Methane emissions by Charolais cows grazing a monospecific pasture of timothy at four stages of maturity

In order to examine the relationship between herbage maturity and methane (CH4) emission under conditions which minimized opportunities for selective grazing, six 8-yr-old, dry and early-pregnant Charolais cows [712 +/- 22.7 kg liveweight (LW)] were grazed on a mono-specific pasture of timothy at four stages of maturity: early vegetative, heading, flowering, and senescence. Daily CH4 production was measured during 7 d using the sulfur hexafluoride (SF6) tracer-gas technique. Organic matter intake (OMI) was determined from fecal output estimated by Yb2O3 dosing and feed digestibility (OMD) estimated using a fecal N index. The OMD (%) decreased (P < 0.05) from 77.6 at early vegetative to 56.2 at senescence, but no cow effect upon OMD was observed. The OMI (14.8 g kg(-1) LW) and CH4 production (273 g d(-1)) were higher (P < 0.05) at heading than at the other stages of maturity. The proportion of gross energy intake (GEI) lost in CH4 did not differ with maturity (mean 6.4% of GEI). Cow effects upon OMI and CH4 emission-were significant (P < 0.05). CH4 production (g d(-1)) was not related to diet chemical composition or OMD, but was correlated with digestible neutral detergent fiber (NDF) intake (kg d(-1)) (r = 0.61, P < 0:01). Across all stages of maturity between-cow variation explained 54 to 70% of the total variation in CH4 production.