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.

中文翻译：

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一种综合生物物理方法，用于确定肉牛养殖场中奶牛及其后代的温室气体排放和碳汇：系统动力学方法

语境

通过自然资源保护、恢复和审慎的土地管理来再生地球，目前必须成为从地方到国际各级的最重要的政策和运营目标之一。肉牛生产部门在这方面也具有重要作用和责任。

客观的

我们估计了一头母牛一生的温室气体排放量，包括其后代在不同情况下相对于生产的肉（胴体重量）的生理阶段的认识，以确定可能的排放量和/或汇的范围来自南非的草原肉牛养殖场。

方法

我们构建了一个系统动力学模型，由一头假设的奶牛及其 8 个后代在 154 个月的整个生命周期中的能量和碳流构成。奶牛及其后代的诞生是对原本稳定的全球牛群的补充。因此，母牛和她的小牛构成了由成年母牛生产和培育的额外碳库。通过为奶牛和她的小牛的生理阶段做好准备，并考虑到这些阶段可代谢能量的典型变化，我们估计了相对于生产的肉类累积量（胴体重量）的累积温室气体净排放量。使用专门构建的系统动力学模型，我们对多个场景进行了建模，根据参数值提供了一系列结果。

结果和结论

在光谱的一端，我们应用了 28 的全球变暖潜能值 (GWP ₁₀₀ ) 甲烷，粪便中含有 10% 的碳被隔离在土壤中。在这种情况下，每吨肉的累积净排放量估计为 19.1 tCO ₂ e。因此，排放源超过汇。在光谱的另一端，该数字变成了大约 12.6 tCO ₂的净汇e 如果使用 8 的全球变暖潜能值 (GWP*) 并且如果粪便中 70% 的碳被隔离在土壤中，那么在具有活跃微生物生命、充足草覆盖和良好土地管理的健康土壤中可以达到这一数字。在这些条件下，将奶牛和她的小牛净添加到原本稳定的全球牛群中会导致温室气体排放量的减少。所有这些数字都不包括通过加速草再生和增加枯枝落叶沉积而产生的额外碳封存，因此可以被认为是保守的。

意义

关于减少牲畜温室气体排放的讨论必须考虑雌性动物及其后代的整个生命周期，包括虚拟（或嵌入）碳，对甲烷的全球变暖潜力的适当估计，承认它是一种短暂的气候污染物，以及牧场的状况。因此，应加大力度改进土壤和土地使用管理，包括奖励措施和知识共享，因为这对减缓温室气体排放和固碳具有相辅相成的影响。