In the coming years, heat waves may occur more frequently and last longer. Beyond reducing the quality and quantity of forage, high temperatures and humidity also affect productivity and the reproductive performance of livestock. The management of heat stress is already a major challenge in animal production, and given the growing demand for food in the world, it will become an even larger strategic challenge in the future.
Like poultry or others mammals, ruminants are homeothermic animals. They are designed to maintain an ideal and stable internal temperature by regulating their various metabolic processes. Generally, their internal body temperature is higher than their surrounding environment and thermoregulation results from endothermic mechanisms that produce heat. Nevertheless, when an organism absorbs or produces more heat than it can dissipate, thermoregulation fails and the animal is unable to maintain its thermal balance. Heat stress occurs.
The Temperature Humidity Index (THI) is a combined value of air temperature (AT (°F)) and humidity (RH). It is usually used to measure heat stress levels. Several formulas exist to calculate THI. As an example: THI = AT-(0.55-(0.55*RH)*(AT-58)) (Vitali et al., 2009). Modern breeders are used to monitoring their THI levels and implementing different actions according to these levels.
A matter of homeostasis - consequences of metabolic adaptation
In conditions which may induce heat stress, animals would change their behaviours to regulate their temperature.
• Usually, increasing water intake and panting help to cool animals by increasing water evaporation. However, the respiratory rate increase also causes blood alkalosis. In order to reduce their blood pH level, animals excrete bicarbonate in their urine. This loss of bicarbonate will eventually lead to rumen acidosis. (See Figure 1)
• Standing increases the effective surface area of the skin able to dissipate heat, but can engender problems such as lameness.
From a performance point of view, all these natural metabolic adaptations cause a reduction in the production of meat or milk, increased sensitivity to metabolic diseases (acidosis, ketosis) and affect reproductive performance. High-yield dairy cows start to decrease production from THI 68.0, a value that can be attained at just 22°C if the relative humidity reached 45% (Zimbelmann et al,. 2009). From a mean daily THI of 73.2, sustenance intake can decrease by 2.3kg/d and, consequently, milk production can decrease by 2kg (Khelil-Arfa et al., 2014).
How do standardized and encapsulated bioactives enhance resistance against heat stress?
Optimized rumen functions
Encapsulated bioactives, which contain active substances such as capsicum oleoresin, cinnamaldehyde and eugenol, optimize the profile of volatile fatty acids produced in the rumen. The supplementation of eugenol and cinnamaldehyde exerts an inhibitory effect on methanogenic microorganisms. This results in a lower molar proportion of acetate and a higher proportion of propionate and butyrate (Busquet et al., 2005). By increasing propionate, a glucose precursor in gluconeogenesis, more energy is provided to the animals and this limits the impact of decreased dry matter intake.
Better feeding behaviour
Rumen acidosis is the primary metabolic disorder which occurs during heat stress and is widely driven by feeding behaviour (Gonzalez et al., 2008). When the number of meals increases, salivation secretion and rumen movements are augmented and, together, these contribute to limiting the ruminal pH variability (Beauchemin et al, 2008). Thus, pH drop is slowed down, and this limits the risk of acidosis during heat stress periods. Maximizing water intake enhances the cooling effect and helps to maintain body temperature and buffer the capacity of rumen.
Moreover, in ruminants, the digestion process generates a large amount of heat due to ruminal fermentations. Regulation of the feeding patterns limits the peak heat which occurs after a big meal eaten over a short period and extends the digestion process over the course of the whole day. This improved distribution of heat production also helps animals to cope with heat stress.
Today, the negative effect of heat stress on ruminants is well known and well documented. Animals adapt their behaviour, but this induces physiological changes and causes metabolic disorders. Bioactives represent a promising natural strategy to alleviate heat stress and support animal metabolism during these challenging times.
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Article made possible through the contribution of Pancosma