The environmental impact of greenhouse gas emissions is a growing concern in society today. The livestock sector is a recognized source of greenhouse gas emissions, and a large amount of research has been completed over many years to develop effective greenhouse gas reduction techniques for use on livestock operations. A three-part review series by an international team of scientists and in collaboration with the Food and Agriculture Organization of the United Nations analyzed the results of hundreds of published studies to evaluate the effectiveness of various emission reduction methods.
The authors of the three-part series reviewed published data related to mitigation of enteric methane (CH4) emissions from ruminant animals (J. Anim. Sci. 2013. 91:5045-5069), manure management practices used to mitigate CH4 and nitrous oxide (N2O) emissions (J. Anim. Sci. 2013. 91:5070-5095), and animal management practices that mitigate enteric CH4 and N2O emissions from animal operations (J. Anim. Sci. 2013. 91:5096-5114). Overall, the authors concluded that a combination of approaches is needed to mitigate greenhouse gas emissions from the livestock sector and interactions among mitigation practices should be considered.
For the first article, more than 900 publications were reviewed to analyze the effectiveness of 14 different categories of feed additives and feeding strategies for mitigating enteric CH4 emissions. Each category was analyzed for potential CH4 mitigating effect, whether long-term effects have been established, the effectiveness of the method, and whether the practice is environmentally safe. Based on these criteria, the authors concluded that 10 of these 14 categories could be recommended.
“Improving forage quality and overall diet digestibility was recommended as one of the most effective practices to reduce enteric CH4 emissions per unit of milk or meat,” stated Dr. Alex Hristov, an author of the reviews. “Addition of lipids to the diet can also reduce CH4 emissions,” added Hristov. Another example of an effective mitigation practice is the use of alternative electron receptors, such as nitrates. However, not enough research has been completed to determine the long-term effects of nitrates. In addition, nitrates can increase nitrogen losses with manure and are potentially toxic to the animal. The toxicity issue can be alleviated if the animals are gradually adapted to the additive, and nitrogen losses can be minimized if nitrates are added to low-protein diets. Other feed management practices with low to medium effectiveness included ionophoric antibiotics, feeding of concentrates, and precision feeding. Even though these practices offer only moderate emission reductions, adequate research has confirmed their long-term mitigation effect.
The second review in the series indicates that a number of manure mitigation practices are feasible and can effectively reduce CH4 and N2O emissions from manure storage and land application. A key factor to be aware of when selecting a practice is “pollution swapping,” which can result in the reduction of one emission while actually increasing another. For example, ammonia emissions will increase with composting of manure, and manure and enteric CH4 emissions may increase if dietary protein is reduced with the goal of mitigating manure N2O emissions.
Opportunities to reduce CH4 and N2O in livestock manure management exist within methods of manure storage, manure application, soil management, plant management, animal diet, and overall animal management. But, no single method is applicable to all farms, and methods must, therefore, be selected at the farm level. Based on this analysis, reducing dietary protein was the most effective method for decreasing potential N2O and NH3 emissions for all farm species. Effective and recommended manure CH4 mitigation practices included, but were not limited to, anaerobic digestion, composting, decreasing manure storage time, solids separation, and manure covers.
The final article in this series addressed animal management options to reduce greenhouse gas emissions. The review suggests that increasing animal productivity is a highly effective strategy for reducing overall greenhouse gas emissions from animal operations. The authors concluded that this could be achieved through improving animal genetics, reproduction efficiency, and adequate nutrition. Recombinant bovine somatotropin (rbST) and growth promotants can also be effective in areas where these compounds are permitted. Furthermore, improved animal health and reduced mortality and morbidity can improve productivity and likewise reduce greenhouse gas emissions.
Taken as a whole, these comprehensive reviews suggest that a combination of methods is required to effectively reduce greenhouse gas emissions from the livestock sector. Selection of mitigation practices must be based on long-term efficacy, animal welfare, and cost-effectiveness.
The articles are titled “Mitigation of methane and nitrous oxide emissions from animal operations: I. A review of enteric methane mitigation options,” “Mitigation of methane and nitrous oxide emissions from animal operations: II. A review of manure management mitigation options,” and “Mitigation of methane and nitrous oxide emissions from animal operations: III. A review of animal management mitigation options.” The articles can be found at journalofanimalscience.org.
A. N. Hristov
Pennsylvania State University