Testosterone metabolism occurs primarily in the liver, and circulating concentrations can be influenced by the amount and type of diet provided. Furthermore, there are reports of the influence of circadian rhythms on secretion of testosterone in ruminants. Based on evidence that supplementation with rumen-protected fat may increase circulating concentrations of steroid hormones (Guardieiro et al. 2010 Pesq. Agrop. Bras. 45, 408–414), this study aimed to evaluate the effect of the addition of rumen-protected fat, antioxidants, or both to the diet on serum testosterone concentration in bulls, and to assess whether there is variation in hormone concentrations at different times of the day after feeding. Forty-eight Nelore bulls were confined and assigned to four treatment groups according to the addition of rumen-protected fat, antioxidants, or both to the standard diet (sugarcane bagasse, citrus pulp, corn gluten meal, urea, and mineral salt): F) rumen-protected polyunsaturated fatty acids [PUFA; rich in linoleic, Megalac E®, QGN-Arm & Hammer, Rio de Janeiro, Brazil; 1.5% on dry matter (DM); n = 12]; A) antioxidant (a source of vitamin C and selenium, EconomasE®, Alltech Biotechnology, Buenos Aires, Argentina; 3 g/head/d; n = 12); FA) Megalac E® and EconomasE® (n = 12), or C) nothing (Control group; n = 12). After 75 days of offering diets, jugular blood samples were collected just before IV injections that were performed three times a day (7:00, 13:00, and 19:00 h) with saline (Control group, n = 24) or gonadotropin-releasing hormone (GnRH; 50 μg Gonadorelin, Fertagyl, MSD Saúde Animal, São Paulo, Brazil; GnRH group, n = 48) and 2 h afterward. Bulls were fed 1 h after the first IV GnRH/saline and the diets remained in the bunk for 17 h. Serum testosterone concentrations were measured by radioimmunoassay. Data were analysed by repeated-measures of GLIMMIX of SAS (SAS Institute Inc., Cary, NC, USA). As expected, the GnRH injections increased concentrations of testosterone after 2 h compared to saline (5.4 ± 0.16 v. 3.2 ± 0.16 ng mL–1; P < 0.001). However, after the last injection of GnRH there were lower (P < 0.001) concentrations of testosterone (2.0 ± 0.57) compared to other times. This fact may be explained by the probable association between the greater clearance of testosterone after feeding and the consumption of stock of pituitary LH by GnRH administration. However, there was no effect of diets on circulating testosterone concentration. We may suggest that males can more efficiently control steroid hormone concentrations than females by feedback mechanisms.
Financial support was provided by CNPq, FAPESP, Arm & Hammer, Alltech, and EMBRAPA of Brazil.
