A short term enteric methane emission measurement is not identical to a measure of daily methane production (DMP) made in a respiration chamber (RC). While RC curtail most variation except that from quantity and composition of feed supplied, all shortterm measurements contain additional sources of variation. The points of difference can include measurement time(s) relative to feeding, feed intake before measurement, animal behaviour in selection of diet and level of activity before measurement. For systems where a short-term emission measurement is made at the same time in the daily feeding cycle (e.g. during twice-daily milking) scaling up of short-term emission rates to estimate DMP is feasible but the scaling coefficient(s) will be diet dependent. For systems such as GreenFeed where direct emission rates are measured on occasion throughout day and night, no scaling up may be required to estimate DMP. For systems where small numbers of emission measures are made, and there is no knowledge of prior feed intake, such as for portable accumulation chambers, scaling to DMP is not currently possible. Even without scaling up to DMP, short-term measured emission rates are adequate for identifying relative emission changes induced by mitigation strategies and could provide the data to support genetic selection of ruminants for reduced enteric emissions.Keywords: ruminant, greenhouse gas, enteric emissions, methane, measurement
ImplicationsThe capacity to estimate daily methane production (DMP) or relative methane production of ruminants on the basis of short-term emission measurements will enable verification of enteric emission budgets and the development and verification of abatement techniques. This will enable more rapid deployment of abatement strategies and verify the emissions data used in carbon accounting transactions. Short-term emissions measurements are uniquely placed to accelerate methane abatement by genetic selection of livestock, which requires emission data for a large number of animals.
IntroductionThe majority of understanding of animal energetics and daily methane production (DMP) has been obtained from indirect calorimetry using open or closed circuit respiration chambers (RC) (Blaxter, 1962;Blaxter and Clapperton, 1965). Increasingly, however, there is a demand to measure ruminant methane emissions in animals within their natural production environment. This demand arises from the need to verify inventories, verify mitigation claims on-farm and measure large numbers of animals to determine the genetic parameters of methane output (Chagunda et al., 2009;McEwan et al., 2012). Although mobile respiration hood systems exist for on-farm use (Kelly et al., 1994), and RC studies using thousands of animals are now being conducted (Arthur et al., 2012), this is extremely slow and expensive. Robinson et al. (2010) identified that short-term measures of methane production were strongly correlated with DMP and can be used to obtain short-term on-farm emission data without need for an RC. This paper summarise...