Dose-response effect of encapsulated nitrate replacing soybean meal on growth performance, ingestive behavior, and blood metabolites of feedlot finishing bulls

This study addressed the dose-response effect of an encapsulated nitrate product (ENP) replacing soybean meal (SBM) in the finishing period (87-d) of feedlot bulls. One hundred twenty Nellore bulls (340 ± 28 kg of body weight, BW; mean ± SD) were allotted to 20 outdoor pens (6 animals/pen) and assigned to a randomized complete block design with 5 blocks based on initial BW and 4 dietary treatments: SBM, as control; ENP-10, 10 g ENP/kg dry matter (DM; 6.5 g NO₃⁻/kg DM); ENP-20, 20 g ENP/kg DM (13 g NO₃⁻/kg DM); and ENP-30, 30 g ENP/kg DM (19.5 g NO₃⁻/kg DM). Finishing diets (90:10 concentrate to roughage ratio) were formulated to provide similar amounts of nutrients. Animals were gradually and simultaneously adapted over 14 d to ENP and concentrate. Dry matter intake (DMI) reduced linearly (from 9.6 to 8.4 kg/d; P = 0.02) with ENP inclusion. Final BW, average daily gain (ADG), carcass ADG, hot carcass weight, dressing percentage, and fat thickness were not affected (P ≥ 0.12) by treatments. There was a quadratic trend for increased carcass gain:feed (0.121, 0.130, 0.128, 0.127, respectively; P = 0.07). Nitrate concentration in meat increased linearly (3.48, 2.73, 3.98, 6.59 mg NaNO₃/kg meat; P = 0.02) with ENP addition, although nitrite was not detected. Time spent eating (min/d) was similar (P ≥ 0.16) among treatments, whereas a linear increase (P = 0.02) was observed for time spent eating per unit of DMI when ENP was fed. There was a trend (P = 0.07) for a quadratic effect in ruminating time per unit of DMI according to ENP increments. Blood methemoglobin before morning feeding increased linearly (P < 0.01) on d 28 and 56 as more ENP was fed, however, values remained ≤ 3.57% of total hemoglobin. Increasing levels of ENP affected further blood metabolites, such as triglycerides (quadratic response, P < 0.01), total cholesterol (linear increase, P = 0.05), urea (linear decrease, P < 0.01), aspartate aminotransferase (linear decrease, P < 0.01), and gamma-glutamyl transferase (linear decrease, P = 0.04). In conclusion, finishing bulls fed ENP performed similarly to SBM, suggesting ENP as a promising nitrogen source. There was evidence of a possible improvement in carcass-based feed efficiency (a decrease in DMI associated with a similar carcass ADG) for bulls fed ENP, reaching a calculated maximum value for the inclusion rate of 17.7 g ENP/kg DM.