Reviews in Agricultural Science
Online ISSN : 2187-090X
The Reproductive Traits of Sheep and Their Influencing Factors
Majeed Hameed Ajafar, Alaa Hasan Kadhim, Tahreer Mohammed AL-Thuwaini
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Keywords: environmental factors, genetic factors, reproduction, sheep
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2022 Volume 10 Pages 82-89

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Abstract

In farm animals, both genetic and non-genetic factors affect reproductive traits. These factors can be categorized as intrinsic and extrinsic. Extrinsic factors relate to the animal’s environment, while intrinsic factors are related to its genotype. Various reproductive traits are studied concerning age, season, management, nutrition, body score, and birth type. In addition, genetic variations of the transforming growth factor-β (TGFβ) superfamily including BMPR-1B, BMP15, and GDF9 have been studied for their effect on sheep reproduction. Numerous other genes that affect reproductive characteristics in sheep have also been identified, including melatonin receptor 1A, SLC5A1, CCNA1, ABCC1, insulin-like growth factor-I (IGF-1), leptin receptor, prolactin, GREM1, and numerous other new candidates. These reproductive traits vary based on differences in environmental conditions and the genetic composition of livestock. Genetic and environmental factors that influence livestock traits can be improved by understanding those factors, but genetic and phenotypic associations that affect livestock traits are scarce. Accurate genetic evaluation and development of breeding goals require a more accurate evaluation of genetic parameters, in particular correlations with phenotypic traits. As a result, understanding how genetic and environmental factors interact becomes critical to developing efficient and effective management plans for sheep. This review provides insight into the factors that influence sheep reproductive performance.

1. Introduction

Livestock is a key component of the economy and livelihood of many people, with sheep being the most common [1]. The consumption of red meat has increased by 5–6% annually in developing countries. Globally, lamb meat consumption has been increasing, and countries like China, New Zealand, and Australia consume a lot of lamb meat [2]. Besides, Asia has the largest sheep population representing 43.6% of the world’s sheep population. A large part of the increase in sheep populations is due to an increase in world meat and dairy consumption [3]. For this reason, sheep are one of the most important livestock animals that affect the economy and industry through their high production and reproduction characteristics [4, 5]. These characteristics of sheep that contribute to their economic value are necessary for the development of livestock breeding programs [6]. Sheep reproductive traits such as age at first lambing, litter size, and lambing interval have high economic value in all sheep production systems [7]. Moreover, reproductive performance is a key factor determining the efficiency of flock production, especially in the developing countries of Africa and Asia where the sheep industry is significant [3, 8]. An increase in the production efficiency of the sheep flock is attributed to good reproductive performance [9].

Several researchers have identified factors that affect reproductive performance, among them season, photoperiod, age, nutrition, management, and the behavior of males as well as genetics [10, 11, 12]. In addition, the reproductive efficiency of sheep can be measured by the number of lambs born annually per ewe that contribute greatly to profits [13]. As a result, an effort is being made in this review to compile the factors that affect reproductive performance and research efforts on associations between genetics and phenotypic traits in sheep.

2. Reproductive performance and the factors affected in farm animals

Factors that correlate with seasonal reproduction in any animal can be divided into intrinsic and extrinsic factors. The intrinsic factors are related to an animal’s genotype, while the extrinsic factors are related to its environment [14, 15]. Besides, there are differences in the reproductive traits of livestock due to management systems and physical environments [16]. Controlling the environmental factors affecting these traits or improving the genetic potential of animals can reduce the age at first lambing, increase the conception rate, and cause twin births in sheep [15]. As well as variations in nutrition and reproductive management practices can be affected reproduction in sheep [17].

2.1 Reproduction and non-genetic factors

The reproductive traits of livestock can be improved through knowing the factors that affect those traits. So it becomes imperative to investigate the impact of non-genetic effects on traits of economic significance, such as reproductive performance [18]. These non-genetic factors can be divided into those that have measurable effects, such as parity, dam weight, damage, and calving season, and those that do not have measurable effects, such as infections [19]. There have been extensive studies on the effect of a dam’s weight on lambs. Mishra et al. [20], Singh et al. [21], and Mohammed et al. [22] all found that the dam’s weight at lambing has a significant effect on birth weight. Prince et al. [23] reported that heavier dams gave birth to heavier lambs because of their better nutrition and more uterine space for growing fetuses.

Additionally, other non-genetic factors that influence reproductive traits include the variation in photoperiods that control melatonin secretion [24]. Melatonin stimulates receptors within the hypothalamus, pituitary, and gonadal axis and can regulate the reproductive seasonality and litter size of ewes by influencing gene expression [25]. In general, sheep are highly sexually active when photoperiods are decreasing and reproductively quiescent when photoperiods are increasing [26]. Moreover, breeds of sheep have a seasonal reproductive pattern that is associated with photoperiod length during the different seasons of the year [27]. Babar and Javed [28] reported significant effects of year and season of birth, age, and birth type on various reproductive traits in Lohi sheep and Khan et al. [29] reported similar findings in Rambouillet sheep. Mellado et al. [30] and Shamsa [31] also observed the effects of the year in Dorper sheep and Awassi ewes. Generally, primiparous ewes are less productive than those with two or more parities, and the herd, region, and season all affected it [32].

Joshi et al. [33] demonstrated that the productive and reproductive traits vary due age of conception, first lambing, lambing interval, and litter size. Age and weight at first mating are considered critical factors for increased litter size in gilts [34]. Breeders commonly use the age of gilts at first mating as a measure of sow reproductive performance [35]. Besides, a heifer’s reproductive performance depends on the age at which it calved for the first time [36]. With the growth in livestock production, animal breeds with larger litter sizes are more desirable [37]. The number of lambs born per ewe lambing is an important indicator of lamb production efficiency, but the number of lambs weaned has a greater impact on sheep production due to both the reproductive capacity of the ewe and the survival of her lambs [38]. Litter size is the most important phenotypic trait in livestock reproduction, which is influenced by ovulation rate and hormones, as well as fecundity genes [39]. Ovulation rate and the number of oocytes released from follicles during ovulation are directly associated with litter size [40]. Moreover, this oocyte is enclosed by granulosa cells and theca cells that secrete estrogen and progesterone hormones during ovulation, which affects litter size [41]. The litter size of sheep also varies between breeds, ranging from single birth in Texel and Suffolk to twin birth in the prolific Booroola Merino breed [42]. Awassi breeds are monoovulatory [43] and have very low incidences of twinning [44], as opposed to other prolific breeds such as the Finnsheep and Romanov with triplet’s birth [45, 46]. In addition to genetic differences, damage and parities affect litter size in sheep [41]. The litter size is heavily influenced by the dam’s age. The litter size can be increased until five years of age or the fourth parity, and then it gradually decreases afterward [11]. On the other hand, an increase in litter size with increasing parity and larger litter size at the fifth parity are reported in other studies [47, 48], in which peak prolificacy is generally achieved between the ages of 4 and 8 years old. Sheep with larger litter sizes contribute significantly to flock productivity and profitability, resulting in increased meat productivity via genetic improvement [48].

2.2 Reproductive performance and genetic factors

Predicting estimated breeding values and reproduction of individual animals requires knowledge of genetic parameters [49]. Ovulation rate and litter size are important reproductive traits in farm animals that have high economic value [38], which are genetically controlled by the fecundity gene (Fec) [15]. Variation in these fecundity genes significantly increased the ovulation rate in sheep [50]. Three of these genes are members of the transforming growth factorβ (TGFβ) gene superfamily [51]. Those genes are bone morphogenetic protein receptor type1B (BMPR-1B), bone morphogenetic protein 15 (BMP15), and growth differentiation factor 9 (GDF9) [52]. Fecundity genes encode proteins that are essential in follicular development in the ovaries, affecting ovulation rate and litter size [53]. These growth factors stimulate the proliferation of granulosa cells, modulate other growth factors and hormones, and influence follicle growth and survival signals [54]. In turn, these proteins are likely able to exert their biological influence by binding to type 1 receptor (BMPR-1A, BMPR-1B or TGFβR1) in the ovaries, which are then combined with type 2 receptors (BMPR-2) [55]. GDF9 and BMP15 stimulate follicle growth and contribute to regulating ovulation rates and litter sizes [56]. Several species of livestock have high levels of these proteins in their oocytes, and they are essential for female fertility and multiple ovulation [57]. Furthermore, variation in BMP15 and GDF9 expression allow more follicles to produce luteinizing hormone receptor (LHR) even when follicle-stimulating hormone (FSH) levels drop and enabling a developing female to release more eggs during its anestrus phase in heterozygous animals [58]. In contrast, ewes homozygous for BMP15 and GDF9 mutations exhibit early inhibition of follicle growth and reduce the sensitivity of granulosa cells to FSH by inhibiting expression of the FSH receptor [59].

Genetic improvement can be achieved by identifying polymorphisms associated with reproductive characteristics to increase litter size and reproduction efficiency [60] and can be very useful in studying animal reproductive genetics and physiology [61]. In some sheep breeds, mutations in BMP15, GDF9, and BMPR-1B have been identified as fecundity genes that are linked to ovulation and follicle development [62]. However, mutations in these fecundity genes affect ovulation rate and litter size in different ways. BMP15 can have limited effects on Hu and Merino sheep fecundity, but it shows a marked effect on Small Tail Han sheep fecundity [63]. BMP15 has been associated with prolificacy in Inverdale, Lacaune, Belclare, and the Small Tailed Han sheep [64]. Eight genetic +variations (FecXL, FecXB, FecXR, FecXI, FecXH, FecXGr, FecXO, and FecXG) are known to have a significant impact on the litter size and ovulation rate in the BMP15 gene [65]. Furthermore, there is a similar expression of GDF9 in the ovary with the BMP15 mutation, although the GDF9 mutation increases ovulation rates in animals even more [66]. Mutations in the FecGE and FecGF of the GDF9 gene affect fecundity traits like ovulation rate and litter size, while mutations in FecGH, FecGT, and FecGV cause increased ovulation rate and litter size in heterozygote ewes and infertility in homozygote carriers [67]. The FecB gene is mutated in position 830 (A to G), leading to arginine to glutamine transition in BMPR-1B expressed in oocytes and granulosa cells [68]. Furthermore, carriers of one or two copies of the FecB gene (Q249R) have an increased ovulation level and litter size when carrying the FecB mutation (Q249R) in the BMPR-1B on Booroola Merino breed [69].

3. Genetic and environmental interactions influence sheep reproductive performance

Genetics and environment interact to influence economically valuable traits (weight at different ages, growth rate, carcass characteristics, and litter size) in small ruminants [70]. Providing the appropriate environment for optimal genetic expression is essential to enhancing the productivity of native breeds. The improved genetic makeup of flocks is required in conjunction with enhancements in flock productivity [71]. Consequently, effective breeding requires the selection of elite animals from flocks that are better genetically. Small ruminant growth is affected not only by the animal’s genetic makeup, but also by its environment, including the age of the dam, the lamb's birth weight, the lamb’s breed, and the lambing season [72]. The development of effective selection indices and accurate estimates of genetic and non-genetic parameters is essential for maximizing genetic progress through selection [70]. Although, several major genes associated with reproductive traits in sheep have been identified, including the melatonin receptor 1A gene in Small-Tail Han sheep [25], the SLC5A1, CCNA1, and ABCC1 genes in Black Sheep [73], the insulin-like growth factor-I gene in Sarda dairy sheep [74], the leptin receptor gene in Bamei Mutton sheep [75], and the OLR1 gene in Awassi sheep [76], and many other new candidates [77]. However, there are limited studies on the association of this genetic polymorphism with phenotypic traits in sheep have been reported such as melatonin receptor 1A gene in Small-Tail Han sheep [25], GREM1 gene in Awassi sheep [50], MTNR1A and AA-NAT genes in Ossimi, Rahmani, and Barki sheep [78], prolactin gene in Awassi sheep [79]. As a result, genetic and environmental improvements offer an opportunity to increase production systems and thus to raise productivity. It can also encourage geneticists and breeders to use these methodologies in selecting the best breeding sources. Fig. (1) summarized the studies that investigate genes, phenotypic traits, and the association of both of these factors in sheep.

Figure 1: The studies that investigate on genes, phenotypic traits, and the association of both of these factors in sheep

4. Conclusions

The reproductive performance of sheep varies not only due to their genetic composition but also due to non-genetic factors. Sheep’s reproductive performance is significantly affected by non-genetic factors; therefore, these factors should be included in genetic analysis models and taken into account when improving herd breeding. Thus, reliable estimates of both genetic and non-genetic parameters as well as effective selection indices must be devised to maximize genetic progress.

References
 
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