Livestock and climate change: impact of livestock on climate and mitigation strategies

The livestock sector requires a significant amount of natural resources and has an important role in global greenhouse gas emissions. The most important greenhouse gases from animal agriculture are methane and nitrous oxide. Mitigation strategies aimed at reducing the emission intensity of this sector are needed to meet the increasing demand for livestock products driven by population growth. To increase the effectiveness of mitigation strategies, the complex interactions among the components of livestock production systems must be taken into account to avoid environmental trade-offs. According to the United Nations (UN, 2017), the world population increased by approximately 1 billion inhabitants during the last 12 years, reaching nearly 7.6 billion in 2017. Although this growth is slower than 10 years ago (1.24% vs. 1.10% per year), with an average increase of 83 million people annually, global population will reach about 8.6 billion in 2030 and 9.8 billion in 2050. Population growth, urbanization, and income rise in developing countries are the main driver of the increased demand for livestock products (UN, 2017). The livestock sector requires a significant amount of natural resources and is responsible for about 14.5% of total anthropogenic greenhouse gas emissions (7.1 Gigatonnes of carbon dioxide equivalents for the year 2005; Gerber et al., 2013). Mitigation strategies aimed at reducing emissions of this sector are needed to limit the environmental burden from food production while ensuring a sufficient supply of food for a growing world population. The objectives of this manuscript are to 1) discuss the main greenhouse gas emissions sources from the livestock sector and 2) summarize the best mitigation strategies. The most important greenhouse gases from animal agriculture are methane and nitrous oxide. Methane, mainly produced by enteric fermentation and manure storage, is a gas which has an effect on global warming 28 times higher than carbon dioxide. Nitrous oxide, arising from manure storage and the use of organic/inorganic fertilizers, is a molecule with a global warming potential 265 times higher than carbon dioxide. The carbon dioxide equivalent is a standard unit used to account for the global warming potential (IPCC, 2013). Figure 1 was adapted from the Global Livestock Environmental Assessment Model (GLEAM) developed by FAO (FAO, 2017) and shows in carbon dioxide equivalents the greenhouse gas incidences that enteric fermentation and manure storage have across the main livestock species raised worldwide. Greenhouse gases incidence of enteric fermentation and manure storage by animal type, expressed as Gigatonnes of carbon dioxide equivalents. Data referred to 2010 (FAO, 2017). In addition to greenhouse gases arising from enteric fermentation and manure storage, feed production together with the related soil carbon dioxide and nitrous oxide emissions is another important hot spot for the livestock sector. Soil carbon dioxide emissions are due to soil carbon dynamics (e.g., decomposing plant residues, mineralization of soil organic matter, land use change, etc.), the manufacturing of synthetic fertilizers and pesticides, and from fossil fuel use in on-farm agricultural operations (Goglio et al., 2018). Nitrous oxide emissions are emitted when organic and inorganic fertilizers are applied to the soil. As shown in Figure 2, feed production and processing contribute about 45% of the whole sector (3.2 Gigatonnes of carbon dioxide equivalents). Enteric fermentation producing about 2.8 Gigatonnes (39%) is the second largest source of emissions. Manure storage with 0.71 Gigatonnes accounts for about 10% of the total. The remaining 6% (0.42 Gigatonnes of carbon dioxide equivalents) is attributable to the processing and transportation of animal products (Gerber et al., 2013). Livestock emissions by source (adapted from Gerber et al., 2013). Direct livestock emissions are shown in red. Feed production (Figure 2) includes all the greenhouse gas emission arising from 1) land use change, 2) manufacturing and use of fertilizers and pesticides, 3) manure excreted and applied to fields, 4) agricultural operations, 5) feed processing, and 6) feed transport. Although these processes result in a large share of the livestock supply chain, in this article, we mainly focus on direct livestock emissions enteric fermentation, manure storage, and manure excreted/applied to the soil. All other emissions are outside the scope of this article. Enteric fermentation is a natural part of the digestive process of ruminants where bacteria, protozoa, and fungi contained in the fore-stomach of the animal (rumen), ferment and break down the plant biomass eaten by the animal. Plant biomass in the rumen is converted into volatile fatty acids, which pass the rumen wall and go to the liver through the circulatory system. This process supplies a major part of the energy needs of the animal and enables the high conversion efficiency of cellulose and semi-cellulose, which is typical of ruminants. The gaseous waste products of enteric fermentation, carbon dioxide and methane, are mainly removed from the rumen by eructation. Methane emission in the reticulorumen is an evolutionary adaptation that enables the rumen ecosystem to dispose hydrogen, which may otherwise accumulate and inhibit carbohydrate fermentation and fiber degradation (McAllister and Newbold, 2008). The emission rate of enteric methane varies according to feed intake and digestibility. Manure acts as an emission source for both methane and nitrous oxide, and the quantity emitted is linked to environmental conditions, type of management and composition of the manure. Organic matter and nitrogen content of excreta are the main characteristics influencing emission of methane and nitrous oxide, respectively. Under anaerobic conditions, the organic matter is partially decomposed by bacteria producing methane and carbon dioxide. Storage or treatment of liquid manure (slurry) in a lagoon or tank promotes an anaerobic environment which leads to an increase in methane production. Long storage periods and warm and wet conditions can further increase these emissions (EPA, 2010). On the other hand, nitrous oxide emissions need a combination of aerobic and anaerobic conditions to be produced. Therefore, when manure is handled as a solid (dung) or deposited on pastures, nitrous oxide production increases while little or no methane is emitted. Nitrous oxide is generated through both the nitrification and denitrification processes of the nitrogen contained in manure, which is mainly present in organic form (e.g., proteins) and in inorganic form as ammonium and ammonia. Nitrification occurs aerobically and converts ammonium and ammonia to nitrites and then nitrates, while denitrification occurs anaerobically converting nitrates to nitrous oxide and nitrogen gas (Saggar, 2010). The balance between ammonium and ammonia is highly affected by pH, with ammonia increasing as pH increases. Almost 60% of the global biomass harvested worldwide enters the livestock subsystem as feed or bedding material (Krausmann et al., 2008). Greenhouse gas emissions from feed production represent 60–80% of the emission coming from eggs, chicken and pork, and 35–45% of the milk and beef sector (Sonesson et al., 2009). As shown in Figure 2, emissions from feed production account for about 45% of the livestock sector. The application of manure as fertilizer for feed crops and the deposition of manure on pastures generates a substantial amount of nitrous oxide emissions representing about half of these emissions (Gerber et al., 2013). Although livestock feed production often involves large applications of nitrogen to agricultural soils, good manure management can reduce the need for manufactured fertilizers. The extreme heterogeneity of the agricultural sector needs to be taken into account when defining the overall sustainability of a mitigation strategy, which can vary across different livestock systems, species, and climates. Generally, no measure in isolation will encompass the full emission reduction potential, while a combination selected from the full range of existing options will be required to reach the best result (Llonch et al., 2017). It is also important to consider the “pollution swapping” effect when evaluating the effectiveness of a mitigation strategy (Hristov et al., 2013). Reduction of methane emissions during enteric fermentation might be counteracted by increased greenhouse gas emissions in applied manure. Reduction of direct nitrous oxide emissions during storage might result in higher nitrate leaching and ammonia volatilization during field application. Mitigation may occur directly by reducing the amount of greenhouse gases emitted, or indirectly through the improvement of production efficiency. The main strategies to mitigate greenhouse gas emissions in the livestock sector have been investigated and are summarized in Table 1. Mitigation potential of various strategies *High = ≥30% mitigating effect; Medium = 10–30% mitigating effect; Low = ≤10% mitigating effect. Mitigating effects refer to percent change over a “standard practice” according to Newell Price et al. (2011); Borhan et al. (2012); Hristov et al. (2013); Montes et al. (2013); Petersen (2013); Battini et al. (2014); Knapp et al. (2014); Llonch et al. (2017); Mohankumar Sajeev et al. (2018). †Inconsistent/variable results. ‡Uncertainty due to limited research or lack of data. Mitigation potential of various strategies *High = ≥30% mitigating effect; Medium = 10–30% mitigating effect; Low = ≤10% mitigating effect. Mitigating effects refer to percent change over a “standard practice” according to Newell Price et al. (2011); Borhan et al. (2012); Hristov et al. (2013); Montes et al. (2013); Petersen (2013); Battini et al. (2014); Knapp et al. (2014); Llonch et al. (2017); Mohankumar Sajeev et al. (2018). †Inconsistent/variable results. ‡Uncertainty due to limited research or lack of data. Decreasing methane emissions from ruminants is one pressing challenge facing the ruminant production sector. Strategies for reducing this source of emissions focus on improving the efficiency of rumen fermentation and increasing animal productivity. A large number of mitigation options have been proposed (e.g., diet manipulation, vaccines, chemical additives, animal genetic selection, etc.) with different efficiencies in reducing enteric methane as shown in Table 1. Forage quality and digestibility affect enteric methane production. Lignin content increases during plant growth, consequently reducing plant digestibility. Therefore, harvesting forage (especially grass) for ensiling at an earlier stage of maturity increases its soluble carbohydrate content and reduces lignification. According to Knapp et al. (2014) practices aimed to increase forage quality have shown a potential enteric methane reduction of about 5% per unit of fat protein corrected milk. Physical processing of forages, such as chopping, grinding, and steam treatment, also improves forage digestibility and mitigates enteric methane production in ruminants (Hristov et al., 2013). However, the reduction potential of this practice was reported to be less than 2% per unit of fat protein corrected milk (Knapp et al., 2014). Improving diet digestibility by increasing concentrate feeding is another effective mitigation strategy, reducing by 15% methane emissions per unit of fat protein corrected milk (Knapp et al., 2014). However, the ratio of forage to concentrate has to be carefully taken into account when applying this strategy. Indeed, although a marked reduction of enteric methane can be expected with rates of concentrate inclusion between 35% and 40% (Gerber et al., 2013). A greater proportion of dietary fermentable carbohydrates could increase the risk of metabolic diseases (e.g., rumen acidosis). Addition of fats or fatty acids to the diets of ruminants can decrease enteric methane emissions by both decreasing the proportion of energy supplied from fermentable carbohydrates and changes in the microbial population of the rumen (Llonch et al., 2017). Although some byproducts (e.g., cottonseed, brewer’s grains, cold-pressed canola meal, etc.) are effective in reducing enteric fermentation (Moate et al., 2011), the mitigation potential of high oil byproducts has not been well-established and in some cases methane production may increase due to increased fiber intake (Hristov et al., 2013). The inclusion of lipids higher than 10% can lead to impairment of ruminal function due to changes to the microbial population which in turn decreases the ability to digest fiber. Lipid diet supplementation between 5% and 8% of the dry matter intake is an effective mitigation strategy (Grainger and Beauchemin, 2011) with a potential enteric methane reduction of about 15% per unit of fat protein corrected milk (Knapp et al., 2014). Feed additives (electron receptors, ionophoric antibiotics, chemical inhibitors, etc.) have also been tested for their ability to decrease methane emissions (Beauchemin et al., 2009). However, the unknown toxicity and the health risks associated with the use of some of these compounds may severely constrain widespread adoption (Herrero et al., 2016). Increased animal density together with continuous inflow of nutrients from imported feeds is likely to increase volumes of manure to be managed. Stored manure accounts for a relatively small amount of direct agricultural greenhouse gases (Figure 2), and it is technically possible to mitigate a very high percentage of these emissions (Hristov et al., 2013). In the following section, some of the most effective mitigation strategies are discussed. As methane production increases with the temperature of stored manure, a reduction of storage temperature has been reported to drop these emissions by 30–50% (Borhan et al., 2012). However, the net greenhouse gas mitigation resulting from this strategy can vary widely, and it is strictly related to the energy used and the cooling system adopted. Frequent removal of manure to an outside storage facility is an effective practice that can be accomplished using grooved floors combined with regular scraping of manure, especially for pigs and some cattle production systems. Indeed, if the channels underneath the stable are emptied regularly, and the manure/slurry are transported to an outside storage facility, this practice has the potential to reduce methane and nitrous oxide emissions by 55% and 41%, respectively (Mohankumar Sajeev et al., 2018). On poultry farms the litter/manure is usually removed at the end of the crop; however, advanced layer housing using belt scrapers can efficiently remove litter/manure continuously and decrease greenhouse gas emissions (Fournel et al., 2012). Solid-liquid separation is a processing technology that partially separates the solids from liquid manure using gravity or mechanical systems such as centrifuges or filter presses. As shown in Table 1, the greenhouse gas mitigation potential of this technique has been reported to be higher than 30% compared with untreated manure (Montes et al., 2013). The organic component with a larger particle size follows the solid stream during the separation process, and it is then stored in stockpiles. The aerated condition of the storage can then limit the potential for methane to be emitted; however, ammonia loss through composting and generating high temperatures can be accelerated. Also, the remaining liquid fraction is still a potential source of indirect nitrous oxide emissions. Indeed, once the fibrous and large pieces of organic material are subtracted, it will not form a crust during storage, leading to increased volatilization of ammonia by increasing the mass transfer coefficient at the surface. Although greenhouse gas mitigation of the solid-liquid separation process can be partially counterbalanced by ammonia emissions, it is important to note that there are many management practices that can overcome these issues, such as covering slurry storage and the use of injection for land application (Holly et al., 2017). Anaerobic digestion is a biological degradation process, which in the absence of oxygen, produces digestate and biogas (mainly methane and carbon dioxide) from manure. Biogas collected from the system is often used to generate electricity, to fuel boilers or furnaces, or to provide combined and into account the greenhouse gas emissions arising from the use of the digestate as and the for the energy anaerobic digestion has been reported to than 30% reduction in greenhouse gas emissions when compared with manure systems et al., 2014). However, further to the management of the digestate the anaerobic digestion is Indeed, mineralization of the organic nitrogen during biological degradation increases the inorganic nitrogen content and pH of the which in turn may increase ammonia volatilization and anaerobic digestion and solid-liquid separation could reduce the amount of ammonia following digestion (Holly et al., 2017). severely of nitrogen in most livestock on the of their feed can in reducing this source of nitrous oxide in the Although a diet could mitigate nitrous oxide emissions from cattle manure storage some must be to dietary nitrogen (Montes et al., 2013). decreasing protein could lead to an increase of fermentable which in turn will likely increase methane production. The diet for all animal species be for acids to avoid a in feed intake and a decrease in animal productivity. acids are used to balance the diet of and the environmental associated with the manufacturing of these must be when acids as a greenhouse gas mitigation strategy. In supplementation of acids in degradation in the a significant increase in animal productivity. On the acids chemical in the rumen and can reach the where are improving milk in feeding protein to the is as an effective mitigation strategy to reduce ammonia and nitrous oxide emissions from manure (Montes et al., 2013). The and of fertilizer applications are important influencing soil nitrous oxide emissions. The nitrogen fertilizer applied is to loss by leaching and denitrification Therefore, ensuring that of nitrogen to the growing and application in wet or major are practices which could in biomass production and reduce soil greenhouse gas emissions. As methane emissions occur manure land decreasing storage can in reducing greenhouse gas emissions However, the resulting soil applications can have a effect on nitrous oxide emissions from field and carbon dioxide emissions from fuel application during periods with wet soil and periods of plant nitrogen could in increasing the effectiveness of this practice (Hristov et al., 2013). storage can provide greater in when to manure to fields, while the use of on-farm manure could the a management and environmental Price et al., The use of nitrification has the potential to reduce nitrogen leaching by the conversion of ammonia to However, this effect is by a reported increase in indirect nitrous oxide emission that can result from increased ammonia volatilization et al., 2016). This the of both gases when evaluating the use of nitrification as an to mitigate nitrification have been as an effective practice to reduce nitrous oxide emissions systems are as a good to increase forage production and reduce nitrous oxide emissions systems are by for the of pastures and can and and nitrogen excreta and A of the reduce the effective nitrogen application which could into a reduction in nitrous oxide emissions et al., 2010). the during wet will reduce and soil In excreta deposition at these times will reduce nitrous oxide emissions and nitrogen leaching et al., 2010). is a direct between greenhouse gas emission and animal efficiency. The the animal the the environmental will be a per unit of management quality and of full genetic potential are to increase production efficiency. for can lead to a reduction of the needed to reach the of production. This is a greenhouse gas mitigation strategy A animal will dietary nitrogen protein and there will less nitrogen in and (Gerber et al., 2013). improvement of and feed conversion that has been in over the last years has the emissions per unit of and 2016). strategies that to change animal to or efficiency may animal health and these effects are and (Llonch et al., 2017). of a selected for increased production will be to this potential on a high system in which resources are In other and can lead to substantial greenhouse gas need to production systems and that may be by limited resources and other that are required in the to meet production and are required to the which in turn increases greenhouse gas emissions. in cattle could lead to a reduction in methane emissions by and nitrous oxide by increasing may increase the metabolic associated with and that could affect animal health and increase the risk of metabolic reduce function and in turn reduce (Llonch et al., 2017). livestock health and are associated with and metabolic which can effect greenhouse gas emissions in an will need energy for A in the United investigated to reduce greenhouse gas emissions by improving cattle that cattle diseases can increase greenhouse gas emissions to per unit of milk produced and to per unit of beef et al., A that reduces feed intake or the ability to digest leads to a in growth which will result in and energy needed to reach the end in and livestock in to global warming through emissions of methane and nitrous oxide. To meet needs of an animal will need to increase and greenhouse gas emission intensity per unit of will need to of the to this environmental standard is to effective mitigation strategies. To increase the effectiveness of these strategies, complex interactions among the components of livestock production systems must be taken into account to avoid environmental trade-offs. there is not a standard to Mitigation practices not be as a component of the livestock production system. The of these strategies to increase of per which in most cases be good of animal health and animal has a mitigating effect in both developed and developing however, the size of the effect will also on such as the genetic potential of the animal and adoption of management is a in the of and at research is on the of greenhouse gases arising from a typical system of the is applying to a in environmental livestock and is a in and systems at has a environmental and has research in the of agricultural and systems. is research on developing to with for greenhouse gas removal from the and for greenhouse gas for agricultural systems and food systems. research to the characteristics of the systems by the and their and is a in Livestock in the of and at of research on the between and the has the effects of on livestock and and the of to global has in the application of systems to livestock production. was in developing the for adaptation to change related to agriculture and food production. has many years in is a leading in the application of systems to agricultural and food production. has the production of all major and livestock species in the United and (e.g., beef in has applied to the greenhouse gas of cattle health as as in and poultry is responsible for developing the beef sector in the agricultural greenhouse gas in the United