Dairy cattle provide a major benefit to the world through upcycling human inedible feedstuffs into milk and associated dairy products. However, as beneficial as this process has become, it is not without potential negatives. Dairy cattle are a source of greenhouse gases through enteric and waste fermentation as well as excreting nitrogen emissions through their feces and urine. However, these negative impacts vary widely due to how and what these animals are fed. In addition, there are many promising opportunities for further reducing emissions through feed and waste additives. The present review aims to further expand on where the industry is today and the potential avenues for improvement. This area of research is still not complete and additional information is required to further improve our dairy systems impact on sustainable animal products.

IntroductionDairy production is considered a major societal asset globally due to its economic and nutritional benefits. In 2019 alone global milk production totaled 851.8 million tons in milk equivalents (Outlook, 2020). This contributes to substantial trade impacts, totaling about 76.7 million tons in 2019, as well as major per capita consumption at about 111.4 kg/year globally (Outlook, 2020). There are over 245 million dairy cows worldwide that on average produce 2,300 kg per year; although average production is less informative as there is such a major disparity between production in different countries (FAO, 2009). This vast amount of milk production has a major global benefit—for human health, society, and the economy. In countries with developing economies livestock serve many purposes including: a source of household income, a financial asset for women, a source of food security, risk management, and a direct link to human health (Herrero et al., 2013). These benefits increase substantially when viewed from a macro lens. Global dairy imports totaled over $42.2 billion in 2014 and global dairy exports expanded 175% from 2005 to 2014 (Davis and Hahn, 2016). There are many dairy commodities being produced and traded as part of these exports with whole milk powder being the highest, followed by skim milk powder, butter, and cheese (Outlook, 2020). India is the largest dairy producer globally, with 22% of global production and 52,841,810 total dairy cattle, the US is second in production, followed by China, Pakistan, and Brazil (FAO, 1997; Knips, 2005). While the US may not be the largest global milk producer, the total economic benefit of the dairy industry is substantial, totaling $628.27 billion dollars in 2018 (O'Keefe, 2018). There are currently 9,336,000 dairy cows in the U.S. that on average produce 10,610 kg of milk each year, which amassed to over 99,056,409 kg of milk in 2019 (USDA, 2020). The US dairy sector also generates over 2.9 million jobs either through direct or indirect support (O'Keefe, 2018). The top five milk producing states in the US are California, Wisconsin, Idaho, New York, and Texas, with California accounting for nearly 20% of national production (Sumner and Matthews, 2019; USDA, 2020). The dairy industry is such a major part of California's economy that in 2019 the associated impact from milk production and processing was about $57.7 billion dollars, providing over 179,900 jobs (Sumner and Matthews, 2019). Dairy is the leading agricultural commodity produced in California, accounting for nearly 13% of the $49.9 billion dollars in cash receipts generated for the top ranked agriculture producing state (CDFA, 2019). Not only is the dairy industry a major driver of the economy but its products serve a substantial nutritional benefit to the growing human population.Milk and dairy products are a well-known source of calcium, vitamins, and other selected minerals as well as being a complete high-quality protein. One of the most well-documented nutritional benefits of dairy products is for bone health—particularly for its ability to prevent osteoporosis and other bone diseases. In particular the calcium in milk positively affects bone mass in children and when coupled with vitamin D, as seen in fortified milk products, will prevent bone loss and osteoporotic fractures in aging populations (Caroli et al., 2011). For low-income countries struggling with nutritional deficiencies in children, studies have shown that supplementation with dairy products causes a significant increase in vitamin B-12 plasma concentrations, improves cognition, growth and activity (Allen, 2003; Siekmann et al., 2003). In addition, maternal milk intake during pregnancy is positively associated with infant birth weight, and subsequent bone mineral content during childhood (Gil and Ortega, 2019). Other milk components, including bioactive peptides present in the whey components of milk were shown to benefit the immune system due to their antimicrobial and immunomodulatory properties (Madureira et al., 2010). Consumption of dairy products has shown an inverse relationship with cardiovascular disease in that consumption of milk and dairy is associated with a lower incidence of type-2 diabetes and improvements in glucose homeostasis (Hirahatake et al., 2014). While milk is relatively high in saturated fat it has been shown that milk intake did not increase cardiovascular risk (Visioli and Strata, 2014). Furthermore, milk intake was associated with reduced risk of childhood obesity as well as improved body composition and weight loss in adults (Thorning et al., 2016). Dairy intake was also shown to be inversely associated with incidence of cancer including colorectal, bladder, gastric, and breast cancer and was not shown to be associated with any other additional forms of cancer (Thorning et al., 2016). Although dairy production serves many benefits to overall nutrition, human health, and the economy, there has been increasing concern about the impact of dairy on the environment.Impact of Dairies on Climate Change and Air QualityThe earth's surface has undergone massive increases in temperature, primarily in the last three decades, and the last 30 years we have seen the warmest period ever recorded (IPCC, 2014). In addition to this temperature increase, there have been other major changes to the climate including trends of increasing ocean temperature, rising sea level, as well as a major increase in greenhouse gas emissions (IPCC, 2014). Another phenomenon that has occurred over the last few centuries is an increase in ocean uptake of CO2, causing ocean acidification and a decrease in surface water pH, as well as a rapid decrease in glaciers and ice sheets around the globe. These major changes in the climate are primarily due to anthropogenic (human caused) emissions of GHGs that have steadily increased since the beginning of the industrial revolution in the 1750s (Place and Mitloehner, 2010). Atmospheric concentrations of CO2, CH4, and N2O are also the highest they have been in at least the last 800,000 years, with about 78% of these CO2 emissions resulting from industrial processes and the combustion of fossil fuels (IPCC, 2014). Several studies have indicated that the production of livestock, including the stages of growing, transport, processing, and consumption have a relatively large impact on climate change (de Vries and de Boer, 2010; Milani et al., 2011). Dairy cattle in particular were shown to impact the environment through their potential negative contributions to air, water, and land (Naranjo et al., 2020).In regards to the environment, the US Dairy industry has seen substantial improvements over the years. In particular it has seen a great increase in milk production 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; Barkema et al., 2015). 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 accounting for each animals age and stage of lactation (National Research Council, 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 continued to improve to 2014 where 1 kg of energy and protein corrected milk (ECM) for California emitted between 1.12 and 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). The dairy industry has continued to still further these improvements. Dairy production systems 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 the industry (Capper and Cady, 2020). Even with these major advancements made over the last century, dairy systems still impact the environment through: GHG emissions from enteric fermentation, manure management, and feed production, water use for feed production and milk processing, water quality with contaminants including nitrogen (N) and phosphorous (P) from manure, as well as the requirement for land used in feed production (Naranjo et al., 2020). In addition to the direct impacts of cattle, such as N and P as a result of dairy production systems, there are also environmental impacts associated with dairy processing and subsequent production (Milani et al., 2011).Manure Emissions From Dairy CattleDairy manure has the potential to negatively impact the environment. Nitrogen not retained by the animal or secreted in milk will be excreted in the urine and feces of the animal (Hristov et al., 2019). Urine is more susceptible to losses of N to the environment from the animal waste as compared with fecal N (Dijkstra et al., 2013, 2018a). Dairy waste is a significant source of N and P that when land applied in excess of crop requirements can cause contamination of surface water (Knowlton and Cobb, 2006). This 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 (Knowlton and Cobb, 2006). 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 hemoglobin and leads to cyanosis (oxygen starvation) (Knowlton and Cobb, 2006).Air quality also affects human and animal health as well as the environment, and dairy cattle have been known to contribute to poor air quality. One such compound that affects air quality produced by dairy cattle is NH3. Ammonia is produced when N in urea from the animal's urine reacts with urease present in feces (Place and Mitloehner, 2010). 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 (Muck, 1982; Sun et al., 2008). In addition to NH3 losses 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 (Bussink and Oenema, 1998). Total losses of NH3 can be between 0.82 and 250 g NH3/cow/day, with the total loss dependent on the amount and composition of animal waste as well as the environment and management conditions of the manure storage (Bussink and Oenema, 1998; Hristov et al., 2011). Dairy waste management strategies greatly influence air emissions of NH3. The greatest NH3 emissions occur after field application, followed by the manure management strategies, for example, separated liquid and solids, aerated, straw covered, untreated, then anaerobic digested (Amon et al., 2006).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 byproduct (Place and Mitloehner, 2010). Land applied dairy manure on cropland as well as the long term storage of manure in lagoons can contribute to emissions of N2O (Velthof et al., 1998; Place and Mitloehner, 2010). The N2O emissions during storage depend on the N and carbon content of the manure (Amon et al., 2006). 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 (Dustan, 2002). The process of long term storage of manure seems to also contribute a larger proportion of N2O emissions compared with land application with aerated, straw covered, digested, separated, and untreated manure contributing decreasing amounts of N2O emissions (Amon et al., 2006).Another substantial GHG produced by dairy cattle waste is methane (CH4). The amount of CH4 emitted by dairy waste is dependent on the amount of carbon, hydrogen, and oxygen present in the waste, making manure storage, diet, and bedding major contributors to total CH4 production (Place and Mitloehner, 2010). A smaller proportion of CH4 is also produced in the hindgut of the animal via post ruminal digestion and fermentation (Ellis et al., 2008). This CH4 is mostly absorbed from the hindgut (89%) and eventually eructated by the animal or excreted with the manure (11%) (Murray et al., 1976; Immig, 1996; de la Fuente et al., 2019). Manure CH4 emissions are substantially higher from long term storage compared with field application (Amon et al., 2006). These emissions are highest from straw covered manure and emissions decrease with untreated manure, followed by separation, aeration, and digested manure management methods (Amon et al., 2006).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 (Place and Mitloehner, 2010). There were 73 detectable VOCs from slurry wastewater lagoons with the most common VOCs being methanol, acetone, propanal, and dimethylsulfide (Filipy et al., 2006; Shaw et al., 2007). As with other waste emissions, VOCs from dairy waste increase with ambient air temperature with summer months having the highest rates of VOC emissions (Filipy et al., 2006). The largest contribution of VOCs on dairy systems come from fermented feedstuffs (i.e., silage) (Place and Mitloehner, 2010).Effect of Nutrition on Emissions From Dairy CattleDairy cattle enteric emissions have been shown to contain a variety of gases. For example 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 the rise in GHGs due to the CO2 having been previously recycled from the atmosphere by fixation during photosynthesis in plants, which are then consumed by the 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 (Casey and Holden, 2005).The most significant enteric emission compound from dairy cattle is CH4. Methane acts as a hydrogen sink in the rumen and is an end 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 also causes the disease state 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 as growth gestation, or lactation (Moe and Tyrrell, 1979).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 and 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, rate 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; Johnson and Johnson, 1995). 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 (Johnson et al., 2002; 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., 2013b). 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 (Mills et al., 2008; Doreau et al., 2012). There are many potential methods to mitigate enteric emissions through alterations to nutrition strategy and composition.