Effect of group size in a Greenfeed system on estimates of enteric methane emissions from goats grazing grass-based pasture

Accurate estimation of methane emissions from grazing livestock is essential for assessing their environmental impact and developing mitigation strategies. The portable GreenFeed (GRF) system is a recently introduced device for measuring methane emissions in ruminants, though there is limited research available on its application for goats. The study evaluated the effects of varying group sizes (12 [G-12], 18 [G-18], and 24 [G-24] goats) on daily methane emissions, methane yield, and visit frequency, as measured by the GRF system, in a grazing setting. The study consisted of seven periods: GRF periods (2, 4, and 6) and calorimetry (CAL) periods (1, 3, 5, and 7). Methane emissions were recorded for 14 days per period using GRF while goats were grazing during GRF period and for two days when they were in the CAL room during CAL periods. While no significant differences in methane emission in g/kg dry matter (DM) or digestible DM intake were detected between the first and last calorimetry periods, confirming no effect of pasture maturity. Methane production (g/day) did not differ significantly across group sizes (P > 0.05), but daily visit frequency (3.23, 2.65, and 2.51 visits for G-24, G-18, and G-12, respectively; P < 0.001) differed among the groups. Methane yield, expressed as g/kg DM intake, however, decreased as group size reduced, indicating differences in methane emissions between group sizes (36.7, 33.4, and 33.0 g/kg DM intake for G-24, G-18, and G-12, respectively; P = 0.010). Methane estimates from smaller group sizes were more variable, highlighting the importance of adequate animal numbers for accurate emission assessments. Pearson correlation analysis also showed stronger relationships between larger group sizes (G-24 and G-18) than between G-24 and G-12 or G-18 and G-12 for methane production (g/day, g/kg DM intake and g/kg body weight). In conclusion, a group size containing 18 to 24 goats per GRF is required for studies involving four spot samplings per day to accurately quantify methane emissions, compare treatment effects, and rank individuals effectively.