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Assessing animal contribution to human food supply

Protein quality and the efficiency of conversion of human-edible foods into animal proteins should be considered when assessing animal contribution to the human food supply

By DR. JIM ALDRICH, Dairy Nutrition Director, CSA Animal Nutrition

Livestock, and in particular ruminants, have been criticized for low efficiency of converting plant protein into animal protein.  However, this argument fails to consider that large portions of feeds that are fed to animals (forages, and by-products) are human-inedible, and the quality of animal protein is superior to plant proteins. In the US, livestock utilize 43 x 109 kg/year of human-inedible food and fiber byproducts converting them to human-edible food, pet food, and industrial products (White and Hall, 2017).  In a comprehensive analysis of the impacts of removing animal products from the food supply, White and Hall (2017) found that plant only - based agriculture would be incapable of supporting the US population’s nutritional requirements.  Because both systems (plant only vs animal and plant) provided excesses of total protein needs in their analysis, specific evaluations of protein quality between the two systems were not addressed in detail.  This article will discuss the efficiency of conversion of potentially human edible protein into animal protein while considering the enhancement of protein (amino acid composition), specifically focusing on dairy.

Researchers (Ertl et al., 2016) used data from 30 dairy farms in Austria including composition of the feeds consumed by cows as well as the production of milk and meat to assess the efficiency of conversion of human edible proteins into animal proteins. Only feeds containing human-edible protein were considered, as protein quality of human-inedible feeds (forages) is not needed for these calculations.

The quality of a protein is a function of the intestinal digestibility and content of indispensable (essential) amino acids (AA) relative to the reference human requirement.  An index or score that combines these two factors (AA digestibility and digestible AA content relative to requirements) is the Digestible Indispensable Amino Acid Score (DIAAS; FAO, 2013) and is calculated as follows:



mg of digestible indispensable amino acid in 1g of the dietary protein


X 100


mg of the same indispensable amino acid in 1g of the reference protein



The AA profile of the reference protein was the requirement for a 6-month to 3-year-old child (FAO, 2013).

The DIAAS for protein ingredients (that are potentially human edible) fed on these farms as well as for the protein produced (milk and beef) are listed in Table 1 [Note that some of the ingredients such as grains (wheat, barley, corn, rye, triticale) are usually not considered protein sources, but nonetheless contribute protein to the animal’s diet and are human edible].  The DIAAS for an ingredient is the score for the first limiting AA in that ingredient relative to the requirement.  For example, in grains, lysine (Lys) is likely first limiting, whereas in soy or other legumes, methionine (Met) is likely first limiting.  Animal sources invariably have higher DIAAS than plant sources.  On average the DIAAS for animal protein outputs was 113.7 vs 61.1% for the human edible sources fed to the cows. A DIAAS score of >100% indicates that all digestible AA in that source exceeded the reference AA requirement pattern.

Table 1. Digestible Indispensable Amino Acid Scores (DIAAS) for Individual Protein Sources

Protein source






Corn grain










Wheat bran


Soybean expeller


Soybean cake


Sunflower expeller


Rapeseed expeller


Rapeseed cake


Corn silage


Whole milk powder




Average plant input


Average animal output


Note: In table 1, the soybean products have much higher DIAAS than in some other published data.

When all human edible inputs and outputs were compared (Table 2) the protein quality output/input ratio (PQR) was 1.87:1. 

Table 2. Digestible Indispensable Amino Acid Score (DIAAS) for Mixtures of Human Edible Protein Inputs and Outputs of Dairy Farms (n=30)



Mixed plant protein input, %


Mixed animal protein output, %


Output-input, % points


Protein quality ratio (output/input)



Combining this qualitative ratio with the quantitative changes in human edible protein (total quantity of human edible protein consumed by cows compared to animal protein output) with an average human edible feed conversion efficiency (heFCE) of 1.15 resulted in one single value (heFCE x PQR) which was, on average, 2.15 for the 30 farms in this study (Figure 1). Ninety percent of the farms achieved a ratio of >1.0. 


Fig 1 Aldrich (002).jpg

Figure 1. Human-edible feed conversion efficiency (heFCE) and human-edible feed conversion efficiency time protein quality ratio (PQR): (heFCE x PQR) for 30 Austrian dairy farms using DIAAS to determine protein quality.

For dairy cows and other ruminants (beef, sheep, goats), part of the efficiency is due to the microbes in the rumen that are able to digest fiber and other non-human edible components and utilize lower quality protein (nitrogen) sources converting them to high quality proteins that the animal, in turn, utilizes to produce milk and meat.

For US dairies, the calculation of heFCE x PQR could be even higher than for the farms in this study because of higher output per animal (annual milk yield averaged 16,600 lb for the Austrian dairy farms) and more use of non-human edible by-product feeds (e.g., ethanol distiller’s grains, corn gluten feed, food industry by products).  See example below. This study only examined dairy farms, but it is likely that the net conversion can be >1.0 for other animals such as pigs and poultry (Wilkinson, 2011) due to the quality of protein produced, their use of non-human edible ingredients, and very high feed conversion efficiencies.

When assessing the efficiency of animal protein production, the following factors need to be considered:

  1. The quantity of potentially human edible protein in the animal’s diet.
  2. The conversion ratio of the human edible protein of the animal’s diet into animal protein.
  3. The transformation of lower quality plant proteins into higher quality animal proteins.

When these factors are accounted for, animal agriculture can provide a net increase in the quantity and quality of protein for human consumption. 

The use of simple input/output models to calculate the efficiency of animal protein production that do not account for all the factors listed above can lead to invalid estimates. 

Applying these efficiency calculations to an example US Dairy

Using the approach described in Ertl et al. (2016), the efficiency for an example US dairy was calculated.  The diet modeled contained the following ingredients supplying protein that would be considered as potentially human edible or derived from a human edible source (e.g. soybean meal from soybeans): corn silage (50% corn grain), corn grain, soybean meal, and heat treated soybean meal.  Other non-human edible sources in addition to forages were soybean hulls, canola meal, distiller’s grains, food industry by-product, blood meal, urea, and added fat.  It was assumed that this single diet was fed to all milking cows.  Milk yield was 90 lb/h/d with 3.80% milk fat and 3.25% milk protein.  Dry matter intake was 58 lb for a gross feed efficiency of 90/58 = 1.55. 

For this example, human edible protein input = 4.19 lb; milk protein output = 2.96 lb; and heFCE = 0.71. 

This was lower than the 1.15 average heFCE for the Austrian farms, but that model also included protein input/output from beef (calves and culled dairy cows).  A measure of protein efficiency in dairy cows is milk nitrogen (N)/feed N. An average efficiency is around 25% with 30% being achievable (LaPierre et al., 2019). This is a valid metric to evaluate N use efficiency on an individual dairy but the efficiency of conversion of human edible protein to milk protein would be more relevant when comparing the economic and environmental impacts of animal versus plant protein sources.

The DIAAS of the modeled ration ingredients were a weighted average of 42.4%. This was lower than the 64% for ingredients fed on the Austrian farms. Not surprisingly the low DIAAS were for Lys in corn products and Met in soy products. As mentioned earlier, Ertl et al. (2016) used a very high DIAAS for soybean products, we used reference values for cooked soybeans (FAO, 2011) which had a lower value of 43% versus 97-100% (Table 1). The DIAAS of milk used here was lower than protein output value of Ertl et al. (2016) 115.9% vs 82.9% with phenylalanine (Phe) being the first limiting1.  The calculated PQR for the modeled dairy was 82.9/42.4 = 1.96, similar to the 1.87 calculated in Ertl et al. (2016). Again, the latter calculation included DIAAS of meat produced from the dairy, whereas in our example only milk output is considered.  Combining heFCE x PQR = 1.38 indicating a net increase in quality adjusted protein for human consumption in the modeled US dairy. 

To estimate the input/output relationship on a whole herd basis for the US dairy, proteins fed to non-lactating animals (calves, heifers, bulls, and dry cows) as well as the protein from beef produced would need to be accounted for.  Except for calves and younger heifers, the human edible proportion of the diet would be less than for the lactating cows because of the higher forage levels typically fed to these groups.  The human edible feedstuff inputs and human edible output (milk and meat) for the Austrian dairy farms (Ertl et al. 2015) was calculated for the whole herd, at the farm gate level.  Including the non-milking livestock in the calculations still resulted in a net increase in quality adjusted human edible protein from these farms. 


Animals are fed ingredients that are potentially human-edible leading to the question about loss of efficiency, environmental impacts, and sustainability when these foods could be directly consumed by humans.  In evaluating animal agriculture’s contribution to the human food supply, the conversion efficiency of human-edible foods combined with a protein quality score are necessary to determine the net contribution to the food supply.  Ertl et al. (2015) integrated these two components into a single value (heFCE x PQR) which can be used to determine the efficiency of animal production systems in converting potentially human-edible inputs into animal proteins.  Applying this calculation to an example US dairy, yielded a value of 1.38, indicating a net increase in quality protein from human-edible inputs into animal protein (milk) outputs.


1Ertle et al. (2016) used 115.9% for the DIAAS of whole milk based on the AA reference ratios for the 4 most likely limiting AA; Lys, sulfur AA (Met+Cys), Thr, and Trp, whereas when all IAA are considered as recommended (FAO, 2013) the DIAAS for whole milk was calculated as 82.9% with Phe being first limiting. This underestimates the AA quality of milk protein as Phe is unlikely to be limiting in mixed human diets. Recalculating the PQR using the higher DIAAS value for milk (115.9/42.4 = 2.73) and heFCR x PQR (0.71 x 2.73 = 1.93) as compared to 1.38 using the lower DIAAS value for milk.


Ertl, P., H. Klocker, S. Hörtenhuber, W. Knaus, and W. Zollitsch. 2015. The net contribution of dairy production to human food supply: The case of Austrian dairy farms. 2015. Agric. Systems 137:119-125

Ertl, P., W. Knaus and W. Zollitscch. 2016. An approach to including protein quality when assessing the net contribution of livestock to human food supply. Anim. 10:11. 1883-1889.

FAO 2011. The assessment of amino acid digestibility in foods for humans and including a collation of published ileal amino acid digestibility for humans.  Report of a sub-committee of the 2011 FAO Consultation on “Protein Quality Evaluation in Human Nutrition.

FAO 2013. Dietary protein quality evaluation in human nutrition – report of an FAO expert consultation. Food and nutrition paper 92. Rome, Italy.

LaPierre P. A., D. Luchini, D. A. Ross, and M. E. Van Amburgh. 2019. Effects of Precision Essential Amino Acid Formulation on a Metabolizable Energy Basis for Lactating Dairy Cows. Cornell Nutr. Conf. Feed Manf. Syracuse, NY.

Wilkinson, J.M. Re-defining efficiency of feed use by livestock. 2011. Anim. 5:1014-1022.

White, R.R. and M.B. Hall. 2017. Nutritional and greenhouse gas impacts of removing animals from US agriculture. Proc. Nat. Acad. Sci. (PNAS). 114:E10301-E10308.

For more information, feel free to contact Jim Aldrich: Jim@CSAAnimalNutrition.com

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