Sustainability of the Dairy Industry: Emissions and Mitigation Opportunities.

Dairy cattle provide a major benefit to the world through utilizing mostly human inedible feedstuffs into milk and associated dairy products. However, as beneficial as this process has become in modern dairies, it does not occur without potential negatives. Dairy cattle are a source of greenhouse gases (GHG) through enteric and waste fermentation as well as excreting nitrogen (N) emissions through their faeces and urine. These negative impacts vary widely due to system and how and what dairy animals are fed, and which then also provide directives for managerial interventions. The study of the authors cited provide an overview of the negatives which also thereby implicate possible mitigation strategies.

Urine, manure and other waste derived: Urine may lose more N per unit to the environment than faeces. In combination, dairy waste is a significant source of N and phosphorous (P) when applied in excess of crop or pasture requirements as they can cause contamination of surface water. Excess N and P in water causes a rapid bloom in the growth of algal populations that consume dissolved oxygen in water, termed eutrophication, which reduces the available dissolved oxygen required for growth of aquatic animal life. Excess N can also contaminate ground water through leaching. This poses a problem for human and animal health as consumed nitrate from drinking water is converted to nitrite in the digestive tract, which replaces oxygen in haemoglobin and leads to oxygen starvation.  

Air quality can also be affected by dairy waste. One such compound that affects air quality produced by dairy cattle is ammonia (NH₃). Ammonia is produced when N in urea from the animal’s urine reacts with urease present in faeces. Ammonia production from dairy waste is dependent on a variety of factors including: urea content in urine, pH, and temperature, as well as the enzymatic activity of urease. In addition to NH₃losses from fresh waste, volatilization can occur during waste application to soil as a fertilizer, as well as during the long term housing and storage of manure.

Nitrogen in waste can also contribute to GHG production through the formation and volatilization of nitrous oxide (N₂O). Nitrous oxide is created during incomplete microbial denitrification process where nitrate is converted to N gas with the potential to create N₂O, an extremely volatile by-product. Land applied dairy manure on cropland or pastures as well as the long term storage of manure in lagoons can contribute to emissions of N₂O. The N₂O emissions during storage depend on the N and carbon (C) content of the manure. Nitrous oxide production and subsequent volatilization is also dependent on environment and management. Higher temperatures as well as surface coverings contribute to increasing emissions, whereas anaerobic conditions, such as those found in lagoon systems, have lower N₂O emissions. The process of long term storage of manure also contributes to a larger proportion of N₂O emissions compared with land application with aerated, straw covered, digested, separated, and untreated manure which contribute less N₂O emissions.

Another substantial GHG produced by dairy cattle waste is methane (CH₄). The amount of CH₄emitted by dairy waste depends on the amount of carbon, hydrogen, and oxygen present in the waste, making manure storage, diet, and bedding major contributors to total CH₄production. Manure CH₄emissions are substantially higher from long term storage, compared with field application. These emissions are highest from straw covered manure and emissions decrease with untreated manure, followed by separation, aeration, and digested manure management methods.

Dairy waste can also produce volatile organic compounds (VOC). Volatile organic compounds are a class of chemicals that when reacted with oxides of N and sunlight contribute to ozone formation. There are a substantial number of VOCs from slurry wastewater lagoons with the most common VOCs being methanol, acetone, propanal, and dimethylsulphide. As with other waste emissions, VOCs from dairy waste increase with ambient air temperature, with summer months having the highest rates of VOC emissions. The largest contribution of VOCs on dairy systems come from fermented feedstuffs (i.e., silage).

Nutrition derived: The most significant enteric (rumen) emission compound from dairy cattle is CH₄. Methane acts as a hydrogen sink in the rumen and is an end product of CO₂reduction by methanogenic bacteria. Methanogens serve an important role in rumen health by removing this hydrogen that can be toxic to some bacterial communities and also causes rumen acidosis, and therefore plays an important role in the rumen.

Dairy cattle diets have a significant impact on enteric CH₄. As there is large variability in the ingredient and chemical composition of diets fed, nutrition and feeding strategies have the greatest potential for reducing CH₄emissions, with potential reported reductions between 2.5 and 15%. The amount of CH₄ produced is dependent on many factors including intake and chemical composition of the carbohydrate, retention time of feed in the rumen, rate of fermentation of different feedstuffs, as well as the rate of CH₄ development. Altering feed digestibility and chemical composition cause a shift in the proportions of volatile fatty acids (VFA) with the predominant VFAs being propionate, butyrate, and acetate. This shift in VFA proportion is important because propionate also acts as a hydrogen sink, so shifting from acetate and butyrate formation to propionate will consume reducing equivalents and help preserve the pH balance in the rumen. An overall reduction in CH₄emissions or a shift in VFAs can be accomplished by feeding more energy dense or more digestible feedstuffs which will result in less CH₄from fermentation. An increase in starch proportion of the diet, such as through an increase in concentrate levels, also results in a more rapid fermentation of these feedstuffs and therefore decreased CH₄production.

However, feeding higher starch diets requires maybe increased grain production, which can cause additional consumption of fossil fuel and fertilizers that will result in an increase in N₂O and CO₂; although results show that this is usually offset by the substantial decrease in overall CH₄emissions. Feeding of cereal forages can also favour propionate production and reduce CH₄emissions due to the higher starch concentration. Higher concentrations of legumes, such as lucerne, when compared with grass forage based diets can also lead to an overall decrease in CH₄emissions. Further, age of harvest of forage has a significant impact on emissions, with advancing maturity resulting in more lignified and less fermentable substrate contributing to increasing emissions associated with higher ruminal acetate. In addition to alterations in forage or concentrate composition and ratio, supplementation of lipids to dairy cattle diets can also mitigate enteric emissions. Replacing concentrates with lipids results in a decrease in fermentable substrate by the microbes in the rumen and can also decrease total protozoa and methanogen populations. Inclusion of high-oil by-products, such as distillers grains or oilseed meals, can result in decreased CH₄ emissions. With ensiled feeds, it is anticipated that maize silage will mitigate emissions due to its higher starch content. This has been confirmed when directly comparing grass-versus maize silage.

Manure emissions are also significantly impacted by various dairy cattle feeding strategies. With higher concentrate diets, the fermentable substrate in the manure can increase. To alleviate this, feeding concentrate with higher lignified fibre has been shown to mitigate both enteric and manure-derived emissions. The greatest impact of diet on waste emissions is with feeding low crude protein (CP) diets, which results in decreased excreted N and therefore NH₃volatilization. Comparing fresh grass with prepared hay at the same CP content, feeding hay causes a higher overall N and C/N ratio excreted but waste from grass fed animals tends to volatilize more NH₃emissions. When comparing maize silage with grass silage, maize silage reduces urinary N excretion. When adding maize silage to lucerne silage based diets, there is an improvement in N efficiency leading to a decrease in N losses in urine and subsequent decreases in available NH₃ and N₂O volatilization. Similarly, higher sugar forages also reduce N excretions, which also have the potential to limit the N available to be volatilized as gaseous emissions.

Feed additives: In addition to changes to the diet ingredient composition, there are also additives to diets that may mitigate enteric emissions. Promising results have been obtained by the methanogenic inhibitor,3-Nitrooxypropanol (3-NOP) and others such as nitrate, condensed tannins and certain essential oils from plants. However, most results are still experimental with a final product tested for safety not yet on the market or not yet registered in particular countries. Thus, they are not discussed here.