One of the communication systems in the body of animals is The Endocrine System, which produces chemical substances that travel by the blood stream. These areÂ Â calledÂ Â hormones and work by sending messages to other cellsÂ Â (Reece 1997). 426 Cells have special proteins called hormone receptors that are located on the surface of the cells(Cupps 1991).62 Hormones are defined as "chemical substances produced by specialized ductless glands that are released into the blood and carried to other parts of the body to produce specific regulatory effects (Reece 1997).426 However, the term was, firstly, used in 1904 by Bayliss and SterlingÂ Â to depict the elaboration of secretion, which is "a hormone produced by the duodenum to stimulate the flow of pancreatic juice" (Squires 2003) . In many kinds of mammals, it is well known that environmental factors are important. They can change the mechanisms that organize the operations of the endocrine glands genital nerve (Thibault et al. cited in Monje et al. 1992).
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A great deal of research has been done to study the effects of hormones on the body of animal's. However, more much of the debate had been sparked in the researches that focused on whether hormones or external factors have a greater direct effect on the reproduction process. This essay will describe the influence of hormones and the external factors on the reproductive process for male animals as well as the five main stages of the reproductive cycle in female animals. It will be argued that regardless of the factors that are responsible for producing the hormones in the body of an animal, hormones have a direct effect on the reproductive cycle of animal.
The sexual reproduction: Is the combination of two reproductive cells, one male and on female, is the core of the sexual reproduction (Blache et al. 2000; Dziuk & Bellows 1983; Evans & Mulligan 2006; Kadokawa 2006; Moore & Thatcher 2006; P. 2008 ; Perera 2008; Roche et al. 2000; Santos 2009; Sjaastad et al. 2003) 622.
2. Male reproduction: Â Â Â
The reproductive functions of the male 'involves the formation of sperm and the deposition of the sperm on the female' (Recce 2004)379. Sperm production is a continuous process as soon as it begins. In some species, it can sometimes change in rates by the amount of daylight (photoperiod).
Nutrient, Metabolites and Metabolic hormones
Sex steroids Inhibit
Fig. 1. Scheme describing interactions between photoperiod, social and nutritional cues in the control of hypothalamo-pituitary-testicular axis in male Merino sheep (adapted from Blache et al. 2000).)
This process is supported by male hormones and the autonomic nervous system (Recce 2004)379. The gonadotrophin control testosterone hormone secretionÂ Â that is produced from leydig cells and, which are known as luteinizing hormone (LH). A decline in testosterone levels causes a rise in LH levels from anterior pituitary (Recce 2004)394. Testosterone is responsible for boosting the meiotic process to preserve spermatogenesis in the seminiferous tubules (Recce 2004) 394. As well as it stimulates male sex characteristics developmentÂ Â and other skeleton and musculature growth (anabolic action). In brief the male animals physiological development and the sex drive are influenced by testosterone (Sjaastad et al. 2003)623. Not only testosterone and LH are important for male reproductive process but also follicle stimulating hormone (FSH) that induces the production of androgen, which is thought that FSH arouses it by the Sertoli cells. However, when the LH is needed continuing basis for spermatogenesis, but FSH is not necessary in maintaining the composition of the sperm once it has been started. FSH needs to stop at the age of puberty and following in the physiologic or pathologic when the spermatogenesis initiated (Recce 2004) 395. In addition, the negative feedback mechanismÂ Â influences the release of GnRH hormones produced by the testesÂ Â (Sjaastad et al. 2003)633.
Â Stress effect directly on male fertility. For example, the libido and fertility rates are declined and can devascularize malesÂ Â due to decrease in the secretion of GnRH by the hypothalamus and subsequent production of LH and FSH by the pituitary and sex steroids by the gonads (Squires 2003) 215. Furthermore, stress causes inhibition of testosterone levels in the blood which is in part to a direct receptor-mediated effect of glucocorticoid in Leydig's cells. On the other hand, chronic stress leads to decrease in the secretion of gonadotropin by the pituitary (Sapolsky cited in Hardy et al. 2005).
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Nutrition is affected the reproductive activity in male sheep. For instance, nutrition in sexually matures by Merino rams are the most powerful factor and the responses to it can be divided into short-term and long-term effects (Blache et al. 2000). The short- term effects are presented fundamentally on the neuroendocrine system controlling testicular activity (Martin et al. cited in Blache et al. 2000).In the contrary, the long-term effects are presented fundamentally on testicular growth and sperm production (Oldham et al. cited in Blache et al. 2000).
Good management can increase the rate of conception in the herd by using male pheromones to detect oestrus for correct timing of AI (Squires 2003) 204. Furthermore, they pulsate with LH in female goats in the early and late luteal phases of the oestrus cycle could be increased by stimulating by male effect.Â Â However, the male effect blocked during mid-luteal phase due to the high concentrations of progesterone in females (Hawken et al. 2009).
3. Female reproduction:
The importance of the female in the re-breeding process is to produce female oocyte and provide an environment conducive to the growth and nutrition of the embryo that develops after fertilization of the mature oocyte by sperm. The final stage of this process is to give birth in a timely manner, and to continue lactation tasks. The coordination of the complicated relationships of hormones and histological changes control the role of the female in prolonging the species (Recce 2004)403.
Fig. 2. A systems representation of factors regulating reproductive success (adopted from Vetharaniam et al. 2010).
The first stage of the reproductive cycle in animals is oestrus. The oestrus cycle is controlledÂ Â by the hormones, FSH, LH, Oestrogen and Progesterone. The commencement of oestrus occurs when the femaleÂ Â animals reach the age of maturation (puberty). In addition, oestrus combines five stages. Each stage determined by the dominance ofÂ Â one hormone. In other words, if that hormone increases this will cause a specific effect on ambient of the ovary. For instance, first stageÂ Â (Pro-oestrus) starts when the concentration of progesterone decreases then the second stageÂ Â (Oestrus) occurs when oestrogen levels rise by producing it from follicles, which suppress FSH production. After that, LH increases dramatically causing ovulation of the ovum which is the third stageÂ Â (Met-ostrus). In the fourth stage (Di-oestrus)Â Â progesterone levels pecked up again by producing it from the corpus luteumÂ Â causing an inhibiting on LH release. By this way progesterone maintains pregnancy. The final stage is (Pro-ostrus). If the cow were not pregnant, prostaglandins will release from her uterus to start a new cycle. If pregnant, anoestrus happens because of corpus luteum existence, which releases progesterone that works on suppressing prostaglandins secretion from the uterine wall which leads to maintaining pregnancy (Dunn et al.). Hormonal changes can causes reduce the fertility of cows due to cystic ovaries because of influence on the uterine tone and, in most of the time, failure to release an ovum (egg) (Hutchinson). However, a number of external factors also affect oestrus.
Fertility during hot weather reduced pursuant to increasing temperatures and humidity, which caused a decline in the expression of overt estrus and lowering animal's appetite that decreases dry matter intake (Rensis & Scaramuzzi 2003). For example, in India heat stress is a main cause for anoestrus in hot summer months causing a rise in blood concentrations of prolactin. This may affect ovarian activity and in the same time sub-fertility can occur and a decline in progesterone levels through repeat breeding (Hsldar & Prakash 2007).
The cessation of the oestrus cycle could happen because of a nutritional shortage or circumstances in the environment (Senger 2005)145. An increase to the highest level of 2.65 in the heat was recorded after the sixth supra maintenance feeding commencedÂ Â in comparison with an average of 1.69 in the conservation level of nutrition (Henniawati & Fletcher 1986) . Furthermore, The period between male exposure and the beginning ofÂ Â the estrous behavior was decreased by short-term nutritional supplementation in goats (De Santiago-Micrometers et al. 2008).
Inability to detect an estrus properly, is a problem associated with mismanagement, which resulting in low conception rates and increased early embryonic death. However, herd reproductive performance can be improved by using coordinated systems of reproductive and nutritional management (Moore & Thatcher 2006).
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The role of hormones during pregnancy is maintained by mastering progesterone that is yielded from the placenta and corpus luteum during gestation. Their input is the main supply to progesterone and is varied from one species to another (Reece 1997) 396. However, during early pregnancy progesterone from CL is necessary for all species except mares and ewes. The need for it for it ends after about 100 days for mares and 60 days for ewes. However, it is needed for much longer in cows, bitches and queens. Furthermore, in sows it is required for the whole period of pregnancy (Reece 1997)396.
Progesterone that is produced from the placenta considered as the core source to maintain pregnancy for the species. The CL progesterone is not necessary in gestation for mares and ewes (Reece 1997)(397). In addition, the membranes of a fetus secrete a different kind of hormone that plays a role in maternal recognition. The endocrine system of the fetus commences at an amazingly early stage. For example, in sheep, pigs, and cattle the gonads are activated by the third week of gestation (Cupps 1991)(377). Earlier, a small amount of nonprotein is produced from the embryo on the tenth day of pregnancy that induces progesterone production by the corpus luteum (Cupps 1991)(455 ).
Moreover, sex differentiation occurs during fetuses stage controlled by hormones. For instance, in cattle the gonads occur from day 45 to day 70 of gestation. Female fetuses show masculinization of the gonads when they are treated with exogesenous testosterone. Furthermore, sexual brain differentiation in cattle occurs after day 60 of gestation when low levels of steroid synthesis in female ovaries from day 70 to day 85, while in male fetus levels of androstenedione are high until day 100, allowing steroid precursors of the male programs from the area of â€‹â€‹the hypothalamus (Squires 2003) 156.
The effect of heat stress on the pregnancy is derived from altering in the endocrine secretionÂ Â Â that minimizes follicular activity and modifies the ovulatory mechanism. This directly causes a reduction in oocyte and embryo quality.Â Â Furthermore,Â Â changes will include the uterine environment. The possibility of embryo implantation will decline (Rensis & Scaramuzzi 2003).
The pregnancy rates in goats were improved when they gave food supplements for both 14 and 28 days starting on the 9th day after introducing the males and managing the grazing conditions (Fitz-Rodríguez et al. 09). Furthermore, there is a potential link between nutrient intake during pregnancy period and development of uterine diseases may be having the impact on the immune status of the cow (Santos 09).
Reducing the costs per unitÂ Â is a major concern for managers to improve the reproduction in cattle. The failure of cows to become pregnant ranked as first reason and perinatal calf losses ranked as a secondÂ Â reason that effect on net calf crop by many researchers. These facts confirm that the same factors reducing net calf crop, regardless of the geographic area (Dziuk and Bellows 1983).
The increase in the contractility of the myometrium occurs before parturition is a result of the increase the levels of oxytocin, which induces uterine contractions (Cupps 1991)( 378).Â Â Estrogen, Progesterone, Oxytocin and Prostaglandins are the main hormones that control parturition.Â Â Calmness of the uterine during pregnancy is derived from increased levels of progesterone hormone and decrease levels of estrogen hormone. However, the response of the uterus will be a change adversely at the end of pregnancy when the contraction of uterine interaction starts again (Cupps 1991)( 379) . On the other hand, Oxytocin hormone has an exact function on the uterus. It is responsible for commencing the uterine contraction. Its level rises 10 fold suddenly before parturition, which increases uterine contraction leading to facility parturition (Cupps 1991)( 379) . Furthermore, Prostaglandins hormone is considered an important synthesis not only to stimulate uterine activity but also the fetus deportation. In other words, it is essential to initiate parturition (Cupps 1991)(380) ForÂ Â Â 'Injection of a 3B-hydroxysteroid dehydrogenase inhibition in late pregnancy goats produced a full in progesterone and cortisol to 20% of the pretreatment levels and led to premature delivery in 44+-2 h' ( Taylor. 1987 cited by Cupps, 1991) (380).
The reproductive efficiency of cattle effected by seasons that plays a role in the induction of the puberty. For example, heifers born in the spring reaching puberty after those born in the autumn. The age of animals that born in the autumn are from 6-12 months when spring arrives, and the hypothalamic-pituitary -ovarian axis stimulates during the spring due to the increase in the photoperiod (Squires 2003) 162.
Hammon et al.; Huzzey et al.; Urton et al. studies cited in Santos (2009) are suggested that the behavior of feeding and consuming forageÂ Â during the pregnancy period and around parturition might mediate some of the increased risk for uterine diseases in dairy cattle. Hammon et al. cited in Santos (2009) observed that cows developing uterine disease postpartum experienced reduced DMI beginning one week before calving. Likewise, Huzzey et al. study cited in Santos (2009) was finding that cows diagnosed with severe metritis after parturition was already consuming fewer dry matter two weeks previously to parturition.
Peripartum period managementÂ Â is very important in early lactation to control the passive energy balance because a greater loss of (BCS) after calving and thus a lower postpartum (BCS)Â Â is a result of high body condition score (BCS) at parturition. This rapid mobilization of excessive fat in the early postpartum period is a major risk factor for prolonged periods to ovulation, and delay to the start of recovery of the energy balance after parturition is positively correlated with a delay of the firstÂ Â ovulation (Kadokawa 2006).
The interval after the birth anoestrous (PPAI) is the period of convalescence in female mammals. ThisÂ Â occurs between birth and breeding behavior in the beginning of the next ovulation, and implies the return of the reproductive endocrine hypothalamic pituitary, as well as uterine involution and restoration (Vetharaniam et al. 2010). The diet of cattle dietary is an important factor influencing the length of anoestrus after natal (Lamond, 1970 cited in Monje et al. 1992). On the other hand, Monje et al. their experiment (1992) indicated that displays of the great bull and the levels of nutritional reaction. By reason of the large number of well-fed cows, which complied to excitation of the corpus luteum activity, and not to the increase in the activity of the corpus luteum. Season changes (photoperiod) have negative effects on some species such as goats. Female goats undergo seasonal anoestrus because of the response to the elevated levels of progesterone (Hawken et al. 2009).
Heat stress impact negatively on the female animals during the postpartum period. Both dry matter intake and appetite are reduced, which leads to increase the interval of calving-conception, especially in high production dairy cows (Rensis & Scaramuzzi 2003).
The variety in the photoperiod according to seasonal differences affects the period of postpartum anoestrus by 10-35 days. The shortest anoestrus period occurs to cows that calving in the period of lengthening photoperiod from late spring to early autumn. In contrast, animals that calve in the winter, when the photoperiod is decreased to have the longer period of anoestrus (Squires 2003) 162.
An inadequate feeding causes a severe weight and body condition score losses That is associated with anovulation and anestrus in dairy cattle (Santos 2009). Santos et al. are finding that (cited in Santos 2009) cows are more likely to be anovular if they were at 65 d postnatal with a low body condition score.Â Â That will influence the reproductive performance at first postpartum insemination.
Reproductive transition period requires effective management. Cows during this period resume normal oestrus cycles in association with high efficiency of detection of oestrus during the designated breeding season; and finally, that pregnancy rates per service are emotional. Because of that the prevention of metabolic and gynecological disease is important. Therefore, high reproductive efficiency requires long-term planning, the implementation of good preventative measures to decrease peripartum diseases, and the interaction of herd managers as a coordinator to keep the coherence of the work is a vital roleÂ Â (Roche et al. 2000).
Lactation is an important component of the reproductive process because the food provision is a necessary factor for survival of the new born. Hormonal changes after parturition are the reason for the commence lactation (Recce 2004)422. Milk secretion is initiatedÂ Â before the delivery of the fetus, but in a small amount. However,Â most of the species, the secretion of copious milk starts much of not long before parturition and in continues for many days after It (Reece 1997) ( 417). They commence of lactation (lactogeneses) and conservation of milk secretion (galactopoietic) are controlled by a central role from the endocrine system (Akers & Denbow 2008)486. The experiment that conducted by Stricker and Grueter in 1928 showed that the virgin rabbits could be stimulated to excretion milk by injecting a pituitary extract. Furthermore, Riddle, and Dykshorn in 1933, they named the protein that responsible for the milk secretion appropriate observed previously by Stricker and Grueter, Prolactin (Prl) (Akers & Denbow 2008)Â Â 486. Hormones necessary for the formation of milk include prolactin, growth hormone, insulin, parathyroid hormone, ACTH, and thyroid hormone stimulation. The importance of ACTH, and thyroid-stimulation hormoneÂ Â are derived from their role in inducing the production of thyroid hormone and glucocorticoid consecutively (Recce 2004)449.
Stress has a negative impact on lactation. It leads to a decline in milk ejection and yields pursuant to decrease in oxytocin secretion because of the chemical materials who are produced during lactation (Squires 2003) 219.
In the event that, the dry ofÂ matter intake in early-lactation reducedÂ where nutrient partitioning to the mammary gland happened. It leads to a passive energy balance and many of the disorders that associated with it (Evans & Mulligan 2006). However, throughout the decades, milk proceeds has increased significantly as a consequence of the clonally selectionÂ Â and nutrition enhancement. For instance,Â Â Holstein herds in the U.S. recorded the highest milk production levels as an average annual production of over 14 000 kg / cow (Kellogg et al. 2001 cited in Sorensen et al. 2008). Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â
Â 3.5.4. Management:
A number of management factors during the lactation period in dairy cattle (such as sub-clinical hypocalcemia and ketosis, endometritis and retained placenta, and lameness) these will lead to the complexity of the simple picture of passive energy balance that stimulates infertility. That have been proven by much research (Evans & Mulligan 2006).
Considering that of the misunderstanding of the many external factors that affect the reproductive cycle, further research is needed to define to what degree nutrition, climate and management impact on it. More research is required to be clarify to the impact on reproduction. At least, the biological pathways related to reproduction should be understood very well (Blanc et al. 2001). Blanc et al. identified the main difficulties (2001) in their review of breeding farm animals in the model. These difficulties: it be supposed to this policy be linked to dynamically different stages of the reproductive life of the animal, and the inclusion of the effects of modulating nutrition, environment and genetics and Administrative on physiological processes of interaction that lie behind the results of reproductive health. Herd reproductive problems have a significant economic effect. TheseÂ Â problems can be reduced by better management for the herd such as the excellent records and skilled working, relationship with the people, flock, resources and other veterinarian. For instance, the use of records, help to find a way to solve such problems of infertility (Hutchinson).
The environments that animals live in are both complicated and continuously undergoing change. As a result animals react against the short term and long term difference in a number of factors., such as photoperiod, nutrition and socio sexual singles. Sophisticated strategies for animal's reproduction process existed before they were domesticated. These strategies enable animals to adapt with these changes and benefits most of the time (Martin et al. 2004). Even though the reproductive cycle is affected by many external factors such as environmental, nutritional, and management. A more crucial partÂ Â isÂ Â played by hormones such as (GnRH, LHRH). The onset ofÂ Â puberty by GnRH began by stimulating gonadotropic cells of the anterior pituitary. In addition, may GnRH immediately act on target cells in the brain to facilitate in the species standard conduct that concur with reproductionÂ Â (F.L. et al. 1982; Moss & S.M. 1973).