In vitro fermentation patterns and methane output of perennial ryegrass differing in water-soluble carbohydrate and nitrogen concentrations

The objective of this study was to determine the effect of perennial ryegrass forages differing in their concentration of water-soluble carbohydrates (WSC) and crude protein (CP), and collected in spring and autumn, on in vitro rumen fermentation variables, nitrogen (N) metabolism indicators and methane (CH4) output using a batch culture system. Two contrasting perennial ryegrass pastures, sampled both in autumn and spring, were used: high (HS) and low (LS) sugar pastures with WSC concentrations of 322 and 343 g/kg for HS (autumn and spring) and, 224 and 293 g/kg for LS in autumn and spring, respectively. Duplicates were incubated for 24 h with rumen inocula in three different days (blocks). Headspace gas pressure was measured at: 2, 3, 4, 5, 6, 8, 10, 12, 18 and 24 h and CH4 concentration was determined. The residuals were analysed for individual volatile fatty acids (VFA) concentrations, total N, ammonia-N (NH3-N) and neutral detergent insoluble N. Another set of duplicates was incubated for 4 h for VFA and NH3-N determination. The HS produced more gas (218 vs 204 mL/g OM), tended to increase total VFA production (52.0 mM vs 49.5 mM at 24 h), reduced acetate:propionate ratio (2.52 vs 3.20 at 4 h and 2.85 vs 3.19 at 24 h) and CH4 production relative to total gas production (15.6 vs 16.8 mL/100 mL), and improved N use efficiency (22.1 vs 20.9). The contrasting chemical composition modified in vitro rumen fermentation tending to increase total VFA production, reduce acetate:propionate ratio and CH4 concentration, and improve N use efficiency through lower rumen NH3-N.