Introduction
The dairy industry continues to provide a nutritious food source to our growing global population. Throughout history this industry has undergone significant evolutions contributing to a “more sustainable” product. But what does sustainability really mean, and in animal agriculture, why do we care? According to the U.S. National Environmental Policy Act (NEPA) of 1969 sustainability is to “create and maintain conditions, under which humans and nature can exist in productive harmony, that permit fulfilling the social, economic, and other requirements of present and future generations” (EPA, 2015). While most consumer and media focus is on environmental sustainability, these three considerations: society, environment, and economy make up the three pillars of sustainability and must all be considered in creating a more sustainable dairy system. Breaking down each of these pillars further, the EPA lists six broad topics related to the environment including Ecosystem Services, Green Engineering and Chemistry, Air Quality, Water Quality, Stressors, and Resource Integrity (EPA, 2015). For society these topics include: Environmental Justice, Human Health, Participation, Education, Resource Security, and Sustainable Communities. The Economic pillar includes Jobs, Incentives, Supply and Demand, Natural Resource Accounting, Costs, and Prices. As agriculture professionals, our work centers around pieces of all three pillars, but we often don’t reflect on, or even address, the bigger picture of how our work plays into a more sustainable dairy system. It is important to consider agricultures contributions as the sustainability conversation continues to press forward and become more prevalent.
As environmental impacts are almost exclusively the center of attention for the sustainability conversation, it is imperative we equip ourselves with additional information on greenhouse gases (GHG) and their associated CO2 equivalents (CO2e). The primary GHGs are carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O), which have varied warming potentials. These global warming potentials are calculated from radiative forcing, or the ability of these gases to trap heat, as well as their persistence in the atmosphere (Rotz, 2020). Currently the most cited global warming potential is the GWP100 which compares these gases against CO2 on a 100-y time horizon. These GWPs are assigned at 34 and 298 kg of CO2 for CH4 and N2O, respectively. However, GWP100 accounts for CH4 on a 100-y scale when in reality CH4 is converted to CO2 within a 10-year period. This becomes an issue when CH4 emissions are changing, as we have exeprienced with cattle populations decreasing over the last 20 years (Place et al., 2022). A more representative emission factor for CH4 has been developed, termed Global Warming Potential Star (GWP*) by Allen et al. (2018) which considers change in CH4 emission rates over time and is defined as:
CO2we = 4.53 × E100(t) − 4.25 × E100(t-20)
where E100 = the CO2e emissions calculated using GWP100, t = the year for which the CO2we are being calculated, and t-20 = the emissions in CO2e emissions calculated using GWP100 20 yr prior (Smith et al., 2021; Place et al., 2022). While this calculation does not yield one number for CH4 CO2e due to the time factor, it does more accurately represent how CH4 impacts the environment. The most recent IPCC report highlights the issue with using GWP100 as it artificially inflates the effect of constant methane emissions, such as those we see from our constant or even reduced dairy cattle herd, on global surface temperature by a factor of 3–4 (Forster et al., 2021). Those of us who touch agriculture should be equipped with this information to share the facts and highlight the benefits of this new system.
Furthermore, animal agriculture should be concerned with gaseous emissions as dairy cattle have been heavily targeted as a scapegoat, even though livestock contribute only 4% of total US GHG emissions (IPCC, 2014). For Canada, emissions from rumination of livestock accounts for less than half of all agricultural emissions (total 8.1%) and represents 3.3% of total GHG emissions for 2018 (Ominski et al., 2021). Dairy cattle emit CO2 as a byproduct of aerobic cellular respiration, which is the GHG with the greatest contribution to climate change (Place and Mitloehner, 2010). However, this gas is not considered a net contributor to GHGs due to the CO2 having been previously recycled from the atmosphere by fixation during photosynthesis in plants, which are then consumed by cattle (Steinfeld et al., 2006). Dairy cattle can also produce N2O from enteric emissions as a result of the NO3 reduction process that takes place by the microbes in the rumen (Kaspar and Tiedje, 1981). Due to the small production of enteric N2O, these emissions are not always considered in dairy emission analyses. The most significant enteric emission compound from dairy cattle is CH4. Methane acts as a hydrogen sink in the rumen and is a product of CO2 reduction by methanogenic archaea (Janssen and Kirs, 2008). Methanogens serve an important role in rumen health by removing this hydrogen that can be toxic to some bacterial communities and leads to rumen acidosis (Beauchemin et al., 2009). In addition to being a potent GHG, CH4 also accounts for a 2-12% loss of potential energy available to the animal that could otherwise be used for maintenance and productive purposes including growth, gestation, or lactation (Moe and Tyrrell, 1979). It is interesting to note that similar to CO2, CH4 from dairy cattle can be attributed to the recycling of CO2 by the plant, termed the biogenic carbon cycle. This is starkly different from the CH4 produced by most other industries in which CH4 is not recycled but rather only flows unidirectionally.
Enteric GHGs are not the only environmental impacts that can be associated with animal agriculture, dairy manure also has the potential to negatively impact the environment. Nitrogen (N) not retained by the animal or secreted in milk will be excreted in the urine and feces. Dairy manure is a significant source of N and P that when land applied in excess of crop requirements can cause contamination of surface water or ground water (leaching) and rapid blooms in the growth of algal populations that divert dissolved oxygen in water (eutrophication) away from aquatic life (Peterson and Mitloehner, 2021). Ammonia can also be produced from excess N in urea from the animal’s urine reacts with urease present in feces. Nitrogen in manure 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 byproduct (Place and Mitloehner, 2010). Manure can also produce CH4 which is dependent on storage, treatment, management practices, and feeding strategy.
While dairy cattle do impact the environment, the Dairy industry has made substantial improvements throughout its tenure. Milk production has greatly improved primarily due to dramatic increases in milk production per cow, increase in average cow numbers per farm, as well as an overall decrease in total animal numbers (Wolf, 2003). Some other major changes over the last 50 years include a shift to a primarily Holstein dairy herd (90%), an increased heifer growth rate, decreased age at first calving, and an increase in the use of artificial insemination (Capper et al., 2009). Nutrition of dairy animals has also allowed for a substantial improvement in production via use of total mixed-rations balanced for nutrient and energy requirements and accounting for animals age and stage of lactation (NRC, 2001). Genetic selection has also been a major driver in increased productivity, longevity, and efficiency of dairy cows, further reducing the environmental impact per unit of milk production (Pryce and Haile-Mariam, 2020). These improvements in nutrition and genetics, in conjunction with improvements to herd management, accomplished primarily through increasing density on dairy farms, have resulted in a fourfold increase in milk yield from the mid-1940s until 2007 (Von Keyserlingk et al., 2013). This efficiency of milk production has only continued to improve. In California, 1 kg of energy and protein corrected milk (ECM) emitted between 1.12 to 1.16 kg of CO2 equivalents (CO2e) in 2014 compared with 2.11 kg of CO2e in 1964, resulting in a 45% reduction in CO2e (Naranjo et al., 2020). Dairy production systems in the US in 2017 compared with 2007 have reduced their inputs by 25.2% for animal numbers, 17.3% for total feed, 20.8% for land, and 30.5% for water of one million metric ton of energy-corrected milk, furthering the exceptional productivity gains and environmental progress of this industry (Capper and Cady, 2020). Over a 30 yr time period (1981–2011), Canadian beef producers have reduced GHG emissions (kg−1 carcass) by 15%, ammonia emissions by 17%, water use by 20%, while using 24% less land. Canadian milk has also undergone an environmental footprint analysis, showing fat- and protein-corrected milk (FPCM; kg·cow−1·yr−1) production increased by 43% while decreasing enteric methane (kg CO2·kg−1 FPCM) and total emission intensity (kg CO2·kg−1 FPCM) by 22% from 1991–2011 (Ominski et al., 2021).
Diet Manipulation Strategies: Carbon Excretion
Although significant improvements have been made to the dairy industry, we are still striving to improve all three of the pillars of sustainability: the economy, society, and the environment. There are many options to mitigate environmental impact to be discussed here, however it is important to note there is no silver bullet or single strategy for the dairy industry. It will be up to individual dairies and their nutritionists to continue to improve upon our dairy systems. Dairy cattle diets have a significant impact on enteric emissions, mostly CH4. As there is large variability in the ingredient and chemical composition of diets fed to dairy cattle, nutrition and feeding strategies have the greatest potential for reducing CH4 emissions, with potential reported reductions between 2.5 to 15% (Knapp et al., 2014). 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, fate of fermentation of different feedstuffs, as well as the rate of methanogenesis (Beauchemin et al., 2009). 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 (Knapp et al., 2014). 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 (Hungate, 2013). An overall reduction in CH4 emissions or a shift in VFAs can be accomplished through a variety of altered feeding strategies. More energy dense or more digestible feedstuffs result in additional energy available to the animal and generate less CH4 from fermentation (Knapp et al., 2014). 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 CH4 production (Moe and Tyrrell, 1979). Feeding higher starch diets requires increased grain production, which can cause additional consumption of fossil fuel and fertilizers that results in an increase in N2O and CO2; however, this system is usually offset by the substantial decrease in overall in CH4 emissions (Lovett et al., 2006). Feeding of cereal forages can also favor propionate production and reduce CH4 emissions due to the higher starch concentration (Beauchemin et al., 2009). Higher concentrations of legumes, such as alfalfa, when compared with grass forage based diets can also lead to an overall decrease in CH4 emissions (McCaughey et al., 1999). Age of harvest of forage also 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 (Pinares-Patiño et al., 2003). In addition to alterations in forage or concentrate composition and ratio, supplementation of lipids to dairy cattle diets can also mitigate enteric emissions (Hristov et al., 2013b). 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 (Ivan et al., 2004). An inclusion of high-oil by-products, such as distillers grains or oilseed meals, can result in decreased CH4 emissions (Hristov et al., 2013). Research on ensiled feeds in relation to enteric emissions is generally lacking, although it is anticipated that corn silage will mitigate emissions due to its higher starch content (Gerber et al., 2013). Furthermore, when directly comparing grass-versus corn silage, a higher inclusion of corn silage seems to mitigate enteric CH4 emissions (Doreau et al., 2012). There are many potential methods to mitigate enteric emissions through alterations to nutrition strategy and composition.
Carbon from manure emissions are also significantly impacted by various dairy cattle feeding strategies. One of the main issues with altering carbohydrate feeding strategies to reduce enteric emissions is that fermentable substrate in the manure can increase, as has been seen with increasing the concentrate to forage ratio in the diet (Beauchemin et al., 2009). This response has also been seen with the supplementation of certain fatty acids (Kreuzer and Hindrichsen, 2006). To alleviate this issue, feeding concentrate with higher lignified fiber has been shown to mitigate both enteric and manure-derived emissions (Kreuzer and Hindrichsen, 2006).
Diet Manipulation Strategies: Nitrogen Excretion
The greatest impact of diet on manure emissions can be seen when feeding low CP diets to dairy animals, which results in decreased excreted N and subsequent NH3 volatilization (Peterson and Mitloehner 2021). Comparing fresh grass with prepared hay at the same CP content, feeding hay causes a higher overall N and C/N ratio excreted but manure from grass fed animals tends to volatilize more NH3 emissions (Külling et al., 2003). Corn silage inclusion in diets has also caused changes to manure emission profiles. For example when comparing corn silage versus grass silage, corn silage tended to reduce urinary N excretion (Mills et al., 2008). When adding corn silage to alfalfa silage based diets there is also an improvement in N efficiency leading to a decrease in N losses in urine and subsequent decreases in available NH3 and N2O volatilization (Gerber et al., 2013). Higher sugar forages also reduce N excretions, which also have the potential to limit the N available to be volatilized as gaseous emissions (Gerber et al., 2013). Overall a variety of feeding strategies can be employed depending on the dairy management and nutrition strategies available to help mitigate N emissions from enteric and manure sources of dairy animals. It is also important to consider the other two pillars of sustainability, economics and society when evaluating diet manipulation strategies for environmental benefit. Return on investment (ROI) of dairy cattle diets needs to be maximized for the most outputs from minimal inputs.
New Frontiers: Additives
In addition to changes to the diet ingredient composition, there are also feed additives that may mitigate enteric emissions. While there are various types of strategies to alter enteric sourced emissions this section will focus primarily on methods to alter CH4. One promising strategy for CH4 reduction is via feed supplementation of the methanogenic inhibitor, 3-Nitrooxypropanol (3- NOP). 3-Nitrooxypropanol is a structural analog to methyl-coenzyme M, which acts on methylcoenzyme M reductase (MCR), a nickel enzyme involved in the final reduction stages of methanogenesis (Duin et al., 2016). In the rumen system 3-NOP was shown to mimic methylcoenzyme M and target the active site of MCR, thus inhibiting the enzymes activity and subsequently causing a decrease in CH4 production (Duin et al., 2016). Research demonstrated that feeding 3-NOP to cattle decreased enteric CH4 emissions up to 95% in vitro (MartínezFernández et al., 2014) and 84% in vivo (Vyas et al., 2016).
Nitrates offer great promise for their potential to mitigate CH4 and have been well studied for their use in beef cattle diets with more recent literature focusing on the potential for use in dairy cattle. Nitrate in the diet serves as a non-protein N source that acts as an electron receptor resulting in effective and consistent reduction of enteric emissions. However, nitrate has the potential to induce methemoglobinaemia and is a known carcinogen (Lee and Beauchemin, 2014). Nitrate toxicity can generally be avoided when the rumen ecosystem is allowed time to adapt (Hristov et al., 2013b). Even with the potential for toxicity, the benefits of 16-50% reduction in CH4 emissions continue to drive research feeding nitrates (Leng and Preston, 2010).
Plant biological compounds have also been explored for their potential to reduce emissions. Condensed tannins are secondary phenolic compounds that generally discourage consumption by herbivories and also concentrate N in the plant (Waghorn, 2008). When consumed by dairy cattle these tannins bind protein in the rumen, which reduces the degradation of protein and enhances protein flow to the intestines (Beauchemin et al., 2009). Tannin source appeared to make a major difference in subsequent mitigation of CH4 emissions from dairy cattle.
In addition to tannins, secondary plant compounds called essential oils have been explored for their antimicrobial properties. Essential oils are naturally occurring volatile components in plants that provide the plant specific color and flavor characteristics (Benchaar et al., 2008). Certain essential oils reduced CH4 production through inhibiting growth and energy metabolism of selected bacteria and archaea including methanogens (Benchaar et al., 2008). Over 250 essential oils have been identified and contain mixtures of terpenoids, a variety of low molecular weight aliphatic hydrocarbons, alcohols, acids, aldehydes, acrylic esters, N, sulfur, coumarins, and homologues of phenylpropanoids (Beauchemin et al., 2009). These essential oils underwent in vitro screening for their potential to reduce rumen CH4 emissions and while 35 were found to be effective only six were found to have significant decreases in emissions without disrupting digestibility (Bodas et al., 2008). It is difficult to directly compare essential oils because of the number of different compounds as well as the difference in study design and species studied. In addition, few essential oils have been thoroughly evaluated in vivo, but is an active area of research.
Manure additives have also been explored to help decrease environmental impacts. Biochar is a general term applied to products produced by thermal decomposition from a variety of biomass substrates for agricultural applications including the added benefit of optimizing the process of composting (Godlewska et al., 2017). Biochar has been shown to have multiple benefits including improving the overall process of composting, improving N conservation, facilitating nutrient transformation, and favoring oxygen supply (Peterson and Mitloehner, 2021). Bacterial inoculums, as well as the supplementation of bacterial produced enzymes, have been well researched in the literature for their potential to alter CH4 emissions. Bacteria are involved in many of the breakdown processes that occur in manure management systems including reactions of hydrolysis, acidogenesis, acetogenesis, and methanogenesis, the latter of which has the potential to increase methane production (Juodeikiene et al., 2017). While increased CH4 may seem in conflict with sustainability goals, this manure management strategy can be applied to systems where CH4 can be captured and transformed into biofuel or other renewable resources, such as anaerobic digesters. Gypsum based products have been applied to dairy manure systems for manure amendments. One of the more common forms of gypsum used for manure amendment is flue gas desulphurization gypsum that is a by-product of wet gas desulphurization from coal-fired power stations (Febrisiantosa et al., 2018). This gypsum has a low heavy metal content and contains high concentrations of S, Si, and Ca that are essential minerals nutrients required by plants (Guo et al., 2016). Gypsum has been well characterized for its reductions to N containing compounds. There are many other early-stage additives such as lime and coal fly ash, zeolite, bentonite, other clays, and medical stone, among others.
Conclusions
There is an increasing amount of literature on strategies to mitigate livestock’s impact on the environment. However, there are still many unanswered questions that require additional research to elucidate the mechanism of actions of compounds to impact enteric emissions and emissions from dairy manure that can contribute to sustainability of the dairy production system. We need to continue to draw additional attention to the immense improvements in dairy: increases in overall production, decreasing inputs of labor, feed and water usage, as well as animal numbers while greatly increasing milk outputs per animal – thanks to increasing scientific involvement. In the sustainability debate it is also important to consider the impacts on society and the economy when choosing which environment-based changes to make. The dairy industry, and agriculture in general, should continue to equip themselves to highlight their continued improvements and be the drivers in the sustainability conversation.
Presented at the 2024 Animal Nutrition Conference of Canada. For information on the next edition, click here. 
