Ruminants - Heat stress

heat stress

Managing heat stress has become more of a challenge than ever before due to rising global temperatures that increase heat potential, both absolute and from a duration perspective, and the intensification of production systems to produce more meat and more milk, thereby indirectly selecting for higher metabolic activity and a greater heat load. Animals naturally change their behaviour to attempt to maintain thermal balance and regulate body temperature when experiencing heat stress. These changes can decrease feed intake, disrupt digestion, increase maintenance requirements, increase inflammation, reduce reproductive efficiency and productive performance, and impair animal health. Heat stress occurs when an animal’s ability to disperse heat produced through normal metabolism is compromised, usually due to the ambient temperature exceeding the thermoneutral zone of the animal. One of the most effective indicators of the potential for heat stress is the temperature-humidity index (THI). Each THI value corresponds to a level of risk of damage linked to heat stress.

Legend: Yellow= Stress Threshold; Orange = Mild-Moderate heat stress; Red = Moderate-Severe heat stress; Purple = Severe heat stress

University of Arizona revised heat stress scale (Burgos & Collier, 2011)

The negative impact of heat stress is linked to both the THI level and to the duration of exposure, both in terms of the number of hours/day and the number of consecutive days the animal experiences heat.

What does this mean in practice?

Cows can start to experience heat stress if exposed for more than 4 hours a day to a 22°C ambient temperature, with 50 % humidity.

Dairy cows:  

the heat threshold is as low as 68, with lactating dairy cows being the most heat-sensitive group of ruminants.

Beef cattle:

the heat threshold is estimated at 72.

Pre-weaned calves:

the heat threshold has been set at 78

Sheep and goats:     

the threshold for dairy sheep and goats is like that of dairy cattle at 72 and 70, respectively, but can vary depending on breed, housing, and production system.

Signs of heat stress

Animal observation is key to detecting early signs of heat stress, allowing one time to implement nutritional and management changes to help preserve animal performance and well-being. The first signs of moderate heat stress include:

  • Increased respiration rate and shallow breathing
  • Reduced feed intake and erratic feeding behaviour
  • Profuse sweating
  • Increased salivation
  • Reduced lying time
  • Decreased rumination activity, with each 10-point increase in THI reducing rumination by 1 hour.

In severe heat stress conditions:
  • Open mouth
  • Panting with tongue hanging out

Heat stress: a rumen challenge

The rumen, which is a true engine of ruminant performance, is the centrepiece of the cascade of events that links heat stress to performance and health. Heat stress imbalances the rumen microbiota, impairs rumen function and increases the risk for acidosis.

During periods of heat stress animals are at risk of sub-acute ruminal acidosis (SARA) as ruminal pH is reduced due to a high concentration and accumulation of lactic acid in the rumen. Factors that contribute to low rumen pH include decreased dry matter intake (DMI) of between 10% – 30%, sorting of feed resulting in a lower proportion of forage and higher levels of readily fermentable carbohydrates, decreased reticular motility and rumination, which lowers saliva production – a source of bicarbonate – as a buffering agent in the rumen.

(Courtesy: Lallemand Animal Nutrition)

Additionally, low rumen pH (pH < 6.0) decreases the abundance and activity of the fibrolytic bacteria thereby impairing the efficiency of fibre digestion. In response to heat stress, total volatile fatty acid (VFA) production is reduced, there is a decrease in the acetate to propionate ratio, and an increase in ruminal lactate, which is responsible for the low rumen pH and increased risk of SARA. An accumulation of high levels of lactic acid in the rumen will damage the rumen wall, leading to papillae erosion, rumen wall destruction and increased permeability, which induces a situation analogous to leaky gut.

How does heat stress negatively impact rumen efficiency in dairy cows?

Examples of problems caused by heat stress

The impact of heat stress on production

A. For dairy cows, the most notable impact of heat stress is the reduction in milk yield and milk components. A 10% to 35% reduction is usually acknowledged during warm summer months, with the extent of milk loss being influenced by both the level of heat stress and duration of exposure. This reduction in milk yield results from a combination of decreased feed intake, along with altered rumen function, hormones, and energy metabolism.

Table 1: Estimated associated milk loss according to the length of exposure to heat stress

Sub-acute ruminal acidosis can lead to increased permeability of the rumen wall, allowing lipopolysaccharides (LPS) and histamine to easily pass from the rumen to the bloodstream triggering systemic inflammation, which can lead to laminitis or an increased risk for mastitis and high somatic cell counts (SCC).

The long-term impacts of heat stress are caused primarily by oxidative stress and negatively impact health, immunity, and fertility. Oxidative stress can increase the incidence of mastitis, increase SCC in milk, embryo mortality, retained placenta, and impact the viability of calves.

B. For beef cattle, fattening cattle are the most sensitive to heat stress. Decreased feed intake and increased risk for acidosis, result in lower growth performance (poorer average daily gains and feed conversion ratio) and behavioural changes (i.e., more nervous animals). Cattle could lose as much as 10kg and in extreme cases have poorer quality meat.

C. For dairy sheep and goats, milk production is reduced by up to 15% depending on breed, production level and lactation stage. There is little data available on the effect of heat stress in fattening sheep.


We have developed a specific approach for heat stress focussed primarily on supporting nutrient utilisation by optimising diet digestion, to compensate for reduced feed intake, while preserving rumen function and reducing the risk of SARA. This is achieved with the rumen-specific live yeast Saccharomyces cerevisiae CNCM I-1077 (Levucell SC®). Our approach also comprises of a range of antioxidant solutions to help strengthen antioxidant status during heat stress and prevent long-term negative impacts on fertility and immunity.

Levucell SC 20
  • Rumen-specific live yeast Saccharomyces cerevisiae CNCM I-1077.
  • Proven to improve rumination behaviour, rumen efficiency, rumen pH and diet digestibility, to support production even under challenging conditions.
  • Concentrated formula designed for use in meal feeds.

Levucell SC 10 ME Titan
  • Rumen-specific live yeast Saccharomyces cerevisiae CNCM I-1077.
  • Proven to improve rumination behaviour, rumen efficiency, rumen pH and diet digestibility, to support production even under challenging conditions.
  • Microencapsulated formulated designed for pelleted feeds and premixes.
  • Innovative TITAN® Technology ensures yeast survivability.

Levucell SC 10% Farm Pack
  • Rumen-specific live yeast Saccharomyces cerevisiae CNCM I-1077.
  • Proven to improve rumination behaviour, rumen efficiency, rumen pH and diet digestibility, to support production even under challenging conditions.
  • Diluted formulation for easy on-farm use.
  • Rumen Efficiency Investigation (REI).
  • An audit service for clients to monitor and track rumen efficiency and the status of dairy and beef herds at various timepoints and to evaluate the impact of nutrition or management changes.

Magniva Range

A range of inoculants to improve the quality, digestibility, and aerobic stability of ensiled roughages providing palatable, nutritious, and cool silage during hot weather.


Burgos Zimbelman R. and. Collier R.J. Tri-State Dairy Nutrition Conference, April 19 and 20, 2011

Additional resource

10 Tips for managing heat stress in cattle

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Caitlyn de Vos

Caitlyn de Vos

Ruminant Development Manager