Farmers are being advised to take preventative steps ahead of late spring and summer to reduce the impact of heat stress on cows and calves, after research highlighted the long-term consequences it can have on health, welfare and productivity1.
A paper published last year reviewing the effects of heat stress on dairy cows1 found it can have significant knock-on effects, not only on milk production but also on fertility and foetal development. Calves born to heat-stressed cows may have lower birthweights and compromised immunity, creating longer-term challenges for health and performance.
Whilst a temperature humidity index (THI) value of 68 has traditionally been used as the threshold for heat stress, fertility begins to be affected at a THI of 65, this can be 18°C – 20°C when humidity is high. Last year, this threshold was widely exceeded by early May in many parts of the country.
The impact of heat stress can be even greater in more vulnerable animals, such as calving cows and young calves. Around calving, cows face a sharp rise in water demand, while calves are more susceptible to dehydration.
Nettex livestock specialist Nia Williams says farmers should be looking at heat stress as more than just a summertime comfort issue.
“As temperatures become more extreme, heat stress is something farmers need to manage proactively rather than reactively,” she says. “There can be immediate effects on performance, but there can also be longer-term impacts on health, fertility and calf development, which makes prevention really important.”
She says practical measures remain the first line of defence, with housing design, airflow and water access all playing a central role.
“Good ventilation, fans, shade for cattle at grass and sprinkler systems where appropriate are all important,” says Mrs Williams. “But alongside that, farmers need to make sure cows have enough drinking space, both indoors and outside, and that calves have access to fresh water from birth. These may seem like basic measures, but they are absolutely fundamental.”
Hydration is also an important aspect, particularly in cattle under added physiological pressure.
Mrs Williams says that while all stock can be affected during periods of hot weather, fresh-calved cows and pre-weaned calves require close attention.
“At calving, cows are already under huge strain and dehydration is inevitable to some extent,” she says. “That can be much more severe after a difficult calving, and when a cow is slow to recover, reluctant to stand or unwilling to eat, the effects can very quickly escalate.”
Cows can lose around 60 litres of fluids and salts during calving, while feed and water intakes often fall around birth. That combination can leave them highly vulnerable in the immediate post-calving period, particularly if temperatures are also high.
“Post-calving rehydration is absolutely critical,” says Mrs Williams. “Water on its own does not provide the electrolytes, energy or calcium a cow may need at that point. The sooner she can drink and restore that balance, the sooner she is likely to recover, eat and get going again.”
This is where targeted rehydration products can support transition management. Nettex Fresh Calver is designed as a palatable supplement for freshly calved cows, providing immediately available energy, electrolytes to aid rehydration, available calcium to meet increased demand, and vitamin E and selenium to support cleansing.
“The aim is not simply to get fluid into the cow, but to encourage rapid recovery at one of the most critical points in the lactation cycle, when the risks of hypocalcaemia, retained foetal membranes, displaced abomasum and negative energy balance are all heightened,” adds Mrs Williams.
Staffordshire farmer Tom Parkinson says using Fresh Calver routinely after calving has delivered clear benefits in his herd.
“Every cow is given an electrolyte solution, Nettex Fresh Calver, as soon as the calf is born,” he says. “It provides energy and calcium and has helped reduce the risk of milk fever. It really gets them going.
“We had a Jersey calve recently and within 20 minutes of giving Fresh Calver she was up and moving. It helps get the cow going and the calf onto colostrum, and we also find it helps support the cow to cleanse properly.”
Calves are also at risk and highly susceptible to dehydration and can deteriorate quickly, particularly during digestive upset or hot conditions. Their body composition, with a high surface area to volume ratio, makes them more vulnerable to fluid losses than adult animals, and the effects can be seen rapidly in reduced milk intakes, droopy ears, sunken eyes and a drop in activity.
Mrs Williams says early recognition is vital. “By the time a calf looks obviously unwell, it may already be significantly dehydrated,” she says. “That is why it is so important to pick up on subtle behavioural changes early and intervene quickly.”
Rehydration support can be particularly useful during periods of digestive disturbance or other stress events, with products such as Nettex Hydra-Power Advanced delivering a six-in-one oral hydration formula for calves, providing electrolytes, energy, an alkalising agent, a natural antioxidant, probiotics and a gelling agent.
Designed to help stabilise water and electrolyte balance and support normal digestion, it aims to improve water absorption from the gut and offer practical on-farm support when calves are at risk.
“Hydration should be seen as part of the wider health and performance picture,” says Mrs Williams. “It is not just about replacing lost fluids. It is about supporting immunity, growth, recovery and resilience.”
With hotter spells expected to become more common, she believes hydration planning needs to sit alongside wider heat stress management strategies, rather than being treated as an afterthought.
“Preparation is key,” she says. “Making sure facilities are right, water access is adequate and the right products are on hand can make a real difference when temperatures rise. Small steps taken early can help avoid much bigger problems later.”
Common signs of heat stress
- Weight loss
- Reduced rumen fill
- Lower milk production
- Changes in feed intake
- Reduced lying time
- Scouring
- Sunken eyes
- A dry mouth and nose
- A fast or unusually slow pulse
- Cold ears and/or legs
- Fluctuating energy levels
- Increased respiration rate
References
- Oliveira CP, Sousa FC, Silva ALD, Schultz ÉB, Valderrama Londoño RI, Souza PAR. Heat Stress in Dairy Cows: Impacts, Identification, and Mitigation Strategies-A Review. Animals (Basel). 2025 Jan 17;15(2):249. doi: 10.3390/ani15020249. PMID: 39858249; PMCID: PMC11758294.
