**2. Climatic indexes and characterisation of the effect of heat stress on milk and reproduction traits in goats**

In order to know the effects of HS on production, a measurement of the climate effect on the animal comfort is needed. The first index, specifically developed for animals, to measure the stress produced by some climatic conditions was the Iberian heat tolerance test (HTC) [8], which was used to assess the heat tolerance of cattle by measuring how much rectal temperature exceeds the normal value of 101.0°F (38.33°C). Heat tolerance was determined by the value of HTC, so the higher the HTC the more heat tolerant the animal is perceived to be. This index was followed by other indices such as the coefficient of adaptability [9], the biochemical index of heat tolerance [10], the discomfort index [11] or the milk production decline index [12].

Johnson et al. [13] showed for the first time the relation of temperature and relative humidity with comfort and dairy performance in cattle, which is the base of the combination of temperature and relative humidity in an index, called temperature humidity index (THI). Some adaptations of the previously described indexes had followed (e.g. Bianca [14] adapted the HTC index to express rectal temperature in °C), Kibler [15] being the first to use an index with the current THI philosophy for livestock. The index was later refined on a number of studies. Berry et al. [12], for example, incorporated dry and wet bulb temperatures to the index. Since then, the THI (in its various formulations) has been widely used to assess the response of animals to heat stress.

Heat balance is a complex phenomenon affected by numerous climatic factors (e.g. ambient temperature, relative humidity, wind speed, radiant heat and other factors such as altitude), animal factors (e.g. age, genotype, hair coat characteristics, degree of acclimatisation, health status, physical activity, level of performance, reproductive state, etc.) and management factors (e.g. housing, provision of shade, fans and others). However, in its usual formulation, the THI index only reflects the influence of the temperature and humidity to which the animal is subjected, without considering other important effects such as thermal radiation (solar and long‐wave), wind speed or the duration of the exposure to these conditions. The reliability of using THI to predict animal responses to thermal stress has been examined in Refs. [3, 16–18], showing all of them some limitations of the index, the major one being that it does not account for solar load or wind speed [19], parameters of great impact on animal physiology, especially in grazing species. In addition, it does not take into account the breed, the genotype or other animal differences (e.g. age, level of production). Gaughan et al. [20] concluded that the THI may not adequately describe the effect of hot climatic conditions on livestock (and much less on the effects of cold conditions). Furthermore, with the exception of the case of cattle, most THIs have not been specifically designed for their own species (and much less for a specific breed exploited in certain geographic and climatic conditions). Thus, in the case of the goat species, the scarce number of studies carried out so far has used THI formulations developed for beef and dairy cattle, with the exception of the works carried out by our research group [21–24] in which a modification of the THI developed for sheep exploited under Mediterranean environmental conditions by Finocchiaro et al. [4] was used. Only very recently the feasibility and the validity of a heat stress score specifically developed for intensive dairy goat farms has been tested [25].

[5, 6]. Temperature increase and rainfall reduction are expected during the next decades in many areas of the world and, particularly, in the Mediterranean region, where the largest part of goats in Europe are raised [7]; therefore, knowledge of the physiological and genetic bases of the response to the consequences of climate change is needed to reduce its impact. The study of physiological indicators and the quantification of the genetic variation of the response to HS, as well as the genomic and other –omic analyses to find candidate genes involved in this response and the changes in gene expression so induced, allow for the identification of animals with a

Research works described in this chapter aimed to ascertain the effects of HS on milk traits and to study the physiological and genetic bases of these effects in order to allow for the inclusion of resilience to HS as a new goal in the selection programmes of three Spanish breeds of goats.

**2. Climatic indexes and characterisation of the effect of heat stress on** 

In order to know the effects of HS on production, a measurement of the climate effect on the animal comfort is needed. The first index, specifically developed for animals, to measure the stress produced by some climatic conditions was the Iberian heat tolerance test (HTC) [8], which was used to assess the heat tolerance of cattle by measuring how much rectal temperature exceeds the normal value of 101.0°F (38.33°C). Heat tolerance was determined by the value of HTC, so the higher the HTC the more heat tolerant the animal is perceived to be. This index was followed by other indices such as the coefficient of adaptability [9], the biochemical index of heat tolerance [10], the discomfort index [11] or the milk production decline index [12].

Johnson et al. [13] showed for the first time the relation of temperature and relative humidity with comfort and dairy performance in cattle, which is the base of the combination of temperature and relative humidity in an index, called temperature humidity index (THI). Some adaptations of the previously described indexes had followed (e.g. Bianca [14] adapted the HTC index to express rectal temperature in °C), Kibler [15] being the first to use an index with the current THI philosophy for livestock. The index was later refined on a number of studies. Berry et al. [12], for example, incorporated dry and wet bulb temperatures to the index. Since then, the THI (in its various formulations) has been widely used to assess the response

Heat balance is a complex phenomenon affected by numerous climatic factors (e.g. ambient temperature, relative humidity, wind speed, radiant heat and other factors such as altitude), animal factors (e.g. age, genotype, hair coat characteristics, degree of acclimatisation, health status, physical activity, level of performance, reproductive state, etc.) and management factors (e.g. housing, provision of shade, fans and others). However, in its usual formulation, the THI index only reflects the influence of the temperature and humidity to which the animal is subjected, without considering other important effects such as thermal radiation (solar and long‐wave), wind speed or the duration of the exposure to these conditions. The reliability of using THI to predict animal responses to thermal stress has been examined in Refs. [3, 16–18], showing all of them some limitations of the index, the major one being that it does not account

positive response to HS and the design of methods to select them.

**milk and reproduction traits in goats**

330 Goat Science

of animals to heat stress.

It is generally recognised that goats are more tolerant to HS than sheep, and both are superior to this respect than cows, due to the morphological and physiological differences between these species related to heat dissipation [26]. However, it is also well known that high temperatures and relative humidity values affect productivity of small ruminants [27, 28]. As opposed to the case of dairy cattle, few woks have dealt with the effects of HS on milk yield and composition in goats. In several studies lately carried out to quantify these effects on native breeds of goats, Murciano‐Granadina and Payoya [21], Florida [22], Malagueña [29], raised mostly in the South of Spain, it has been observed that the animals are exposed to stressing climatic conditions, due to high temperatures, during 45–55% of the year (**Figure 1**), generating losses of 1.9 and 3.1% of annual fat plus protein yields in Murciano‐Granadina and Payoya goats, respectively (**Figure 2**). Sano et al. [30] found milk yield losses of 3–13% in dairy Saanen goats exposed to moderate or severe HS for 4 days (THI, 81 or 89), respectively. Brown et al. [31] reported that

**Figure 1.** Evolution through the year of THI values in the South of Spain (from Ref. [22]).

HS situation (THI = 77–85), results shown in Refs. [34, 35] indicate that although a substantial reduction in DMI (22–35%) coupled with an increased rectal temperature (+0.58 °C) and respiration rate (+48 breaths/min) were observed, reduction in milk yield was relatively low (3–10%) with reduced contents of fat, protein and lactose. As in cattle, the reduced intake was not accompanied by increasing levels of non‐esterified fatty acids (NEFAs), which is typical in feed‐restricted animals under thermal neutral conditions. In cattle, this seems to respond to a shift in the energy metabolism from using fat to using glucose as main fuel under HS [32, 36]. In dairy goats, the lack of fat mobilisation was not accompanied by decreased glucose levels and increased levels of insulin as it is in cattle [34, 35]. These authors launched several hypotheses to explain this different behaviour in goats, one of them being that the pancreas of HS goats is less sensitive, which could be a way to maintain normal glucose levels in blood. Overall, the effect of heat stress on goats seems milder than in highly producing dairy cattle

Characterisation of Goats' Response to Heat Stress: Tools to Improve Heat Tolerance

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and the metabolic consequences may be attenuated with respect to those in cattle.

**4. Quantitative genetic analyses of individual reaction norm**

of proliferation and survival of pathogens [26].

Kelly and Bond [47].

The effect of HS on reproduction takes place through a reduction of oestrus duration and intensity [37, 38], malfunction of the axis hypothalamus‐pituitary‐ovary and low quality of the oocyte [39], anomalous spermatogenesis [40] and a bad embryo development [39, 41, 42]. Among the effects of HS on the health status is a higher risk of mastitis [43], but it is not clear that this is due to a direct action of the stress on the animal immune system or to higher rates

According to Silanikove [28], rectal temperature is the best physiological indicator of HS. Heritabilities between 0.12 and 0.22 were estimated for rectal temperature by Prayaga and Henshall [44] in Australian beef cattle and an estimate of 0.17 was obtained by Dikmen et al. [45] in dairy cattle. These heritabilities permit to expect a positive response to decrease rectal temperature under HS conditions, as it was proven by Burrow and Prayaga [46] in a selection experiment also carried out in Australian beef cattle. However, rectal temperature is not an easy trait to be routinely registered in a large population; therefore, most of the quantitative genetic analyses of the response to HS have used bioclimatic indexes reflecting the level of thermal comfort of the animal. One of the most used is the formerly described temperature humidity index (THI) combining dry bulb temperature and relative humidity, proposed by

The current approach to the quantitative genetic analysis of the response of milk traits to HS uses frequently random regression models based on the concept of norm of reaction: the different phenotypic expressions of a gene in different environments [48]. The magnitude of the change in the value of a given trait from one environmental condition to another measures the plasticity of an individual for a given trait and it is also a measure of the genotype by environment interaction. According to their norm of reaction, animals can be classified as stable or robust for a certain trait if the value of that trait remains constant through the range of values

of the environmental variable and unstable or plastic if the value of the trait changes.

**Figure 2.** Graph of daily kg of milk yield (DMY) and g of fat plus protein yield (DFPY) as a function of THI for Murciano‐ Granadina (MG) and Payoya (PY) goats (from Ref. [29]).

the exposure of dairy goats to moderate HS conditions (THI = 79) decreased milk yield in Alpine but not in Nubian goats. Differences in the genetic potential for adaptive traits and also for production might explain these differences.
