stressful periods can certainly help immune function and promote disease resistance. Preparing the immune system of the dairy cow to mobilize large numbers of neutrophils could therefore be an important advantage against infectious organisms, such as those associated with mastitis.
To achieve this, some modifications to the control room such as the addition of fans and sprinklers are to be considered. Other important factors to consider are feeding during cooler periods, frequently cleaning feeding areas, and using more palatable forages are also important factors to consider. At the ration level, increasing energy density, maintaining adequate dietary fiber intake, increasing potassium intake and positioning certain food additives to maximize immune system functions are also good ideas. Thermal stress has significant repercussions several days after the heat. In the United States, the effects of this stress are estimated to cost the dairy industry between $ 897 million and $ 1.5 billion a year, so it is very relevant to pay attention to it and ensure that the system Immune is optimized.

Gabrielle L. Dumas
Technical Representative, DCL Nutrition and Animal Health

Written in collaboration with Annie Pelletier, agr., Phibro Animal Health.
References have been omitted but are available upon request.

Scroll to Top