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Review

Recent Advances in Understanding the Impact of Environmental Heat Stress on Sheep Production and Reproductive Performance: A Subtropical Climate Perspective

by
Jabulani Nkululeko Ngcobo
1,*,
István Egerszegi
2 and
Khathutshelo Agree Nephawe
1
1
Department of Animal Sciences, Faculty of Science, Tshwane University of Technology, Private Bag X680, Pretoria 0001, South Africa
2
Department of Animal Husbandry Technology and Animal Welfare, Institute of Animal Sciences, Hungarian University of Agriculture and Life Sciences, Páter Károly 1, 2100 Gödöllő, Hungary
*
Author to whom correspondence should be addressed.
Climate 2025, 13(6), 130; https://doi.org/10.3390/cli13060130
Submission received: 5 April 2025 / Revised: 15 June 2025 / Accepted: 17 June 2025 / Published: 18 June 2025
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Abstract

:
The subtropics are affected by severe climate change, which may induce heat stress in animals. Moreover, the region is significantly seasonal; hence, mitigating climate risks and implementing climate adaptation measures are necessary. Sustainable Development Goals 1, 2, and 13 call for no poverty, zero hunger, and climate action. These are the most severe problems affecting food security in the modern world. Food security refers to a situation in which all people have physical, social, and economic access to sufficient, proper, and healthy food that satisfies their dietary requirements. Nevertheless, the projected increase in the human population implies a greater demand for employment opportunities; hence, developing countries are building more industrial areas. The burning of fossil fuels in various industries potentiates climate change and environmental pollution. It is predicted that the ecological temperature will increase by almost 2.3–4.8 °C by 2100 due to climate change. Agriculture and animal products remain vital in Africa as drivers of the economy and transformation for sustainable livelihood and development. Sheep production has long been used as a source of income and livelihood and provides jobs for people who live in rural areas. It is also sometimes used for ritual ceremonies and to pay penalties to local authorities. Nevertheless, sheep have been identified to be sensitive to heat stress, characterized by low reproductive performance, low microbiota quantities, and poor general health. There are different strategies for mitigating heat stress; however, many smallholder farmers have limited access to education and lack the financial support required to incorporate artificial shade and plant trees for shade to limit heat stress. In this review, we aim to understand the effects of environmental heat stress on sheep production and reproductive performance. Based on this review, it can be concluded that heat stress can threaten food security if not addressed, especially for farmers who depend on sheep rearing. As a result, future studies are recommended to understand different adaptation methods that can be used to mitigate the heat stress effect on sheep productivity, reproductivity, and general health.

1. Introduction

The human population is projected to reach 9.7 billion by 2050, necessitating double the effort in order to double the current amount of food produced to meet the corresponding food demand [1]. It also appears that this increase in the human population means more jobs will be required; hence, developing countries are developing more industrial areas. These factors resulted in the United Nations’ creation of the Sustainable Development Goals 2030 [2] to keep up with the demands of the growing human population. Ending poverty and achieving zero hunger are amongst the top Sustainable Development Goals and should be attained by 2030. However, the increase in environmental temperatures is predicted to be between 2.3 and 4.8 °C by 2100 [3,4]. Among numerous causes of climate change, human activities involving greenhouse gas production have been identified as the primary causes of global warming, with the potential to increase due to human population growth. This projected temperature increase has significant implications for agriculture, particularly in the context of food security and climate resilience. There has been a call for innovation in agricultural technology in order to expand adaptation to heat stress [5]. This call follows a report that the global temperature is projected to rise by 2.46–4.10 °C in comparison to current levels, with a median increase of 3.28 °C [6,7], threatening food security. Food security refers to a situation where all people have physical, social, and economic access to enough proper and nutritious food to meet their dietary requirements [1]. Worldwide, there are several threats to food security, such as the growing human population, the lack of economic access in rural areas, climate change, and consequent heat stress.
Sheep farming in poor rural communities remains a backbone for food production, job security, and income [8]. Moreover, sheep are also raised mostly in arid or semi-arid areas, where they are exposed to different climate conditions, leaving them susceptible to heat stress [9]. Several factors, such as nutrition, management practices, and environmental conditions, may alter sheep production [10]. Romo-Barron et al. [10] defined ecological stress as a phenomenon where an animal cannot cope with its surrounding environment. Reproductive wastage is a primary concern for sheep producers and can be induced by physiological and environmental conditions such as heat stress [11]. Reproductive wastage could be defined as the loss of potential offspring due to failed pregnancies, stillbirths, or early lamb deaths. This wastage reduces sheep productivity and can result from disease, poor nutrition, or breeding issues. Sheep serve as a notable source of protein, and sheep production generates income and ensures food security and employment for poor rural farmers in underdeveloped and/or developing countries [12].
One of the largest hurdles to sheep production is heat stress, and temperatures are continuing to rise in countries like South Africa [13], increasing at a rate twice that for the rest of the world [3]. Heat stress poses a serious challenge to sheep production due to the hostile influences of climate change [14]. Heat stress decreases reproductive activity in sheep worldwide by reducing overall fertility, restricting fetal development, and causing growth retardation [15,16]. In general, sheep are raised extensively, so they are commonly affected by heat stress [17]. Heat stress occurs in sheep when high air temperatures and relative humidity render the body incapable of balancing metabolic heat production and releasing heat into the surrounding atmosphere [18,19]. Moreover, heat stress caused by increased air temperatures due to global warming has become a daily stressor for sheep producers, exerting a severe negative impact on the reproductive performance of sheep [20].

2. Methodology

This review aimed to assess the advanced understanding of the heat stress effect on sheep’s general health, production, and reproductive performance. Studies from countries along subtropical regions were included in this study. Scientific articles were collected and evaluated from different Web of Science sources/databases. The keywords that were used to search for articles included: “heat stress on sheep in subtropical countries”, “sheep production under heat/thermal stress under subtropical region”, “sheep reproductive performance under subtropical region”, “progress towards achieving sustainable development goal 1, 2 and 13”, “microbial development in sheep exposed to heat/thermal stress”, “candidate genes for heat stress adaptation in sheep”, “heat stress on growth performance, production of volatile fatty acids (VFAs), and rumen pH”, “mechanism for adaptation to heat stress”, “sheep rural farming and heat stress” and “exogenous antioxidant sources to ameliorate the influence of heat stress in sheep”.

Literature Search

A literature search was conducted from different databases or engines such as the Tshwane University of Technology library, Google Scholar, PubMed, Scopus, and ScienceDirect. These literature searches were then saved to Zotero, and duplicates were merged and some deleted. Studies addressing heat stress from other regions and not from subtropical regions were not included.

3. The Geographical Region of the Included Studies: The Subtropics

The subtropics are affected by severe climate change [21], which may lead to heat stress in animals. Moreover, the subtropics (Figure 1) appear to be significantly seasonal; hence, there is a need to mitigate climate risk and implement climate adaptation [22].
Sheep rearing is still of significant socioeconomic importance for smallholder farmers worldwide [17]. Sheep are seasonal breeders, reproducing during certain periods of the year, and they are heavily affected by different environmental factors, including heat stress. The seasonality of the subtropical region forces sheep to produce during certain seasons. For instance, in South Africa, sheep breed from late March to May, when day length decreases [23]. Climate conditions can induce heat stress, which may result in reduced animal productivity [9]. Therefore, understanding the impact of climate on animal productivity in different regions remains vital [24].

4. Overview of Sustainable Development Goals 1, 2, and 13

Sustainable Development Goal 1 calls for the alleviation of poverty, the most problematic situation in the modern world. According to the Food and Agriculture Organization [25], poverty is defined as a state in which people cannot afford necessities and, therefore, cannot adhere to a balanced diet. SDG 2 calls for the eradication of hunger in all communities. SDG 2 is closely related to SDG 1 and requires a substantial understanding of income and distribution to end poverty and all its forms and ensure zero hunger through balanced nutrition.
Previous studies have found that in African communities, most rural poor people are undernourished and do not consume a balanced diet [26]; hence, Moyer and Hedden [27] mentioned that poverty can be calculated as a function of average income and its distribution per household. Nevertheless, in countries like South Africa, the unemployment rate hiked to over 31% in 2025, and this factor has been found to be a significant contributor to poverty elsewhere [28]. Therefore, unless they employ sound strategies for poverty alleviation, it will be difficult for African nations to achieve SDG Goal 1.
In Africa, agriculture remains vital as an economic driver and transformation agent for sustainable livelihood and development [29]. Sheep production has long been used as a source of income and livelihood, and provides jobs to rural people [8,30]. Therefore, this sector shows promise with respect to achieving SDGs 1 and 2. Nevertheless, a great deal of work should be conducted to teach rural farmers about sheep rearing and management, as they currently lack knowledge on this topic. Moreover, the people engaged in farming are primarily adults [8], and their limited education, because they have worked as African rural farmers, has also been identified as a hindrance to achieving SDGs 1 and 2 [28].
Some breeds can be used as climate-smart breeds (i.e., livestock breeds that can tolerate drought); however, this approach has received less support than that of exotic breeds due to the market standard, growth, and reproductive performance. Some African regions, such as the Southern and the Eastern ones, are more exposed to climate change, negatively impacting rural livelihoods, food security, and balanced nutrition [31,32]. The Inter-Governmental Panel on Climate Change projected that Africa would suffer significantly due to further water stress, frequent droughts, floods, and variable rainfall, leading to lower agricultural yields; this African water crisis has come to pass in South Africa [33].
Climate change can also reduce the available land suitable for agricultural production. Nurturing native forests, using climate-smart plants and pasturelands, and crop cultivation might assist in clearing carbon dioxide from the air. Climate change is a substantial threat to the survival of numerous species and ecosystems, and even the sustainability of livestock worldwide [34].

5. A Synopsis of Heat Stress, with an Emphasis on Temperature and Humidity

Temperature increases have been observed even in regions known for their cold weather, like Ireland and Scotland, with temperatures in South Africa increasing at twice the rate of the entire globe [13]. Moreover, it has been predicted that animal productivity will decrease by 25% in tropical and subtropical regions due to global warming [35], leaving livestock production vulnerable worldwide, which will affect food security. The temperature required for sheep to produce and reproduce adequately ranges from 12 °C (lower) to 25–31 °C (higher) [36,37]. Ruminants cannot cope with heat stress, even if their thermoregulation physiology is well developed [38]. The global surface temperature has increased by 1.1 °C since 1880, with the majority of the rise in temperature starting in 1975, rising at a rate of about 0.2 to 0.3 °C per decade [39]. The historical highest temperature rise since 1850–1900 was observed between 2011 and 2020 [3,40] (see Figure 2). A method for estimating the severity of heat stress using both ambient temperature and relative humidity was proposed, termed the temperature–humidity index (THI) [41]. The temperature index can be used as an indicator of heat stress, and the formulas developed so far for sheep are as follows:
T H I = 1.8 × T + 32 − 0.55 − 0.0055 × R H × ( 1.8 × T − 26 )
where THI is the temperature–humidity index, RH represents the relative humidity in percentage, and T is the ambient temperature in degrees Celsius (°C) [41]. The term 1.8 × T + 32 is there to convert from degrees Celsius (°C) to Fahrenheit (°F).
T H I = 0.81 A T + R H ( A T − 14.4 ) + 46.4
where THI denotes the temperature–humidity index, AT denotes ambient temperature (°C) (sometimes called dry bulb temperature), and RH stands for relative humidity expressed as a decimal [15,42].
T H I = d b   ° C − { 0.31 − 0.31 R H d b   ° C − 14.4 }
Here, THI is the temperature–humidity index, db °C is the dry-bulb (air temperature measured using a regular thermometer) temperature in degrees Celsius (°C), and RH is relative humidity in decimal form (RH%)/100 [43,44]. The term (0.31–0.31) RH expresses the influence of humidity; when RH increases, this term decreases, denoting a reduced cooling effect.

6. The Threat Heat Stress Poses to Food Security and Poverty Alleviation

Sheep, particularly South African sheep, are capable of enduring various climate conditions and can be reared on marginal land with poor-quality forage or land with crops not suitable for conversion into quality animal protein [45,46]. These factors favor extensive rearing by rural poor people for subsistence or self-sufficiency. Environmental heat can affect ruminants, particularly sheep, in various ways, such as by impacting their overall health and reproductive rates, increasing mortality rates, and affecting the overall physical quality of the animals [47]. In sheep, environmental heat stress lowers feed intake, increases respiratory rates and water intake, and decreases glucose, protein, and cholesterol levels, leading to a decline in reproductive performance (see Figure 3). When reproductive performance is lowered, it is likely that there will be no productivity on a farm, which will cause farm owners to retrench their employees, and thus, food insecurity will strike.

7. Effect of Heat Stress on Sheep Growth Performance

Livestock rely on the dry matter (DM) pasture yield and the nutrient composition of the dry matter available to reproduce under harsh environmental conditions [48]. This DM value varies by locality, season, and physiology in reproductive phases. Moreover, feed proficiency is a vital economic trait in sheep production, determining the financial benefits of sheep farming [49]. Heat stress is the primary stressor for both animals and the nutrient value of pastures, and this can impact livestock performance, health, and fertility [48].
Environmental heat stress triggers a decline in dry matter intake due to adaptive responses to higher environmental temperature [50]. For instance, when the temperature rises beyond an optimum point, feed intake declines significantly because of the appetite control center located in the hypothalamus [51]. Increased environmental temperatures trigger capsaicin receptor 1, whose expression leads to a dramatic decrease in feed intake [52]. It has been reported that heat stress may also induce the production of chyme, hence the decrease in appetite due to the delay in gastric emptying time [53].

8. The Influence of the Microbiota–Testis Interaction and Environmental Heat Stress on Sheep Fertility

In sheep, the rumen is anaerobic and provides a suitable environment for microflora to ferment nutrients (fermenting structural carbohydrates, recycling urea, and detoxifying toxic components in forage) from consumed feed. This fermentation results in three major volatile fatty acids: acetate, butyrate, and propionate. In heat-stressed animals, a reduced energy supply to microbes (Figure 4) has been reported, resulting in a reduction in microbial protein synthesis [4].
Few studies address the effect of heat stress on the sheep rumen microbiome. According to Wang et al. [20], heat stress alters the activity of the gut microbiome and its metabolites, influencing both the immune system and the central nervous system. This interaction leads to the suppression of steroidogenic genes, ultimately affecting the production of follicle-stimulating hormone (FSH), luteinizing hormone (LH), cortisol, and/or testosterone and thereby impairing spermatogenesis in rams. Moreover, Hyder et al. [4] mentioned that in sheep, heat stress may also lead to ruminal acidosis and an increase in ruminal enteric methane concentrations. Testosterone is a hormone required for spermatogenesis, and it is known as a secondary sex characteristic in males. Therefore, any factor disrupting testosterone production may also disrupt spermatogenesis. For instance, a study by He et al. [53] found a clear impairment of spermatogenesis when gut microbiota was disrupted.
Numerous microbiota (Ruminococcus, Quinella, Rikenellaceae, and Lactobacillus) have been associated with sperm motility and the production of FSH and GC in the testes [54,55]. Heat stress leads to dysregulation of microbial and metabolite rhythms; reduced sperm motility; decreases in cortisol, testosterone, and LPS levels; and antioxidant disorder, indicating reduced sperm motility [20]. Cortisol levels have proven to be a good stress indicator in rams [56]. According to Wojtas et al. [57], in Polish Merino sheep the level of heat stress is linked to increases in the levels of the hormone cortisol. Moreover, Wang et al. [54] reported a higher cortisol concentration that prevented testosterone production in Sertoli cells. Testosterone is a vital steroid hormone that assists spermatogenesis, sperm growth, and sperm movement via androgen receptor pathways. Furthermore, testosterone reduction may lead to impaired spermatogenesis.
Sertoli cells are vital in supplying nutrients to developing spermatozoa and maintaining the immune system within the testicles [58]. Heat stress can also increase cortisol levels, associated with lower fertility, compromised antioxidant acute-phase protein activity, and antioxidant defense responses in Brucella-seropositive dromedary camel bulls [59]. A low antioxidant defense response may cause a build-up of reactive oxygen species and oxidative stress in semen. This happens because antioxidants are vital in harnessing or scavenging deleterious ROS and lowering oxidative stress [60].
Heat stress has several negative effects on in vitro fertility, as noted in Table 1. Ewes mate seasonally, preferably during autumn–winter months due to the decrease in daylight length [15]. Heat stress has been associated with great losses of embryos after artificial insemination and/or embryo transfer [11]. For instance, Narayan et al. [11] found that the exposure of ewes to higher temperatures compromises the quality of oocytes and embryos. Moreover, heat stress reduces the expression of signs of estrous and steroid hormone (progesterone and luteinizing hormone) production [61]. Summer heat stress in Pelibuey ewes did not affect estrous signs but decreased the functionality of the corpus luteum. The corpus luteum is important because it produces progesterone after fertilization to maintain pregnancy [42].
Heat stress interferes with the hormonal equilibrium essential for the regulation of physiology, production, and reproduction in female livestock, affecting oocyte maturation, estrous behavior, early embryo growth, fetal growth, and lactation [62]. Nevertheless, few studies have investigated the effect of heat stress on oocyte maturation and embryo development in sheep in comparison to bovines. In bovines, Feng et al. [63] found that heat stress impacts the expression of genes responsible for oocyte development, cytoskeleton maintenance, mitochondria function, and epigenetic modification.
Mammalian testicles, including those of ruminants such as rams, require temperatures 3–5 °C lower than body temperature for proper spermatogenesis [64]. Many studies have reported the deleterious effects of testicular heat stress on spermatogenesis [65]. Moreover, when Hu and Wugu crosses were subjected to testicular insulation, sperm motility and transcriptome were downregulated and greatly influenced [66].
Table 1. Effect of heat stress on sheep semen and in vitro fertility.
Table 1. Effect of heat stress on sheep semen and in vitro fertility.
BreedEffectReference
Pelibuey ewesReduced corpus luteum functionality[42]
Australian Merino ewesOocyte and embryo quality were compromised[11]
Unidentified sheepHigher embryo mortality[10]
Unidentified sheepReduced placental and fetal weight[10]
Unidentified sheepA 1.13 °C increase in body temperature in pregnant ewes[10]
Unidentified sheepIncreased scrotal temperature, reduced sperm motility, decreased testicular weight and seminiferous tubule diameter, and reduced sperm count[64]
Hu and Wugu crossesSperm motility and transcriptome were downregulated and greatly influenced[66]
Australian MerinoIncreased sperm abnormalities, including tailless sperm and proximal droplets[67]

9. Physiological Mechanism for Adaptation to Heat Stress

Sheep use different physiological responses to adapt to heat stress, and they vary depending on the humidity and temperature of their surroundings. All these factors determine the stress level (Table 2). According to McManus et al. [12], various physical characteristics are involved in the adaptation to climate change, such as body size and shape, hair type and color, skin structure and color, appendages, and fat rump or tail availability. Physiological adaptations are changes that occur over generations (from generation to generation) that enhance animals’ fitness for certain environments [68].
Table 2. Spectrum of THI and panting scores, with corresponding images adapted from various studies [13,69,70].
Table 2. Spectrum of THI and panting scores, with corresponding images adapted from various studies [13,69,70].
THI Spectrum and ImageHumidity (%)Temperature (°C)Breaths/MinutePanting ScoreStress LevelDescription
Climate 13 00130 i00120–29<2340–600No stressNo panting, normal respiratory rate
30–4024–25.560–801Mild stressSlight panting, mouth closed, and rapid chest movement
30–4024–25.560–801.5Mild stressMouth still closed, with fast chest movement
41–5926–28Between >1202ModerateRapid panting, with the mouth opened slightly
60–70>30>120–2003Severe stressMouth opened, neck extended, head held up, tongue extended, and rapid panting rate
>70>35>2004Extreme stressOpen mouth with tongue fully extended for a longer period, head lowered, and deeper breathing may occur, with a reduced panting rate for a shorter period

10. Challenges Associated with Rural Farming and Heat Stress

A rural, impoverished farming setup does not offer suitable resources, such as adequate kraals (traditional livestock enclosures common in southern Africa) and handling pens, and high-quality grazing areas. Farmers graze their animals on natural veld (open grassland typical of southern Africa) with questionable-quality grazing grass. In this farming setup, animals are confined in kraals at night [8] without separating males and females, leading to uncontrolled crossbreeding and inbreeding.
Some farmers breed animals that can be easily accessed without considering their adaptability [71]. There are educational interventions in place, such as Kaonafatso Ya Dikgomo, led by the Agricultural Research Council (ARC) [72]. However, these interventions might be insufficient and require more resources for farmers to ensure implementation.
In developing countries like South Africa, water scarcity remains a huge challenge. Previous studies on the effect of heat on livestock animals have suggested using water sprinklers to cool the animals [73]. This technique has been proven to dissipate heat effectively; however, some countries, such as South Africa, face water scarcity challenges. Water scarcity has been identified as a major challenge in livestock production in subsistence-oriented communal farms in dryland areas [74].

11. Prospective Solutions

11.1. Shelter, Shades, and Afforestation

Building shades in camps has been identified as a possible solution (Table 3) to mitigating heat stress. Heat stress poses a serious threat to sheep welfare, particularly when the animals are reared outdoors without access to shade [75]. Despite the numerous strategies employed on sheep-producing farms to reduce its influence, heat stress continues to be problematic. However, shelter and shade provide a promising heat stress mitigation strategy. For instance, a study by Shorten and Schütz [76] reported a slight 24 BPM decrease in respiratory rate, with the shaded model for respiration rate having a validation RMSE of 47 BPM.
Shade also has a negative influence on grazing behavior in sheep. For instance, unshaded sheep spent more time grazing during the daytime compared to when shaded (62.6 vs. 57.7% of observations, SED = 11.92, p = 0.080) [77]. One of the best solutions to global warming is planting trees; hence, the South African government has called for the implementation of afforestation initiatives, such as community tree planting, through the Ministry of Forestry and Fisheries and other NGOs. This may help ameliorate the effects of climate change and environmental conditions while also providing shade to sheep on sunny days.
Table 3. Effect of shading camps or grazing land as a heat stress mitigation strategy.
Table 3. Effect of shading camps or grazing land as a heat stress mitigation strategy.
BreedCountryType of TemperatureEffectReference
Border Leicester × MerinoAustraliaSummerReduced respiratory rate[78]
-New ZealandSummer (Temperate)Improved sheep welfare[77]

11.2. The Use of Antioxidants to Ameliorate Heat Stress in Sheep

There are different types of antioxidants that animals should have to better their immune systems, including superoxide dismutase (T-SOD), catalase (CAT), and glutathione peroxidase (GPx), termed endogenous antioxidants, and total antioxidant capacity (T-AOC) should also be monitored [79]. The effects of the exogenous supplementation of these antioxidants can be found in Table 4. These endogenous antioxidants are responsible for neutralizing any harmful substances, such as built-up reactive oxygen species and oxidative stress throughout the entire body of an animal. Prolonged exposure to heat has been found to lower most of the antioxidant defense systems in sheep. For instance, Shi et al. [80] found that exposing male lambs to prolonged heat stress may reduce T-SOD, CAT, and GPx levels and T-AOC.
Table 4. The use of exogenous antioxidant sources to ameliorate the influence of heat stress in sheep.
Table 4. The use of exogenous antioxidant sources to ameliorate the influence of heat stress in sheep.
BreedAntioxidant UsedType of Supplementation EffectReference
Ossimi ramsL-carnitine-Ameliorates testicular hemodynamic disruption[81]
Ossimi ramsZinc sulphateDietary supplementationIncreases testicular volume, testosterone levels, and semen quality[82]
Merino × Poll DorsetVitE + Se; 100 IU vitamin E and 1.20 mg Se/kg DMDietary supplementation Significantly decreases respiration rate and rectal temperature[83]
FarafraAstaxanthin (Keto antioxidants)Oral administration (0.25 mg)Improves estrus response, conception, and twinning rate[84]
Barbarine100 µL/day/animal of thyme essential oilOral supplementationDoes not increase fertility[85]

11.3. Studying Candidate Genes Associated with Heat Stress

The genetics of livestock regarding environmental adaptability are important. This ability was developed through natural selection, wherein indigenous breeds survived local environmental conditions (see Table 5). Nevertheless, due to economic demand, these indigenous breeds are crossbred with exotic breeds to improve their growth rates and meat quantity, and quality [48]. Abnormally high temperatures lead to decreases in production; hence, heat stress is associated with financial loss and requires different mitigation strategies. Genetic manipulation is a long-term strategy for heat stress mitigation, deemed more rewarding than other strategies such as management and nutrition [38]. Heat stress hampers sheep productivity, health, and reproductive performance, provoking farmers and researchers to investigate different strategies in order to lessen its consequences and prevent economic losses [86]. Therefore, studying and reviewing genes associated with heat stress in sheep will help us understand heat stress and develop strategies for helping sheep adapt to it.
Table 5. Candidate genes associated with heat stress adaptation in sheep.
Table 5. Candidate genes associated with heat stress adaptation in sheep.
BreedCandidate GenesFunctionReference
Iranian sheepSIK2, FER, ATP1A1, CDK5RAP3, and TLR4Associated with heat stress tolerance[87]
CD109, CR2, EOMES, and MARCHF1Promote immune response under arid and warm conditions
ZEP1, PLCB1, and PDGFDInduce response to drought stress and adaptation
HTR4, TRHDE, and ALDH1A3Induce a response to heat stress by controlling digestive metabolism
Barki and AboudeleikCAST, LEP, MYLK4, MEF2B, STAT5A, TRPV1, HSP90AB1, HSPB6, HSF1, ST1P1, and ATP1A1Correlated with growth and heat tolerance[88]
Sarda sheepFCGR1A, MDH1, UGP2, MYO1G, and HSPB3Associated with heat tolerance[89]
Turban Black sheepSYCP2, TDRD9, BRDT, CEP120, and BRCA1Protect spermatogenesis for normal production of sperm after heat stress[90]

11.4. Adopting Climate-Smart Agricultural Practices

According to the FAO [91], the agricultural sector contributes 40% of the world’s gross domestic product (GDP), employs 1.8 billion people, and accounts for one billion of the livelihoods of people living in poverty. In developing countries, livestock are also a source of renewable energy for drafting and are essential for producing fertilizer for crops [91]. Indigenous people are the first to suffer greatly from the increasingly harsh and erratic environmental conditions [92]. A great proportion of these people make their living in vulnerable environments, such as mountainous areas, jungles, and even dry land [92]. In regard to livestock, heat stress has been identified as one of the main factors responsible for reducing milk yields and reducing conception rates [93], threatening food security. Kunene et al. [30] found that indigenous breeds have adapted to South Africa’s climate change with minimal feed, veterinary, and water input in comparison to exotic breeds [8].
Adaptation is the ability to survive and reproduce under extreme living conditions [45]. A number of different sheep adaptation mechanisms have been documented, including morphological, behavioral, physiological, and blood biochemical responses, with a genetic basis for adaptation (see Table 6). Climate-smart agriculture has been a topic of interest, and some farmers are now adopting this strategy to adapt to the adverse effects of climate change [72]. Among the noticeable climate-smart agricultural strategies used, the use of adaptable, preferably local breeds is outstanding. Breeding strategy changes will improve the health, adaptability, and subsequent reproductive performance of sheep. Previous studies listed indigenous breeds, such as Namaqua Afrikaner, as being tolerant to heat stress [94,95]. Therefore, teaching smallholder farmers to change their breeding strategies to adapt to heat stress is necessary. This endeavor will require these smallholder farmers to prioritize indigenous ecotypes such as Zulu, BaPedi, Namaqua Afrikaner, and Damara sheep [46]. All these breeds are known for their unique body features, including fat tails, which are responsible for energy storage for use during dry seasons.
Table 6. Comparison of the adaptive responses of different sheep (indigenous and exotic breeds) to heat stress.
Table 6. Comparison of the adaptive responses of different sheep (indigenous and exotic breeds) to heat stress.
Adaptative Response to Heat StressIndigenous SheepExotic SheepReference
Morphological response
Body sizeSmall body (±39.1 kg) (e.g., Zulu sheep) and slow growthLarge bodies (±50 kg) (e.g., Dohne Merino) and high growth performance[96]
Body shapeLong leggedShort legs
Coat and skin colorMulticoloredPure color (e.g., white Dorper)
Hair or woolHair (e.g., Zulu sheep)Fine wool[96]
MobilityNomadicStationary, sometimes transhuman[96]
Fat storageFat-tailedThin tail
Behavioral response
Food intakeLower feed intake, consumed in small portions more frequentlyHigh feed intake, in large portions[97]
Quality of feed intakeAble to utilize feed with low nutritive valueRequire balanced nutritional value
Water intakeLower water intake and can walk long distances in search of waterHigh water intake and inability to walk long distances in search of water sources
Physiological response
Heart rate->107.79 beats/min, Merino sheep[98]
Respiratory rateIncreased by only 84% from a cool morning (18.9 °C) to a warm afternoon (30.2 °C) (e.g., Namaqua Afrikaner)Spikes by 181% from cool morning (18.9 °C, 203%, and 278%) to warm afternoon (30.2 °C) (e.g., Dohne Merino, Dormer, and Merino)[99]
Rectal temperature<38 °C in the morning and ±39 °C afternoon (e.g., Namaqua Afrikaner)±38.8 °C in the morning and ±39.3 °C in the afternoon (e.g., Dohne Merino, Dormer, and Merino)[99]
Blood biochemical response
Red blood cells (million/cubic mm)10.28-[100]
Hemoglobin (g/percent)9.80-[100]
White blood cells (thousand/cubic mm)9.039.12[100]
Packed cell volume (%)31.47-[100]
Heat stress implicationsTolerate heat stressNeed more prevention measures to mitigate heat stress-

12. Research Gaps

The phenotypic traits for adaptation to heat stress in sheep are still not well investigated [87]. Despite the various studies conducted on candidate genes related to heat stress, there is still a need for more studies on sheep to understand the interrelationship between microbial genome sequences and rumen microbes [101]. Rural farmers are often not well informed about recent technologies that can improve production, and research stations are commonly in urban or semi-urban areas [101]. Therefore, studies investigating the role of research stations in rural farms are still needed to improve production.

13. Conclusions

Environmental temperatures are increasing, subsequently increasing the influence of heat on sheep. Sheep are sensitive to heat, hence their reduced reproductive performance following exposure to heat stress. One mitigation strategy consists of adopting adapted breeds that can easily dissipate heat and remain productive under heat stress. Studying candidate genes associated with heat stress adaptation will also help, although numerous rural farmers cannot access research stations. However, these breeds mostly exhibit low growth performance and have low carcass weight, which might threaten the achievement of SDG 1, poverty alleviation. Therefore, other heat stress mitigation strategies, such as the use of shelters, might also help, though they are more expensive depending on the type of shelter (artificial or natural) used. Future studies must address the interrelationship between microbial genome sequences and rumen microbes. It also appears that rural farmers are not well informed about the recent technologies that can improve production, and research stations are located in urban or semi-urban areas. Therefore, it can be concluded that heat stress can threaten food security if not addressed, especially for farmers who depend on sheep rearing. As a result, future studies are recommended to understand different adaptation methods that can be used to mitigate heat stress effects on sheep productivity, reproductivity, and general health.

Author Contributions

Conceptualization, J.N.N.; writing—original draft preparation, J.N.N.; writing—review and editing, J.N.N., K.A.N. and I.E. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Tshwane University of Technology under grant number SEEDFUND. The APC was funded by the Department of Animal Husbandry Technology and Animal Welfare, Institute of Animal Sciences, Hungarian University of Agriculture and Life Sciences, Páter Károly 1, 2100 Gödöllő, Hungary.

Data Availability Statement

Not applicable.

Acknowledgments

The Tshwane University of Technology and the Hungarian University of Agriculture and Life Sciences are acknowledged for providing seed fund and APC fund for this work.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. The geographical area that the included studies address. Countries highlighted in grey are countries along the subtropical region, and where included studies were conducted. Four lines highlight the subtropical region.
Figure 1. The geographical area that the included studies address. Countries highlighted in grey are countries along the subtropical region, and where included studies were conducted. Four lines highlight the subtropical region.
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Figure 2. Increase in temperature from 1880 to 2020 [3,40] (Graph adapted from National Aeronautics and Space Administration (NASA)) [39].
Figure 2. Increase in temperature from 1880 to 2020 [3,40] (Graph adapted from National Aeronautics and Space Administration (NASA)) [39].
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Figure 3. Impact pathways through which heat stress leads to food insecurity and poverty.
Figure 3. Impact pathways through which heat stress leads to food insecurity and poverty.
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Figure 4. A schematic of the influence of heat stress on sheep rumen microbiota. Note: HT—heat stress.
Figure 4. A schematic of the influence of heat stress on sheep rumen microbiota. Note: HT—heat stress.
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MDPI and ACS Style

Ngcobo, J.N.; Egerszegi, I.; Nephawe, K.A. Recent Advances in Understanding the Impact of Environmental Heat Stress on Sheep Production and Reproductive Performance: A Subtropical Climate Perspective. Climate 2025, 13, 130. https://doi.org/10.3390/cli13060130

AMA Style

Ngcobo JN, Egerszegi I, Nephawe KA. Recent Advances in Understanding the Impact of Environmental Heat Stress on Sheep Production and Reproductive Performance: A Subtropical Climate Perspective. Climate. 2025; 13(6):130. https://doi.org/10.3390/cli13060130

Chicago/Turabian Style

Ngcobo, Jabulani Nkululeko, István Egerszegi, and Khathutshelo Agree Nephawe. 2025. "Recent Advances in Understanding the Impact of Environmental Heat Stress on Sheep Production and Reproductive Performance: A Subtropical Climate Perspective" Climate 13, no. 6: 130. https://doi.org/10.3390/cli13060130

APA Style

Ngcobo, J. N., Egerszegi, I., & Nephawe, K. A. (2025). Recent Advances in Understanding the Impact of Environmental Heat Stress on Sheep Production and Reproductive Performance: A Subtropical Climate Perspective. Climate, 13(6), 130. https://doi.org/10.3390/cli13060130

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