Nutritional, ruminal, and metabolic parameters of beef bulls fed high-energy diets as a function of dietary addition of a magnesium oxide blend associated or not with monensin

This study aimed to evaluate the impact of a magnesium oxide blend (MG) and monensin (MON) on the nutritional, ruminal, and metabolic parameters of finishing bulls fed high grain diet. Six ruminally cannulated bulls (542 ± 33.6 kg) were used in a 6 × 6 Latin square design with treatments arranged as a 2 × 3 factorial, with 6 periods of 23 days. The MG was included at 0, 2.5, or 5.0 g/kg of dry matter (DM) - MG0, MG2.5, and MG5.0, respectively, without or with MON (30 mg/kg of DM). During each experimental period, days 1–10 were designated for dietary additive adaptation. Days 11–20 (feeding period), were used to assess DM and nutrient intakes, digestibility, ruminal pH, volatile fatty acids (VFA) profile, and blood parameters. Days 21–23 (corn challenge period), included supplementing all treatments with 2 kg/day of ground corn, replacing 2 kg/day of forage, to observe responses in ruminal pH and DM intake (DMI) in the presence of increased fermentable carbohydrates in the rumen. The ruminal pH increased linearly (P = 0.01) with the MG level during the feeding period. In the corn challenge, bulls fed diets without MON and MG5.0, as well as those fed MON and MG2.5 or MG5.0 showed a marked increase in rumen pH value during the first day. In the feeding period, the DMI was greater (quadratic: P = 0.05) for MG2.5 compared to other tested doses. The DM and nutrient intakes were greater (quadratic: P ≤ 0.05) for the MG2.5 group. Bulls fed MON diets had 0.5 kg lower DM and nutrient intakes than those without MON (P ≤ 0.05). The ruminal and intestinal digestibility of diet compounds did not differ among treatments (P ≥ 0.08). The DM and organic matter (OM) apparent total-tract digestibility tended to be (quadratic: P = 0.06) and were greater (quadratic: P = 0.05) for the MG2.5 group. The microbial crude protein synthesis (MCP) was greater for bulls fed the MG2.5 diet (quadratic: P = 0.03). Bulls fed diets without MON had a lower valeric acid (MON × Time: P = 0.03) and those fed diets without MG had a lower iso-valeric acid (MG × Time: P = 0.03) molar proportion 12 hours post-feeding than 1 hour pre-feeding. Glucose levels increased linearly with MG doses (P < 0.001). In summary, a dosage of 5.0 g/kg of DM of MG promotes a superior response in ruminal pH, whereas beneficial effects on digestive parameters are achieved with 2.5 g/kg of DM. Furthermore, MON and MG showed no significant associative effects.