Heat stress in dairy cows: A review of abiotic and biotic factors, with reference to the subtropics

 

 

L.M. Erasmus^#; E. van Marle-Köster

Department of Animal Science, Faculty of Natural and Agricultural Sciences, University of Pretoria, Pretoria, Gauteng, South Africa

 

 

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ABSTRACT

Heat stress has been identified as one of the major challenges for livestock production. Global temperatures are steadily increasing, with South African temperatures increasing at nearly twice the global rate. Of the livestock used for food production, dairy cows are the most sensitive to thermal changes, which have detrimental effects on their health, welfare, and overall productivity. Several abiotic factors that influence the heat load experienced by the cow are not commonly included in thermal indices used to measure heat stress; these include solar radiation, wind speed, and soil quality. Furthermore, the thermal comfort zone of cows has been altered by years of intense selection for increased milk yield, causing cows to become heat stressed at lower temperatures. Considering the abiotic and biotic factors affecting the cow's heat load, it can be argued that dairy cows in tropical and subtropical climates are experiencing constant heat stress. In this review, the abiotic and biotic factors influencing the heat load experienced by dairy cows are reviewed, along with the available thermal indices that can be utilised at farm level.

Keywords: climate change, heat load, Holstein cattle, Jersey cattle, thermal indices

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Introduction

Dairy cows are homeotherms that strive to maintain their body temperature by regulating their thermal energy balance, and this can be threatened during periods of high temperatures (Ouellet et al., 2021). Heat stress is a physiologically driven response that occurs when an animal produces more heat than it can dissipate (Cartwright et al., 2023). Of the livestock species used for food production, dairy cows are the most sensitive to thermal changes (Herbut et aí., 2018; Cheruiyot et al., 2022). Exposure to either short- or long-term high ambient temperatures will thus have detrimental consequences for the health and production of dairy cows (Schüller et al., 2016; Amamou et al., 2019).

The consequences of climate change are multifactorial, with heat stress highlighted as one of the major burdens on livestock production (Lees et al., 2019). Even regions known for more temperate climates, such as Ireland and Scotland, are experiencing periods of heat stress (Haskell et aí., 2023). The Southern African region is experiencing intensified heat conditions, with temperatures increasing at nearly twice the global rate (Scholes & Engelbrecht, 2021). South Africa is regarded as a semi-arid country with a unique blend of climates, biomes, and rainfall patterns, and has been identified as one of the sectors of the world that will be most affected by climate change, with an estimated average temperature increase of 1.5 to 2.0 °C (Williams et al., 2016). This increase in temperature will be characterised by an increase in the occurrence of droughts, coupled with exposure to extreme summer temperatures, which may be as high as 40 to 45 °C (Roffe et al., 2021).

A number of abiotic factors can influence the degree of heat stress experienced by dairy cows, with ambient temperature and relative humidity being among the most important because of their significant impact on the heat abatement abilities of cattle (Ji et al., 2020). In addition, biotic factors such as size, production potential, and stage of lactation influence the cow's ability to respond to heat stress (Polsky & von Keyserlingk, 2017). Several studies have highlighted the billions in financial losses resulting from the direct and indirect effects of heat stress on the dairy production industry (Moore et al., 2024). Unwanted physiological responses triggered by exposure to heat stress contribute to reduced quality of life, behavioural changes, and, in severe cases, death (Oliveira et al., 2019; Godde et al., 2021).

Dairy farming is a highly intensive industry, producing more than 965 million tonnes of milk annually (FAO, 2023) and employing an estimated 240 million people worldwide (Bojovic & McGregor, 2023). In addition, this industry plays a vital role in the livelihood of communities in developing regions, - such as Africa, India, Brazil, and Pakistan - where a wide spectrum of production systems are employed, ranging from subsistence and small holder farmers to intensive production systems (Bang et al., 2022). In sub-Saharan Africa, indoor housing is limited and 75% of milk is produced using extensive production systems (Hernández-Castellano et al., 2019). In South Africa specifically, 70% of dairy production systems are pasture-based, with the remaining 30% defined as total mixed ration systems, which entail dirt lots with shade and/or open-sided houses (Williams et al., 2016). The effect of heat stress is expected to be more severe for extensively managed, pasture-based cows because of the higher temperatures and increased exposure to radiation (Veissier et al., 2018). Heat stress will also be exacerbated by the long walking distances often required for grazing in extensive systems, which can cause an increase in body temperature (Saizi et al., 2019).

Heat stress poses a threat to the sustainability of dairy production. The global dairy industry is already expected to increase milk yield without utilising any additional resources, such as land and water (Cartwright et al., 2023). However, sustainability may decrease because of the pressures of heat stress on production efficiency, health, and welfare (Polsky & von Keyserlingk, 2017). A major concern is the prolonged effects of heat stress on the physiology and overall well-being of dairy cows (Perano et al., 2015). In this review, abiotic and biotic factors influencing heat stress in dairy cows are reviewed, with reference to the available thermal indices that can be applied as monitoring tools.

 

Abiotic factors and heat stress

The abiotic environment includes a range of factors, as shown in Figure 1; these factors interact and contribute, either directly or indirectly, to the potential heat abatement and overall comfort of dairy cows.

Ambient temperature affects relative humidity, and vice versa, so these two factors should be considered together (Herbut et al., 2018; Islam et al., 2021). However, several other abiotic factors that are not included in the temperature-humidity index (THI) model also play a key role in the heat load carried by the cow (Lees et al., 2019).

Solar radiation affects the severity of heat stress experienced by cows (Ji et al., 2020), and several consequences of dairy cows' exposure to solar radiation have been reported. These include an increase in respiration rate (Becker et al., 2020), increased panting score (Veissier et al., 2018), increased occurrence of DNA damage due to chromosome dissociation and fragmentation and altered DNA-repair signalling (De Abreu et al., 2020), increased metabolic rate (Broucek et al., 2020), decreased feeding behaviour and increased shade-seeking behaviour (Oliveira et al., 2019), increased body temperature (Tucker et al., 2008), increased rectal temperature (Shephard & Maloney, 2023), and decreased fertility and follicular activity (Sesay, 2023). Research has shown that access to shade and shelter can alleviate some heat stress by decreasing the heat load and improving heat dissipation capabilities (Zhou et al., 2022).

Wind speed may have a positive effect by reducing the ambient temperature experienced by the animal, and may thus provide some short-term relief. Wind improves heat loss through convection by replacing hot air near the surface of the cow's skin with cooler air, whether the cow is standing or lying down (Wang et al., 2018; Becker et al., 2020). Studies have found that moderate wind speeds (0.2-0.5 m/s) are preferable (Hill & Wall, 2015), and can play a major role in the thermoregulation of dairy cows. Researchers have thus argued for the inclusion of wind into the THI model, and housing systems try to imitate wind through the incorporation of airflow and ventilation (Zhou et al., 2022). Wind can assist in alleviating the heat load of cows, and thereby decrease rectal temperature, improve feed intake, and increase the display of normal behaviour, resulting in improved milk yields and animal welfare (Dikmen & Hansen, 2009; Haskell et aí., 2023).

The geographical locations of cows determine several environmental factors that can influence heat load (Figure 2), including photoperiod, atmospheric pressure, and rainfall patterns. Photoperiod is generally defined as the duration of time that an animal is exposed to light within a 24-hour cycle, and this changes seasonally (Hut & Beersma, 2011). A shorter day length is associated with cooler temperatures and decreased incidences of heat stress (Velasco et al., 2008). A short-day photoperiod also increases the secretion of melatonin, which is known to improve heat dissipation methods and lower internal heat production (Collier et al., 2006). Melatonin, in turn, influences the secretion of several other hormones, including cortisol, prolactin, and gonadotropin-releasing hormone (Pal et al., 2022). Studies have found that cows require exposure to long-day photoperiods to increase their dry matter intake and water intake, with this being correlated with weight gain and improved production (Macmillan et aí., 2018; Tang et al., 2022). Cows therefore require a balance of short and long days for adequate growth and development, and individuals that are underweight or underdeveloped will be at risk of heat stress (Suarez-Trujillo et al., 2020).

Areas with higher altitudes are characterised by lower atmospheric pressures, and therefore cooler temperatures and lower incidences of heat stress (Martí-Herrero et al., 2015). The evaporative cooling abilities of cows are improved at lower atmospheric pressures, as the moisture-holding capacity of the air increases (Schüller et al., 2016; Broucek et al., 2020). Tropical and subtropical areas are primarily found at low to moderate altitudes, and are characterised by high temperatures and minimal relief from heat (Jeelani et al., 2019).

Rainfall patterns are dependent primarily on geographical location, including aspects such as latitude, altitude, and ocean proximity (Oettli & Camberlin, 2005). Rainfall can decrease the ambient temperature and therefore the heat load of the cow, with the added benefit of cloud cover, which decreases the amount of solar radiation (Tucker et al., 2008).

It is well known that South African soils are deficient in phosphorus, but several of the highest milk-producing tropical countries, including India and Brazil, also have soils that are deficient in phosphorus (Balemi & Negisho, 2012; Magnone et al., 2022). This phosphorus-poor soil can aggravate the effects of heat stress in cows by impairing energy metabolism, further increasing their energy requirements and making it even harder for them to cope under heat stress conditions (Goselink et al., 2015). Phosphorus deficiencies can also lead to poor bone growth and development, resulting in weaker animals that are unable to survive under heat stress conditions (Hill et al., 2008), and can reduce feed intake, impair digestion, and increase fluid loss, further exacerbating the effects of heat stress (Keanthao et al., 2021).

Plants grown in poor-quality soils are of inferior nutritional quality because of the limited absorption of nutrients (El-Ramady et al., 2014). The composition of ingested feed plays a vital role as it determines the amount of internal heat generated by the animal; providing an appropriate diet will thus support improved production during episodes of heat stress (Herbut et al., 2021). Diets containing large quantities of concentrates will increase the risk of ruminal acidosis during heat stress (Cartwright et al., 2023). Feed and water intake, along with the composition of the feed and expected rumination time, will influence the risk of heat stress occurrence in cows (Herbut et aí., 2018).

Considering these abiotic factors - with temperature being a major factor - it can be argued that dairy cows in subtropical regions are under constant heat stress, especially with warmer average temperatures being recorded over the past decade (Hernández-Castellano et al., 2019).

 

Biotic factors and heat stress sensitivity

The thermoneutral zone (TNZ) is the range of ambient environmental temperatures at which the cow is not required to expend energy to maintain her normal body temperature (Shephard & Maloney, 2023). The TNZ of a lactating dairy cow is reported as ranging from -5 to 22 °C, with some researchers reporting a discrepancy of ± 5 °C (Müschner-Siemens et aí., 2020). Each individual cow has a heat stress threshold beyond which a decline in performance will be observed (Saizi et al., 2020; Herbut et al., 2021). As a result, upper and lower critical limits are identified, which are temperatures beyond which the cow's ability to thermoregulate will become challenged (Pinto et al., 2020; Ouellet et al., 2021). The ideal habitat for cows is defined as one with suitable air flow, temperatures below 25 °C, and relative humidity values of 50% to 80% (Herbut et al., 2018). However, this perfect habitat, based on data from Dragovich (1979), does not account for the effects of production-focused breeding objectives or intense climate change (Kadzere et al., 2002; Sesay, 2023). Decades of intense selection for cows with increased milk yield have resulted in an altered TNZ, causing cows to experience heat stress at lower ambient temperatures (Cartwright et al., 2023). Several individual and breed-based factors can influence the sensitivity of the dairy cow to heat stress, as shown in Figure 3.

Genetic factors

A cow's genetic production potential has a direct impact on her ability to tolerate hot climates (Becker et aí., 2020). The TNZ shifts to lower temperatures for higher-producing cows (Hill & Wall, 2015), because of the inverse relationship that exists between the metabolic production of internal heat and heat tolerance (Cartwright et al., 2023). Once the upper critical limit has been exceeded, the cow has to use more energy to cool down, thereby depleting the energy reserves needed for milk production (Williams et al., 2016; Archer et al., 2021).

Breed plays a significant role in determining the heat dissipation capacity of cows (Hoffmann et aí., 2020; Ji et al., 2020). Some breeds simply dissipate heat more effectively, allowing them to have a higher critical limit (Correa-Calderon et al., 2004; Fabris et al., 2019). Bos taurus breeds are typically less tolerant of hot climates than their Bos indicus and Zebu conspecifics (Kadzere et al., 2002; Polsky & von Keyserlingk, 2017). Holstein cows are particularly sensitive to heat stress and exhibit a significant decrease in production performance under conditions of moderate (THI = 72-75) heat stress (Liang et aí., 2013; Amamou et al., 2019). In contrast, Jersey cows only experienced a decrease in production performance during severe (THI = 75-80) heat stress, and remain unaffected during moderate heat stress (Smith et al., 2013). Breed can also affect basal body temperature, as demonstrated by the lower reticulorumen temperatures exhibited by Jersey cows under heat stress conditions (Liang et al., 2013). Research has found that some milk breeds (Jersey and Brown Swiss) and cross-breeds (Holstein-Gyr and Holstein-Boran) have a higher rate of cutaneous evaporation than the Holstein breed, supporting a lower internal temperature (da Cruz et al., 2016; Galán et al., 2018).

Several breed-specific factors affect the rate of energy exchange, including the type of hair coat (length and thickness), hair colour, and skin pigmentation (Anzures-Olvera et aí., 2019). Cattle breeds with short hair have a higher tolerance to heat stress than those with long hair, mainly due to their increased evaporative ability (Galán et aí., 2018). The thickness of a cow's hair is positively correlated with rectal temperature (Hansen, 2020), with thick hair acting as a layer of insulation that reduces the ability of the cow to dissipate heat (Dikmen et al., 2014).

Cows with dark-coloured coats experience higher rates of solar absorption, which compromise their ability to lose heat through convection or evaporation (Tucker et al., 2008). Consequently, these dark-coloured breeds often exhibit higher respiratory rates, panting scores, and skin surface temperatures (Kadzere et al., 2002; Becker et al., 2020). Holsteins typically have thicker and darker coats, and, as a result, they tend to experience a greater degree of thermal discomfort (Anzures-Olvera et al., 2019).

Cattle have apocrine sweat glands (one sweat gland per hair fibre), and hair density thus directly affects the number of active sweat glands and the cow's ability to lose heat evaporatively (Collier et al., 2008). Sweating is the primary mode of heat loss for cattle (Hansen, 2020). The sweating response of Bos indicus cattle is triggered at temperatures 8 °C higher than in other breeds, making them less efficient at dissipating heat (Islam et al., 2021). Evaporative heat loss is facilitated by sweating until heat stress becomes too severe, at which point heat loss through respiratory mechanisms is needed (Dahl et al., 2020).

A frameshift mutation in the bovine prolactin receptor gene has been associated with a short, sleek coat (Hansen, 2020). This mutation, called the SLICK1 mutation, has been mapped to chromosome 20 (Scheffler, 2022). SLICK1 is a dominant allele and, therefore, for progeny to exhibit the associated phenotype, only a single parent needs to be a homozygous carrier (Dikmen et al., 2014). This specific mutation has caught the attention of researchers because of its improvement of the thermoregulatory abilities of cows, including enhanced heat dissipation from reduced coat thickness, increased sweating rates, and an improved ability to regulate core body temperature (Cheruiyot et al., 2022). Dairy cows with the SLICK1 allele exhibit lower vaginal temperatures, lower skin temperatures, and increased sweating, enabling them to cope with higher thermal temperatures (Carmickle et al., 2022). Therefore, the introduction of the SLICK1 allele has the potential to improve efficiency by minimising the reduction in milk yield in warm climates, as well as enhancing welfare through reduced discomfort and suffering (Pozzebon et al., 2024). The SLICK1 allele has already been introduced into the Senepol, Red Angus, and United States Holstein cattle breeds (Hansen, 2020).

The breed of cow genetically dictates the size of the animal (Godde et al., 2021). Cow frame size is positively correlated with intake capacity and, as a result, larger cows have higher feed intakes and higher maintenance energy requirements, which impacts their ability to regulate their body temperature (Walker et al., 2015). Larger cows also have a higher metabolic rate and produce a larger amount of internal heat (Ji et al., 2020). The heat load of large cows is exacerbated further when they need to walk long distances while grazing to fulfil their maintenance requirements (Polsky & von Keyserlingk, 2017; Pontiggia et al., 2023). Smaller cows also have greater sweating rates and a higher surface area-to-volume ratio, making them lose heat more efficiently through evaporation (Saizi et al., 2019; Zhou et al., 2022). Moreover, smaller cows can increase their respiratory rate more efficiently than larger cows can (Scharf et al., 2012).

Physiological factors

Several factors influence the thermoregulatory abilities of cattle (Herbut et al., 2018). Previous exposure to heat stress will affect the cow's susceptibility to subsequent heat stress exposure, with the extent of this effect depending on the intensity and duration of the exposure, as well as the acclimatisation ability of the cow (Godde et al., 2021). The ability of a cow to adapt to extreme weather conditions depends on her specific endocrine, metabolic, and immune system, all of which are directly related to the mechanisms of heat loss in dairy cows (Aggarwal et aí., 2013; Tang et al., 2022). Cows have an increase in core body temperature mid-oestrus, limiting their ability to thermoregulate efficiently, and this is further exacerbated by the increase in physical activity associated with the onset of oestrus (Suthar et al., 2011). The circadian and seasonal rhythms of cows become altered during episodes of heat stress, causing variation in the production and metabolism of glucose, non-esterified fatty acids, urea, and cholesterol, thereby decreasing the energy available to dissipate heat (Shehab-El-Deen et al., 2010). Seasonal fluctuations can also disrupt the autonomic nervous systems of cows, resulting in altered internal signalling and cardiac fluctuations (Kovács et al., 2016).

A higher body condition score is indicative of a higher amount of body fat. Cows with high body condition scores have an increased rate of metabolic activity, which generates a larger amount of internal heat and places them at risk of experiencing heat stress (Shephard & Maloney, 2023). Not only do these cows produce more internal heat, but they also have a reduced ability to dissipate heat (Lees et al., 2019). Fat is an insulator that traps heat within the cow's body, making it more difficult for the cow to regulate her internal temperature (Kadzere et al., 2002). Furthermore, fat cows become insulin-resistant, which is correlated with a reduced ability to regulate body temperature, further restricting the cow's ability to dissipate heat through panting and altered blood flow (Dunshea et al., 2013; Zeng et al., 2023). Heat stress alters the pattern of blood flow in cattle, increasing cutaneous blood flow that carries heat from the core to the periphery, and thereby facilitating heat loss (Dahl et al., 2020). In addition, under heat stress conditions, blood flow to the epithelium decreases, hindering reticular motility and rumination (Sesay, 2023), and blood flow to the skin increases, enhancing the sweating rate (Dunshea et al., 2013).

Cow age plays a significant role in determining susceptibility to heat stress - determining not only the upper critical limit, but also the thermoneutral range (Becker et al., 2020). The parity of a cow is normally closely linked to her age because of the intensive breeding practices used in dairy production systems. In South Africa, for instance, heifers are bred when they reach 60% of their mature body weight, and this typically occurs at approximately 14-16 months of age (Muller, 2017). Most studies thus prefer to describe cow age in terms of parity number, as it allows for easier classification. Multiparous cows (cows that have calved more than once) are significantly more susceptible to heat stress than their primiparous counterparts (Ji et al., 2020). This susceptibility is evident through a larger decrease in milk production (Adriaens et al., 2021), lower quality colostrum due to decreased concentrations of immunoglobulin G, solid non-fat, protein and fat (Avendaño-Reyes et al., 2023), and a higher increase in the somatic cell count (Smith et al., 2013). Heat-stressed multiparous cows also tend to have a greater decrease in the time spent ruminating (Müschner-Siemens et al., 2020), a lower threshold for respiration rate and rectal temperatures (Yan et al., 2021), and a higher proportion of cows that have to be re-bred because of failed insemination (Biffani et al., 2016).

The physiological state of a cow will determine the severity of heat stress that she experiences (Pontiggia et al., 2023). Pregnant cows have higher energy demands than dry cows and, as a result, they have higher metabolic heat production (Becker et al., 2020). The stage of lactation will also affect the cow's heat dissipation rate, as lactating cows have more heat to dissipate than non-lactating cows do (Ji et al., 2020). A study by Calamari et al. (2007) found that mid-lactation cows experienced heat stress more severely than cows in early- or late-lactation stages, which Galán et al. (2018) supported with the observation that early-lactation cows produce less metabolic heat per kilogram of milk yield because of the mobilisation of stored tissue. Even though dry cows are more tolerant of heat, exposure to heat stress will affect their subsequent lactation period, primarily by decreasing milk production (Fabris et al., 2019).

 

Thermal indices

The temperature-humidity index (THI) was developed to quantify the intensity of heat stress experienced by dairy cows (Hoffmann et al., 2020). Initially designed for humans, the THI model was adapted by Berry et al. (1964) for cows and is still accepted as the global standard (Ji et al., 2020). The most important abiotic factors influencing heat stress are relative humidity and air temperature, or a combined effect thereof (Islam et al., 2021). The THI has been utilised to assess the degree of heat stress experienced by dairy cows under various conditions, including in indoor and outdoor housing, and in different climates and production systems (Hill & Wall, 2015). However, the THI has some limitations that must be considered.

Several studies have reported that THI predictions underestimate the severity of heat stress experienced by dairy cows (Pinto et al., 2020). For many years, a THI of 68 was accepted as the point of heat stress in both lactating and dry cows, but this has recently been described as outdated (Perano et al., 2015; Ouellet et al., 2021). The THI values for different degrees of heat stress reported by different researchers (listed chronologically) are summarised in Figure 4. Unfortunately, these values do not correspond to the most recent literature, which indicates that cows experience heat stress at lower temperatures and thus lower THI values (Pontiggia et al., 2023). Researchers agree that the higher the THI value, the larger the extent of stress and discomfort experienced by the cow (Herbut et al., 2018). However, there is no consensus on the THI value at which production and behaviour will become altered.

The THI simplifies complex environmental factors but omits vital abiotic and biotic elements that impact the effect of heat stress on dairy cows (Dikmen & Hansen, 2009; Moore et al., 2024). Individual cow factors vary significantly and, as a result, biotic factors should be the focal point of heat stress measurements in order to obtain accurate and reliable results (Pontiggia et al., 2023). Numerous thermal indices have been developed to evaluate the degree of heat stress experienced by cows during hot weather, as alternatives to the inadequate THI system, and these are summarised in Table 1 (Wang et al., 2018; Cheruiyot et al., 2022). These modern indices are not commonly employed at farm level because of their complexity, the need for specialised equipment to measure the necessary variables, and the unwillingness of dairy producers to use alternative measurements (Polsky & von Keyserlingk, 2017; Dado-Senn et al., 2023). The THI therefore remains the thermal evaluation method of choice for dairy farmers, despite its limitations. Archer et al. (2021) thus suggested that the THI be used as a management tool to guide mitigation strategies and thereby ensure the optimal production of dairy cows.

The health, welfare, and overall productivity of dairy cows is severely affected by heat stress and, as a result, short-term amelioration strategies are employed to modify cows' microclimates through mechanical means (Ji et al., 2020). However, dairy farms worldwide employ pasture-based systems, and many of the suggested mitigation strategies (both physical and nutritional) are regarded as impractical for these systems. Long-term, holistic strategies should thus be considered (Sesay, 2023). Nevertheless, dairy production systems remain focused on increasing production, despite the negative association between the level of production and heat tolerance and, as a result, researchers are striving to develop breeding values for heat tolerance that can be incorporated into selection indices (Cheruiyot et aí., 2022). Future research should focus on improving heat tolerance, to provide dairy cows with the greatest chance of surviving under increasingly harsh conditions.

 

Conclusion

Heat stress has become a focal point of research endeavours, livestock production industries, humanitarian organisations, animal welfare and conservation groups, and even broadcasting and media agencies. Even though heat stress is not a new challenge, it has become a severe threat to the future livelihood of Africa and its residents. Dairy cows are the most susceptible of livestock species to heat stress and, as a result, it is essential to consider and address the abiotic and biotic factors influencing the susceptibility of dairy cows to heat stress. Because of the large variation that exists within the use of thermal indices based on abiotic factors, it is essential to focus on cow-based factors to ensure accurate heat stress assessments.

 

Authors' contributions

L.M. Erasmus (ORCID: 0000-0002-2364-5886) wrote the initial draft of this manuscript; E. van Marle-Koster (0000-0002-3672-6976) designed the structure and edited drafts. Both authors have read and approved the finalised manuscript.

 

Conflict of interest declaration

The authors have no conflicts of interest to declare.

 

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Submitted 27 August 2024
Accepted 27 November 2024
Published January 2025

 

 

# Corresponding author: lizemari.erasmus@up.ac.za