Livestock

Livestock comprise 62% of mammal biomass (humans are 34%) and 71% of bird biomass in the world.^[1] Among mammals, cattle account for 35% of world biomass (Fig 1). Meat is an energy-dense, and nutrient-dense food in a world in which approximately a quarter of children under the age of 5 still exhibit stunting due to malnutrition.^[2] But meat’s role in diets and the role of livestock in environmental impacts are complex.

Livestock production and impacts

Global meat production has more than tripled since 1961, with the largest increase occurring in Asia (Fig 2). In 2022, the average meat supply per person per year, globally, not counting fish, was over 40 kg per person (97 lbs), with Mongolia, a country with a strong pastoral tradition leading, and the US not far behind (Fig 3). In general, wealthier nations produce more meat per person. In the developing world, livestock are important for food security and income, particularly for those living in poverty.^[3]

Life-cycle analyses of meat production reveal a number of processes associated with impacts. Agricultural impacts from production of animal feed, methane production during digestion, impacts associated with wastes from pastures and feedlots, transportation for processing and to market, and packaging all have related impacts. Impacts vary significantly both between subsistence and industrial production and by location.^[4]

Currently, livestock production uses approximately 30% of Earth’s land area and 70% of overall agricultural area is used for grazing and livestock feed production. Approximately 60% of harvested biomass used by humans goes to livestock.^[5] In 2022, approximately 40% of cereal production went to livestock (Fig 4),^[6] and about 75% of soybeans, by weight, are fed to livestock directly or in processed feeds.^[7]

The leading cause of deforestation for commodities during 2001-2015 was land for livestock pasture.^[8]. In the US, the EPA estimates that burping livestock (primarily from ruminants – cattle, sheep, goats; sometimes classified as “enteric fermentation”) account for approximately 25% of agricultural GHG emissions, and manure management accounts for another 14%. Overall, the agricultural sector was responsible for 10% of US emissions in 2022. Burping livestock produced 36.6% of total methane emissions across all economic sectors in that year.^[9]

Animal feeding operations

In the developed world, most livestock are raised in confined-animal feeding operations (CAFOs), also called, less formally, factory farms or megafarms. Dairy cattle, pigs, and chickens may spend most of their lives in such operations. Beef cattle typically are on pasture for their earlier life and on CAFOs for 6-12 months before they are slaughtered. Because many animals can be raised in a small, carefully monitored space, with controlled access to food, CAFOs are efficient, potentially reducing costs for meat, eggs, and dairy products. They reduce the direct footprint of livestock agriculture. In 2020, the EPA listed over 21,000 CAFOs in the US.^[10]

However, CAFOs have significant drawbacks, as well, in terms of sustainability. for all three pillars of sustainability. Although the confined nature of the operations means that animal wastes are concentrated and can be managed efficiently, water pollution, particularly nutrient pollution such as nitrates, is a constant problem associated with the operations.^[11] Although the manure can be a welcome source of organic fertilizer, the feeding operations are often not located near appropriate fields, which increases transportation costs and GHG emissions. When manure is located close to fields, it may be overapplied in order to dispose of it, although laws can limit application rates. During rainfall events, waste collection areas may flood, washing wastes, pathogens, and associated pollutants including livestock pharmaceuticals into groundwater and surface waters. In the US, CAFOs are not covered by point-source, water-quality regulations, despite their concentrated nature. In the EU, they fall under the Industrial Emissions Directive and must meet the relevant standards.

Ammonia fumes from concentrated waste collection areas on CAFOs can exceed normal levels by 40-fold and can cause respiratory and other health issues.^[12]  Odor, generally, is also an issue, and one that is less regularly regulated. Greenhouse gas emissions from livestock operations (see above) are also unregulated in the US; the EU covers them under the Industrial and Livestock Rearing Emissions Directive, requiring best practices, but does not set numerical limits per operation.

If manure can be contained, then it can be used to generate methane (natural gas) as an energy source. Anaerobic digesters are used to capture the gas, and can also be used with food waste and solids from sewage. However, the nutrient-rich material that remains after methane production must still be disposed of.

Animal welfare and related ethical concerns are also an issue with confined animal feeding operations. Animals experience higher levels of stress and injury than in more naturalistic settings and incidents of abuse have been documented.^[13]

Grazing operations

Grazing of livestock herds is a practice that dates back approximately 10,000 years, both nomadically and with fenced enclosures. Grazing is a good means of producing food on land that is marginal for agriculture. These rangelands (comprising mostly dry grasslands, shrublands, woodlands, and deserts) cover 50% of the land surface of the planet, however, production on them is low and can only support low population densities of humans – they produce an estimated 16% of food, globally. Of course, livestock can also be pastured on more productive lands, but these typically can produce more food under intensive agriculture than under grazing. In the developed world, smaller operations including some family farms, organic farms, and parts of beef cattle operations using grazing systems. In the developing world, grazing is of particular importance to the  poorest inhabitants, providing food and income.

Although animal densities are lower in grazing systems than on CAFOs, animal wastes can still contribute to water pollution, particularly if animals are allowed direct access to water bodies and can deposit urine and manure directly into them. Overgrazing degrades land, resulting in soil compaction and erosion, further contributing to water pollution. In 2021, the FAO estimated that perhaps 35% of grasslands are at risk of degradation and other rangelands up to 26-27%.^[14]

Sustainability of meat-based diets 

In Chapter 1, we learned the basics of food webs – that each successive level of a food web has available to it only about 10% of the level below it. There is a lot of variability around that 10% figure, but the general lesson of decreasing energy at successfully higher levels of a food web is inescapable.

By feeding food to animals to create food for people, we are pushing at least part of the human diet up the food web, and strongly limiting the quantity of food that can be produced, in the process. The American Soybean Association states that more than 90% of US soybeans are fed to animals.^[15] Figure 4 above shows us that the developed world feeds over half its cereal to livestock. These are food resources from which we will see only a small fraction emerge as food for people, because of food-web realities.

Livestock researchers study food conversion ratios – how much meat a given quantity of food produces when fed to livestock. Commercially bred chickens are most efficient, with food conversions ratios around 1.5-1.9: it takes between 1.5 and 1.9 kilograms of feed to produce a kilogram of chicken. Food conversion ratios aren’t the most accurate instrument – ratios vary with the specifics of the feed, and it’s important to know how much of the chicken is edible. But they are the instrument in use at the moment. The conversion ratio for pigs averages about 3.3, and for cows it’s 4.5-7.5.^[16]

Beyond the inefficiency of meat-based diets in using agricultural products to feed the world, are a suite of other environmental impacts. In a 2018 study, across GHG, land use, acidification of land, eutrophication of water resources, and freshwater use, even the animal-based foods with the lowest impacts had more impacts than almost all of the vegetable products examined. ^[17] Land-use comparisons are illustrative; beef from cattle raised in a beef herd used almost 100 times as much land as tofu, a complete, plant-based protein (Fig 5).

The FAO estimated that meat production produces 12% of global GHG emissions.^[18] Cattle production, alone, is estimated to drive 36% of tree loss associated with agriculture.^[19] Meat-based diets cause considerable environmental harm, as well as being linked to clear risks to human health.^[20]

Aquaculture

The raising of aquatic plants and animals for food, like grazing, is an ancient practice dating back thousands of years. Its use has increased in intensity and in overall use from perhaps 2 million tons in 1960 to over 100 million tons in 2022 (Fig 5); Asia’s contributions to its growth are much larger than its contributions to the growth of land-based meat production. Aquaculture surpassed so-called capture fisheries – catches of wild fish – in the early 2010s (Fig 6). Aquaculture relieves pressure on wild fisheries and shellfisheries, but not all aquatic species are readily domesticated, so relief varies among target species.

Aquaculture takes a variety of forms, with species raised indoors, in inland ponds or raceways, in coastal salt-water or brackish-water lagoons, and in netted enclosures in open water (Figs 7, 8). For some species, different systems are used at different life stages. Freshwater species can be raised in ponds created for that purpose or in enclosures in rivers or lakes. Shrimp are often raised in coastal lagoons (Fig 7). Marine fish are often raised in coastal enclosures in open water (Fig 8), although intensive operations may use indoor facilitites. More vulnerable life stages and more valuable species are most likely to be raised in the most protective settings.

Food-web issues in aquaculture

Fish species vary in how efficiently they can be produced through aquaculture. In the worst cases, wild-caught fish that are less valuable as food for humans (but often still eaten by humans – anchovies, for example) are reduced to fish meal and fed to fish that are more attractive to humans, such as salmon or swordfish, with an overall loss in protein and biomass during production. Whereas chickens and cows are fed plant food, and so occupy one level up from plants, fish that are fed other fish are feeding at least one additional level up the food web. Such losses are mostly characteristic of predator fishes and early use of these species in aquaculture. More sustainable approaches use fish that can be fed vegetable feeds, keeping them down on the first level above plants.

Environmental impacts of aquaculture

In general, aquaculture has not resulted in the kind of forest loss that occurs when land is converted to agricultural use. However, coastal mangrove forests have been targeted for aquaculture ponds for marine species. Shrimp farming, in particular, was responsible for clearing of mangroves throughout the Asia-Pacific region during 1980-2000s. During this time, over a third of world mangrove stands were lost, with approximately half of this owing to aquaculture. Strict regulations against destroying mangroves have reduced loss of this important coastal barrier to sea-level rise and storm waves, but regulations have not spurred restoration efforts. Mangrove clearing continues, at a much slower pace, but mangrove-aquaculture systems are also being introduced, which provide some progress towards restoration. ^[21]

Significant aquaculture impacts stem from wastes – wasted food and bodily wastes of farmed organisms; escape of other inputs – antibiotics, pesticides, pharmaceuticals; and escape of the target organisms, themselves. These issues are least severe with indoor and inland-pond operations, where access to natural water systems is reduced. However, indoor operations must dispose of wastes and polluted water, and inland ponds can be flooded during storms, sending polluted water into nearby receiving waters. Polluted water from coastal ponds and lagoons is more likely to reach nearby water bodies. Finally, caged operations in freshwater or marine locations exchange polluted water freely with rivers, lakes, or seas and generally have no means of recovering pollutants.

Farmed fish, themselves, can harm wild fish populations. Due to high densities of fish and nutrients, floating fish farms support diseases organisms and parasites in high numbers, which can also attack wild fish. Sea lice levels in wild salmon, attributed to salmon farms, are associated with an 18% increase in mortality of the wild fish.^[22] In addition, farmed fish, which are bred to grow quickly and efficiently in captivity, can escape from their netted enclosures during storms or due to damage to the nets. When they interbreed with wild salmon, the resulting offspring are less well adapted to the wild.

A study of hatchling salmon found decreases in survival over 2 years of 49% and 70% in two year-classes for which both interbred (introgressed) and pure wild individuals were available. To date, there are no data on impacts of genetically modified aquaculture fish. Modified salmon were in production in the US and Canada from 2021-2023, but major retail chains declined to offer them for sale, and the company closed in 2024.^[23]

Knowledge Check

Take a moment to complete the short quiz below to assess your understanding of this section. Read each question carefully and refer to the section content as needed. This quiz is not graded – it’s simply an opportunity for you to reflect on what you’ve learned and reinforce key concepts.