Environmental stresses and livestock reproduction

Abstract

Reproductive fitness may be regarded as the most important criteria for studying or evaluating animal adaptation. Body systems activated by stress are considered to influence reproduction by altering the activities of the hypothalamus, pituitary gland, or gonads. Activation of stress pathways may directly affect the activity of Gonadotropin-releasing hormone (GnRH) neurons within the hypothalamus or higher neural centers which in turn affects the synthesis or secretion of GnRH into the hypophysial portal blood. It is also possible that stress directly influences the responsiveness of gonadotrophin cells in the anterior pituitary gland via the action of GnRH. A further potential action of stress is to alter the feedback actions of sex steroids in the hypothalamus or pituitary and inhibin in the anterior pituitary gland. Reproduction processes in animals may be impacted during heat exposure and glucocorticoids are paramount in mediating the inhibitory effects of stress on reproduction. Glucocorticoids are capable of enhancing the negative feedback effects of estradiol and reducing the stimulation of GnRH receptor expression by estrogen. Glucocorticoids may also exert direct inhibitory effects on gonadal steroid secretion and sensitivity of target tissues to sex steroids. Heat stress (HS) influences estrous incidences and embryo production. The birth weights of lambs of heat stressed ewes are generally lower than the unstressed animals. This could be attributed to the fact that HS may cause a temporal impairment of placental size and function, resulting in a transient reduction in fetal growth rate. Secretion of the hormones regulating reproductive tract function may also be altered by HS. Further, HS can inhibit 3-beta-hydroxysteroid dehydrogenase (3β HSD) thereby minimizing progesterone secretion from luteal cells. Aromatase is an enzyme that converts androgens into estrogens and is present in the granulosa cells. By inhibiting the expression of this enzyme, HS may induce follicular atresia and consequently anestrus. Effects of steroid hormones on reproductive tract tissue could be reduced during exposure to HS due to increased synthesis of heat shock proteins (HSPs)-HSP 70 and HSP 90. Increased synthesis of HSP might alter assembly, transport, or binding activities of steroid receptors. Further, increased magnitude of these stresses will increase secretion of prostaglandin and reduce the secretion of interferon tau which affects the maternal recognition of pregnancy. In male, HS adversely affects spermatogenesis by inhibiting the proliferation of spermatocytes. This chapter will address the effect of environmental stresses on livestock reproduction.