Nutrition and reproduction

Abstract

Through their influence on embryonic and early fetal development dietary nutrients affect the size, vigour and viability of the newborn and, in the case of sheep, adult ovulation rates. Nutritional regimens that impair post-natal growth reduce ovarian follicular development and delay puberty but have little, if any, effect on either N-methyl-D-aspartate stimulated GnRH release or the pituitary content of gonadotrophins. Nutritional effects on ovulation rate in ewes are not always accompanied by shifts in circulating FSH concentrations. Instead they may operate through altered ovarian hormone feedback mechanisms that influence the duration of exposure of the gonadotrophin-dependent follicles to FSH or through threshold-reducing actions of specific nutrients and dietary-induced metabolites on the amount of FSH needed to support the gonadotrophin-dependent follicles. Recent results suggest that the enhanced faecal excretion of oestradiol by well-fed ewes may increase ovulation rate by reducing ovarian oestradiol feedback. In the post-partum period, the key hypothalamic and pituitary secretions for successful ovulation develop normally despite undernutrition but ovulation is prevented by failure of GnRH release with, in the case of suckler beef cows, a delay in oestrus arising from the repeated development and atresia of dominant ovarian follicles. This effect is accentuated in cows that are in poor body condition at calving. Alleviating the energy deficits of high-yielding dairy cows in early lactation by giving supplements of dietary fat can reduce the interval to first oestrus but subsequent fertility can be impaired. Loss of body protein delays oestrous cyclicity; paradoxically attempts to correct this loss in ruminants by feeding diets high in rumen undegradable protein can stimulate body fat mobilization and milk production with detrimental effects on rebreeding. Excess rumen degradable protein and high feeding levels reduce embryo survival, the latter by suppressing circulating progesterone concentrations. In superovulated ewes diet-induced reductions in progesterone concentrations during oocyte maturation can impart a legacy of developmental retardation leading to decreased embryo survival. In late pregnancy, undernutrition decreases uterine blood flow, fetal insulin and IGF-1 concentrations and impairs growth and development. In the newborn, low insulin and a low selenium and iodine status inhibit thermogenesis from brown adipose tissue. Insulin may also be important in conveying information on nutritional status to the GnRH neurones thereby initiating the cascade of events that is required for successful reproduction in both sexes.