Impact of Climate Change on Small Ruminants Production: A Review

The adverse impact of climate change and consequential heat stress affecting the wellbeing of the livestock and further their production and reproduction efficiency is well known at present. As per the prediction of the intergovernmental panel on climate change, by the year 2100, the global surface temperature may be increased by 3.7-4.8 °C [1]. At a time when the emphasis is being laid on the increasing production and productivity of livestock to meet the increasing demand for animal protein at global levels, climate change has been posing a major threat to the sustainability of livestock production systems. It has been an established consensus that animal domestication began with goats and then sheep [2], which happened in the ‘Fertile Crescent’ of the Middle East about 10, 500 years ago before established worldwide [3]. Globally sheep and goats represent 58% of the domestic ruminant population, with 1209 million sheep and 1045 million goats [4].

Both sheep and goats are found to be well adapted under different environmental conditions including harsh climates showing better performance than other domesticated ruminants [5-7]. As per the available information, more than 1,000 sheep and 600 goat breeds are available worldwide [8] and these breeds possess the varied capacity to overcome extreme climatic conditions [9]. The adaptability of small ruminants to the hotter climatic condition can be recognized from their large-scale (over 50%) presence in the arid region, which further indicates the suitability of these animals to higher temperature regimes in the future too [10]. Further, it has been recognized that sheep is one of the most resistant species against any extreme climatic condition, particularly to high environmental temperatures [11]. Heat stress, however, is one of the complex factors which makes the task of sheep management and husbandry a challenging one in several geographical regions in the world [12,13].

Sheep has been one of the most resistant species to climatic extremes, especially to elevated environmental temperatures [11]. However, the biological functions of the sheep were found to undergo several changes with higher ambient temperature, which include reduced feed intake and utilization, disturbances in the metabolism of protein, energy, and minerals, and secretion of hormones [14], which lead to reduced growth. Further, it is known that the growth of animals occurs due to cell multiplication, which is controlled by genetic and environmental factors. According to [12] exposure to higher temperatures (30-40 °C, 40% humidity) during the early embryonic life stage in sheep results in a significant reduction of total cell number and placentome size, and also a marginal decrease in cell size, compared to thermo-neutral temperatures (18-20 °C, 30% humidity). The heat exposure during placental growth further restricts early foetal development.

Exposed to heat stress, the animals, in general, reduce feed intake and increase water intake, and further increase their maintenance requirements, thereby leading to reduced growth performance [15]. The production potential and physiological functions of native sheep breeds were found to be less affected at higher environmental temperatures or solar radiation during hot summer months in the hot semi-arid areas compared to the nonadapted exotic and crossbred sheep [16,17]. Working with Bharat Merino sheep, [17] observed increased respiration rate and body temperature at higher ambient temperature. [18] also reported temperatures of over 31 oC lead to increases in rectal temperature and also heartbeat rate in Santa Ines sheep. Studies of three different Indian sheep breeds, viz., Chokla, Magra, and Marwari of the arid zone under heat stress conditions showed that the Magra breed had the highest adaptability followed by Marwari and Chokla, although they did not differ significantly [19].

High environmental temperature has been the major constraint for the productivity of sheep in tropical and sub-tropical areas. Higher temperature leads to a reduction in feed intake and an increase in energy demands due to activation of thermoregulation mechanisms, which negatively affect the development and productivity in sheep [20]. The high temperature together with high humidity found to aggravate the effect of heat stress [21,22]. The Temperature-Humidity Index (THI), therefore, is considered a major indicator of heat stress. Working on Mediterranean dairy sheep (Valle del Belice), [23] reported heat stress to result in decreased production, although the breed originated from the hot environment. In this case, the THI ≥23 was found to affect the production. A study on Comisana dairy sheep and Australian Merino sheep showed the effects of heat stress at THI of ≥27 and ≥32 respectively [24,25]. [26] reported a reduction in production performance in Sarda sheep by 20% with THI passing from 60-65 to 72-75. Working under a semi-arid tropical environment, [27,28] reported a more detrimental effect of combined stressors of heat and nutritional factors on growth and reproductive performance in Malpura ewes, compared to animals subjected to separate stressors.

The extent of the heat stress impact on the productivity in sheep largely depends on the adaption level of the breed to higher temperature regimes. Studies have shown that the tolerance level to heat stress, in general, are found to be more in hair breeds [29,30]. Similarly, [31,32] reported a higher level of heat tolerance in the hair sheep breeds in hot agro-ecological regions in Mexico and the authors did not find a substantial adverse effect on the growth and reproductive capacity in lambs. Adapted to hot climates, such breeds exhibit physiological and metabolic plasticity, which do not impact the productivity of these animals [33]. In the case of hair sheep, the phenotypic characteristics provide them the adaptability to heat stress [20]. The presence of hair is an advantage in terms of heat loss both by non-evaporative and evaporative means, compared to that of wool [29,33]. [34] reported a significant reduction in body weight and height in different sheep and goat breeds from north to south of the Mediterranean area connected to the period of dry months. Such an impact of high ambient temperature on size reduction is further expected to pose a huge risk on the reduction in average carcass weight in the ruminants in European, Asian, and African Mediterranean areas with global warming [35]. Further, the increased climatic variability and high ambient temperature are projected to exert a strong influence on pastoral systems, particularly in Africa, Australia, Central America, and Southern Asia. Further, the reduction of availability of biomass is expected to have a direct impact on the production system of the small ruminants.

There have been several studies demonstrating the adverse effect of heat stress on yield and quality of milk in ewes and goats, which include the decrease of milk yield, reduction in total protein, fat and casein contents, increase of saturated fatty acids and decrease of oleic, rumenic, vaccenic, linoleic and linolenic acids, and reduction of milk coagulating ability [24,36-43]. Studying the impact of heat stress on Mediterranean dairy sheep, [23] reported the milk production and fat-plus-protein yields were consistently negatively correlated with temperature and THI. Further, the extent of the decline in milk production was quite different among different sheep breeds.

The physiological and production performance of lactating sheep is found to be affected at temperatures higher than the upper critical point [11]. [24] recorded a reduction of milk yield in Comisana ewes exposed to temperatures over 35 °C, even for a short period. The sheep subjected to the heat-stressed condition found to have a significant reduction of fat and protein contents in milk, affecting the production of high-quality cheese which is the principal product [11]. Working with Sarda ewes, [26] recorded a 15% reduction in milk yield at maximum environmental temperatures of over 21-24 °C, and a reduction of 20% milk yield with an increase of minimum temperatures from 9-12 °C to 18- 21 °C. [35] reported the reduction of milk yield and the content of milk components in goats at high air temperature. [42] reported a decrease of milk yield in dairy goats with an increase of THI index value, and with the increase of 1-unit t of THI, there was a decrease of 1% in milk yield. The lactating Saanen goats exposed to moderately hot (30±2 °C) and severe hot (35±2 °C) environments for 4 days at Relative Humidity of 70±5% reported the reduction of the milk yields by 3% and 13% respectively in moderate and severe heat exposure as against thermos-neutral (20±2 °C) environments [44]. Experimenting with Murciano-Granadina dairy goats in the late lactation stage, [45], however, did not observe any variation in the milk yield between the heat-stressed and a thermo-neutral group of animals. But the milk of the heat-stressed goats resulted in a reduction of protein and casein levels by 12.5% and 11.5% respectively than the thermo-neutral ones. Under heat stress conditions, a reduction of protein content and protein fractions in the milk of goat was reported by [45]. Further, dairy goats under heat stress conditions have been also observed to produce milk with less fat content [42]. Studying the effect of heat stress on dairy goats, [46] observed a modest effect on milk yield at THI 80-85, the severe effect at THI 85-90, and extreme with the threat of death at THI≥90. With the exposure of dairy goats to moderate heat stress conditions (34 °C, THI=79) for 5 weeks, [47] reported a reduction in milk yield, solids, fat, and N levels in Alpine goats compared to Nubian goats, indicating a response to the heat stress is breedspecific.

The heat stress was shown to affect both the male and female reproductive functions in livestock in several ways, viz., fertilization rate, estrous activity, embryonic survival, sperm motility, and abnormalities and mortality in spermatozoa [12,48,49]. According to [50], most of the reproductive processes are influenced by environmental stressors, either by directly affecting the functions of reproductive organs or blocking the cellular functions of the hypothalamic-pituitary-gonadal axis. Working with Australian merino ewe, [49], reported heat stress to result in a reduction in embryo production during artificial insemination or embryo transfer, which was attributed to the disruption of the physiological and cellular features of the reproductive function of the animals and early embryo development. The ewes exposed to a hot condition during the first 3 days after artificial insemination shown to affect the oocyte and/or embryo quality [51].

The susceptibility of pregnant and lactating ruminants to heat stress is found to be much higher than those of non-pregnant and non-lactating ones [52,48]. [53] reported heat stress affecting the reproductive performance in sheep through impaired reproduction. [54] observed thermal stress to harm the embryo quality during the pre-ovulatory period in Bharat Merino sheep. With the induction of heat stress to Malapura ewes in a climatic chamber at 40 °C and RH of 55% for 6h per day for two estrous cycles and restricted feeding, [28] observed reduced body weight, oestrus duration, birth weight of lambs and oestradiol 17‐β, and increased oestrus cycle length and progesterone levels.

Exposure of Ossimi ram to severe heat stress during hot summer and at THI of over 84, reported increasing the rectal and scrotal skin temperature, abnormalities in sperm and semen, and further reduction of conception rate and lambing [55]. Studies with ewes of different breeds viz., Ossimi, Rahmani, and Ossimi x Suffolk have shown to have negative relationships between conception rate, and ambient temperature and daylight length [55-57]. Further, exposed to high ambient temperature, the reproductive functions of the females are found to be adversely affected, with a reduction in the length of the estrous cycle and effect on ovulation [14]. The authors further reported the production of dwarfed lambs and hairy appearance in ewes subjected to elevated ambient temperatures during early pregnancy, especially in the case of lambs of the wool breeds. [58] reported an increase in early embryonic mortality and a decrease in fecundity in ewes subjected to heat stress. Studying crossbred ewes of Targhee x Suffolk under heat stress conditions for a short and long duration of 25 days and 53 days respectively, [59] observed a smaller size of lambs at birth for both duration of treatments.

Meta-analyses carried out by [22] with 20, 626 ewes through 36 studies demonstrated that heat stress decreased the duration of estrus in cycling in ewes by 7.09h but increased the cycle length by 0.57 days. The study further showed that the heat-stressed cycling ewes resulted in higher embryo mortality and decreased impregnation. Such heat-stressed pregnant ewes also resulted in reduced placental and fetal weights. [60], however, reported decline of the susceptibility of embryos to maternal heat stress with the progress of the development.

Several studies have been carried out to understand the impact of heat stress on changes in rectal temperatures, respiration rates, and other physiological functions in sheep [11,24,25,45,61-65]. Higher body temperature and increased respiration rate are the important indicators of heat stress in sheep and goats [66,67]. At higher ambient temperature, the physiological responses of sheep largely comprise increases in rectal temperature, respiration rate, and heart rate [11]. [12] reported the rectal temperatures of sheep to vary between 38.3 oC and 39.9 oC under thermo-neutral conditions. Owing to the presence of a wool coat that hinders sweating, the heat loss in sheep, is largely taken place through the increase in respiratory rate [12]. In the case of hair sheep, the phenotypic characteristics provide them the adaptability to heat stress [20]. The presence of hair is an advantage in terms of heat loss both by non-evaporative and evaporative means, compared to that of wool [29,33]. The ewes and goats subjected to heat-stress were found to decrease the feed intake in an attempt to create less metabolic heat [42,44,45,68,69]. According to [70], heat stress not only depresses the growth rate of sheep through the reduction of food intake but also by affecting digestion and metabolism.

Study to assess the effect of multiple stresses viz., thermal, nutritional, and walking stress on the adaptive capability of Malpura ewes showed these stresses to affect significantly on the body weight, respiration rate, pulse rate, rectal temperature, sweating rate and several other biochemical parameters [71]. Experimenting with a new heat stress model, i.e., subjecting the Malapura sheep to a different temperature ranging 38-44 °C at different hours of the day and thereby simulating the natural heat stress condition for the sheep reared under a hot semiarid environment, [72] found less severe physiological strain, which could be seen from lower variations in the heat stress markers such as respiration rate, rectal temperature, and plasma cortisol. The authors further suggested the present model for heat stress study to be more ethical than that of the constant heat stress model. [48] reported the heat stress is qualified as cytotoxic, as it alters biological molecules, disturbs cell functions, modulates metabolic reactions, induces oxidative cell damage and activates both apoptosis and necrosis pathways.

Several methods are suggested for the small ruminant farmers to combat the negative effects of heat stress, which include the use of proper shades, selection of the appropriate site for animals’ housing, use of fans and evaporative cooling, availability of adequate drinking water and adoption of appropriate feeding and grazing strategies. [73] suggested possibility reduction of the effect of thermal stress through three basic management schemes, viz., physical modification of the environment, improving thermostolerance by genetic modifications and improving nutritional regimes.

Provision of shade, especially during the summer months, is the simplest, effective and economical method to minimize the heat stress for the animals [9,74-76]. In extreme heat periods of the day, the animals were reported to decrease their grazing time and spend more time in the shade [7]. The trees and shrubs often serve as shelters for these animals in case of extensive grazing systems. It is well known that the sheep and goat flocks are rarely housed permanently, while most of them are housed only during the night hours [69]. Simple shelter with straw or hay or aluminum sheets as the rooftop can be very effective for even semi-intensive rearing practices. Provision of shade to the sheep and goats was found to improve body weight gain, milk production, and reproductive performance [77]. Installation of fans or other cooling systems in the sheds can minimize heat stress [77]. Because the water requirements of sheep and goats increase under heat stress conditions, access to adequate cool freshwater, therefore, is one of the best practices to reduce heat stress [74,77]. Experimenting with German Fawn × Hair crossbred dairy goat genotypes, [78] reported consumption of 18% more feed, 7% more water intake and 21% more milk yield in experimental group which was sprayed and ventilated for 1 h a day to reduce the heat stress over the control group.

Ration modifications, including changes in feeding schedules and feed composition, also reported helping in reducing the adverse impact of heat stress. [79] reported that increased feeding frequency helps to minimize the diurnal fluctuation in ruminal metabolites and increase feed utilization efficiency in the rumen of the animals. While appropriate management measures including that of nutritional strategies can be important remedial measures to overcome heat stress conditions, it would be appropriate to give increase emphasis on the development of more heat-resistant varieties through the approach of genetic selection for wider adaptability in the scenario of climatic change.

The ever-increasing demand for meat, in particular, necessitates the increased emphasis on sheep and goat farming in the coming years. In this context, the understanding of the negative impact of heat stress on the productivity and well-being of sheep and goats is of paramount importance today to take up appropriate alleviation strategies. It is, therefore, necessary that the livestock keepers are given adequate exposure for improved understanding of the impact of such elevated ambient temperature which is likely to be intensified further in the coming years with the increasing impact of climate change. In this endeavor, there is a need to intensify the linkage between the associated researchers, extension workers, and the livestock keepers to improve the knowledge and skills of the later for ensuring all necessary measures of good management practices, and for taking appropriate strategic and operational management decisions to improve production systems in such heat stress condition. It would be also necessary to continue the research efforts on the subject to improve our understanding of such climate extreme situations for providing necessary guidance to the stakeholders associated with the farming of these small ruminants for harnessing maximum production and ensuring the wellbeing of the animals.

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