Seasonal variation in methane emission from dairy cows and breeding ewes grazing ryegrass/white clover pasture in New Zealand

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Daily methane emission from 12 Romney‐cross‐bred ewes and 10 lactating Friesian dairy cows, rotationally grazed on perennial ryegrass/white clover dominant pastures, was measured during four seasons of a year (September, November, March, and June/July). Methane emission was measured from each animal for 5 consecutive days in each measurement period using the sulphur hexafluoride tracer gas technique. The pastures varied significantly in chemical composition between seasons, generally decreasing in protein, soluble sugars, and digestibility and increasing in acid detergent fibre (ADF) and neutral detergent fibre (NDF) as the grasses flowered in November, with an increase in protein and a decrease in soluble sugars in March, a trend that continued through to June/July. Methane emission (g/day) from dairy cows was significantly different (P < 0.001) between seasons, being high at peak lactation in September (430.6) and declining with milk yield and feed intake in November (247.6) and March (181.5) and maintaining its level in June (137.4) when the cows were not lactating. Methane emission expressed per unit of feed intake was significantly higher in September and June than in November and March, suggesting some inhibition in the latter 2 months. There was less seasonal variation in methane emission (g/day) from the breeding ewes: March (27.0) and July (27.9) were significantly lower (P < 0.05) than in November (33.2), but neither differed from September (30.6). July was significantly lower (P < 0.001) than the other periods in methane emission per unit of feed intake, presumably because of its higher feed intake. It is suggested that for inventory purposes a methane emission factor of 26 g/kg digestible dry matter intake (DDMI) would be suitable for sheep and dairy cows grazing fresh temperate pasture.