9. The Role of Early Life Nutrition in the Biological Programming of Growth and Reproductive Development of Female Ruminant Livestock Species.

Abstract Growth and reproductive efficiency is fundamental to the biological efficiency and profitability of livestock production systems, and is heavily influenced by nutrition, and, in particular, early life dietary management, which directly impacts sexual development, fertility, and fecundity. Nutrition plays a central role in the post-natal development of both cattle and sheep, with an enhanced plane of nutritional generally hastening the onset of puberty and sexual maturation. The Gonadostat Hypothesis has long been thought to explain the trajectory of sexual development in female ruminant animals including heifers and ewe lambs, where onset of puberty is regulated by a decreasing sensitivity of the hypothalamic–pituitary axis to the negative feedback of oestradiol (E2). Metabolic hormones including IGF-1, insulin, and leptin have been identified as key metabolic gatekeepers with enhanced nutrition typically increasing the systemic concentrations of these hormones, which, in turn, stimulate the release of gonadotropin-releasing hormone (GnRH) and promote ovarian follicular development. Achieving a target liveweight is essential for successful breeding of young replacement females. Heifers are more likely to be pubertal and conceive early in the breeding season if they reach a higher percentage of their mature bodyweight. While improved nutrition before the breeding season increases the chances of early pregnancy, it does not always impact overall pregnancy rates. For ewe lambs, a target liveweight of 65–70% of mature weight is recommended for breeding with a positive relationship between body adiposity at mating and subsequent fecundity. Despite this target weights can often be weak predictors of pubertal status in well-grown ruminant females. Instead, physiological markers like IGF-1 concentrations at 8 months of age in heifers, have been found to be superior predictors of the age at puberty than bodyweight per se. While much research has been conducted on the potential of dietary supplementation with specific nutrients such as polyunsaturated fatty acids or trace elements to improve growth and reproductive efficiency in ruminant livestock, results have often been inconsistent for effects on reproductive outcomes. This paper will also examine the impact of enhancing early life on metabolic and endocrinological responses in young female ruminants with particular emphasis on gene and protein expression profiling across a range of key organs and tissues including muscle, fat, liver and tissues of the hypothalamic-pituitary-ovarian axis. Ultimately, advancing the age of puberty through early nutrition can shorten the generation interval and improve the economic and environmental sustainability of ruminant based livestock systems