An integrative bio-physical approach to determine the greenhouse gas emissions and carbon sinks of a cow and her offspring in a beef cattle operation : a system dynamics approach

Abstract

CONTEXT : Regenerating the planet through natural resource protection, restoration and prudent land management must currently be one of the most important policy and operational objectives at all scales from local to international level. The beef cattle production sector has both an important role and a responsibility to this end as well. OBJECTIVE : We estimated the greenhouse gas emissions of a cow over her entire life inclusive of that of her offspring taking cognisance of her physiological stages under different scenarios relative to the meat produced (carcass weight) to determine the range of emissions and/or sinks that are possible from a grassland beef cattle farming operation in South Africa. METHODS : We constructed a system dynamics model populated by the energy and carbon flows of a hypothetical cow and her 8 offspring over their entire life cycle of 154 months. The birth of the cow and her offspring represents a marginal addition to an otherwise stable global herd. The cow and her calves thus constitute an additional carbon pool birthed and nurtured by the mature cow. By making provision for the physiological stages of the cow and her calves and allowing for typical variations in the metabolisable energy during these stages, we estimated the cumulative net greenhouse gas emissions relative to the cumulative amount of meat (carcass weight) produced. Using a purpose-built system dynamics model, we modelled several scenarios providing a range of outcomes depending on the parameter values. RESULTS AND CONCLUSIONS : On the one end of the spectrum, we applied a global warming potential (GWP100) of methane of 28 and that 10% of the carbon contained in the manure is sequestered in the soil. Under this scenario the cumulative net emissions are estimated as 19.1 tCO2e per ton meat. Thus, the sources of emissions exceed the sinks. At the other end of the spectrum the figure turns into a net sink of approximately 12.6 tCO2e if a global warming potential (GWP*) of 8 is used and if 70% of the carbon in the manure is sequestered in the soil, a figure attainable in healthy soils with active microbial life and sufficient grass cover and good land management. Under these conditions the net addition of the cow and her calves to an otherwise stable global herd lead to the reduction in greenhouse gas emissions. All these figures exclude additional carbon sequestration through accelerated grass regrowth and increased litter deposits and could thus be deemed conservative. SIGNIFICANCE : Discussions with respect to the reduction of greenhouse gas emissions from livestock must consider the full life cycle of the female animal and her offspring, inclusive of the virtual (or embedded) carbon, the appropriate estimate of the global warming potential of methane acknowledging that it is a short-lived climate pollutant, as well as the condition of the rangelands. Much more effort should therefore be directed towards improving the soil and land use management, including incentive measures and knowledge sharing, as it has a mutually reinforcing impact on the mitigation of greenhouse gas emissions and the sequestration of carbon.