Invited Review: Effect of early-life nutrition on the molecular and physiological regulation of puberty onset in the bull

The advent of genomic selection has accentuated interest in procuring saleable semen from young genetically elite bulls as early in life as possible. However, the timing of availability of semen for commercial use will be determined by the age at which these young animals reach puberty and subsequent sexual maturity. Enhancing early-life nutrition stimulates the functionality of the hypothalamic–pituitary–testicular axis, mediated through complex biochemical interplay between metabolic and neuroendocrine signals and culminating in enhanced testicular growth, steroidogenesis, spermatogenesis, and ultimately, earlier onset of sexual maturation. Indeed, recent evidence indicates that the timing of sexual precocity, which is dictated by an early gonadotropin rise (8–20 wk of age) in the bull, is determined by prevailing metabolic status during calfhood and is not compensable even where prior undernutrition is followed by dietary augmentation. However, despite this, the precise neuronal mechanisms regulating these developmental processes remain to be elucidated for the bull. Although early-life nutrition clearly affects the sexual maturation process, there is little evidence for latent effects on postpubertal semen characteristics. Equally, postpubertal fertility, measured as in vitro fertilization and early embryogenesis, is not influenced by nutritional status during early life. Current efforts employing high throughput nucleic acid and proteomic sequencing and targeted immunofluorescence coupled with systems biology–based gene network analyses can provide further insight on how nutrition may mediate the biochemical interaction between neuroendocrine and testicular cellular processes. Such information can be harnessed to identify potential genomic targets as part of genomically assisted breeding programs as well as facilitate development of strategic nutritional regimens to optimize sexual maturation and subsequent semen availability from genetically elite young bulls.