Mitigation of enteric methane emissions from ruminants in subtropical production systems

Abstract

Globally agriculture and livestock producers have come under increasing pressure over the environmental impact of production systems. The objectives of this study were to re-calculate the direct methane (CH4) and nitrous oxide (N2O) emissions of livestock production systems in South Africa, taking into consideration the uniqueness of the South African scenario and to identify and evaluate possible greenhouse gas mitigation strategies for extensive production systems. It is important to generate accurate greenhouse gas (GHG) baseline figures to develop South Africa’s capacity to understand and reduce GHG emissions emitted from the livestock sector. Livestock produce GHG’s in the form of methane from enteric fermentation and nitrous oxide and methane from manure management and manure deposited on pastures and rangeland by grazing animals. Agriculture, forestry and land use (corrected for carbon sink values) emitted an estimated 4.9% of South African GHG gases in 2004, which makes it the third largest GHG contributor in South Africa after the energy industry and industrial processes. Livestock produced approximately 27% of the national methane emissions and 98% of the agricultural sector’s methane emissions in 2004. Methane is a potent GHG that remains in the atmosphere for approximately 9 to 15 years and is 28 times more effective in trapping heat in the atmosphere than carbon dioxide (CO2) over a 100-year period. Nitrous oxide has an atmospheric lifetime of 150 years and a global warming potential of 265 times that of CO2 over a 100-year period. South African livestock production is based on a unique combination of commercial (intensive and extensive) and emerging and communal (subsistence) production systems. The levels of productivity and efficiency in these production systems vary greatly in certain areas and it is important to distinguish between them when calculating GHG emissions. Previous inventories were conducted on a national scale utilizing IPCC default values (Tier 1 approach) for some or all of the emission calculations. These emission factors do not distinguish effectively between classes of animals, production efficiencies, and production systems. They are often based on assumptions of animals utilizing diets which are not representative of South African production systems. The IPCC Tier 2 methodology seeks to define animals, animal productivity, diet quality and management circumstances to support a more accurate estimate of feed intake for use in estimating methane production from enteric fermentation. It was also considered important to do separate calculations for each province as provinces differ in vegetation or biomes and production systems which may require different approaches to mitigation recommendations. Due to the heterogeneity of available feed types within South Africa it was considered important to use methodologies that could reflect such differences and was developed under similar conditions. The methodology utilized is based on the Australian national greenhouse account’s National Inventory Report, which contains Australian country-specific and IPCC default methodologies and emission factors. Emission factors specific to South African conditions and management systems were calculated where possible. A Tier 2 approach was adopted for all major livestock categories including privately owned game in accordance with the IPCC Good Practice requirements. Recently game farming has become a recognized commercial enterprise in the agricultural sector which needs to be included as an anthropogenic emissions source. Methane emissions from South African livestock were estimated at 1328 Giga gram (Gg) during 2010. Dairy and beef cattle contributed an estimated 964 Gg or 72.6% of the total livestock methane emissions in South Africa during 2010. Beef cattle in extensive systems were the largest contributor (83.3%), followed by dairy cattle (13.5%), and feedlot cattle (3.2%). The estimated direct enteric methane emission factors for dairy and beef cattle were higher than the IPCC default factors for Africa. The Eastern Cape recorded the highest dairy and beef cattle methane emissions, whereas Gauteng showed the highest feedlot methane emissions primarily due to cattle numbers. Small stock was responsible for 15.6% of the total livestock emissions contributing an estimated 207.7 Gg, with sheep producing 167 Gg and goats producing 40.7 Gg. Calculated enteric methane emission factors for both commercial and communal sheep were higher than the IPCC default values for developing countries. A similar tendency was found with goat emission factors. The highest sheep and goat methane emissions were reported for the Eastern Cape province. The pig and ostrich industry both contributed approximately 8 Gg CH4 during 2010. The North- West province produced the highest commercial pig GHG emissions with the highest communal pig emissions originating from the Eastern Cape. The poultry industry was the largest direct N2O producer of the non-ruminant livestock industries, contributing 2.3 Gg or 92.8% of the total nonruminant N2O emissions. The privately owned game industry contributed an estimated 131.9 Gg of methane emissions with the provinces of Limpopo, Eastern Cape and Northern Cape being the three largest contributors with 43.4, 37.3 and 21 Gg methane, respectively. The total privately owned game population was estimated at 2 991 370 animals, utilizing 20.5 million hectares. Beef cattle are the major contributors to livestock GHG emissions in South Africa followed by sheep, privately owned game, dairy cattle, goats, pigs, ostriches, equine, and poultry. The IPCC default values for Africa underestimate emission factors across all livestock categories. The methane emission factors calculated for commercial livestock production systems are more comparable to emission factors from developed countries and the emerging/communal production systems to those of developing countries. This emphasizes the need to develop country-specific emission factors through quantitative research for livestock in all provinces and on all types of production systems to produce accurate baseline figures, which is critical to future mitigation protocols. As part of this study fourteen tropical grass species typical of transitional rangeland regions of South Africa were characterised in terms of chemical composition, in vitro total gas and in vitro methane production. The results of the study demonstrated that in vitro methane production varied between tropical grass species typical of transitional rangeland in South Africa. The variation between species allows for the potential to identify and select species with a lower enteric methane production potential. Panicum maximum, Eragrostis curvula and Elionurus miticus were the three species which produced the lowest in vitro methane production but which also had a crude protein (CP) concentration of more than 3.5% of dry matter (DM) and with an in vitro organic matter digestibility (IVOMD) above the group average for the study. Furthermore, the results of the study revealed that in vitro methane production was higher in Decreaser species compared to Increaser species. Improving the quality of available forages through the use of cultivated pastures and fertilization is known to improve ruminant production efficiency. The effect of level of nitrogen (N) fertilization on certain qualitative parameters and in vitro total gas and methane production of improved grass species commonly utilised in South Africa was evaluated. Treatments included seven grass species divided into two photosynthetic pathways (C3 and C4) with three levels of N fertilization (0, 50 and 100 kg N/ha). No effect was found for N fertilization on in vitro total gas or methane production. The CP concentration increased (P < 0.05) and the NDF concentration tended to decrease (P < 0.1) as the level of N fertilization increased for both C3 and C4 species. Increasing the level of N fertiliser increased (P < 0.05) the methanogenic potential of Dactylis glomorata, Festuca arundinacea and Cenchrus ciliaris after the 24 hour incubation period but no effects (P>0.05) were found after the 48 hour incubation period. Results suggests that the stage of physiological development of forages might have a greater influence on the methanogenic potential of forages compared to the effect of N fertiliser application.