Tuesday, October 16, 2018

Veganism and Energy Efficiency


In this post, I discuss how veganism relates to energy efficiency, evaluate a few specific livestock common in the U.S., and conclude that factory farming techniques are significantly wasteful of food energy. In closing, I suggest some ideas for how people can adjust their diets to be more environmentally sustainable, especially if they are not interested in veganism.


I've been vegan since May 2017. I made the switch for a few reasons:

  • My brother adopted veganism about a year prior, and convinced me that the diet could be as healthy, or healthier, compared to an omnivorous diet, overturning my misconception that vegans were inherently nutritionally deficient.
  • A growing body of informal and academic work supports the idea that veganism is more environmentally sustainable that omnivorism.

Now about 1.5 years later, I've noticed many benefits: increased speed, stamina, energy, and near-elimination of common colds. Contrary to popular misconceptions, my food spending has decreased slightly (despite inflation), and I've naturally maintained my pre-veganism weight while becoming leaner and more muscular with increased exercise.

Effect of veganism on my combined grocery + restaurant spending

My interest in veganism is primarily as it relates to environmental sustainability, and I've read a goo
d deal on the topic. Though the big-picture argument is cohesive and compelling, much of the writing on it is lacking in technical rigor. Here are some typical examples:
"Eating lower down on the food chain provides a massive savings in terms of how much energy and resources you need. If you're on the third trophic level and you eat herbivores, the animals you eat contain only 10 percent of the energy originally stored by the plants they consumed...[In] general, eating lower on the food chain is always a more efficient practice."

Seems reasonable, but herbivores can digest food sources that humans can't, like grass and stovers, so not all food is equally valuable to all trophic levels. The food chain is more like a web rather than a neatly-ordered chain. Another example:

"It takes more than 2,400 gallons of water to produce 1 pound of meat; 1 pound of wheat takes 25 gallons."

Maybe so, but pound-for-pound, meat is much more nutritious and energetic (i.e. higher in calories) than wheat, so maybe the additional water is a good investment? From this statistic alone there's not enough information to make a meaningful conclusion.

A Litmus Test for Veganism

Unsatisfied with writing on veganism, I sought to develop my own metric: a litmus test to gauge the efficacy of veganism in improving environmental sustainability. The food chain starts with plants, and consequently all other trophic levels depend on plants. Growing plants is non-negotiable. So in effect, a referendum on veganism is a referendum on livestock cultivation. Should we do it, or not?

In an abstract sense, livestock can be viewed as energy conversion machines that convert plant products like corn and hay to animal products like meat and eggs. The figure of merit for an energy conversion machine is its efficiency - that is, its output divided by its input. The outputs are straightforward - meat, milk, and eggs. The inputs are the animal's diet - typically, some combination of corn, grains, legumes, and forages (i.e. grass, hay, stovers, etc). This was my starting point that I refined as I learned more about the modern food supply system.

Discourse on environmental impacts of mass agriculture tends to focus on land/water use, carbon emissions, and pollution, and these are all important considerations. However, I decided to focus on energy efficiency instead, for a few reasons:
  1. To clarify a perceived gap: pro-vegan sources usually ignore the fact that livestock are significant consumers of agricultural waste with digestive systems that are fundamentally different from humans', thereby underestimating their practical efficiency.
  2. To reduce the problem to its most essential form: energy is arguably the most fundamental agricultural resource, as it can be traded for water (e.g. through desalination), land (e.g. through terracing), carbon emissions (e.g. through sequestration), or pollution cleanup.

Effect of Diet on Energy Efficiency

All energy transfers are less than 100% efficient, so eliminating unnecessary intermediate trophic levels, and their associated energy transfers, increases overall energy efficiency. Cultivating the world's staple crops like wheat, corn, beans, and rice produces a great deal of agricultural waste that humans can't digest, but are readily digested by ruminant livestock. As there are no intermediate trophic levels between livestock and forages, and no other uses for forages, it seems reasonable that they be fed for livestock.

However, in the factory farming model which dominates U.S. agriculture, forages are a secondary food source for modern livestock, with the primary food source being, most typically in the U.S., corn. These reasons for this are complex, but include heavy tax subsidies for corn, high meat demand, and consumer preference for marbling in meat. Further, non-ruminant livestock consume no forages at all.

From an energy-efficiency perspective, this is sub-optimal as corn and other livestock feed staples are human-edible. In economic terms, feeding human-edible food to livestock is essentially an "investment" that only makes sense if the payout in animal products exceeds the initial investment. In this formulation, forages are excluded from the "investment", since they are not human-edible and have no human value. To quantify this exchange, I developed a metric called "Return on Useful Energy Input" (RUEI), which is calculated over a livestock's lifetime by:

RUEI = (E_ou
tput, all sources - E_input, human-edible sources) / E_input, human-edible sources

By design, RUEI has som
e useful properties: it's a dimensionless percentage, enabling direct comparison of disparate livestock and diets, and it's meaningfully signed: positive RUEI indicates a net gain, and negative RUEI a net loss. Unlike thermodynamic efficiency, which is always 0-100%, RUEI can be negative or exceed 100%, since it is not an efficiency but rather a normalized return.

World Livestock Overview

RUEI indicates if any given livestock is a good idea or a bad idea from an energy-efficiency perspective. Which livestock to analyze? Here's what the global herd looks like by population:

And by total biomass:

To limit sc
ope, I restricted my analysis to three animals with sufficient data available and that are of primary importance in the U.S.: cows, pigs, and chickens.

Assumptions and Limitations

Before proceeding, some notes on my simplifying assumptions used to calculate RUEI:
  • Feed ingredient proportions do not change with time 
  • Feed amount is proportional to animal mass 
  • Animal mass increases linearly with time up to mature mass, after which it is constant 
  • Animal products not included in the dressing percentage yield no food 
  • Forages and distiller's grains are considered human-inedible and are not included in RUEI 
  • All other food sources are considered human-edible and are included in RUEI 

And a few notes on overall scope:
  • Other environmental impacts like carbon emissions, land usage, water usage, and pollution are neglected to focus on energy efficiency 
  • Nutrition content, aside from calories, is neglected 
  • Energy costs upstream and downstream of livestock are neglected; system boundaries are drawn strictly around the livestock only


Cows b
reeds are specialized for either milk or beef production. Both cases are analyzed below.

Dairy cows

A typical dairy cow is first mated or inseminated at about 13 months old. After a 9 month gestation, the cow gives birth and produces milk for about 10 months, the "calving period". On average a cow has about 1.7 calving periods before it is no longer profitable, after which it is slaughtered. Over its lifetime, a dairy cow produces about 3,300 gallons of milk and yields about 400 kg of beef when slaughtered, with a dressing percentage of 60%. I considered two diets describing TMRs (total mixed rations), the daily food allotted to each cow, found on farm blogs:

IngedientMass, lb
Alfalfa hay28.00
Corn stover35.00
Cotton seed6.00
Oat flour4.50
source: Dairy Carrie: "What Do Cows Eat"

This diet is representative of more-traditional, non-factory-style feeds, with forages as the primary feed and grains secondary. For obvious reasons, factory farms do not publish their feed recipes as transparently as small farms. The energy breakdown over the cow's lifetime, in round numbers, is:
  • Milk output: 9,000,000 Cal
  • Meat output: 1,000,000 Cal
  • Feed input, all sources: 198,000,000 Cal
  • Feed input, human-edible sources: 43,000,000 Cal
As can be seen from the data, the cow produces significantly less useful energy than it consumes, even after accounting for its forages. The RUEI is:

RUEI = (E_output, all sources - E_input, human-edible sources) / E_input, human-edible sources
RUEI = ((9,000,000+1,000,000) - 43,000,000) / 43,000,000
RUEI: -77%

In other words, the cow is a net useful energy loss; the return is -77% of the useful input, i.e. an input of 100 useful plant calories yields only 23 animal calories. Repeating the same analysis for an alternate diet from a similar-size farm:

IngredientMass, lb
Alfalfa hay12.00
Canola meal2.00
Corn stover62.00
Cotton seed4.00
Soybean meal3.00
Wheat stover12.00
source: Slow Money Farm: "What Do Cows Eat"

RUEI: -68%

Beef cows

Data on specific beef cow diets proved difficult to find, so I re-used the diet information for the dairy cows. Beef cows produce no milk and yield a similar amount of meat, though their lives are much shorter at around 18 months.

RUEI: -88%


Pigs are slaughtered at around 4 months old, with a dressing percentage of 72%, and consume about 2.6% of their body weight daily. Most pigs are fed only corn, soybean meal, and sometimes wheat. Specific pig diets were found abundantly, and seven specific diets were analyzed. Representative highlights are shown below:

"[Baseline] reference diet"
IngredientMass, lb
Soybean meal1.51
source: Niche Pork Production: "Example Pig Diets"

RUEI: -27%

"Example [diet] including cooked, full-fat soybeans"
IngredientMass, lb
Soybean meal0.37

RUEI: -7%

"High-forage diet for gestating sows"
IngredientMass, lb
Soybean meal0.31
Alfalfa hay4.81

RUEI: 130%

This is the only diet found to have a positive RUEI, owing to it being based on forages. Feed varies significantly between farms, and it's unclear how common this type of feed is, but it does suggest an interesting gray area that will be discussed further.


Like cows, chickens breeds are specialized for either egg or meat production, with the latter termed "broilers". A laying chicken typically lives to be 18 months old and produces around 170 eggs, being slaughtered at a weight of around 2.2 kg, with a dressing percentage of around 70%. Broilers live to be about 1.5 months old, and are slaughtered at around 2.0 kg, though some broilers can reach as much as twice this weight. Both types of birds eat about 5% of their weight daily in the form of corn and soybeans. Chickens are not significant consumers of agricultural waste products.

Laying chickens

"Corn-soy-based diet (CS)"
IngredientMass, lb
Soybean meal0.09
Soybean oil0.01
source: Poultry Science: "Energy and nutrient utilization of broiler chickens..."

A chicken's egg production constitutes the majority of its food output, about 73%.

RUEI: -87%

"Corn-based diet (CN)"
IngredientMass, lb
Soybean meal0.05
Soybean oil0.01

RUEI: -83%

Broiler chickens

Applying the same diet proportions to broilers, noting that their daily food intake is slightly higher:

RUEI: -32% to -13%


This figure shows the food outputs of the livestock analyzed, broken down by the output type in terms of days of food, assuming a typical 2,000 calorie/day diet. Incredibly, a single dairy cow produces enough milk in its brief life for about a decade's worth of human caloric intake.

Feed varies significantly by livestock, farm size, and geographic region, and is also usually proprietary, so these figures cannot be precise. Nevertheless, they capture some important trends about the extent to which livestock are recyclers or consumers of food products. All of these livestock originally existed near 100% on this chart, consuming mostly scraps and forages, but modern farming techniques have dramatically pushed the numbers lower by replacing large portions of feed with grains and legumes.

RUEI was negative for all livestock analyzed except forage-fed pigs, which consume mostly alfalfa hay or corn stovers. The other livestock shown could also be raised in a positive-RUEI manner, but this is at odds with current factory farming and consumption habits.


The data supports the view that factory farming, with its livestock feed characterized by low forage and high human-edible feed, is environmentally unsustainable, or at minimum, energetically wasteful. Mainstream methods for cultivation of cows, pigs, and chickens in America cause a net useful energy loss, even when considering the fact that some animals consume agricultural waste that is human-inedible.

However, an interesting gray area is found in the case of high-forage diets characteristic of free-range agriculture. Further, within this framework, livestock that consume human-inedible/waste products exclusively would have an infinitely high RUEI. Interestingly, this aligns with the pre-industrial model of farming, before factory farming, when crop cultivation was the primary focus and livestock existed in a secondary capacity, grazing freely, consuming occasional food waste, and in some cases serving as labor. In these times, meat was rare and weighty, a far cry from its commodity status in modern America.

We are unlikely to ever return to this idyllic past, but we would be well-served to learn from its wisdom. Those who are not prepared to give up meat should at least better understand its environmental impact and reduce consumption, while opting for local, free-range, forage-fed options whenever possible. Veganism is too extreme for most people, but that's okay. In practice, it's far more beneficial, and socially conscientious, to reduce livestock consumption and/or improve cultivation standards than trying to convert non-vegans, which is usually a losing battle and risks stigmatizing the movement. For example, reducing the meat consumption of 10 people by 10% is far better and easier than reducing the meat consumption of 1 person by 100%, since the ask is more reasonable while the impact is equal yet more socially broad. Purism should not stand in the way of pragmatism.


Those interested in the data, calculations, and sources may view my full spreadsheet, the basis of this post. I welcome any corrections and criticisms.

Books that shaped my general thinking on this topic are "Who Will Feed China?" by Lester R. Brown and "America's Food" by Harvey Blatt.

Update 2018-10-24

The Economist ran an article last week called "Why people in rich countries are eating more vegan food", which includes a nice graph showing "feed to food loss" for various livestock. They use protein as their intermediate quantity to compare between crops and livestock instead of energy as I've done here, but the concept and conclusions are similar.

Graph from The Economist comparing protein gains and losses for crops vs. livestock

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