Methane emissions from lactating and non-lactating dairy cows and growing cattle fed fresh pasture

Currently, a fixed methane (CH4) emission factor is used for calculating cattle emissions in the national greenhouse gas (GHG) inventory of New Zealand, independent of age, cattle class, physiological stage or diet composition. The objectives of this study were to determine methane emissions from lactating and non-lactating dairy cows fed fresh pasture forage in respiration chambers. This information was used to determine the relationship between methane emissions and DMI, feed quality and physiological stage in dairy cows and in comparison with available NZ data from growing cattle fed fresh pasture forage. Methane emissions were measured in open-circuit respiration chambers from 118 dairy cows (81 lactating and 37 non-lactating) over 10 periods and twice from 36 growing beef cattle (3 groups of 12) and 12 growing dairy heifers. All animals receive fresh pasture that was cut daily as the sole diet. Dry matter intake alone explained 89.5% or 89.1% of the variation in pCH4 (g/d ) with (pCH4 = 21.6 DMI + 13.6) or without (22.9 g/kg DMI) intercept, respectively. Regression analysis was not improved by including dietary composition, cattle class or physiological status in the relationship between pCH4 and DMI. In conclusion, methane emissions form cattle fed freshly cut pasture was accurately predicted by DMI only.