Ruminants’ prolonged exposure to high heat conditions results in significant changes in the physiological and biochemical parameters in the animal, making heat stress one of the most concerning issues facing producers. This is particularly relevant in the case of high-producing dairy cows. Mitigating heat stress is crucial to achieving efficient, profitable and sustainable animal agriculture operations.
Ruminant Technical Sales Support – EMEA
ADM Animal Nutrition
CLIMATE CHANGE AND HEAT STRESS IN RUMINANTS
Farm animals are commonly exposed to various stressors that can negatively affect their health and well-being. Heat stress is a condition in which the body has problems dissipating excess heat, impacting animal function, including growth, health, digestibility, lactation and reproduction (Wang, J., Li, J., Wang, F. et al., 2020). Although most common during summer months, a heat wave can occur at any period of the year, with significant effect when animals are not adapted to changes in ambient temperature, relative humidity, wind speed and solar radiation (Habeeb, A.A. et al., 2018).
Heat stress is one of the most significant stressors at a time when ever-changing climate conditions are impacting ruminants of all sizes, from sheep and goats to beef and dairy cattle. While detrimental to all agriculturally important species, effects on the dairy industry are the most economically severe (St-Pierre et al., 2003). Analysts have noted rising cattle mortality rates because of heat waves in the mid-central United States (5-10% in July 1995, Hahn, 1999) and France (12-24% during 2003 and 2006, Morignat et al., 2014).
An animal’s thermoneutral zone (TNZ) is the temperature range in which they don’t have to expend energy to maintain normal body temperature. For healthy cattle, that range depends on a variety of factors, including breed, age, adaptation or animal status, among others, and is influenced by other environmental conditions such as relative humidity and wind velocity. For example, a lactating dairy cow’s TNZ is in the range of 5° to 25°C (Kadzere et al., 2002). Above 25°C, the animal must modify physiology and behavior. Consequently, animal productivity and feed efficiency decrease.
Despite remarkable advances in the construction and design of animal housing facilities and cooling technology, high heat events are a costly issue for the dairy industry [estimated at $900 million annually in the United States (St-Pierre et al., 2003)]. The financial impact of heat waves on the industry will certainly become more of an issue in the future as high-yielding animals have more natural metabolic activity and will be more affected by high temperatures as they produce more body heat (Jones and Stallings, 1999).
BEHAVIORAL AND PHYSIOLOGICAL CHANGES DURING HEAT EVENTS
The behavioral coping strategies of dairy cows facing heat challenges include increased standing time, shade seeking and decreased activity and movement (Schutz et al., 2009). Furthermore, cattle may increase water consumption, and change or reduce feed consumption. If rough and concentrated feeds are provided as alternatives, animals will prefer concentrated feeds, as they induce less metabolic heat to digest (Sireli et al., 2017).
However, the overall effect of heat stress on growth performance cannot be explained solely based on reduced feed intake (Slimen et al., 2015). A study by Rhoads et al. (2009), provides data that indicate heat-induced reductions in nutrient intake only account for approximately 35% of the decrease in milk synthesis.
As for physiological changes, heightened environmental temperatures are shown to alter the basic mechanisms of the rumen, with increased risks of metabolic disorders and health concerns (Soriani et al., 2013). For example, the rumen’s microbial population changes and its pH fluctuates (Hall, 2009). Shifts in the cow’s acid-base balance in the blood may reduce production of bicarbonate and secretion of this buffer into the saliva. Plus, water consumption may be inadequate, which reduces acid dilution in the rumen. Declines in rumen pH can predispose cattle to sub-acute rumen acidosis.
When TNZ conditions are exceeded, the cow’s body attempts to adapt to the new environmental situation. The animal can radiate body heat through increased respiration rate, panting and sweating, otherwise known as the principles of convection, conduction, radiation and evaporation. During extreme heat events, however, the cow’s adaptive mechanisms may fail to remove the excess heat generated.
Heat stress is one of the most significant stressors at a time when ever-changing climate conditions are impacting ruminants of all sizes, from sheep and goats to beef and dairy cattle. Multiple approaches have been suggested to mitigate the impact of heat stress in dairy cattle, including housing modifications, genetic selection for heat-tolerant cows and nutritional management.
Animals experiencing heat stress lose body fluids (sweat) and need to control dehydration and blood homeostasis (Das et al., 2016). Heat challenges can also result in an altered endocrine status and changes in body composition (Rhoads et al., 2013). Reduced blood flow can limit cellular gut activity and may affect the gut’s ability to maintain barrier functions, such as mucous secretion. This barrier function is part of the cow’s defense system and is designed to prevent pathogens and toxins from entering the circulatory system.
On a cellular level, heat stress is responsible for a number of molecular responses, and is linked to oxidative stress in livestock animals (Ganaie et al., 2013; Nizar et al., 2013), resulting in cytotoxicity in various forms, such as alteration of biological molecules, disturbance of cell functions, modulation of metabolic reactions, induction of oxidative cell damage and activation of apoptosis and necrosis pathways (Du et al., 2008; Pandey et al., 2012). An increase of the intestinal cells’ permeability also increases the cow’s inflammatory status and could potentially lead to sepsis. Thus, the more stress the animal is under, the more immunity is compromised.
Removing excess body heat is essential to prevent the animal from entering a stage of hyperthermia. In addition, maintaining optimal temperatures for cows is a crucial condition for their high productivity and general health.
ALLEVIATION STRATEGIES FOR DAIRY CATTLE
Multiple approaches have been suggested to mitigate the impact of heat stress in dairy cattle, including housing modifications, genetic selection for heat-tolerant cows and nutritional management.
The first step dairy farmers can take is providing shade to effectively block solar radiation, as exposure is a key concern for ruminant health and welfare. Additional evaporative cooling and air ventilation can also be implemented; however, in contrast with confinement dairies, optimal cooling strategies for grazing cattle have not yet been identified (Dahl et al., 2020).
From a husbandry standpoint, the breed, age and lactation phase are known to influence the risk of occurrence of heat stress in cows (Bernabucci et al., 2014). Proper breed selection is thus a valuable tool for sustaining animal production in an increasingly challenging environment (Silanikove, 1992). Genetically altering the thermal tolerance of high-production animals has, however, proven to be difficult (Collier et al., 2019).
Additionally, strategic nutrition management is increasingly of interest to help improve the animal’s response to high heat. For example, it has been found that feeding dietary fat (rumen inert/rumen bypass) reduces rectal temperature and increases milk yield in dairy cows (Wang et al. 2010). Studies also indicate the importance of antioxidant supplementation in both in vivo and in vitro trials, such as the thermo-protective role of vitamin E in buffalo cows (Megahed et al., 2008).
Furthermore, certain feed additives can be used to alleviate heat stress. Much attention has been placed on the use of live microorganisms and yeast products to help modulate the immune system, helping the animal fortify its defenses against stressors. Another area of research has been with plant extracts such as capsaicin. Capsaicin may help mitigate heat stress through vasodilation, improvements in feed eating patterns and water intake.
ADM’s FreshUp approach is a heat stress solution for ruminants, leveraging the synergistic effects of specialty nutrition formulations to counter the negative impact of high temperatures on animal outputs. This unique blend of natural ingredients, including plant extracts and Pichia guilliermondii inactivated yeast, can improve animal thermoregulation and has been tested at university and field locations across North America for more than 15 years. Experimental trials show greater feed efficiency in dairy cows and higher milk yields in cows (ADM internal research, 2006-2020).
CONSEQUENCES OF CLIMATE CHANGE
Ruminants’ prolonged exposure to high heat conditions results in significant changes in the physiological and biochemical parameters in the animal, making heat stress one of the most concerning issues facing producers. This is particularly relevant in the case of high-producing dairy cows.
Mitigating heat stress is crucial to achieving efficient, profitable and sustainable animal agriculture operations. However, this may become increasingly costly and difficult to implement. According to the World Meteorological Organization (WMO), global temperatures are expected to set new records in the next five years. The annual mean global near-surface temperature for each year between 2023 and 2027 is predicted to be between 1.1°C and 1.8°C higher than pre-industrial levels (i.e., the 1850-1900 average). Climate change will be an ongoing challenge, given the projected magnitude of potential heat stress conditions as global temperatures continue to climb.
