Does biochar inclusion affect the in vitro ruminal fermentation of tropical beef cattle diets?

We evaluate the effects of biochar ( BIO ) on in vitro ruminal fermentation, methane production, and digestibility of tropical beef cattle diets. The BIO effects were assessed in three different studies: I) Determination of the inclusion level in beef cattle diets (Exp. 1 and 2); II) Comparative effects of BIO with monensin ( MON ) and nitrate ( NIT ) on ruminal fermentation parameters (Exp. 3 and 4); III) Comparison of the effects of BIO with MON and nitrate NIT on the extension of ruminal in vitro digestibility (Exp. 5 and 6). In each study, we evaluated a forage-based and concentrate-based diet ( FOR and CON , forage-to-concentrate ratio of 80:20 and 20:80, corresponding to odd and even experiments, respectively). For the Exp. 1 and 2, five inclusion levels of BIO were tested (0, 10, 20, 30, and 40 g/kg dry matter, DM ) in four independent incubation runs to evaluate gas production ( GP ), in vitro organic matter digestibility ( IVOMD ), and enteric methane ( CH 4 ). The BIO inclusion in FOR (Exp. 1) linearly decreased (P < 0.01) IVOMD, CH 4 production (mL/g DM), and yield (mL/g organic matter digested, OMd ). On the other hand, a cubic effect of BIO (P = 0.01) was verified on CH 4 yield when CON was evaluated (Exp. 2), which 20 g/kg DM presented lowest value. The addition of 40 and 20 g of BIO/kg DM at Exp. 1 and 2 decreased CH 4 yield by about 15 and 11% compared with the control, respectively. For study II and III, BIO level reduced CH 4 yield in Exp. 1 and 2 (i.e., 40 and 20 g/kg DM for FOR and CON diets, respectively) compared to control, MON (30 mg/kg DM), and NIT (20 g/kg DM). MON resulted in 18% lower IVOMD (P < 0.05) compared to control and NIT when FOR diet was incubated (Exp. 3). Nevertheless, GP and CH 4 yield were not affected by the treatments (P = 0.14). Treatments did not affect IVOMD, CH 4 production or yield (P > 0.13) when compared to CON diet (Exp. 4). However, NH 3 -N concentration increased with BIO and NIT compared to control and MON (53.3 and 55.9 vs. 45.8 and 44.9 mg/dL, respectively; P < 0.05). In Study III, the same treatments for Study II were evaluated in jar incubators to evaluate the in vitro digestibilities. For FOR (Exp. 5), only IVADFD was affected by the treatments (P = 0.02), with BIO presenting greater values (P < 0.05) than the others. For CON diet (Exp. 6), greatest IVNDFD and IVADFD were observed when BIO and NIT were included (P < 0.001). We concluded that 40 g of BIO/kg DM inclusion level decreased CH 4 yield by about 12% when high-forage diet was incubated. However, subsequent studies did not present significant CH 4 reductions with BIO addition, regardless of diet type. BIO promoted positive effects on NH 3 -N concentration and fiber digestibility.