PS13-7. In Vitro Ruminal Methane Production and Fermentation Responses To Narasin In Low- and Medium-Quality C4 Forage Substrates.

Abstract Narasin is an ionophore that has been shown to modify ruminal fermentation and reduce enteric methane (CH4) emissions; however, its effects vary depending on diet characteristics and forage quality. The objective of this study was to evaluate the effects of narasin inclusion on in vitro fermentation and gas and CH4 production using two C4 forage substrates, differing in nutritive value. Thus, two experiments were conducted using an automated in vitro gas production system. In experiment one (Exp 1), a medium-quality forage substrate (Brachiaria hybrid; Brachiaria sp. approximately 12% CP and 60% NDF) was used as a substrate, whereas in experiment two (Exp 2), a low-quality forage was used (Limpograss, Hemarthria altissima; approximately 4% CP and 75% NDF). In both experiments, treatments were: control (CON; no ionophore inclusion), monensin (MON; 20 ppm, positive control), and narasin (NAR; 13 ppm). Treatments were incubated in three consecutive 48 h fermentation runs, with five replicate bottles per treatment and two blanks per run. Bottles were incubated at 39 °C, and total gas and CH4 production were continuously recorded. Ruminal pH, the concentration and proportion of volatile fatty acid (VFA), and the concentration of NH3-N were determined after 48 h of incubation. In both experiments, data were analyzed as a completely randomized design using the mixed procedure of SAS (PROCMIXED), considering treatments as fixed factors and bottle and incubation run was considered random. In both experiments, NAR resulted in lower total gas and CH4 production compared with MON and CON at 24 and 48 h (P < 0.001). Also, NAR reduced CH4 production expressed per unit of dry matter incubated in Exp 1 and 2. In Exp. 1, NAR increased ruminal pH and the proportion of propionate and decreased the proportion of acetate and the acetate:propionate ratio (P < 0.01). The concentration of total VFA and NH3-N was not affected by treatment. In Exp 2, NAR reduced the concentration of total VFA and the acetate:propionate ratio and increased the proportion of propionate compared with CON (P ≤ 0.03), but no differences (P > 0.01) were observed in the concentration of NH3-N. Inclusion of MON generally resulted in intermediate responses between CON and NAR on ruminal fermentation, gas, and CH4 production. These results indicate that narasin consistently decreased in vitro CH4 production and shifted ruminal fermentation toward increased propionate production in both low- and medium-quality C4 forage substrates, suggesting its potential as a CH4 mitigation strategy across warm-season forages.