Methane emission from dairy cows and wether sheep fed subtropical grass-dominant pastures in midsummer in New Zealand

Methane emission was measured from 10 dairy cows and 12 wether sheep grazing kikuyu grass- (Pennisetum clandestinum) dominant pastures at Waimate North, Northland, in February 1997 and March 1999, and from 10 dairy cows grazing summer grass- (Digitaria sanguinalis) dominant pasture at Edgecumbe, Bay of Plenty, in March 2000. Methane emission was measured from each animal for 5 consecutive days in each measurement period using the sulphur hexafluoride (SF6) tracer gas technique. Analysis of variance of the kikuyu grass chemical composition with sheep and cow data combined showed that the 1999 pastures were significantly higher in protein (P < 0.01), soluble sugars (P < 0.001), lipid (P < 0.01), and dry matter (DM) digestibility (P < 0.001) and lower in ash (P = 0.023), acid detergent fibre (ADF) (P < 0.001), and neutral detergent fibre (NDF) (P < 0.001) than the 1997 pastures, presumably as a consequence of urea topdressing in 1999. The chemical composition of the summer grass-dominant pasture grazed by cows in 2000 was similar to the kikuyu grass, except that DM digestibility was higher. Daily methane emissions from kikuyu grass were 363 and 167 g/day for the cows and 15.6 and 4.4 g/day for the sheep in 1997 and 1999, respectively. These corresponded to methane yields (MYs, methane energy as a percentage of gross energy) of 7.1 and 3.8% for the cows and 6.3 and 1.9% for the sheep in 1997 and 1999, respectively. There was clearly a significant inhibition (P < 0.001) of methane production in 1999 in both species fed kikuyu grass. Methane emission was 422 g/day and MY 6.7% in the cows fed summer grass, values that were similar to the kikuyu-fed cows in 1997. Methane emitted in g/kg digestible DM intake was 33.8 and 38.2 for the 1997 kikuyu grass-fed cows and sheep and 33.3 for the summer grass-fed cows. This suggests that methane emitted per unit of digested DM is higher for ruminants fed subtropical (C4) grasses than those fed temperate (C3) grasses and we believe it to be related to the higher cell wall content of C4 grasses.