Читать книгу Bovine Reproduction - Группа авторов - Страница 97

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 of age. Dietary energy affected sexual development in Simmental bulls, but not in Hereford bulls. Simmental bulls in the high‐energy group were heavier and had greater SC and testosterone concentrations than bulls in the low‐energy group (in general, the medium‐energy group was intermediate). However, increased dietary energy did not hasten age at puberty. The only semen trait affected by dietary energy was semen volume, which was depressed in Simmental bulls in the medium‐energy group. Serving capacity was greater for Hereford bulls on the high‐energy diet, but medium‐ and high‐energy diets were associated with a decrease in the number of services between two testing periods in Simmental bulls. There was a trend for lower sperm motility and proportion of normal sperm in Simmental bulls fed the low‐energy diet [97, 98].

In Holstein bulls producing semen for artificial insemination, high energy intake was associated with visual evidence of weakness of the feet and legs and increased reaction time after three years of age [85]. Under field conditions, post‐weaning high‐energy diets are frequently associated with impaired reproductive function in bulls, likely related to altered testicular thermoregulation due to excessive fat deposition above and around the testes in the scrotum. In one report, sperm motility decreased and the proportion of sperm defects increased with age in Hereford bulls fed to gain more than 1.75 kg/day, which was significantly different from bulls fed to gain approximately 1 kg/day (control). Even after the high‐nutrition diet was changed to a control diet, bulls previously receiving high nutrition continued to have lower semen quality. There was greater deposition of fat around the testicular vascular cone in the scrotal neck in bulls in the high‐nutrition group and the difference between body and testes temperature was reduced in this group compared with bulls in the control group. This difference was still present after the high‐energy diet was changed and the bulls had lost a considerable amount of weight, indicating that fat accumulated in the scrotum is more difficult to lose than other body fat [99].

In another series of experiments, Angus, Hereford, and Simmental bulls were fed high nutrition (80% grain and 20% forage) or medium nutrition (forage only) from approximately 6.5 until 12–24 months of age. In general, bulls receiving high nutrition had greater body weight and backfat, but paired testes weight was not affected by diet. Moreover, bulls receiving high nutrition had lower daily sperm production and epididymal sperm reserves and a greater proportion of sperm abnormalities. The authors indicated that increased dietary energy may adversely affect sperm production and semen quality due to fat deposition in the scrotum, which reduces the amount of heat that can be radiated from the scrotal skin, thereby increasing the temperature of the testes and scrotum [100–103]. Observations from a different study indicated that bulls fed high‐nutrition diets had greater SC and scrotal weight than bulls fed medium‐nutrition diets, but paired testes weight was not different between the two groups [104]. Growth rate between 6 and 16 months of age did not affect sexual development and reproductive function in Angus and Angus × Charolais bulls. However, greater body weight at various ages was associated with reduced age at puberty and maturity and with larger testes at 16 months of age, indicating that improved nutrition might be beneficial, but only when offered before six months of age. Average daily gains of 1–1.6 kg/day did not result in excessive fat accumulation in the scrotum, increased scrotal temperature, or reduction in sperm production and semen quality, and could be considered “safe” targets for growing beef bulls [61].

In summary, low nutrition has adverse effects on sexual development and reproductive function regardless of the bull's age. However, most research indicates that high nutrition is only beneficial during the first six months of life, which presents a challenge to bull producers. Beef bull calves are usually nursing until six to eight months of age and very little attention is paid to their nutrition, whereas nutrition offered to dairy bull calves is often suboptimal. Efforts to obtain maximum weight gain during the first months after birth by offering high‐nutrition diets and adopting management practices like creep‐feeding will be compensated by reduced age at puberty and greater sperm production capacity in adult bulls. It is also clear that although high nutrition diets after six months of age might be associated with greater SC, this effect is likely the result of fat accumulation in the scrotum and not actually greater testicular size. Moreover, sperm production, semen quality, and serving capacity are all compromised in bulls receiving excessive nutrition after this age. Adjusting diets accordingly to maximize growth but to prevent overconditioning after the peripubertal period is advisable.