| Summary: | Animal supplementation during the background phase may increase greenhouse gas emissions (GHG). The inclusion of tannins in the diet of Nellore bulls can mitigate nitrous oxide (N<sub>2</sub>O), methane (CH<sub>4</sub>), and ammonia (NH<sub>3</sub>) production. The objective of this study was to quantify the effect of energy supplementation associated with sources of tannins in the diet of young Nellore bulls backgrounded in pastures with N<sub>2</sub>O, CH<sub>4</sub>, and NH<sub>3</sub> emissions. Two experiments were conducted in a completely randomized design. The treatments were three supplementation strategies: (1) soybean hulls 0.3% of body weight (BW), (2) sorghum grain 0.3% of the BW, and (3) peanut peel 0.3% of BW, the last two being sources of tannin. The static closed chambers method was used to quantify N<sub>2</sub>O and CH<sub>4</sub> emissions and the semi-open chamber technique to estimate NH<sub>3</sub> volatilization. Supplementation strategies did not affect the N<sub>2</sub>O emissions (<i>p</i> = 0.9116). The soil water-filled pore space explained the variation in the N<sub>2</sub>O fluxes (<i>p</i> = 0.0071). The treatments did not change the total CH<sub>4</sub> emissions (<i>p</i> = 0.3599), and no explanatory variable was correlated with the CH<sub>4</sub> fluxes. The NH<sub>3</sub> volatilization did not vary according to the supplements or tannin inclusion (<i>p</i> = 0.5170). However, the type of excreta affected the NH<sub>3</sub> volatilization (<i>p</i> < 0.0001). Ammonia volatilization averaged 14.05, 4.16, and 2.25% of the applied N for urine, urine + dung, and dung, respectively. The energetic supplementation of Nellore bulls containing sources of tannins in the evaluated dosages was not a mitigation strategy for the emissions of N<sub>2</sub>O, CH<sub>4</sub>, and NH<sub>3</sub>.
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