Heat stress is no stranger to New Zealand dairy farmers, with drier regions of the country such as Northland, King Country, and the Bay Of Plenty each averaging over 80 sweltering hot heat stress days every year.

Cows known to have suffered from heat stress will not only produce less and poorer quality milk, but also suffer from reduced fertility, and increased susceptibility to disease. Further to this, extended bouts of heat stress are shown to have a drastic impact on the number of cattle that lose their pregnancy mid-term, along with a significant increase to lameness in the herd, numerous metabolic issues, and potentially even heat stroke and death.

The prevention of heat stress is no easy feat – you can’t control the weather after all – but you can put systems and structures in place to help mitigate the risk to your herd and profit margins.

The definition of heat stress in dairy cattle

Defined as the inability to regulate body temperature resulting in overheating, heat stress is a major contributor to poor milk production yields and overall health of dairy cattle herds. Heat stress occurs not just when it is hot, but when it is too hot and too humid for cows to cool down properly.

Scientifically speaking, heat stress is caused by a number of prevailing conditions, with factors such as relative humidity, ambient temperature, and evaporation rate all taken into account as part of the temperature humidity index (THI).

THI is the industry standard best practice measure to evaluate discomfort in cattle and the likelihood of heat stress rearing its ugly head on any given day. Heat stress occurs in dairy cows between a THI of 72 and 99, with a THI of 100 often resulting in death.

Here are a few examples of various THI readings:

  • On a day with a temperature of 25°C and 50% relative humidity, THI is rated at 72 which is enough to induce mild heat stress (and this weather is not uncommon in New Zealand)
  • Any days where the THI is rated 78 or above, the negative impacts of heat stress are significant and enough to drastically reduce milk yield and start causing severe health issues
  • On a sweltering hot 37°C day with 100% relative humidity (highly uncommon in New Zealand, but not impossible) THI is rated at 99, which induces the most severe heat stress symptoms and negative health impacts
  • Heat stress will not occur in dairy cattle at a THI reading of 72 or below.

What are the symptoms of heat stress in cows?

While the symptoms of heat stress in dairy cattle depends on the breed of cow and the severity of the weather, a common sight in cows suffering from heat stress is that their mouths are agape, wide open and struggling to regulate their internal body temperature. This is further emphasised by the frequent panting that often accompanies such a condition, as cattle attempt to improve their air intake to further cool the body.

Another obvious sign is that heat stress-affected cows will be comparatively lethargic to normal times, appearing to be less active, and, by all accounts “sluggish”, but will often stand and refuse to lie down in order to improve airflow around the body.

Heat stress will often drive cows to seek out shade where available, or a body of water to stand in if there is no shade but a creek or pond nearby. It is not uncommon to also observe a significantly decreased feed intake – which can sometimes lead to not eating altogether – along with an uptake in water consumption.

Dairy cattle undergoing heat stress will almost always have an increased heart rate compared to normal times, as their bodies have to work that much harder in an attempt to regulate the extreme heat.

Heat Stress Calculator

To help farmers calculate how heat stress can negatively impact productivity and profit margins, we have created an interactive Heat Stress Calculator which allows for customisable inputs including cow breed, herd size, milk price, and number of heat stress days per year in each region (with an easy to access guide as a reference point).

Calculate your herd’s maximum and minimum heat stress loss estimates with our calculator Heat Stress Calculator today.

List of heat stress symptoms in dairy cows

For clarity, here is a list of all heat stress symptoms that have been seen in New Zealand dairy cows:

  • Lethargy
  • Animals seeking shade and/or wind
  • Agitation, restlessness and a refusal to lie down
  • Accelerated and laboured breathing through an open mouth
  • Sweating and excessive drooling
  • Greatly increased drinking, decreased eating
  • Increased urination (caused by electrolyte loss)
  • A drop in milk yield and lower milk quality
  • Emaciation
  • Increasing instances of infection, disease and other health issues
  • Lower fertility, increased embryo mortality and birth defects
  • Reduced weight gain in growing animals
  • Convulsions, collapse and instances of coma.

This list was previously published in our ‘Preventing Heat Stress For Cattle’ article in 2020.

How heat stress can negatively impact productivity and why you should do something about it

In light of the negative health effects to cattle and the impact on milk production yields, heat stress can significantly change the profitability of a dairy farm when not actively managed or prevented, meaning there is more work to be done to achieve the same result, not less.

And then there is the Dairy Cattle Code Of Welfare and Animal Welfare Act 1999, which governs the humane treatment and care of all cows in New Zealand. The Code – as the Dairy Cattle Code Of Welfare is commonly referred to – is actively enforced by several government agencies and can provide farmers with a considerable legal challenge should they not remain compliant, further hampering the operational efficiency of a farm.

The Code has a raft of proposed changes currently (at the time of writing) before government legislators and likely to take effect in the coming seasons, some of which directly speak to the need to provide adequate shelter and protection from heat stress. Further reading on these proposed changes can be found here.

Here at SmartShelters we have a range of suitable shelters for dairy cattle, providing cows with a warm, dry environment to rest in, helping to maintain high milk production yields while keeping the negative effects of heat stress at bay.

All in all, heat stress needs to be taken seriously in order to keep productivity levels and profit margins pointing in the right direction, with the provision of purpose-built shade structures just one part of the solution towards preventing heat stress and keeping your herd cool and productive all year round.

Safeguard your herd against the negative effects of heat stress

Good animal welfare management is not just the kind and humane thing to do – it is also good business practice. Improvements to the welfare of dairy cattle have been shown to improve dairy yields, leading to greater sustainability of the industry as a whole, and providing a long-term financial incentive to those who prioritise the welfare of their herd.

SmartShelters provides a range of high quality, built-to-last farm shelters for dairy cattle, increasing the productivity and income of your farm, all the while protecting the health of your herd and keeping you compliant with the Dairy Cattle Code Of Welfare and Animal Welfare Act 1999, and the proposed upcoming changes to The Code.

Our team of dedicated agricultural industry experts are on hand to help you get and stay compliant ahead of the proposed changes to The Code, and will be happy to assist you with any questions you may have.

Contact us today.