A limiting factor to genetic progress in the cattle industries is that the most popular, and oftentimes most genetically superior, sires cannot produce enough semen at an early enough age to meet demand. The growing trend of using genomically-tested sires exacerbates this problem. In this series of experiments, both dietary and direct endocrine manipulations were used in growing bulls with the goal of positively affecting several physiological changes that precede puberty.
The first experiment was conducted in order to examine the effect of a high energy (HE) diet on prepubertal endocrinology, age at puberty, mature sperm production, and testicular characteristics of Holstein bulls. It was hypothesized that the HE diet would cause bulls to attain puberty sooner, and produce larger testes that produced more sperm. In order to test this hypothesis, Holstein bulls were either placed on a HE diet (HE, n = 9, targeted ADG 1.5 kg/d) or a control diet (CONT, n = 10, targeted ADG 0.75 kg/d) from 58 to 230 d of age. HE bulls experienced increased systemic LH concentrations at 125 d, increased testosterone concentrations from 181 to 210 d, and increased scrotal circumference from 147 to 367 d of age. Semen collections beginning at 241 d of age to assess puberty on a subset of bulls (HE, n = 8; control, n = 7) revealed no difference in the age at puberty between treatments (310 ± 10 d), and mature sperm production did not differ between groups during thrice-weekly collections obtained in the four weeks preceding slaughter at 569 d of age. Testis weight (318.0 ± 13.5 vs. 267.5 ± 14.4 g), epididymal weight (31.6 ± 1.1 vs. 28.0 ± 1.2 g), and testis volume (305.0 ± 11.9 vs. 244.9 ± 12.9 cm3) were greater (P < 0.05) in the HE treatment at 579 ± 4.7 d of age (post mortem), but seminiferous tubule diameter and area comprised of seminiferous tubule did not differ between treatments (252.23 ± 2.44 µm; 72.1%). These data suggest that a HE diet initiated at 2 mo of age accelerated many aspects of sexual maturation and resulted in bulls with larger testes. However, no differences were noted in age at puberty or production of semen as adults using the approach employed in this experiment.
Two subsequent experiments were conducted to examine the effects of a slow-release exogenous FSH treatment on the endocrinology and testicular development of prepubertal bulls. In the first experiment bulls were treated at 50 and 53 ± 3 d of age with 0.75 ml containing either 30 mg NIH-FSH-P1 in 2% hyaluronic acid (FSH-HA, n = 5) or saline (control, n = 5). Blood was collected every 6 hours for 24 hours after treatment and every 12 hours thereafter. Bulls treated with pFSH had significantly greater (P < 0.05) concentrations of FSH 6 h after treatment on d50 and 53, and a tendency (P = 0.08) for greater FSH concentrations 12 h after treatment administration on both days. There were no differences in serum concentrations of FSH between treatments from 18 to 84 h after each treatment. In a second experiment, a new set of bulls (35 ± 6.5 d age; FSH-HA, n = 11; control, n = 11) were given the same treatment as described above every 3.5 d from 35 to 91 d of age, with blood collected to measure testosterone and FSH before each treatment and BW and SC measured weekly. FSH concentrations increased compared to previous levels in the FSH-HA treatment bulls beginning at 70 d of age, and this increase was maintained and greater than control bulls through 91 d of age. All bulls were castrated at 93 d of age to acquire gross measurements and histological data, and to perform immunohistochemical staining to visualize Sertoli cells. Concentrations of intratesticular testosterone, BW, SC, testis weight and volume, percent of parenchyma comprised of seminiferous tubules, tubule diameter, or testosterone concentrations did not differ between treatments. However, number of Sertoli cells as evidence by GATA-4 nuclear staining was greater (P < 0.05) in the FSH-HA than control bulls (28.27 ± 0.9 vs. 33.35 ± 0.9 cells/round tubule). In summary, exogenous pFSH treatment enhanced endogenous FSH secretion beginning at 70 d of age and increased the number of Sertoli cells at 93 d of age. Therefore, exogenous FSH supplementation was able to alter the mechanisms regulating endogenous FSH secretion as well as augment Sertoli cell proliferation in young bulls.
These experiments employing nutritional and direct endocrine manipulation during the prepubertal period show promise in their ability to change the testis in terms of overall size and cellular composition. Such changes have potential in causing differences in sperm production when bulls reach maturity. However, in the nutritional study, the effect of larger testes as a result of the HE diet on mature sperm production or age at puberty may have been diluted due to a switch in diet upon the move to Select Sires, Inc. that put HE bulls on nutritional restriction. In the experiment where bulls were administered exogenous FSH, all bulls were castrated before the time when they would attain puberty, but changes induced by treatment were promising for increasing mature sperm production. A longer study would have to be conducted in order to examine these effects, however. Overall, these technologies represent useful ways in which cause changes early in the life of bulls that may positively affect their mature productivity.