Heat Stress in Non-Milking Cattle

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Allaster Dallas (FCG Midwest)

There has been a lot of focus on the effects of heat stress in dairy cattle and its impact on milk production. It is worth noting its effect on the other stock on the farm, in particular calves and youngstock. But how is heat measured, and if your farm doesn’t have the technology or run an intensive system, does it matter?

What is THI?

The Temperature-Humidity Index (THI) for cattle is a single numerical value that combines ambient air temperature and relative humidity to measure the total heat load on an animal. Because cattle cannot sweat effectively, high humidity blocks their ability to cool down through panting, meaning they experience heat stress at much lower temperatures than humans do.

Because the UK has an average relative humidity that routinely exceeds 70%, cattle reach a “mild heat stress” threshold (THI of 68) at ambient temperatures as low as 21°C-22°C.

Practically, how can you tell the THI?

There are many ways to collate data, and farmers are increasingly turning to technology to provide specific farm data. For some systems that rely on grazing or don’t collect information daily, there may be a disconnect in understanding how heat affects the cattle. Below are a few ways to estimate the THI on farm.

Direct Visual Panting Scores: When digital tools are unavailable, UK producers rely on physical signs of panting to estimate the practical heat impact. The AHDB Dairy Assessment Protocols categorise heat levels based on a standardised 0-4 panting score, with 4 being the most severe and 2 indicating a need to act.

Live Industry Frameworks: Other collaborative tools using internet-enabled data loggers and environmental readings can be accessed by farmers who don’t have similar systems on their farms. See for example https://www.thi-live.com/en#map

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Because maintaining the cow’s body temperature increases an animal’s maintenance energy requirements by 7% to 25%, the animal is hit doubly hard: it burns more calories trying to cool down while consuming drastically less energy. This dynamic can cause an immediate stall or drop in average daily liveweight gain.

Prevention

Proactive management is critical during the summer months to maintain animal welfare and productivity. This may include:

  • Water Access: Double water availability, as water needs can increase by up to 2.5 times. Youngstock and beef animals may stand over small troughs to drink and cool down. This will restrict others’ drinking. Having another source of drinking water will alleviate this pressure.
  • Shade Provision: Provide adequate natural or artificial shade. It may be worth parking a trailer in the field to provide shade if none is available or moving livestock to an area with shade. It is worth noting that flies tend to bother cattle more, particularly in the summer, and are more prevalent in areas with tall hedges and trees. Preventive treatments may be advisable.
  • Handling: Avoid working cattle during the hottest parts of the day. Schedule necessary handling early in the morning. This is particularly important when disbudding calves.
  • Feeding Times: Feed cattle in the late afternoon or evening so the heat of digestion occurs during the cooler nighttime hours.
  • Ventilation: Use fans or well-ventilated structures for calves.

Dietary Adjustments:

Consult with a nutritionist to ensure appropriate mineral intake (such as trace mineral salts) and avoid excess dietary protein. Processing excess unused protein requires the animal to expend energy converting nitrogen to urea, which generates additional metabolic heat and further taxes an already stressed liver.

When grazing cattle experience heat stress, their dry matter intake drops by 8% to 15% under mild-to-moderate conditions and can plummet by 25% or more during severe heatwaves. This drop is driven by two distinct mechanisms:

  • Fibrous and low-quality forages, including late summer grazing, generate significantly more heat during rumen fermentation. Cattle instinctively select against roughages when hot, which drastically alters their normal feeding patterns and lowers total intake.This can be an issue in summer- and autumn-calving herds, when maximising intake of a low-energy, low-density diet before calving is important for promoting intake after calving.
  • Cattleactively decrease daytime grazing hours, choosing instead to stand or huddle in whatever shade is available. Because walking and harvesting forage generate physical heat, they limit movement to avoid raising their core body temperature.

Heat-stressed cattle pant heavily, which causes them to drool and lose vital salivary buffers (bicarbonate). Combined with slug feeding when temperatures cool at night, this can trigger subacute rumen acidosis (SARA), suppressing their appetite for days even after the weather breaks. Adding a buffer such as sodium bicarbonate may reduce risks at high temperatures.

Whilst the focus will always be on the milking herd, some thought on how to mitigate the effects of the increasing heat waves on the performance of other stock will pay off in the longer term.

To discuss the impact of heat on your cattle, please contact Allaster Dallas at allasterdallas@fcgagric.com or your local FCG Office.

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