There is no essential need for food of animal origin for human beings, but the consumption of milk, eggs, meat and fish may substantially contribute to a more balanced and palatable human diet. Therefore one of the main goals of animal husbandry is the production of food of animal origin. Such food contributes substantially to meeting the human requirements in essential amino acids (e.g., [
74,
75,
76]) because of the high content in essential amino acids (such as lysine, methionine and cysteine, threonine, leucine,
etc.; see [
77]). Furthermore, such food contains important minor nutrients like major and trace minerals (such as Ca, P, Cu, Fe, I, Se, Zn) and vitamins (e.g., A, E, some B-vitamins, especially B
12) and has a considerable enjoyment value [
78]. Human nutritionists [
79,
80] recommend that about one third of the daily protein requirement (0.66–1 g per kg body weight and day; [
75,
80,
81]) should originate from protein of animal origin to guarantee a more balanced diet, especially for “risk groups” such as pregnant and lactating women, infants and children. Therefore, one endpoint, or the outcome of specific animal yields could be edible protein or essential amino acids and should be clearly defined. Otherwise there will be discrepancies in calculations and variations in the results between various working groups as shown in
Table 1 and
Table 2.
3.2. Food from Slaughtered Animals
It is much more difficult to quantify and characterize the yield from the animal body after slaughtering and processing. The GHG balance per kilogram body weight gain can only be calculated on the farm level. Mialon
et al. [
82] carried out a feeding study with Blond d’Àquitaine bulls in the finishing phase (400–650 kg body weight) including various feeding systems and weight gains between 1,494 and 1,862 g/d and Doreau
et al. [
83] calculated a CF between 3.6 and 4.7 kg CO
2eq per kg body weight gain. Those values are similar to high daily weight gains as shown in
Table 5, but much lower than for lower weight gains. Normally, the GHG emissions for the whole beef system include also emissions of cows, calves and heifers, needed to produce beef. They are much higher than in the system dairy cow—growing/fattening bulls for beef (see allocation).
Mostly the term “meat” is used, but it is not clearly described, what it means (real meat or meat plus bones). Peters
et al. [
37] introduced the term “hot standard carcass weight” (HSCW) as the weight at the exit gate of the meat processing plant. It varies between 50–62% of the live weight of the cattle to be slaughtered, but it may vary between 50% in the case of sheep and up to 80% for fattening turkeys (e.g., [
12,
37,
83]).
In the case of animals for meat/fish production the following endpoints can be measured:
− Weight gain of the animal (per day or per growing period) during the whole life span
− Weight gain of animal without gastro-intestinal tract
− Empty body weight (or carcass weight; meat and bones; warm as HSCW or cold)
− Meat (empty body minus bones)
− Edible fraction (meat plus edible organs and tissues)
− Edible protein (edible fractions of the carcass multiplied with their specific protein content).
Therefore it is really difficult to find an adequate CF for meat or edible products from slaughtered animals. Various authors used different bases to calculate CF for products from slaughtered animals. Williams
et al. [
12] estimated the killing out percentages for beef and poultry with 55 and 70% and 72, 75 and 77% for pigs with live weights of 76, 87 and 109 kg, respectively. Lesschen
et al. [
7] used fixed values to calculate the carcass fraction from the final body weight of animals (e.g., 58% for beef; 75% for pork and 71% for poultry). Most authors used a fixed fraction of 0.9 for all animal species for conversion of carcass weight to edible “meat”. De Vries and de Boer [
5] used calculation factors to determine the amount of edible product per kg live weight by 0.43; 0.53 and 0.56 for beef, pork and poultry.
Table 4 shows potential outputs for growing/fattening cattle under consideration of various endpoints as mentioned above.
Calculation of CF may base on various outputs. For practical reasons carcass weight or weight gain (warm or cold) would be the most important endpoint to measure the yield of slaughtered animals because this weight is measurable in the abattoir [
37] and can be used for further calculations. Based on the values derived from
Table 4, CF is calculated for various endpoints under consideration of differences in feeding and greenhouse gas emissions and is shown in
Table 5.
Table 4.
Model calculation to show various endpoints for growing/fattening bulls (150-550 kg body weight; calculation based on data collected by [
84]).
Table 4.
Model calculation to show various endpoints for growing/fattening bulls (150-550 kg body weight; calculation based on data collected by [84]).
Gross weight gain (g/day) | Weight gain without content of intestinal tract (g/day) | Carcass weight (warm; % of weight gain) | Carcass weight gain (warm; g/day) | Meat gain (% of weight gain) | Meat gain (g/day) | Edible fraction gain 1 (g/day) | Edible protein (g/day; 19% protein in edible fraction) |
---|
500 | 438 | 50 | 250 | 40 | 200 | 250 | 48 |
1,000 | 900 | 53 | 530 | 44 | 440 | 490 | 93 |
1,500 | 1,385 | 56 | 840 | 48 | 720 | 770 | 146 |
Table 5.
Model calculations for CF of beef (150-550 kg body weight
1) depending on feeding, weight gain, methane- and N
2O-emissions and N-excretion [
28]
Table 5.
Model calculations for CF of beef (150-550 kg body weight 1) depending on feeding, weight gain, methane- and N2O-emissions and N-excretion [28]
Weight gain (g/day) | Feed intake (kg DM/ (animal x day) | Portion concentrate (% of DM-intake) 1,2 | Methane emissions (g/kg DM) | N-excretion (g/day) | N2O-synthesis (% of N-excretion) | Carbon footprints (kg CO2eq/kg) |
---|
Weight gain | Empty carcass weight gain | Edible fraction gain | Edible Protein |
---|
500 (Pasture, no concentrate) | 6.5 | 0 | 26 | 110 | 2 | 11.5 | 23.0 | 28.0 | 110 |
1,000 (Indoor, grass silage, some concentrate) | 7.0 | 15 | 24 | 130 | 1 | 5.5 | 11.0 | 13.8 | 55 |
1,500 (Indoor, corn silage, concentrate) | 7.5 | 30 | 22 | 150 | 0.5 | 3.5 | 7.0 | 9.0 | 35 |
3.3. Edible Protein as Most Important Objective of Animal Husbandry
The production of protein of animal origin is one of the most important goals of animal husbandry [
5]. On the other hand, the efficiency and the emissions of animal products can be also compared on the basis of edible protein. The N or protein content of various foods of animal origin may vary from values used for calculations in
Table 6 (data by [
84] on the basis of own studies). Our data agrees with values used by Lesschen
et al. [
7], and it does not substantially disagree with values from human food tables (see
Table 6). De Vries and de Boer [
5] used for their calculations 190 g protein/kg edible beef, pork and poultry meat; 30 g per kg milk products and 130 g per kg eggs.
Considering various influencing factors such as animal yields, feeding, edible fractions and protein content in the edible fractions, the yield of edible protein per day and per kg body weight of animals is given in
Table 7.
Table 6.
Published data regarding the protein content of some edible animal products (in g per kg edible product).
Table 6.
Published data regarding the protein content of some edible animal products (in g per kg edible product).
Product | References |
---|
| [7] 1 | [77] | [84] | [85] | [86,87,88,89] |
Cows milk | 34.4 | 33.3 (30.8-37.0) | 32 | 34 | 34 |
Beef | 206 | 220 2 (206-227) | 190 | 206-212 | 170-200 |
Pork | 156 | 220 2 (195-240) | 150 | 183-216 | 157 (129-178) |
Broiler | 206 | 199 | 200 | 182-242 | n.d. |
Eggs | 119 | 125 | 120 | 125 | 121 (110-124) |
The feeding may influence CF of food of animal origin. In the case of ruminants, higher amounts of concentrate are required for higher animal yields. The proportion of by-products [
90,
91] used in animal feeding does not only have nutritional implications, but it also affects the results of calculations on land use [
92]. There are large differences in protein yield per animal per day or per kg body weight and day depending on animal species and category as well as their performances and the fractions considered as edible (see
Table 7).
Table 7.
Influence of animal species, categories and performances on yield of edible protein [
84].
Table 7.
Influence of animal species, categories and performances on yield of edible protein [84].
Protein source (Body weight) | Performance per day | Dry matter intake (kg per day) | Roughage to concentrate ratio (on DM base, %) | Edible fraction (% of product or body mass) | Protein in edible fraction (g per kg fresh matter) | Edible protein (g per day) | Edible protein (g per kg body weight and day) |
---|
Dairy cow (650 kg) | 10 kg milk | 12 | 90/10 | 95 | 34 | 323 | 0.5 |
20 kg milk | 16 | 75/25 | 646 | 1.0 |
40 kg milk | 25 | 50/50 | 1292 | 2.0 |
Dairy goat (60 kg) | 2 kg milk | 2 | 80/20 | 95 | 36 | 68 | 1.1 |
5 kg milk | 2.5 | 50/50 | 170 | 2.8 |
Beef cattle (350 kg) | 500 g 1 | 6.5 | 95/5 | 50 | 190 | 48 | 0.14 |
1,000 g 1 | 7.0 | 85/15 | 95 | 0.27 |
1,500 g 1 | 7.5 | 70/30 | 143 | 0.41 |
Growing/fattening pig (80 kg) | 500 g 1 | 1.8 | 20/80 | 60 | 150 | 45 | 0.56 |
700 g 1 | 2 | 10/90 | 63 | 0.8 |
1,000 g 1 | 2.2 | 0/100 | 81 | 1.0 |
Broiler (1.5 kg) | 40 g 1 | 0.07 | 10/90 | 60 | 200 | 4.8 | 3.2 |
60 g 1 | 0.08 | 0/100 | 7.2 | 4.8 |
Laying hen (1.8 kg) | 50% 2 | 0.10 | 20/80 | 95 | 120 | 3.4 | 1.9 |
70% 2 | 0.11 | 10/90 | 4.8 | 2.7 |
90% 2 | 0.12 | 0/100 | 6.2 | 3.4 |
Table 7 shows the highest protein yields per kg body weight for growing broilers as well as for laying and lactating animals and the lowest values for growing/fattening ruminants. Based on those values, emissions per kg edible protein are given in
Table 8. Higher proportions of edible fractions or higher protein content (e.g., 50 g protein per kg camel milk, [
77]) as shown in
Table 6 and
Table 7 may increase the protein yield and reduce the CF per unit of product.
Apart from protein food of animal origin also contains fat and some carbohydrates which contribute to human nutrition and which may replace energy of plant origin in human diets.
At high levels of performance there are remarkable differences in CO
2 emissions due to human consumption of 1 g protein from food of animal origin (eggs and meat from broiler < pork < milk < beef, see
Table 7). But here it has to be emphasized that this protein intake is accompanied—willingly or not—by an energy intake from the protein itself, but also from further nutrients like lactose and fat in milk or from fat in meat or eggs. Therefore, it should be avoided to attribute the CO
2 burden to the protein fraction (“edible protein”) exclusively. To prevent that this fact is neglected, there are different alternatives:
In a first simple method, the CO
2 emission due to 1 kg edible protein could be used as CO
2 burden of consumed energy (for example: 1 kg edible protein of eggs corresponds to about 8 kg egg corresponding to 51.6 MJ energy (calculated by [
77]); these combined intakes are related to 3 kg CO
2).
One alternative could be a “nutritional allocation” (as described before for economic allocation), meaning that the CO2 emissions are attributed to different functions of the food (source of protein/source of energy/source of further essential nutrients).
Table 8.
Influence of animal species, categories and performances on emissions (per kg edible protein, own calculations).
Table 8.
Influence of animal species, categories and performances on emissions (per kg edible protein, own calculations).
Protein source (Body weight) | Performance per animal per day | N-excretion (% of intake) | Methane emission (g per day) 3 | Emissions in kg per kg protein |
---|
P | N | CH43 | CO2eq |
---|
Dairy cow (650 kg) | 10 kg milk | 75 | 310 | 0.10 | 0.65 | 1.0 | 30 |
20 kg milk | 70 | 380 | 0.06 | 0.44 | 0.6 | 16 |
40 kg milk | 65 | 520 | 0.04 | 0.24 | 0.4 | 12 |
Dairy goat (60 kg) | 2 kg milk | 75 | 50 | 0.08 | 0.5 | 0.8 | 20 |
5 kg milk | 65 | 60 | 0.04 | 0.2 | 0.4 | 10 |
Beef cattle (350 kg) | 500 g 1 | 90 | 170 | 0.30 | 2.3 | 3.5 | 110 |
1,000 g 1 | 84 | 175 | 0.18 | 1.3 | 1.7 | 55 |
1,500 g 1 | 80 | 180 | 0.14 | 1.0 | 1.2 | 35 |
Growing/fattening pig (80 kg) | 500 g 1 | 85 | 5 | 0.20 | 1.0 | 0.12 | 16 |
700 g 1 | 80 | 5 | 0.12 | 0.7 | 0.08 | 12 |
900 g 1 | 75 | 5 | 0.09 | 0.55 | 0.05 | 10 |
Broilers (1.5 kg) | 40 g 1 | 70 | Traces | 0.04 | 0.35 | 0.01 | 4 |
60 g 1 | 60 | 0.03 | 0.25 | 0.01 | 3 |
Laying hen (1.8 kg) | 50% 2 | 80 | Traces | 0.12 | 0.6 | 0.03 | 7 |
70% 2 | 65 | 0.07 | 0.4 | 0.02 | 5 |
90% 2 | 55 | 0.05 | 0.3 | 0.02 | 3 |
In a first simple step it is recommended to diminish the CO
2 emission per 1 kg edible protein (
Table 8) by the CO
2 amounts that would occur at an identical energy intake from food of plants (energy from carbohydrates and fat). It means that an intake of 1 kg protein from eggs (corresponds to 8 kg eggs; see
Table 7; and corresponding to 51.6 MJ energy) saves high amounts of other food (and their CO
2 burden). A more sophisticated way of an “allocation” within the foods could be to differentiate between “protein derived energy” and “non-protein derived energy”. In milk and eggs more than 50% of the total energy content is related to the non-protein-fraction (lactose/fat), therefore, it is questionable whether the entire CO
2 emission should be attributed only to the protein intake. Due to the very low CO
2 emission caused by energy intake of carbohydrates and fat from plants/seeds [
12,
15,
17] it would avoid/save high amounts of CO
2 emissions, if the production of food of animal origin focussed on “edible protein” and not on energy of non-proteinaceous fractions.
Furthermore animal products are not only used as food or respectively, as protein/amino acids, and energy sources; they also offer some other important side-products such as skins or hides, fish meal or meat and bone meal, etc. A kind of combined “nutritional/further purposes allocation” may contribute to a more scientific assessment of CF for nutrient and energy supply as well as further uses.
Advantages and weaknesses of endpoints (outputs) of various types of animal production are summarized in
Table 9. All endpoints are characterized by some advantages and disadvantages. From nutritional and scientific points of view the edible protein seems to be the most favourable measurement, but its measurement is not easy and requires some analytical work (see
Table 9). Land requirements (e.g., arable land and/or grassland) as well types and intensities of food production may be calculated on the basis of various protein sources for human nutrition. Such calculations can contribute for better understanding of various conflicting aims in the field of food production, human nutrition, use of unlimited and limited resources and resource efficiency, emissions and further points in public discussion.
Table 9.
Advantages and disadvantages of various outputs/endpoints of animal yields.
Table 9.
Advantages and disadvantages of various outputs/endpoints of animal yields.
Animal yields | Advantages | Disadvantages |
---|
Milk, Eggs | Easily measurable, almost complete edible | Variation in protein, fat and energy yield, analyses may be useful |
Body weight gain | Easily measurable | High portion of non edible fractions in the gains |
Carcass weight | Easily measurable | Contains still fractions which are not edible (e.g., bones) |
Meat, edible fraction | Completely edible | Categorization and separation not easy |
Edible protein | Most important objective of animal production; comparison of various methods and sources to produce protein of animal origin | Categorization of various fractions as edible and difficulties to measure; additional analytical work; variation in N/protein content |