Effects of Nutrition on Sexual Development
In early studies evaluating the effect of nutrition from birth to maturity on sexual development and reproductive function in bulls, Holstein bulls receiving low nutrition (approximately 60–70% of requirements) were older at the time the first ejaculates containing motile sperm were collected and had smaller testes, whereas bulls receiving high nutrition (approximately 160% of requirements) had earlier puberty and larger testes when compared with bulls receiving control nutrition (100% of requirements) [84, 85]. Several recent studies involving manipulation of nutrition during prepubertal and pubertal periods in both beef and dairy bulls have provided unequivocal evidence that (i) most pronounced effects of nutrition occur during the prepubertal period, (ii) effects of nutrition during the prepubertal period cannot be compensated with changed nutrition during the pubertal period, and (iii) high nutrition during the peripubertal period hastens puberty and results in larger testes (SC and testes weight), whereas low nutrition produced opposite results. With optimal prepubertal nutrition, puberty can be hastened by 1–1.5 months and testicular mass at approximately 1.5 years of age can be increased by 12–18% (Table 6.4) [86–91].
Table 6.4 Effect of prepubertal nutrition (before 6 months of age) on age at puberty (transformed from days from original reports) and paired testes weight at 16–19 months of age in bulls. Values reported as ranges indicate bulls received varying nutrition in the pubertal period.
| Breed | Age at puberty (months) | Paired testes weight (g) | Reference | ||||
| Low nutrition | Control | High nutrition | Low nutrition | Control | High nutrition | ||
| Angus and Angus × Charolais | 10.3–10.8 | 9.6 | – | 528–553 | 600 | – | [86] |
| Angus and Angus × Charolais | – | 10.7 | 10.3 | – | 531 | 611 | [87] |
| Angus and Angus × Charolais | 10.5 | 9.8 | 9.4 | 520 | 549 | 655 | [88] |
| Holstein | – | 10.6 | 9.9 | – | 535 | 636 | [89] |
| Holstein | 12.1 | 10.7 | 10.6 | 562 | 611 | 727 | [90] |
| Holstein | 10.4–10.6 | – | 9.3–9.8 | 594–627 | – | 658–660 | [91] |
The effects of nutrition on sexual development and reproductive function in bulls are mediated through the hypothalamic–pituitary–testes axis. Nutrition affects the gonadotropin‐releasing hormone (GnRH) pulse generator in the hypothalamus, since differences in luteinizing hormone (LH) pulse secretion in bulls receiving different nutrition can be observed even in the absence of differences in pituitary LH secretion capability as determined by GnRH challenge [87, 88]. Interestingly, though, when low nutrition was imposed on bulls by limiting the amount of nutrients in a ration fed ad libitum, only reduced LH pulse frequency was observed, whereas reduced LH pulse frequency, mean and peak concentrations, and secretion after GnRH challenge were observed when nutrition was controlled by restricting the availability of food [86]. These results seem to indicate that the inhibitory effects of limited availability of nutrients on LH secretion appeared to be exerted only on the hypothalamus, whereas the combination of limited availability of nutrients and the sensation of hunger experienced by bulls with restricted intake affected both hypothalamic and pituitary function, producing a much more severe inhibition of LH secretion. The effect of nutrition on Leydig cell number and/or function in bulls receiving different diets was demonstrated by differences in testosterone secretion after GnRH challenge even in the absence of differences in LH secretion after the challenge [87, 88].
Differences in yearling SC due to age of the dam in beef bulls could also be interpreted as an indication that nutrition during the pre‐weaning period affects sexual development, although possible in utero effects cannot be completely ruled out. SC in B. taurus beef bulls increases as age of the dam increases until five to nine years of age and decreases as dams get older. Adjustment factors of 0.7–1.4, 0.2–1.0, 0.1–1.0, and 0.3–0.75 cm for yearling SC have been suggested for bulls raised by 2‐, 3‐, 4‐, and ≥10‐year‐old dams, respectively [19–23, 92]. In these studies, the inclusion of weight as a covariate in the models describing SC resulted in decreased effects of age of the dam, indicating that the effect of age of the dam on testicular growth seems to be primarily the result of age of the dam effects on bull's body weight, likely related to differences in milk production. This theory is also supported by reports that, similarly to that observed in bulls receiving low nutrition, LH secretion after GnRH challenge was greater from 3.5 to 6 months of age in bulls raised by multiparous than in bulls raised by primiparous females [93].
Several studies reported in the literature describe the effects of nutrition only during the pubertal period, in other words after the initial hormonal changes that regulate sexual development have occurred. In general, these studies indicate that low nutrition has adverse effects on growth and sexual development. In one study, bulls receiving one‐third of the amount supplied to their twin controls had lower body and vesicular gland weights, vesicular gland fructose and citric acid contents, and circulating and testicular testosterone concentrations, whereas circulating androstenedione concentrations were increased [94]. In another series of experiments, beef bulls 8–12 months old receiving diets with low levels of crude protein (8, 5, and 1.5%) for periods of three to six months had markedly reduced testes, epididymis, and seminal glands weights compared with control bulls fed diets containing 14% crude protein. Moreover, seminiferous tubule diameter and seminiferous epithelium thickness were smaller in bulls with a restricted protein intake [95, 96].
Although low nutrition during the pubertal period has adverse effects on reproductive function, the potential beneficial effects of high nutrition after weaning are questionable at best. Effects of energy on sexual development were not consistent in a study with Simmental and Hereford bulls fed diets with a low, medium, or high energy content (approximately 14, 18, and 23 Mcal/day, respectively) from 7 to 14 months