Carolyn F. Walker

METHANE EMISSIONS MEASURED DIRECTLY FROM GRAZING LIVESTOCK IN NEW ZEALAND

Abstract We report measurements of methane emissions from individual ruminant livestock-both sheep and dairy cows-grazing pasture typical of New Zealand lowlands in the temperate southwest Pacific. These are the first measurements reported from grazing sheep, and among the first from grazing cattle. The measurement technique, developed at Washington State University, enables emission rates to be determined from analyses of “breath” samples collected while grazing. More than 250 measurements of daily methane emission from 50 sheep (8 months old) were made, with flock-mean emission 18.9 ± 0.8 g hd −1 d−1. Although emissions were weakly correlated with feed intake, they represented a 4.6 ± 0.1 % average loss of gross dietary energy. The corresponding mean emission based on 40 measurements of daily emissions from 10 lactating dairy cows was 263 ± 10 g hd−1 d−1, approximately 6.2% of estimated gross energy intake. A notable feature was the large inter-sheep variability in daily methane emission (factor of 1.4 range) that could not be attributed to variable intake. This would appear to suggest an appreciable diversity of methanogenetic response to digestion, and may be significant in the search for strategies to control emissions of this greenhouse gas.

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

Abstract 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 (SF 6 ) 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.

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

Abstract 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.

On the performance of SF6 permeation tubes used in determining methane emission from grazing livestock

In a technique for measuring methane emitted by grazing ruminant livestock, a calibrated source of inert tracer sulfur hexafluoride (SF6) is inserted into the rumen of each participating animal prior to collection of “breath” samples for gas analysis. Each source comprises a “permeation tube” from which an SF6 charge slowly escapes through a permeable membrane, to be sampled with the breath. This paper reports analyses of the permeation characteristics of such tubes and provides evidence that the permeation rate slowly changes rather than stays constant as the technique supposes. This feature has been observed routinely over several generations of tube fills in our laboratory. Failure to take account of a changing permeation rate can lead to a systematic error in the inferred methane emission rate of up to about 15%. A quality control strategy is proposed that enables permeation rates to be extrapolated with confidence, based on the monitored performances of control tubes as proxies for inserted tubes.

Methane emission from sheep grazing four pastures in late summer in New Zealand

Abstract Four groups of sheep were grazed on four late summer/autumn pastures: southern North Island summer moist hill country (Ballantrae); good quality perennial ryegrass/white clover dominant pasture in the Manawatu (Aorangi); severe late summer drought pasture in Hawkes Bay (Poukawa); and after drought conditions in Canterbury (Springston). Mature ewes were used at Springston, while young wethers were used at all the other sites. Methane emission from each animal was measured using the sulphur hexafluoride (SF6) tracer technique and feed intake was also determined. The pastures used were chosen for their expected chemical compositions at that time of the year. However, unseasonal rain fell just before the measurements were made at Ballantrae, Aorangi, and Springston. Those three pastures, although different in botanical composition, were similar in chemical composition and dry matter (DM) digestibility. The Poukawa pasture was dead and had low protein, soluble carbohydrate and DM digestibility, and high cell wall content. For the Ballantrae, Aorangi, Poukawa, and Springston pastures respectively, methane emissions were: 19.3, 21.9, 21.4, and 35.2 g/day; 13.8, 12.9, 17.8, and 21.1 g/kg DM intake; and the methane yields (methane energy as a percentage of gross energy intake) were 4.1, 3.9, 5.3, and 6.3%. The results support the view that young wether sheep have a lower methane yield than mature sheep and that methane yield is higher from pastures of poor feeding value.

Dimethylsulfoniopropionate (DMSP) and dimethylsulfide (DMS) cycling across contrasting biological hotspots of the New Zealand Subtropical Front

The oceanic frontal region above the Chatham Rise east of New Zealand was investigated during the late austral summer season in February and March 2012. Despite its potential importance as a source of marine-originating and climate-relevant compounds, such as dimethyl sulfide (DMS) and its algal precursor dimethylsulfoniopropionate (DMSP), little is known of the processes fuelling the reservoirs of these sulfur (S) compounds in the water masses bordering the subtropical front (STF). This study focused on two opposing short-term fates of DMSP-S following its uptake by microbial organisms (either its conversion into DMS or its assimilation into bacterial biomass) and has not considered dissolved non-volatile degradation products. Sampling took place in three phytoplankton blooms (B1, B2, and B3) with B1 and B3 occurring in relatively nitraterich, dinoflagellate-dominated subantarctic waters, and B2 occurring in nitrate-poor subtropical waters dominated by coccolithophores. Concentrations of total DMSP (DMSPt) and DMS were high across the region, up to 160 and 14.5 nmol L−1, respectively. Pools of DMSPt showed a strong association with overall phytoplankton biomass proxied by chlorophyll a (rs= 0.83) likely because of the persistent dominance of dinoflagellates and coccolithophores, both DMSP-rich taxa. Heterotrophic microbes displayed low S assimilation from DMSP (less than 5 %) likely because their S requirements were fulfilled by high DMSP availability. Rates of bacterial protein synthesis were significantly correlated with concentrations of dissolved DMSP (DMSPd, rs= 0.86) as well as with the microbial conversion efficiency of DMSPd into DMS (DMS yield, rs= 0.84). Estimates of the potential contribution of microbially mediated rates of DMS production (0.1–27 nmol L−1 day−1) to the near-surface concentrations of DMS suggest that bacteria alone could not have sustained DMS pools at most stations, indicating an important role for phytoplankton-mediated DMS production. The findings from this study provide crucial information on the distribution and cycling of DMS and DMSP in a critically under-sampled area of the global ocean, and they highlight the importance of oceanic fronts as hotspots of the production of marine biogenic S compounds.