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 (CH 4 ) 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 CH 4 production was measured during 7 d using the sulfur hexafluoride (SF 6 ) tracer-gas technique. Organic matter intake (OMI) was determined from fecal output estimated by Yb 2 O 3 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 CH 4 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 CH 4 did not differ with maturity (mean 6.4% of GEI). Cow effects upon OMI and CH 4 emission were significant (P < 0.05). CH 4 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 CH 4 production.