Animal production is increasing with the ever-increasing human population. Feed land use and feed production are generally in balance with animal production through dynamic changes in feed price, while feed production is ultimately limited by the availability of feed land. Increasing feed prices has resulted in continuous improvement in feed production efficiency and animal production efficiency. Improvements in production efficiency are the major opportunities to positively affect the balance among feed land use, feed production, and animal production. Improving feed production efficiency can reduce feed land use per unit of feed production. Improving animal production efficiency can reduce feed required per unit of animal production. Therefore, improving productivity is not contradictory but instead is a major contributor for mitigating GHG emissions from feed production. For GHG emissions from manure, solutions include prevention of manure from entering anaerobic state, while the ultimate solution is manure energy recovery through capture and use of biogas. However, adoption of manure energy recovery is limited by barriers in technology cost, which may be affected by regulatory compliance requirements and economics of manure transportation/application, and will ultimately be decided by land capacity for accepting manure. The emission factors (emissions per head) for GHG from enteric fermentation are relatively stable. Improving consumer stewardship, reducing food waste and thus reducing animal protein demand are always a practical and bottom-line strategy for mitigating GHG from enteric emissions.                                         Read more ......

Mitigating enteric emissions

Other than reducing animal protein demand, many strategies have been proposed to mitigate GHG from enteric emissions by reducing emissions per kg meat produced. Most of the mitigation strategies were based on dietary manipulation, feed management or feed supplements, and they are summarized in Table 1.

Table 1. Strategies and technologies for mitigating enteric GHG emissions

Increasing feed intake and feed digestibility both can reduce CH₄ conversion factor on a digestible energy basis. This can be achieved by increasing proportion of concentrate in feed. Improved forage quality in forage-based diets can also result in increased feed intake and feed digestibility. Concentrates or high quality forage generally provide more digestible nutrients, and thus increase animal productivity, resulting in lower GHG emissions per animal product. Feed processing can also be an effective mitigation practice through its effect on feed digestibility. Dietary supplementation with fat is a potential mitigation strategy, while the long-term effects have not been well-established and there are challenges to identify fat sources in a cost-effective manner. High-oil by-product feeds, such as distiller’s grains, may be an economically feasible alternative to fat supplementation. But their higher fiber content needs to be evaluated to not to counteract the GHG mitigating effect, depending on diet composition. Supplementation with tannins or nitrates has also been reported effective. Other attempts in modification of rumen function have had very little success for sustained reduction in enteric CH₄.

Based on the LCA results, the majority of enteric CH₄ was from breeding stock in the cow–calf phase. When considering mitigation of enteric GHG emissions, breeding stock should be given the first priority. The largest reductions are achieved when mitigation practices target breeding animals. In addition to the strategies and technologies summarized in Table 1, genetic selection could be another potential strategy to improve feed efficiency and to mitigate enteric GHG emissions.

Mitigating emissions from manure

Major strategies and technologies for mitigating GHG emissions from manure are summarized in Table 2.

Table 2. Strategies and technologies for mitigating GHG emissions from manure

Optimizing the animal diet to improve N use efficiency and reduce excreted N is effective to reduce manure NH₃ and indirect N₂O emissions, although the effect on direct N₂O emissions were not consistent in literature. The choice of manure management systems has significant effect on GHG emissions. GHG emissions from anaerobic liquid manure systems (lagoons, tanks) are larger than from dry manure systems (stacks, piles). Manure treatment for dairy and swine operations deserve more attention due to the wide use of liquid manure systems. Composting and use of storage cover are two common practices that can be used to mitigate GHG emissions in addition to the benefit of odor control during manure storage. Reducing storage time, reducing manure temperature, and preventing anaerobic conditions all help to minimize GHG emissions. Manure acidification may reduce CH₄ and NH₃ emissions, but it might increase H₂S emissions as well as N₂O emissions following land application. Biofilters can be effective to reduce CH₄ and NH₃ from mechanically ventilated animal housing, however, careful management is required to mitigate N₂O and the overall GHG emissions. Optimization of manure application method has been shown to control the amount of N available for nitrification and denitrification in soil, to promote the aerobic metabolic path and reduce CH₄ emission from land application. Use of cover crops could also be an effective tool for GHG mitigation through improved soil quality, enhanced soil organic C sequestration, and reduced N fertilizers use.

Anaerobic digestion (AD) with combustion of the biogas produced is probably the most effective end-of-pipe methods for mitigation of GHG emissions from manure. Compared to conventional manure management practices, an AD system usually costs more to install and manage, but it can also generate additional revenue. Whether an AD system is feasible for a livestock operation depends on type and scale of the operation, how the manure is handled, the frequency of manure collection, the potential uses for the recovered biogas, and the local market for the end products. Smaller operations may make AD feasible through special design, such as including co-digestion of manure and other organic substrates such as food waste.

Mitigating emissions from feed production

The GHG emissions from animal feed production contributed 11 to 33% to carbon footprint (CF) of beef cattle, 20.3% to CF of dairy cattle, and 26% to CF of swine. Avoiding feed waste and improving feed efficiency are obvious choice for mitigation GHG emissions from feed production. Continuous improvements in feed production efficiency may reduce feed land use per unit of feed production; and improvements in animal production efficiency may reduce feed required per unit of animal production. They both represent great opportunities for mitigation GHG emissions from animal feed production.