167. Effects of Early Life Nutrition on Development and Age at Puberty.

Abstract Early life nutrition is a primary determinant of the rate of sexual development and the age at which puberty is attained in both heifers and bulls. Research consistently shows that enhanced nutrition in early postnatal life hastens the onset of puberty, while nutritional restriction during this critical window can lead to significant delays. The potency of nutritional intervention is greatest the earlier it is applied in postnatal life. For bulls, prevailing plane of nutrition offered before six months of age is the most important determinant of the age at puberty. Indeed, early-life nutritional restriction in bulls cannot be fully compensated for by providing an enhanced diet later. Similar to bulls, improved nutritional status during early juvenile development (specifically between 2 and 6 months of age) has a far more potent impact on advancing the maturation of the reproductive axis in heifers than interventions applied later. Nutrition influences the hypothalamic–pituitary–gonadal (HPG) axis through metabolic hormones that act as “gatekeepers”. For example, enhanced nutrition increases systemic concentrations of insulin-like growth factor 1 (IGF-1), insulin, glucose, and leptin. These hormones have receptors in the brain that stimulate the release of gonadotropin-releasing hormone (GnRH). In bull calves, enhanced early nutrition (between 12 and 20 weeks) specifically increases the secretion of luteinising hormone (LH) and the synthesis of testosterone. While in heifers, puberty is triggered by a progressive reduction in the sensitivity of the hypothalamus to the negative feedback of oestradiol. A high-plane of nutrition accelerates this shift, leading to earlier GnRH pulsatility and ovulation. Early-life plane of nutrition directly impacts the growth of both metabolic and reproductive organs and indeed the onset and progression of gametogenesis for both genders. Recent studies have identified specific molecular shifts in an array of metabolic and reproductive tissues in cattle induced by early nutrition. For example, high-plane nutrition in heifers downregulates orexigenic genes (AGRP and NPY) and upregulates the anorexigenic gene POMC in the arcuate nucleus (ARC). These changes are consistent with advanced sexual maturation and increased GnRH secretion. In bull calves, enhanced nutrition upregulates genes related to cholesterol biosynthesis and androgen signalling in the testes, as well as CLDN11, which regulates the formation of Sertoli cell tight junctions necessary for spermatogenesis. There is also evidence that a high plane of nutrition during the first six months of a bull's life can induce latent modifications to the sperm methylome (DNA methylation), specifically on genes associated with testis development and reduced age at puberty. This paper will utilise the results of various molecular based network analyses together with physiological and endocrinological evidence to examine the underlying biological control of sexual maturation and onset of puberty in male and female cattle.