In Vitro Ruminal Fermentation and Enteric Methane Production of Tropical Forages Supplemented with Nitrogen or the Combination of Nitrogen and Starch

This research was conducted to evaluate in vitro ruminal fermentation parameters (CO₂ emissions, NDF degradability, pH, VFA, and NH₃-N) and enteric methane production of medium quality tropical grass hay (TGH; Brachiaria decumbens, 6.8 % CP), supplemented with N only (urea:ammonium sulfate), or the combination of N and starch. The 7 treatments evaluated were: control (TGH only, 6.8 % CP), N supplementation (TGH + N at 90.1 and TGH + N 130 g/kg), N + starch supplementation (low startch, 10% of forage weight; and high starch, 20% of forage weight). Samples were incubated in serum bottles (50 mL) at 39^°C, each run being replicated 4 times. Methane, CO₂, and VFA were evaluated after 24-h incubation. The measurements of NDF degradability, NH₃-N concentration, and pH were performed at 3, 6, 9, 12, 24, 36, 48, 72 and 96 h. Values of pH, NH₃-N, VFA, CH₄, and CO₂ obtained for the different incubation times were evaluated as repeated measures design. Data were analyzed using PROC MIXED of SAS (SAS Inst., Cary, NC) and differences were declared significant at P≤0.05. Treatment was considered fixed effect whereas treatment within run considered random. When expressed in mL/g DM, no effect was found on CH₄ (P=0.498) and CO₂ (P=0.538) production, potentially degradable fraction of NDF (P=0.429), nondegradable fraction of NDF (P=0.429), and fractional degradation rate of NDF (P=0.568). Yet, compared to control, supplementation did affect total VFA concentration (P<0.05), more specifically, with starch affecting (P<0.05) total acetate production, and decreasing A:P ratio (P<0.05). The pH was found to be higher for supplemented diets (P<0.05). Supplementation also affected NH₃-N, after 36 h, with high starch inclusion being the highest (P<0.05). Overall, the N supplementation alone does not improve TGH fermentation dynamics, and independent of the level of N supplementation, the inclusion of starch onto TGH supplementation strategies is likely to affect only VFA profile rather than total production.