Development of a new methane inventory

Key messages

Feed intake is the over-riding factor driving methane production by lactating and non-lactating cattle and sheep across a broad range of diet types and productive states.

It is concluded that between-animal variation is greater than between-breed variation.

Sheep

• Smaller sheep tended to eat less than larger sheep, and therefore produced less CH₄ per day, but produced the same amount per unit of feed intake; however, body weight was not a good proxy of intake – there was a positive, but relatively poor, relationship between body weight and daily CH₄ emissions
• However, within-genotype differences between improved and unimproved breeding lines of Scottish Blackface sheep were found; high genetic merit animals produced more CH₄ (per day and per unit intake) than lower genetic merit animals
• Diet tended to have a much greater effect on CH₄ emissions, and Ym was affected by diet.  On relatively good quality diets, such as improved ryegrass swards, Ym was found to be close to the IPCC default value of 6.5%.  On poorer quality diets, such as permanent pasture and upland rough grazing, Ym was lower

Beef

• In beef cattle, as with sheep, there was limited effect of breed and breed type on enteric CH₄ emissions (daily emissions and CH₄ yield from feeds)
• There was a much greater effect of diet type, with animals consuming more higher-quality (lowland) forage than poorer-quality (upland) forages and consequently producing more CH₄ each day on the better quality feeds
• As digestibility of forage fibre increases methane output per kg DM tends to increase
• Similarly, animals fed concentrate-based diets, compared with those fed high-forage diets, produced less CH₄ per day and per unit feed intake, with less effect of cattle breed
• Most of the breed effects seen, including effects of beef animal sire, were due to differences in feed intake, sometimes driven by genetic potential for growth
• There were no major effects of gender, other than any effect related to feed intake which can sometime be related to genetic potential for growth
• In youngstock, emissions increased with increasing age as the animals consumed more feed
• Differences due to physiological state (dry vs lactating) may be due to differences in passage rate of digesta which in turn maybe related to the fact that as feed intake increases the proportion of feed energy lost as methane reduces
• Differences in methane output of individuals within a breed were often bigger than differences found between breeds
• With beef cattle an effect of diet quality was evident, as starch content of the diet increased methane output tends to reduce in terms of both total output per day and per unit of feed input or production

Manures

• Although the majority of CH₄ emissions from ruminant livestock emanate from the gut, a significant proportion can be released from manures
• Acidification of cattle slurry was effective at reducing emissions of both CH₄ and ammonia, while the use of floating clay granules reduced ammonia but not CH₄ emissions

The Problem

As reported by the UK’s 2011 greenhouse gas (GHG) inventory returns, agriculture accounts for approximately 8% of UK total GHG emissions.  The main GHG produced by agricultural activities are nitrous oxide (N2O) and methane (CH4), making up 62% and 38% of direct GHG emissions respectively in 2011.  The current UK National GHG Inventory largely estimates emissions from agriculture using the most simplified approach to accounting (Tier 1).  This approach uses generic assumptions and factors about livestock management to estimate GHG emissions, and uncertainties associated with the national estimates for GHG emissions from enteric fermentation are approximately ±20% of the estimated annual mean.

The Challenge

The overall objective of this project is to deliver a set of emission factors (EFs) for methane from different livestock species (focussing on cattle and sheep) and breeds/genotypes, under a range of different farm systems and representative business structures. These will be generated through a number of measurements within representative breed/system approaches that will also assess the effects of nutrition (basal forages, concentrate supplements, and feed additives), and which will be applicable to the development of an improved UK greenhouse gas inventory reporting structure.

EBLEX is part of the project advisory group (PAG) which also includes DairyCo, HCC, QMS, LMC and AgriSearch.

This programme of work included literature reviewing, modelling and animal studies to collect data sets not available from the literature.