MITIGATION STRATEGIES TO TACKLE AIR POLLUTION FROM LIVESTOCK FARMING
Air pollution is a major problem for the health of people on the earth and for the environment, and livestock production contributes significantly to the emission of several air pollutants. Particularly the emission of ammonia (NH3) is related to agricultural production. According to the European Environment Agency (EEA, 2015) agriculture is responsible for 93 % of ammonia emission in EU-28 and four of the five most important key emission categories are related to animal production. Similarly, livestock production leads to emission of greenhouse gasses (GHGs), which contribute to global warming. Agriculture accounts for approximately 10 % of Europe’s total GHG emissions when excluding emissions coming from Land Use, Land Use Change and Forestry (LULUCF). Out of these 10 % of the total emissions of CO2-equivalents (CO2-eq), enteric fermentation accounts for 42 % and manure management for 15 % (Fernandez et al. 2015). The main livestock-related GHGs are methane (CH4) from enteric fermentation and manure and dinitrogen oxide (N2O) from manure. As we know the ammonia, methane and N2O are air pollutants, which have effect on climate, ecosystems and human health. Livestock production is also related to other environmental impacts like nutrient leaching to watercourses or nitrate leaching to ground water. On the other hand cattle also has positive impacts on society and environment. For example they are able to transform types of biomass, which are inedible to humans, into nutritionally and economically valuable beef and dairy products. Another example is the preservation of certain ecosystems, where grazing is necessary to maintain the present biological balance. Within Europe there are large differences in agricultural emissions, which is a reflection of different livestock production patterns and varying roles of livestock production in society as a whole . Emission quantities, emission patterns and potential mitigation measures vary, depending on whether cattle farms are dairy or beef farms, whether they are large, medium or small scale and whether production is intensive or extensive.
Cost-efficient measures for reducing emissions ———
Therefore, a major challenge for the agricultural sector is to increase the production and at the same time decrease the environmental impact. This calls for cost-efficient methods of reducing net emissions. Fortunately, there are already techniques, practices and strategies in the pipeline or already being applied around the world that may also be of benefit to the farmers. Many of these approaches actually stem from the wish to save money because saving resources and minimizing waste are an advantage for the farmers. This also goes for emissions from cattle. Basically, all emissions of enteric methane from cattle are a loss of potential energy for the cow. The feed is literally going up in the air instead of sustaining milk or beef production. Reducing the amount of methane emitted therefore makes sense from a farming perspective. Similarly, all emissions of ammonia or N2O are a loss of nitrogen, which could otherwise fertilize plants in the field or sustain animal production, as illustrated in figure 5. It therefore also makes sense to safeguard nitrogen – both from an environmental and from an economic perspective. For a mitigation measure to be cost-effective, there must however be a balance between the effect on emissions, the effect on production, the investments that will have to be made and possibly also side effects. Emission efficiency (emission per kg of milk or beef) is an obvious way to compare different production systems but does not tell the whole story on cost-effectiveness.
Measures at animal level ———–
The rumen is a complex microbial ecosystem within the even more complex organism of the whole ruminant. By affecting the rumen or the rest of the digestive system, it is possible to reduce GHG emissions or lower the nitrogen concentration in the manure. A simple measure is to alter the feed composition. By switching to roughage with a higher digestibility or increased amount of concentrates, it is possible to reduce emissions of enteric methane from cattle, if not overall then relatively to the production of milk or meat. The same effect can be achieved by feed processing, which increases the digestibility of roughage or concentrate. Reducing the dietary content of protein for cows can also decrease ammonia, methane and nitrous oxide emissions. The tricky part is not to impair the animal production or to shift emissions to later stages in the production chain, e.g. during manure application. Adding different additives to the feed can also change emissions of enteric GHGs. The idea is that the additives change the microbiology or biochemistry in the rumen so that less methane is produced. Examples of additives are nitrates, ionophores, fats, plant- or fungal-derived bioactive compounds, e.g. tannins, and direct-fed microbials like yeast. Attempts have also been made to directly affect the composition of the microbial community in the rumen and thereby reduce GHG emissions. This can be done by vaccinating the animals to suppress the methaneproducing archaea, by inhibiting protozoa (defaunation) or by affecting the initial microbial colonization of the digestive system of the ruminants shortly after birth. Without understanding the underlying mechanisms completely, it is a fact, that emissions vary a lot between individual animals, even when they are in the same herd, of the same breed and consuming the same feed. An interesting approach is therefore to select animals for breeding on the basis of low emissions. Breeding for higher production is well known and will often result in a lower emission per kg of product. If this can be coupled with measurements of emissions, breeding can be even more focused on improving emission efficiency.
Measures in stable and manure storage —————
Moving further on in the production chain, manure management is very important in order to control emissions from livestock. This goes for all livestock production systems, and some lessons and advice can therefore be gathered particularly from the pig and poultry production. There are, however, substantial differences between the different animal production systems, both regarding the properties of manure and factors like housing systems. An important factor is the choice of animal housing system with the most common systems being deep litter and slurry-based systems. The risk of N2O production is higher in solid manure/deep litter because of the alteration between aerobic and anaerobic conditions. Generally, there is an inverse relationship between emission of N2O and ammonia from manure – aeration reduces N2O formation, but increases ammonia evaporation. In slurry-based stables, an effective measure against ammonia emissions is to minimize the surface area of manure from which ammonia can evaporate. This can be done by building stables that quickly lead the manure into storage. A proven technique is grooved floors combined with regular scraping of manure. Another simple way of reducing the surface area is shorter storage time of slurry in the stable. If the canals underneath the stable are emptied regularly and the slurry is transported to a closed storage facility, the emissions will be reduced. Cooling liquid manure has an effect on both ammonia and methane emissions. With lower temperatures the evaporation of ammonia is reduced and the microbial production of methane is slowed down. Natural cooling outside the stable can be used if the climate is cool. Technical solutions are based on active cooling of the manure canals. If the heat extracted from the manure can replace other heat sources, slurry cooling can even be profitable for the farmer. This is the case for new stables for piglet production in a temperate climate like Denmark. The heat from the sow manure is used to warm the stable for the piglets. Another option is acidification of manure, which can be done already in the slurry canals underneath the stable floor in new stables or in the slurry tank. Acidification lowers the pH of the slurry, which prevents ammonia evaporation and inhibits the methane producing microorganisms. During the storage a good cover of the manure can reduce ammonia emissions substantially. There are various forms of manure covers ranging from tent structures on slurry tanks to natural covers (crusts) formed by fibers in the manure, possibly supplemented with added fibers. In the technical end of the range of mitigation options chemical or biological cleaning of the air from the stable can be applied, if the animals are housed in closed systems. This is well known from production of pigs and poultry but cattle are often housed with natural ventilation, which prevents this approach to emission mitigation.
Post stable and manure storage ——–
The use and treatment of manure after it has left the stable and manure storage facility also has a large effect on emission of particularly ammonia and N2O. This part of the production chain is closely related to both feeding initiatives and choices regarding housing and is therefore a part of the whole picture. The scope of this focus group is however on the animal and housing related processes. Measures directed at manure handling will therefore not be discussed directly, but are a part of the perspectives of all reduction measures. Existing measures to reduce emission of ammonia include technical solutions that incorporate liquid manure into the soil or deposits it directly on the surface with e.g. trailing hoses. This reduces emissions compared to broadcasting of manure. For solid manure ammonia emissions are reduced if the manure is incorporated into the soil immediately after spreading. As mentioned above acidification can be used to reduce emissions from the stable or manure storage. It can however also be done later on in the production chain, i.e. directly before spreading of the manure. The earlier in the process it is applied, the larger is the overall effect on ammonia and methane emissions. Acidification applied directly before spreading has no effect on methane emissions. On the other hand, acidification of manure in the stable or storage is not compatible with another mitigation option: anaerobic digestion of manure for biogas production, because it inhibits biogas formation. If the manure is only stored for a short- term at the farm and instead transferred to anaerobic digestion facilities, the production of methane can be collected and exploited instead of polluting the environment.
Farm systems approaches ————
From a management point of view there are also various tools, which can help reduce emissions from cattle. These are for instance management tools that take several of the parameters mentioned above into account and estimate overall emissions. Depending on the complexity of the models behind the tools interactions between different mitigation measures can also be included in the estimate. Assuming that the aim is to improve emission efficiency (reduce emissions per kg of product), rather than reducing emissions from individual cows or calves, a straight forward approach is to improve production. Producing a lot more milk or meat with the same or only slightly higher emissions, means that the demand for livestock products can be fulfilled with fewer total emissions. Focusing on increased production has the benefit that it corresponds well with most farmers’ general aims. There is however a large range of factors, which can affect production and the importance of factors for emission efficiency, differs from farm to farm. Examples of factors with relevance for emission efficiency are milk production, feed utilization, nitrogen utilization and number of heifers per cow. A possible tool for farmers is to benchmark these factors against standards or against other farmers. If a farmer can see that his or her cows emit more enteric methane per kg milk than the average it will be natural to focus attention on e.g. feed composition and utilization. On an animal level the replacement rate of milking cows is very important for the overall emission efficiency at the farm. Even though emissions from a heifer are lower than from a high-yielding dairy cow, the heifers are not producing any milk and therefore reduce overall emission efficiency. The use of grazing as a tool to reduce emissions is also a management approach. Grazing is mostly applied for other management reasons but the effects on emissions are also worth quantifying and the correlation with period of grazing or grazing pressure is not straight forward. Nitrogen budgets are another way of visualizing where an effort to reduce emissions will have the greatest impact. By setting up a roadmap for the recycling of nitrogen at the farm any losses of nitrogen can be spotted and mitigated. Potentially this can improve plant production in the field and thereby benefit the farmer. Such budgets are normally based on models where average factors are used to estimate nutrient flows for the individual farm. In theory this can be supplemented with actual measurements of for instance nitrogen content in the manure or ammonia emission from the stable. Cattle is however often housed in naturally ventilated stables or grazing parts of the year, which makes actual measurements impractical as part of a daily management tool.
Compiled & Shared by- Team, LITD (Livestock Institute of Training & Development)
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