Physiological Relationship between Stress and Reproductive Efficiency

AGRICULTURE AND BIOLOGY JOURNAL OF NORTH AMERICA ISSN Print: 2151-7517, ISSN Online: 2151-7525, doi:10.5251/abjna.2013.4.6.600.604 © 2013, ScienceHuβ, http://www.scihub.org/ABJNA Physiological Relationship between Stress and Reproductive Efficiency NseAbasi N. Etim1*, Edem E. A. Offiong1, MetiAbasi D. Udo2, Mary E. Williams1 and Emem I. Evans1 1 Department of Animal Science, Akwa Ibom State University Obio Akpa Campus, Akwa Ibom State, Nigeria 2 Farm Services Department, Akwa Ibom State University, Obio Akpa Campus, Akwa Ibom State, Nigeria *Corresponding Author’s E-mail: etimbobo@yahoo.com ABSTRACT The effects of both environmental and management related stressors on reproduction were examined. Reproduction is the ultimate measure of an animals ability to adapt to an ever changing external milieu as well as forming the basis of life-time production. Thus, one of the main goals in animal production is to achieve the highest possible reproductive success. Reproductive efficiency is the major factor affecting profitability in many livestock production systems. Reproductive success in livestock is essential for the economic livelihood of producers and ultimately affects the consumer cost of meat and other animal products. In many livestock production systems, poor fertility is a major factor that limits productivity. Unpleasant stress impairs reproduction. Reduced reproductive efficiency can occur as a result of environmental and management factors or stressors associated with animal housing, human-animal relationship/animal handling and management, modern production methods, temperature extremes or changes in photoperiod (day and night cycles). These stressors cause deviation in hormonal pattern and clinical manifestations. Reduction of stressful situations allows for greater well-being, growth and reproductive efficiency of the animal. To maximize reproductive efficiency during the summer/hot seasons, measures must be taken which reduce heat gain and facilitate heat loss to the environment. Animals should be exposed to positive handling and management practices. Further research aimed at obtaining greater clarity of the hormonal interactions involved should be carried out as well as further studies to provide input into management procedures that should be used in intensive production units for optimal fertility and productivity. Keywords: Physiological, relationship, stress, reproduction, efficiency. INTRODUCTION Reproductive efficiency is the major factor affecting profitability in many livestock production systems. For example, the fertility of domestic ruminants (cattle and sheep), even under optimal conditions is about 50%. Inefficient reproduction may be caused by numerous factors, which include environmental stressors such as temperature extremes or changes in photoperiod (day and night cycles), light intensity, humidity, rainfall and wind speed (National Institute of Food and Agriculture, 2009). According to Lewis et al. (2006) reproductive efficiency has a greater influence on the economic sustainability of commercial sheep production than does any other performance related traits. This is because reproductive efficiency is a composite trait that affects the total weight of lambs weaned from a flock and because commercial sheep are currently marketed on a live-weight basis. Lewis et al. (2006) also reported that flock reproductive efficiency is an integrated measure of age at puberty, capacity to produce and deliver adequate numbers of fertile spermatozoa, ovulation rate, ovum fertilization rate, embryo and fetal survival to weaning, interval between pregnancies, reproductive lifespan, and ability to cope with a variety of environmental stressors. Dobson et al. (2000) stated that stress is revealed by the inability of an animal to cope with its environment, a phenomenon that is reflected in a failure to achieve Agric. Biol. J. N. Am., 2013, 4(6): 600-604 related to the requirements of modern production methods (Coubrough, 1985; Dobson et al., 2001). According to Dobson et al. (2001), there is growing concern in many parts of the world that fertility in dairy cattle is reducing as milk yields increase, stress could be one important cause. Dobson and Smith (2000) stated that field data from dairy cows show that stressors such as milk fever or lameness increase the calving to conception interval by 13-14 days and an extra 0.5 inseminations are required per conception. Changes in social groupings greatly increase the number of insemination required per pregnancy (Dobson et al. 2001). Moreover, Dobson et al. (2001) documented that fertility is lower after caesarian operations. Delayed uterine involution after dystocia is associated with abnormal ovarian cyclicity and prolonged intervals to the next pregnancy. There is a greater reduction as the clinical conditions of lameness, milk fever or mastitis worsen. According to Coubrough (1985) these stressors cause deviations in hormonal pattern and clinical manifestations. Dobson and Smith (2000) reported that a variety of endocrine regulatory points exists whereby stress limits the efficiency of reproduction. Stressors affect reproductive functions through actions at the hypothalamus as well as impairing pituitary LH release induced by GnRH (Dobson and Smith, 1995). genetic potential (e.g. growth rate, milk yield, disease resistance or fertility). Animal environment and environmental stress: Animal environment is affected by climatic factors that include temperature, humidity, radiation and wind (Gwazdauskas, 1985). Environmental stress is not limited to climatic factors but extends to nutrition, housing and any stimuli that demand a response from the animal to adapt to new circumstances (Lee, 1993). Extremes in climate alter energy transfer between the animal and its environment and can affect deleteriously reproduction (Gwazdauskas, 1985). According to Lee (1993), low energy and low or excessive protein levels in the diet are detrimental to reproduction. Gwazdauskas (1985) reported that seasonal variation of environment, nutrition and management alters estrous activity and duration of estrous. Conception rates are reduced under stress of heat and cold. High ambient temperatures and humidity alter the intricate balance of endocrine profiles, leading to lower intensity of estrous behaviour, anestrous, embryonic death and subsequent infertility (Lee, 1993). In hyperthermia, adrenal function is reduced, and this may allow the animal to cope with the environment because of the lower calorigenic actions of glucocorticoids. Estrogens are lower during late gestation and appear to manifest their physiological actions through shorter duration of estrous and lower calf birth weights, respectively. Season alters endocrine profiles and influences fertility of males. Spermatogenesis is impaired and testosterone is lower during early exposure to hyperthermia. Environmental modifications can alleviate stress of heat and cold to some extent (Gwazdauskas, 1985). According to Lee (1993) most of these stress factors can be managed with modern technologies to achieve maximum production and that further research in vitamins and minerals under heat stress may add to the knowledge of efficient livestock production. Moreover, experimentation using indices of environmental measures is needed to assess interactive effects of environment on reproduction (Gwazdauskas, 1985). Most researchers believe that general stress exerts its influence through the endocrine system. This mechanism is still being debated, but an involvement of the hormones of the adrenal cortex has received considerable attention. It is known that stress will cause the release of ACTH from the anterior pituitary which, in turn, stimulates release of cortisol (Skull, 1997; Frandson, 2003) and other glucocorticoids from the adrenal cortex. Glucocorticoids inhibit the release of LH. Therefore, if an animal is under stress during a critical period of the oestrus cycle (late proestrus or oestrus) a glucocorticoid induced suppression of LH is likely to either delay or prevent ovulation and may reduce libido in males (Moberg, 1976). Physiological distress that can be caused by movement of animals to new environments or caused by abusive treatment will elicit release of ACTH and glucocorticoids. Also, research has shown that embryos are more likely to be retarded and/or abnormal when collected from female animals that were subjected to heat stress during estrus when compared to embryos from those that were not stressed. Another factor related to the low fertility seen during heat stress is the evidence that the Stress and reproduction: Reproduction is the ultimate measure of an animals ability to adapt to an ever-changing external milieu, as well as forming the basis of life time productivity (Coubrough, 1985). While environmental heat as a stressor is significant in disrupting normal reproductive cyclicity as embryo collected from heat-stressed donors are less viable and have delayed trophoblast function. Management induced stress is becoming more important when 601 Agric. Biol. J. N. Am., 2013, 4(6): 600-604 embryo loses its ability to alter prostaglandins synthesis in a manner that favours the maintenance of the corpus luteum when under such conditions. These effects, combined with the other endocrine changes which occur during heat stress, accounts for the more pronounced effect of heat stress on reproduction than is seen with other stressors (Moberg, 2000). are some quite surprisingly consistent effects on reproductive endocrinology (Dobson and Smith, 2000). From a series of experiments conducted in the past years, Dobson and Smith (2000) suggested that stressors reduce fertility by interfering with the mechanisms that regulate the precise timing of events within the follicular phase. Acute stressors – either transport or hypoglycaemia imposed at precisely defined times have been investigated for effects on different parts of the reproductive control mechanism. In addition, in understanding the variable reproductive response to stress, one must understand that animals will adapt to specific stressors. Animals maintained in cold environment will usually reproduce normally. On the other hand, adapting to cold stress may make them more susceptible to other stressors. If in addition to cold stress, estrus females are subjected to wind and rain or are transported to a new location for insemination, the physiological mechanisms that interfere with reproduction may be triggered. It is important to recognize that stress may interfere with reproduction. Frequently, simple modifications in management will lessen the likelihood of stress occurring around the time of estrus and insemination (Gwasdaukas et al., 1975). Transport for 4 or 8 hours reduced the frequency and amplitude of LH pulses especially within the first few hours in ovariectomised ewes or intact animals in the late follicular phase (Dobson et al., 1999b; Phogat et al., 1996b). Similar effects have been observed during insulin-induced hypoglycaemia even though glucose concentrations decrease after insulin but increase during transport. The reduction in LH pulse frequency suggests an effect of both these stressors on GnRH pulsatile secretion mediated through effects at the hypothalamus or higher centres in the brain; whereas effects on LH pulse amplitude could be mediated by the hypothalamus, or at pituitary level. Direct proof of the suppressive effects of an acute stressor on GnRH secretion has been provided by Battaglia et al. (1997) after endotoxin administration. Moenter et al. (1990) reported that in the follicular phase of a normal oestrus cycle, the correct pattern of gonadotrophin-releasing hormone – GnRH secretion from the hypothalamus leads to increased pulsatile release of Luteinizing hormone – LH from the pituitary gland. In concert with follicule stimulating hormone, this dictates the rate of follicular growth and oestradiol production, ultimately leading to a preovulatory LH surge and ovulation (McNelly et al., 1991). In order to achieve a perfectly timed LH surge, a series of closely controlled events must occur within the hypothalamus and pituitary gland. After removal of the suppressive effects of progesterone during luteolysis, GnRH – and thus LH pulses are secreted with increasing frequency, to culminate eventually in continuous secretion at the onset of the LH surge in response to the positive feedback effects of oestradiol (Evans et al., 1995). In addition, there is evidence from both in vitro perifusions and in vivo experiments to show that exogenously increased ACTH concentration or transport reduce the amount of LH released by challenges with small doses of GnRH (Phogat et al., 1997, Grandin, 1993; Grandin, 1998a,b; Phogat, 1999a,b). This provides support for additional effects at the pituitary level. Clearly, activation of the hypothalamus-pituitary-adrenal axis by stressors reduces the pulsatility of GnRH-LH actions at both the hypothalamus and pituitary gland, ultimately depriving the ovarian follicle of adequate LH support. This will lead to reduced oestradiol production by slower growing follicles. Such a hypothesis is supported by the marked decrease in oestradiol secretion observed after reducing the frequency of exogenous LH pulses driving follicular growth in an ovarian auto transplant model (Dobson et al., 1999a). In view of the complications incurred with repeatability, habituation and duration of stressors as already highlighted, these aspects have to be standardized as much as possible when examining the influence of stress responses on physiological mechanisms such as reproduction. Furthermore, the effects of more than one stressor must be investigated in order to avoid the dangers inherent with stressor specific artifacts. However, in spite of some differences between stress responses, there A combination of the above effects on LH pulsatility at hypothalamic and pituitary levels no doubt contribute to the delay and reduced magnitude of the LH surge observed after transport or insulin administration in the follicular phase just prior to the expected LH surge (Dobson et al., 1999c). This effect on LH surge control mechanism could be exerted 602 Agric. Biol. J. N. Am., 2013, 4(6): 600-604 heat stress and may include problems in detection of estrus, conception, and fetal growth, a more basic understanding of the animal’s response to heat is needed. This will help the animal manager adopt practices to increase reproductive efficiency during hot weather/climate. To maximize reproductive efficiency during the summer, measures must be taken which reduce heat gain and/or facilitate heat loss to the environment. The response to environmental stressors often compromise the health, vigour and reproductive efficiency of animals. Development of a vaccine that selectively neutralizes ACTH, a central player in the endocrine cascade that is activated during stress, may be useful in lessening the impact of unavoidable stress in animals. The role of a stockperson in animal welfare and productivity should not be underestimated. Quiet calm handling at an early age will help produce calmer, easier-tohandle adult animals. The fact that stressors can be deleterious to such an important function as reproduction, emphasizes that stress is important and should be minimized whenever possible. directly via influence of GnRH on production of its own receptors, or indirectly by the induced reduction in oestradiol which, in turn will alter the balance of systems controlling LH surge release. Thus, another level of interference at the ovary has been revealed to play a part in the multi-centered effects of stress on reproductive control mechanisms (Dobson and Smith, 2000). Reproductive success and animal production: The highest possible reproductive success is one of the main goals in animal production and forms the basis for economically profitable animal production. Reproductive success can also be used as a measure of welfare, since unpleasant stress may impair reproduction. Reduced reproductive efficiency can be the result of environmental factors or stressors associated with animal housing, humananimal relations and management (Ahola, 2008). Living conditions may indeed have an effect on reproduction through stress mechanisms (Broom and Johnson, 1993; Moberg, 2000). REFERENCES Reproductive success in livestock is essential for the economic livelihood of producers and ultimately affects the consumer cost of meat and other animal products. In many livestock production systems, poor fertility is a major factor that limits productivity (National Institute for Food and Agriculture (NIFA), 2011). Ahola, L. (2008). Environmental factors on reproduction in farmed blue fox (Vulpes lagopus) vixens. Kuopio University Publications C. Natural and Environmental Sciences. 242. Battaglia, D. F., Bowen, J. M., Krasa, H. B., Thrun, L. A., Viguie, C. and Karsch, F. J. (1997). 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