How to detect early signs of heat stress in ruminants

Long before milk production or growth rates are affected, heat stress can be detected and recognized by changes in animal behavior. Detecting the signs of heat stress as early as possible is essential to preserving its full production and health potential by adjusting the heard management. Here are some tips.

Watch the weather

Table 1: THI table (from Burgos & Collier)

The temperature-humidity index (THI) is a common indicator of heat stress risk (table 1). That’s because the temperature is important but also the air relative humidity level, which exacerbates the effect of heat.

The THI in the barn could be higher than the THI based on meteorological data. On the other hand, solar radiation could increase the actual heat stress in grazing animals. By placing a thermo-hygrometer — or a new generation of sensors that continuously monitor THI within the barn, — close to the animals, producers can monitor and anticipate heat stress risks.

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

• For dairy cows: the heat stress threshold is as low as 68 (Burgos and Collier, 2011). This means that, for example, at 50% humidity, cattle begin to experience the negative effects of heat stress at 22°C.
• For beef cattle: the heat stress threshold is estimated at 72. This means that at 50% humidity, cattle are exposed to heat stress at 25°C.
• For pre-weaned calves: a heat stress threshold has been set at 78 (Kovacs et al., 2020)
• The thermal neutral zone for sheep is between 12-32°C and for goats is between 12-24°C. The actual threshold for heat stress in dairy sheep and goats, based on THI, is more or less comparable to dairy cattle but can vary depending on breed, housing, and production system.

Animal observation is key to detect early signs of heat stress

The first signs of moderate heat stress include:

• Shallow breathing
• Increased respiration rate
• Profuse sweating
• Reduced feed intake
• Reduced lying time: to increase heat dispersion, the animal will spend less time lying quietly (and ruminating). Research has shown a significant drop in lying time under the heat stress threshold.
• Decreased rumination activity: there is a correlation between rumination time and rumen function, hence the lowered production performance. In dairy cows, it has been shown that for each 10-point increase in the THI, daily rumination could be reduced by 1 hour and dairy production by 2.7 kg/day (6 lbs.) (Haan, 2016). We estimated that a good target is to have 50-60% of dairy cows lying in their stalls ruminating at the same time. Under heat stress conditions, rumination is suboptimal in 50% of farms (Lallemand Animal Nutrition internal data, REI audits¹).
• Erratic feeding behavior: as digestion produces heat, the animals will tend to eat less at the coolest hours of the day to reduce heat production.

Look out for signs of reduced rumen function

The following signs can indicate poor rumen function: decreased lying and rumination time, loss of saliva through panting, a reduction in meal frequency, and larger meal size, also known as “slug feeding.” The experts at Lallemand can also help producers measure rumen function on their operation¹ by assessing signs such as:

• Manure color and consistency are effective ways to assess rumen function. The presence of undigested grain or cotton seeds, for example, are signs of sub-optimal rumen function, where precious (and expensive) nutrients are not being fully digested by the animal.
• Locomotion. Lameness can be a sign of poor rumen function and Sub Acute Ruminal Acidosis (SARA) events. Heat stress can exacerbate poor rumen fermentation and function and negatively affect animal behavior resulting in even worse locomotion scores and a higher incidence of lameness.

In small ruminants, heat stress has a large impact on rumen health too.

Consequences on animal performances and health

Table 2: Estimated associated milk loss according to the length of exposure to heat stress.

The reduced rumen function and lower feed intake lead to a reduced energy supply, which impacts performance in all ruminant types:

• For dairy cows: milk yield and milk components are reduced. Milk loss is linked to both heat stress levels and duration of exposure: only 4 hours exposure/day to low heat stress levels already leads to a milk production decrease by 1.1 kg/day (table 2).
• For dairy sheep and goats, the decrease in milk production depends on the breed, production level, and lactation stage. A reduction in milk production of 15% is possible (Cannas, 2007, Salama, 2014). In addition, the decrease in feed intake is often partially compensated by the mobilization of body tissue.
• For beef cattle: average daily gain drops with increased exposure to heat stress.

It has also been shown that heat-stressed animals experience increased oxidative stress combined with a lower antioxidant status. This could also have a negative effect on animal health, immune defenses, reproductive performance as well as meat quality.

Importance of prevention

In conclusion, it is crucial to detect the first signs of heat stress as early as possible, by monitoring both the environment and the animals. During a risky time, it is important to prepare the animals and support proper rumen function and rumen pH to help maintain the performance level. During hot weather, it is essential to ensure a sufficient intake of fiber in the diet and stabilize the rumen with the rumen-specific yeast Saccharomyces cerevisiae CNCM I-1077 (Levucell SC)— which was demonstrated to improve both nutrient digestion and rumen efficiency (Fustini et al., 2013; Perdomo et al., 2020). Supplementation with this live yeast should be also continued in the period after the heat stress event to support the recovery because the rumen buffer capacity is not yet sufficient directly after a heat stress event.