3.1 Enteric Fermentation
3.2 Enteric Fermentation Case Study: Strategic Supplementation
3.3 Livestock Manure
3.4 Manure Facility Case Study: Covered Lagoon
Many viable opportunities exist for reducing methane emissions from enteric fermentation in ruminant animals and from livestock manure management facilities. To be considered viable, these emissions reduction strategies must be consistent with the continued economic viability of the producer, and must accommodate cultural factors that affect livestock ownership and management.
The primary method for reducing methane emissions from enteric fermentation is to improve production efficiency, which reduces methane emissions per unit of product (e.g., methane emissions per kilogram (kg) of milk produced). As part of the improvement in production efficiency, a greater portion of the energy in the animals feed is directed toward the creation of useful product (milk, meat, draft power), so that methane emissions per unit product are reduced. This increase in production efficiency also leads to a reduction in the size of the herd required to produce a given level of product. Because many countries are striving to increase production from ruminant animals (primarily milk and meat), improvements in production efficiency will help these goals be realized while simultaneously avoiding increases in methane emissions.
Because the conditions under which animals are managed vary greatly by country (and by regions within countries), especially between developed and developing countries, methane emissions reduction strategies must be tailored to country-specific circumstances. Nevertheless, the following general framework (shown graphically in Exhibit 29) can be established to assess the attractiveness of emissions reduction projects, particularly in developing countries.
· Development Priority: Improving the production efficiency of the ruminant livestock sector must be identified as a development priority. In many developing countries, the livestock sector represents a significant portion of overall agriculture production. The economic development benefits derived from improving production efficiency in this sector can be significant. These development benefits must be the principal driving force behind the investments undertaken to reduce methane emissions.As shown in Exhibit 29, these attributes are strongly inter-related, with economic development priorities being the central theme to the entire effort. The most attractive emissions mitigation projects will balance the needs in all of these areas, so that no one factor creates a constraint on continued improvement in production efficiency, and the resulting methane emissions reductions.
· Product Demand: Increases in product demand generally lead to increased production, and hence increased herd sizes and increased methane emissions. In some areas, there is unmet demand due to inadequate production and/or inadequate production distribution. Under situations of increasing or unmet demand, improvements in production efficiency (which will reduce methane emissions) can be particularly attractive investments.
· Infrastructure: In order for producers to benefit from increases in production efficiency, sufficient infrastructure must be available to move the individual producers product to market. Through transportation, processing, and retailing facilities, the infrastructure must provide a reliable and cost effective link between producers and consumers. In addition to physical infrastructure, economic infrastructure in the form of reliable and fair pricing and contracting mechanisms must be in place: i.e., producers must have a ready and reliable method for getting a fair price for their product.
· Livestock Resources: The livestock resources must be capable of improved production efficiency. In some cases, improved genetic potential for production is needed to improve the local livestock resource base (see below). Most importantly, the livestock must be well suited to their production environment, which often requires a significant contribution of genetics from the indigenous breeds.
· Local Resources: Local resources must be available for improving production efficiency, including feed resources, credit for establishing advantageous cash flow conditions, and expertise. Efforts that require significant imports of costly materials will be unlikely to be sustainable economically under most conditions.
Within this framework, methane emissions mitigation options for enteric fermentation can encompass a wide range of activities across these areas. However, underlying these activities must be specific options for improving the production efficiency of the livestock. Without these options, methane emissions cannot be reduced. In many cases, increased attention to basic livestock management principles offers the best opportunity for improving production efficiency. Examples of these steps would include providing basic veterinary care for disease prevention and treatment, undertaking adequate feed preparation and preservation, and matching livestock production to underlying grazing resources. Within the context of these fundamental livestock management principles, specific techniques for improving production efficiency and reducing methane emissions include the following (USEPA, 1993):
Improved Nutrition Through Mechanical & Chemical Feed Processing: Improved nutrition reduces methane emissions per unit product by enhancing animal performance, including weight gain, milk production, work production, and reproductive performance. Mechanical and chemical feed processing options include wrapping and preserving rice straw to enhance digestibility, chopping straw to enhance animal intake, and alkali treatment of low digestible straws to enhance digestibility. These options are applicable to accessible ruminant animals with limited or poor quality feed, and may decrease methane emissions per unit product on the order of 10 to 25 percent (assuming feed digestibility is increased by 5 percent), depending on animal management practices.The fact that various techniques can be applied to reduce methane emissions per unit product is demonstrated by the diversity in emissions per unit product shown for each of the production systems displayed in Exhibits 17 and 18. Within each of the production systems there is a wide range of emissions rates, which indicates the range of production efficiency that can be achieved. It should be recognized that the lower levels of emissions per unit product shown in the exhibits were achieved, not as part of deliberate emissions reduction strategies, but as the result of efforts to improve production efficiency and economic viability. As discussed above, improved economic viability and economic development should remain the central motivating factors for investments in emissions reduction techniques, particularly in developing countries.
Improved Nutrition Through Strategic Supplementation: Strategic supplementation provides critical nutrients such as nitrogen and important minerals to animals on low quality feeds. Additionally, it may include providing microbial and/or bypass protein to the animal. Methane emissions per unit product may be reduced by 25 to 75 percent due to substantial increases in animal production efficiency, depending on animal management practices. In particular, applying molasses/urea multinutrient blocks (MNBs) and bypass protein techniques in tropical areas with chronic feed constraints can produce emissions reductions per unit product near the high end of the range. The use of chemicals (ionophores) and defaunation are also possible options, though further efforts to develop better agents and to demonstrate practical methods of defaunation are necessary.
Production Enhancing Agents: Certain agents can act directly to improve productivity. These agents are generally most applicable to large-scale commercial systems with well-developed markets. Emissions reductions per unit product of 5 to 15 percent have been demonstrated. Additional reductions may be achieved by shifts in rumen microbial patterns. Options include the use of bovine somatotropin (bST) and anabolic steroids.
Improved Production Through Improved Genetic Characteristics: Genetic characteristics are limiting factors mainly in intensive production systems. Continued improvements in genetic potential will increase productivity, and thereby reduce methane emissions per unit product. Emissions reductions from these options remain to be quantified.
Improved Production Efficiency Through Improved Reproduction: Large portions of the herd of large ruminants are maintained for the purpose of producing offspring. Methane emissions per unit product can be significantly reduced if reproductive efficiency is increased and fewer animals are required to provide the desired number of offspring. Options such as artificial insemination, twinning, and embryo transplants address reproduction directly. The nutritional options described above can also improve reproduction.
Improved Grassland and Rangeland Management: In some regions production efficiency is hampered by inadequate management of grazing lands. In particular, over-grazing has led to reductions in grazing land productivity which in turn reduces livestock productivity. Approaches for better matching livestock grazing loads to seasonal grazing resources will help improve production efficiency and reduce methane emissions.
8 This case study is derived from material presented in EPA (1993) and Bowman et al. (1992).The efficiency of digestion in the rumen requires a diet that contains essential nutrients for the fermentative microorganisms. When the available feed lacks these nutrients digestion will be less efficient, lowering productivity and raising methane emissions per unit product. Strategic supplementation of missing nutrients can greatly improve the efficiency of digestion without requiring a change in the basic diet. The use of molasses/urea multi-nutrient blocks (MNBs) is a proven and cost effective diet supplementation strategy.
For grazing animals and those fed low quality diets, the primary limitation on efficient digestion is the concentration of ammonia in the rumen. It has been accepted that the optimum level of ammonia in the rumen is 50-60 mg/l. However, more recent studies have shown that digestibility is maximized above 80 mg/l, and feed intake increases at levels up to 200 mg/l (Perdok et al., 1988). Supplying ammonia can therefore greatly increase digestive efficiency and utilization of available energy.
Ammonia can be supplied by urea, chicken manure, or soluble protein that degrades in the rumen. Urea is broken down in the rumen to form ammonia, and adding urea to the diet has been the most effective method of boosting rumen ammonia levels demonstrated to date. Chicken manure, which has a high uric acid content, has been used in some regions, where available. While protein in the feed can provide rumen ammonia, sources of protein are often scarce, and where possible should be processed and used as a bypass protein in conjunctions with the MNBs.
In addition to ammonia, there are numerous nutrients that must be present in the diet to support the microbe population in the rumen. The most common nutrients required are sulphur and phosphorus, although this will vary greatly by region.
Urea and other supplemental nutrients are mixed with molasses to make it palatable to livestock. In addition, molasses provides the energy needed in order to realize the improved microbial growth that can result from enhanced ammonia levels. Demonstration projects have added the molasses/urea mixture directly into the feed or have supplied the supplement as a pre-mixed lick block. The molasses/urea multinutrient block is easy to use and can be produced in large quantities and transported easily.
The exact composition of the blocks will depend on local needs and available materials. Important factors are: 1) the level of urea, which must be effective but not toxic; 2) the quantity of molasses, which must cover the bitter taste of the urea and provide adequate energy; and 3) the hardness of the block, which must deter chewing, and yet be soft enough to allow easy intake. In addition to molasses, urea, and supplemental nutrients, blocks contain a binding agent to ensure correct consistency, and often contain a locally available source of soluble protein such as wheat or rice bran. A typical MNB composition is shown in Exhibit 30.
The application of MNBs has been extremely successful in improving productivity, such as milk yields, growth rates, and reproduction (International Atomic Energy Agency, International Symposium on Nuclear and Related Techniques in Animal Production and Health, 1991). MNBs have been used as a supplement in many countries including India, Pakistan, Indonesia, and Bangladesh (Habib et al. (1991); Hendratno et al. (1991); Leng (1991); and Saadullah (1991)). Typical results have been: milk yield increases of 20-30 percent; growth rate increases of 80-200 percent; and increased reproductive efficiency. Based on these results, methane emissions per unit product are expected to be reduced by up to 40 percent.
To be cost-effective, the use of MNBs should be implemented in situations where producers can realize the value of the increased productivity caused by the blocks. Therefore, the blocks are most applicable to situations where adequate cash flow is generated through the sale of animal products such as meat, milk, wool, calves, or draft power. Even in these circumstances, however, acceptance of new animal management techniques will always face institutional and cultural barriers. These can be overcome through successful demonstration projects and grassroots level education. Where investment in new techniques or more expensive feed may not pay off for several years, or are subject to risk from animal losses, favorable finance and insurance schemes can overcome initial resistance. Also, local ingredients must be available to manufacture a suitable block.
The analysis was performed using LAM_100.WB1 and assumptions on page 69 of Bowman et al (1992). The results are saved in two spreadsheets: CASE01.WB1 and CASE01a.WB1. A key assumption affecting the results is that the level of draft power provided must not decline. This was modeled by making the draft power constraint binding in the MNB case (so that the MNB population characteristics are given to the modeled draft population). Using data in Bowman et al. (1992), it is estimated that strategic supplementation of dairy animals with MNBs will reduce methane emissions by 25 percent, while allowing milk production to increase by 35 percent.9 Overall, methane emissions per unit of milk production is reduced over 40 percent. The MNBs are estimated to: reduce age at first calving (48 months to 44 months); reduce the inter-calving interval (21 months to 16 months); and in increase milk production per lactation by about 80 percent. Total herd size is reduced by about 25 percent, while maintaining the same number of mature males for draft power. If the number of draft males are permitted to decline, the herd size and emissions would be reduced further.
9 This analysis was performed using the Livestock Analysis Model, described in USEPA (1995).The principal factors leading to the reduced emissions per unit of output are the improvements in production efficiency. The improved overall herd structure, including the faster rate of maturity of the animals, is an important driving influence. Feed intake is adjusted upward to reflect the more rapid growth and increased milk production per lactation. In fact, emissions per mature diary animal are estimated to increase by nearly 10 percent per year. The overall emissions are reduced, however, because production does increase by as much as the emission per unit of output is reduced.
While strategic supplementation is generally applicable to many conditions in which ruminant livestock are consuming poor quality feeds, the specifics of such initiatives will vary depending on the availability of local supplementation resources. Assessments of the type presented in Bowman et al. are required to design the preferred strategy for country-specific conditions.
Methane emissions from anaerobic digestion can be recovered and used as energy by adapting manure management and treatment practices to facilitate methane collection. This methane can be used directly for on-farm energy, or to generate electricity for on-farm use or for sale. The other products of anaerobic digestion, contained in the slurry effluent, can be utilized in a number of ways, depending on local needs and resources. Successful applications include use as animal feed and aquaculture supplements, in fish farming, and as a crop fertilizer.
Additionally, managed anaerobic decomposition is a very effective method of reducing the environmental and human health problems associated with manure management. The controlled bacterial decomposition of the volatile solids in manure reduces the potential for contamination from runoff, significantly reduces pathogen levels, removes most noxious odors, and retains the organic nitrogen content of the manure.
The selection of successful methane emissions reduction options depends on several factors, including climate; economic, technical and material resources; existing manure management practices; regulatory requirements; and the specific benefits of developing an energy resource (biogas) and a source of high quality fertilizer. Because most of the manure facility methane emissions occur at large confined animal operations (primarily dairies and hog farms), the most promising options for reducing these emissions involve recovering the methane at these facilities and using it for energy. The primary approaches for recovering this methane include the following:
Covered Lagoons: The treatment of manure in lagoons is associated with relatively large scale intensive farm operations. Manure solids are washed out of the livestock housing facilities with large quantities of water, and the resulting slurry flows into an anaerobic primary lagoon. The average retention time for the manure in the lagoon is about 60 days. The anaerobic conditions result in significant methane emissions, particularly in warm climates. Placing an impermeable floating cover over the lagoon and applying negative pressure effectively recovers the methane which can be used as energy. Lagoons are most commonly used at large confined dairy and swine facilities in North America, Europe, and regions of Asia and Australia.Large dairies and hog farms with significant energy requirements will typically find these systems to be cost effective. The amount and price of the energy displaced by the recovered methane are the main determinants of the profitability of these systems.
Large Scale Digesters: Large scale digesters are engineered vessels into which a mixture of manure and water is placed. To keep the cost of the vessels low, the vessels are engineered to provide an average retention time for the manure of about 20 days. To facilitate as much gas production as possible during this relatively short retention time, the digester is heated to about 60° C. The gas is drawn off and used for energy. The two main digester designs are Complete Mix and Plug Flow.1010 A Complete Mix digester is a large temperature-controlled insulated tank with a mechanical mixing mechanism. A Plug Flow digester is typically a long temperature-controlled insulated tank, with no mixing mechanism. Manure is added regularly to each digester (e.g., daily). In the Plug Flow design, the manure takes about 20 days to flow the length of the digester, from the inlet to the outlet.
Small scale livestock producers can also use digesters, albeit on a smaller and less complex scale. Small scale digesters typically require a small amount of manure to operate and are relatively simple to build and operate. As such, they are an appropriate strategy for producers with technical, capital, and material resource constraints. Due to the rising cost of commercial fertilizers, the recovery of high quality fertilizer from digesters can be an even more important benefit than the energy supplied from biogas. A number of different digester designs have been developed. These small scale designs typically do not include heating the digester, and while they can operate in colder regions, they are more appropriate for small scale operations in temperate and tropical regions.
11 This case study is drawn from USEPA (1995).Covered lagoons provide a method for integrating methane recovery into every-day manure management and treatment practices. In temperate and tropical climates, anaerobic lagoons provide excellent manure treatment. Manure is flushed into the lagoon from where it is initially deposited. With an average retention time of at least 60 days, the bacteria in the lagoon stabilize the waste and produce biogas, which is about 50 percent methane and 50 percent carbon dioxide. By covering a portion or all of the lagoon with an impermeable membrane, the biogas is recovered and used for energy.
In California (USA) a 1,400 sow farrow-to-finish hog farms has successfully demonstrated this technology for over 10 years. Royal Farms has a 42,600 square foot (approximately 4,000 square meters) primary lagoon that is covered with an industrial fabric (see Exhibit 31). A small gas pump draws the biogas trapped under the cover through a series of perforated pipes. Gas recovery currently ranges from about 50,000 cubic feet per day (1,400 cubic meters per day) in winter to about 70,000 ft3/d (2,000 m3/d) in summer.
The gas is used to operate two engine-generators with a combined capacity of 175 kW. The entire system cost about $220,000 to install in the early 1980s, including the costs for the engines. Annual energy savings, including electricity and heating, are about $84,000. Annual operating costs are about $8,000, including engine maintenance. Overall, the system paid for itself in less than four years.