Impacts and Actions Following Heat Stress in Cattle Farming

Heat stress refers to the inability of cattle to maintain a normal temperature when temperature and humidity are high. The rumen being a large fermenter, it generates heat. The thermal optimum for the dairy cow is therefore between 5 and 15°C. Beyond these values, the animal will implement compensatory mechanisms to maintain a constant body temperature. Beyond a certain threshold, these mechanisms are overwhelmed and the rectal temperature rises : the cattle is then in heat stress.

The consequences of this heat stress are numerous, which is why it is important to detect and limit it.

Detecting heat stress in your herd

Observing your herd

Ideally, heat stress is assessed by measuring the temperature of animals that are apparently healthy. Normally, this temperature is around 38-38.5°C and rises in case of heat stress. It is possible to observe heat stress by counting respiratory movements.

Panting score and body temperature (T°C) of dairy cows ^[1]
Panting score	Description	Breaths per minute	Heat stress
0	Normal without panting.	<60	-
1	Light panting, mouth closed without salivation.	60-90	Light to moderate stress T°C > 38.5
2	Rapid panting with salivation. No open-mouth panting.	90-120	Moderate to severe stress T°C > 40
3	Open-mouth panting and hypersalivation. Neck stretched and head often raised.	120-150	Severe stress T°C > 41
4	Open mouth with tongue fully extended for long periods and hypersalivation, often associated with neck stretched forward.	>150	Very severe stress

This only gives an indication of temperature but cows are also very sensitive to humidity.

THI (Temperature-Humidity Index)

THI (or in English THI: temperature-humidity index) takes into account temperature and relative humidity of the air. It is not the most comprehensive indicator but it has the advantage of being easy and large-scale to calculate, these two parameters being measured at every weather station and publicly available. Signs of heat stress appear as soon as the THI reaches 68. For more details, see the annexes.

It is possible to make a forecast curve of THI to anticipate heat waves and implement preventive actions.

There are also mobile applications that estimate THI day by day and thus predict heat stress episodes : Thermotool^[2] by CCPA and Heatstress ^[3] by Phileo.

Risks of heat stress

Metabolic changes

The maintenance needs of the cow increase with temperature. The metabolism will indeed be modified to maintain body temperature around 38.5°C. Thus, there will be an increase in respiratory rate, more abundant sweating and an increase in heart rate. If these regulatory mechanisms are insufficient, rectal and vaginal temperatures will increase.

Changes in cow behavior

Cows will reduce lying time by about 30% to stay standing to better dissipate heat. They will also actively seek watering points and shade. If they drink more, they eat less and ruminate less to limit heat production. This decrease in intake (10 to 35%) will be more pronounced in animals fed with a ration composed mainly of forages. Moreover, rumination stimulates saliva production which allows proper rumen function. A decrease in rumination and intake will therefore be accompanied by a limitation of the cow's production capacity.

Reduction in milk production

The decrease in production is linked to the decrease in intake but especially to metabolic changes. The intensity of this decrease will depend on:

The intensity and duration of the stress : the higher the THI and the longer the stress, the more significant the production drop.
The initial production level of the animal : the more productive the cow, the greater the decrease.
Its lactation stage : production of cows in mid-lactation is more impacted than those in early or late lactation. According to a study by Brian Lang in 2011, for cows producing more than 45 kg of milk per day, the THI calculation should take into account a temperature 5°C higher than the actual temperature.

Alteration of milk composition

Lactose content is not affected by heat stress.

Fat content is sometimes unaffected and sometimes lower in case of heat stress. A monitoring of the fatty acid content can be a good indicator of animal health.

Protein content tends to decrease in case of heat stress.

An increase in somatic cell count is also often observed during periods when THI is high.

Reproductive impairment

In females, several changes in the functioning of the reproductive system can be observed :

Decrease in follicle and oocyte quality.
Decrease in intensity and duration of estrus.
Lower conception rates.
Increased embryonic mortality.

All this compromises fertility and fecundity of cows.

Compromised welfare and health

In case of heat stress, one can observe :

Competition behaviors when available shade is limited and for water points.
Disruption of rumen function : cows more prone to ketosis.
Increased risk of hoof problems (lameness).
More frequent cases of clinical mastitis.

Suffering of dry females, young stock and fetuses

Calves whose mothers were subjected to heat stress late in gestation have lower birth weight and access to lower quality colostrum, which can threaten their survival.

Moreover, lactations will be lower for females whose mothers were subjected to heat stress late in gestation.

Activating levers to limit heat stress

Adapting daily herd management

If animals go out to pasture, keep them indoors during the day and let them out at night. Otherwise, trees in the pasture can provide shade points. Agroforestry will create a microclimate beneficial to animal comfort and welfare.

Watering is a key point to limit the effects of heat stress. Consumption can easily increase by 50% and go from 85L to more than 120L/day. It is therefore necessary to ensure water supply to drinkers, clean them daily and add additional water points if needed. It is also important to ensure that water temperature does not rise too much, in other words, avoid drinkers being in full sun. It is recommended that each cow has access to at least 10 cm of drinker length.

Ideally, multiply feedings to offer smaller quantities each time. This will avoid ruminal fermentation peaks and prevent cows from sorting their feed. If the number of feedings is to be limited, the ration should be mostly consumed in the evening and early morning. Feedings should preferably take place late in the day to avoid heating and increase intake.
It may be interesting to add 3 to 5 liters of water per cow in the ration to improve palatability and increase water intake. The use of a forage stabilizer such as propionic acid also helps limit heating of the feed bunk.

Adapting feeding

Since intake is lower and energy needs are increased, it is necessary to densify the ration in energy and protein while maintaining a sufficient fiber content (18%) to preserve rumination and rumen flora balance.
Mineral losses are significant during heat stress, notably via urine, sweat, and respiration. A temporary increase in mineral and vitamin supplementation helps compensate for these losses while providing additional vitamins and trace elements that limit oxidative stress and thus immune deficiency.
The risk of sub-acidosis is increased on these days. Bicarbonate supplementation helps reduce this risk.
Nutritional supplements based on yeasts, appetite-stimulating plants, trace elements and vitamins help maintain adequate performance even during heat waves. You can use a dietary supplement such as ThermoSan®^[4], available from San’Élevage, which helps dairy cows better withstand temperature peaks.

Having an adapted building

The frequency of heat waves forces us to consider effective means to improve herd comfort and welfare. Before starting building modifications check the following points :

Does the building allow sufficient air circulation? Are air inlets and outlets well positioned?
Is the bedding suitable? Does it remain clean and dry?
If the natural ventilation system is inadequate, is it possible to install a dynamic ventilation system such as fans or misting?

Cows must encounter conditions inside buildings that mitigate the impact of high heat. In summer, the building should resemble a parasol without any obstruction to air circulation.

Limiting radiation from walls, roof and building environment

Limit the height of walls exposed to the sun as they will radiate heat in the late afternoon and thus slow building cooling.
Grass-covered green areas around the building should be maintained as much as possible.
Do not install translucent roof panels on sun-exposed sides (if any, they should be covered inside with shading paint).
Insulate the roof, especially when close to animals. A 4 cm thick insulating panel is sufficient. The benefit of insulation in large buildings is reduced but in small volume buildings it reduces the perceived temperature by an average of 1.5°C.
Choose light colors for the roof to reduce heat accumulation.

Benefit from natural ventilation

Ventilation is more effective in narrow buildings exposed to winds. Long sides should not be obstructed by constructions such as silos, milking parlor, nursery...

Opening gates is beneficial except when exposed to the sun.
Open buildings more : dairy cows are very little sensitive to cold, the construction of a very open building is feasible in many regions. On the East, South and Southwest sides, openings must be protected from sun rays in summer by roof overhangs or light shade nets. On the West side, windbreak curtains can be installed, when the sun declines, offset from the facade to provide shade while facilitating air circulation.
To provide air velocity at animal level, the natural ventilation solution is to create free openings as low as possible along the long side.

Mechanical ventilation to complement

When previous solutions are insufficient, mechanical ventilation can be used. For more details on mechanical ventilation, click here.

Fan equipment priority orders (Source : Cniel)
Priorities	Conventional milking	Robotic milking
1	Waiting area	Lying areas, feed access area and waiting area in front of robots
2	Lying areas and feed access area	Traffic alleys between rows or behind cubicles
3	Traffic alleys between rows or behind cubicles

Sprinkling and/or misting can be interesting. It is not widely developed but can be very useful especially in waiting pens to relieve cows when they are crowded waiting for milking. They should only be used in open buildings or properly equipped with fans.

Nursery

For calves, it is essential to ensure quality ventilation, without drafts to prevent illness. Managing the atmosphere of your nursery during heat stress periods is crucial.