Heat stress in dairy cattle and youngstock is a severe issue affecting productivity, health, fertility, and welfare. And with temperature and humidity levels continuing to rise in the UK, it’s a problem more farmers will already be facing or likely to face in their herds. In this blog, we look at how it can affect stock, symptoms to look out for, and how to prevent it to stay a step ahead.
Cattle are sensitive to factors which impact their thermal exchange with the environment. These factors include:
- Air temperature
- Radiant temperature
- Air velocity
- Relative humidity
Heat stress occurs when cows and calves generate and absorb more heat than they can get rid of by respiration, sweating, and air blowing by them. This leads to increased respiration rates, body temperatures, sweating, and time standing and is a significant welfare concern.
Air and radiant temperature directly influence the heat exchange ability of the animal. Air velocity increases the amount of heat transfer from the surface of the cow whilst air movement can also improve evaporation, which assists in heat loss. However, humidity can decrease the heat exchange and have debilitating effects on the cow.
When does heat stress occur?
Dairy cows are most comfortable (thermoneutral zone) when temperatures are between -15°C (Lower Critical Temperature (LCT)) to +25°C (Upper Critical Temperature (UCT)). However, humidity plays a significant part because, as the relative humidity increases, the temperature at which a dairy cow exhibits signs of heat stress falls.
Cows begin to experience heat stress at much lower temperatures than humans. This will depend on the animal’s age, production, and humidity. Generally, mild heat stress in cows starts around 22°C with 50% humidity, though high-yielding cows are more susceptible to heat stress as they eat more and generate more heat. They can begin to experience heat stress in well-ventilated sheds at air temperatures as low as 18°C.
Calves will likely start to feel the effects of heat stress at 21⁰C. Above this, they will have to use additional energy to maintain a normal body temperature of 38 – 39°C.
Using the Temperature Humidity Index (THI), it’s possible to determine whether your cows/ calves are at risk of heat stress. THI considers both temperature and humidity to estimate the level of heat stress cows will experience based on environmental conditions.
Cow Plan has developed a nifty calculator to generate real-time THI https://www.cowplan.com/thi .
Table 1 highlights when cows may be heat stressed based on THI.
|Heat stress level
||Temperature humidity index (THI)
||Respiration (breaths per minute)
||Body temperature (centigrade)
|No heat stress
||Less than 68
|Mild to moderate
|Moderate to severe
Signs of heat stress
When a cow’s temperature exceeds the UCT, they have two main ways to maintain their thermal balance:
- Increasing heat dispersion– evaporation through panting, drooling, and increased subcutaneous blood flow.
- Limiting heat production– by reducing activity and decreasing her appetite.
Cows that are heat stressed will have reduced production, fertility, and health issues. Research suggests mild to moderate heat stress can reduce yields by 2.6L a cow a day and moderate to severe by almost 4L a day.
Heat stress can also affect fertility and embryo development. For example, according to one study in the Journal of Dairy Science1, cows subjected to heat stress were 63% less likely to get pregnant than those not.
Early growing embryos are also likely to die within the first three days of fertilisation from prolonged periods of heat stress. Heat detection also becomes more difficult due to a cow’s or heifer’s lack of activity or movement.
Calf production can also be severely impacted by heat stress. Youngstock can experience poor growth rates from reduced feed intakes and lethargy, which can cause strain on metabolic processes in the body and increase the risk of disease/illness.
Signs of heat stress in cows and calves include:
- High respiratory rates (normal = 30-60 bpm for calves/ 40-60 bpm for cows)
- Reduced feed intakes
- Open-mouthed panting
- Increased water consumption
- Sweaty coats
- Higher rectal body temperatures
- Calves lying flat out/lethargic
- Poor growth rates in growing heifers
- Smaller breeding size
- Poor fertility
- Coughing/health problems
- Reduced milk production
- Rumination problems/acidosis
9 steps to managing heat stress
Heat stress can be managed by simple system changes, including:
- Monitoring shed temperatures daily – use a min/max thermometer and record temperature fluctuations. Recording humidity is also beneficial to calculate THI.
- Adjusting the diet – when feed intakes reduce due to heat stress, the nutrient concentration should be increased to maintain the equivalent nutritional level of the diet. This can be done by increasing the energy density, using high-quality forages and feeding more concentrates. However, care should be taken to balance diets properly to avoid digestive disorders such as acidosis and displaced abomasums.
- Changing feeding times – encourage higher intakes by feeding 60-70% of your herd’s diet between 8pm and 8am – the coolest part of the day.
- Provision of water –water intakes can increase by 10-20% in hot weather, so it’s essential that yards, buildings, grazing areas, and dispersal areas are well supplied with clean and fresh water. When cows are out in the field, consider adding additional troughs as their activity is reduced. Cows can consume 100L a day in hot weather2.
- Assessing air flow – by using smoke bombs. If the flow is inadequate, consider adjusting the ventilation by installing an open/protected ridge in the roof and increasing the number of air inlets. Mechanical interventions such as fans and/or sprinklers can also help. Airflows as low as 10km/hour can reduce respiration rates in heat-stressed animals by as much as 50%.
- Reduce sun exposure and improve airflow – look at opening inlets and outlets, prevent hot spots under sunlights, and consider mechanical ventilation, if necessary
- Handling – ensure any handling or routine work is carried out in the coolest part of the day
- Stocking rates – if possible, consider reducing stocking rates in the shed
- Increasing milk intakes – for calves, consider increasing milk by 0.5-1L per day
- -K. Schüller, O. Burfeind, W. Heuwieser,Effect of short- and long-term heat stress on the conception risk of dairy cows under natural service and artificial insemination breeding programs, Journal of Dairy Science, Volume 99, Issue 4, 2016, Pages 2996-3002.