Protein Quality and Amino Acid Digestibility - Poultry [Skip to Content]

Illinois Livestock Trail
Protein Quality and Amino Acid Digestibility
by Carl M. Parsons

Multi-State Poultry Meeting, May 25-27, 1999


Animal protein meals such as feather meal (FM), meat and bone meal (MBM) and poultry by-product meal (PBPM) are important feedstuffs in poultry nutrition. These ingredients are high in protein but their protein quality can vary. For example, Sibbald (1) and Parsons (2) reported that the digestibility of amino acids, particularly lysine (Lys) and cystine (Cys), in MBM varied greatly among samples. There are several factors that have been reported or hypothesized to influence protein quality of animal protein meals. This paper will summarize some of our recent research to quantitate the effect of factors such as processing systems, processing temperatures, processing pressure and ash on the protein quality of animal meals, with the emphasis being on MBM.


Before discussing the factors affecting protein quality, I would like to first discuss which amino acids are most deficient and most important in animal meals. The order of amino acid limitation or deficiency in MBM and PBPM determined in my lab are shown in Table 1. These orders of limitation were assessed using two-week chick growth trials in which either MBM or PBPM provided the only source of dietary protein. The three most deficient amino acids were the same for both MBM and PBPM, namely, Cys, Trp and Thr. Cystine and Trp were much more deficient than the others and Cys was more limiting than Trp in PBPM but was equally first limiting with Trp in MBM. The more severe deficiency of Trp in MBM is probably due to the higher ash or bone content in MBM. Bone protein is very deficient in Trp. When looking beyond the first three limiting amino acids, the order varied for MBM vs PBPM, particularly with Ile and Met being more deficient in MBM than PBPM.

Table 1. Order of Amino Acid Limitation in Meat and Bone Meal and Poultry By-Product Meal for Chicks1

Amino acid Meat & bone meal Poultry by-product meal
Cystine 1 1
Tryptophan 1 2
Threonine 3 3
Isoleucine 4  
Phe + Tyr 4 6
Methionine 6  
Lysine 7 3
Valine 8 5
Histidine 8 6
1From Wang et al. (3) and Wang and Parsons (4).


We have recently confirmed earlier reports that the variation in amino acid digestibility among samples of MBM is substantial. The results of an evaluation of 14 random industry samples of MBM are summarized in Table 2. Lysine digestibility varied from 69 to 88% and Cys digestibility varied from 37 to 72% for four selected samples. It is further interesting that the variability in digestibility was greater for Lys and Cys than for Thr and particularly Met. Moreover, even though both Met and Cys are sulfur amino acids, the variation in Cys digestibility is greater than all other amino acids, whereas the variation in Met digestibility is generally less than all others. Thus, there are considerable differences in digestibility among specific individual amino acids.

Table 2. Variation in Amino Acid Digestibility Coefficients (%) for Different Samples of Meat and Bone Meal1

Sample Lysine Cystine Threonine Methionine
5 88 72 86 89
12 69 37 72 81
13 86 68 86 91
16 77 55 79 84
1Parsons et al. (5).

Effect of raw material source, processing system and processing temperature

A large study was conducted in cooperation with the Fats and Proteins Research Foundation in attempt to identify the major commercial factors affecting protein quality of FM, MBM, and PBPM. Six samples of FM, 32 samples of MBM, and 12 samples of PBPM were processed in different rendering systems at two different temperatures (low vs high). The raw material source also varied for the MBM (beef, pork, mixed species). The latter variable was not found to have any substantial effect on amino acid digestibility. The type of processing system and processing temperature significantly affected amino acid digestibility of FM, MBM, and PBPM. The most striking affects were observed for MBM and a small portion of the data from that study are summarized in Table 3. The digestibility of Lys and Cys was considerably higher for MBM produced in processing System B than in System A. Moreover, System B generally yielded very high Lys digestibility of 90% or greater. In addition, amino acid digestibility was higher when the MBM was processed at the lower temperature in both Systems A and B, but the temperature effect was greater in System A. At least part of the probable explanation for the lower amino acid digestibilities for System A is that the MBM was processed at higher temperatures than in System B. Cystine was the amino acid that was most affected by processing system and temperature, with the high temperature used in System A yielding very low Cys digestibilities. Finally, it is important to note that the results for System B show MBM with very high amino acid digestibility can be produced with good processing procedures.

Table 3. Effect of Processing System and Temperature on Amino Acid Digestibility of Meat and Bone Meal1

    Digest. Coefficient (%)
Processing system Processing temperature (°C) Lysine Cystine
A 132 85 39
A 152 78 20
A 132 81 50
A 132 71 31
B 110 92 71
B 140 90 62
B 110 91 59
B 140 87 51
1From Wang and Parsons (6).

Effect of pressure processing

Recently, we have conducted research with MBM to assess the effects of pressure processing on AA digestibility. The reason for evaluating pressure processing is due to concerns of bovine spongiform encephalopathy (BSE). The feeding of BSE-infected MBM to ruminants may cause BSE. It is suspected that the consumption of meat from BSE-infected cattle may, in turn, cause Creutzfeldt-Jakob Disease (CJD) in humans. Consequently, extreme restrictions have been placed on the feeding of MBM in the United Kingdom and the feeding of MBM containing ruminant tissue to ruminants has been banned in the U.S. BSE and CJD are caused by heat-stable prion proteins that can be at least partially inactivated by pressure cooking. The European Union requires that MBM be processed at 3 atmospheres (30 gauge psi) for 20 minutes at 133° C (271° F) to reduce the risk of BSE and CJD. Future requirements/regulations for MBM processing are unknown. It is possible that MBM may have to be pressure processed in the U.S. in the future.

Processing conventionally-rendered MBM at 15, 30, 45 or 60 psi for 20 min. influenced AA digestibility (Table 4). Pressures of 15 and 30 psi produced moderate depressions in digestibility of most AA, including Thr, Lys and Met. The reductions in digestibility of Cys were greater than those for the other AA. Increasing the pressure to 60 psi produced large decreases in AA digestibility for all AA, with by far greatest reduction occurring for Cys. The large reduction in digestibility at 60 psi was due both the destruction of AA and decreased digestibility of AA that were not destroyed.

Table 4. Effect of Pressure Processing on Amino Acid Digestibility Coefficients (%) for Meat and Bone Meal1

Gauge pressure (psi)2 Thr Lys Met Cys
0 80a 75a 79a 65a
15 74b 65b 74b 46b
30 73b 64b 75b 44b
45 71b 60b 73b 44b
60 58c 45c 58c 14c
a-cMeans within a column with no common superscript differ (P < .05).
1Shirley and Parsons, unpublished data.
2Meat and bone meals processed for 20 min. at the specified pressure.

Effect of ash content

Another variable that is alleged to influence protein quality and amino acid digestibility of animals meals such as MBM and PBPM is ash content. Meals that contain higher ash are generally considered to be of lower protein quality and have lower amino acid digestibility. However, when one reviews the literature, there are limited data to support the effect of ash on protein quality and little or no data to support an effect of ash on amino acid digestibility. The results of one of our initial studies to evaluate the effect of ash for MBM and PBPM are summarized in Table 5. Increased ash content had no negative effect on amino acid digestibility of MBM or PBPM. In contrast, increased ash did have a negative effect on protein quality as measured by protein efficiency ratio (PER) in a 10-day chick growth trial. In the PER trial, 10% CP diets were fed in which the MBM or PBPM provided the only source of dietary CP. The PER values were calculated by dividing the weight gain (g) by the protein intake (g). The reduction in protein quality (PER values) due to increased ash was not due to reduced amino acid digestibility but was due to poorer total amino acid balance or profile; that is, the level of sulfur amino acids and Trp per unit of CP decreased as the ash content increased. The latter effect is probably due to the increased bone content in the higher ash samples. Bone contains approximately 25 to 30% CP and the protein is of very low quality due to extreme deficiencies of sulfur amino acids, particularly Cys, and Trp. Thus, our initial results indicate that the protein quality of MBM and PBPM do, indeed, decrease with increasing ash content, but the reduction is due to a poorer balance of total amino acids, not decreased amino acid digestibility.

Table 5. Digestibility of Total Essential Amino Acids (TEAA) and Protein Efficiency Ratio (PER) of Meat and Bone Meals (MBM) and Poultry By-Product Meals (PBP) Varying in Ash Content1

Meal Ash (%) Digest. Coeff. for TEAA (%) PER2
MBM 24 73 1.43
MBM 35 78 0.68
PBP 7 75 2.35
PBP 16 78 2.10
1From Johnson and Parsons (7) and Johnson et al. (8).
2PER = chick weight gain (g) divided by protein intake (g).


It continues to be of great importance to nutritionists to be able to predict protein quality of animal meals with laboratory assays so that expensive, time consuming animal trials do not have to be conducted. We have evaluated many different types of assays but have only found a few that are useful. Both ash and CP level are significantly correlated with PER values (Table 6). The reasons for the ash relationship are discussed in the previous section and the CP correlation is probably due indirectly to the ash level (i.e., samples with high CP are low in ash). The pepsin N digestibility assay is also useful for MBM if the pepsin level is reduced from.2% to .002% or .0002% (Table 6). We found no benefit to reducing the pepsin level from .002% to .0002% for MBM.

Table 6. Correlation Coefficients Among Different Assays for 14 Meat and Bone Meals1

Assay r value
Ash vs protein efficiency ratio -.80*
CP vs protein efficiency ratio .68*
.2% pepsin vs Lys digestibility .25
.002% pepsin vs Lys digestibility .69*
.0002% pepsin vs Lys digestibility .62*
1From Parsons et al. (5).
*r value is significant ( P < .001).


Due to variation in amino acid digestibility values among samples of animal meals, formulation of poultry feeds containing animal meals on an available or digestible amino acid basis should be superior to formulation on a total amino acid concentration basis. The development of the precision-fed cecectomized rooster assay has resulted in a large increase in digestible amino acid data and also provided a means of obtaining ingredient digestibility data within a reasonable amount of time and at a reasonable cost. A number of recent studies (e.g. Fernandez et al., 9; Rostagno et al., 10) have shown definite benefits to formulating diets on a digestible amino acid basis versus a total amino acid basis. The results of a study from my lab for low and high-quality MBM are shown in Table 7. Inclusion of 10 or 20% of either low or high-quality MBM into a corn-soybean meal diet on a total amino acid basis resulted in depressed chick weight gain and/or feed efficiency. When diets were formulated on a digestible amino acid basis, 10% low-quality or 10 or 20% high-quality meal had little or no negative effect on performance. These results clearly illustrate the superiority of formulating diets on a digestible or available amino acid basis. A negative effect was observed from 20% low-quality MBM even on an available amino acid basis. Further experiments showed that the latter negative effect was not associated with amino acids but was due to some other unknown characteristic of the meal.

Table 7. Dietary Formulation with Low-Quality (LQ) and High-Quality (HQ) Meat and Bone Meals (MBM) on a Total Versus an Available Amino Acid (AA) Basis1

Dietary treatment Formulation method Weight gain (g) Gain:feed ratio
Corn-SBM diet 326 .690
10% LQ MBM Total AA 313 .644
20% LQ MBM Total AA 288 .595
10% HQ MBM Total AA 322 .656
20% HQ MBM Total AA 310 .653

10% LQ MBM

Available AA 323 .669
20% LQ MBM Available AA 304 .641
10% HQ MBM Available AA 335 .688
20% HQ MBM Available AA 332 .682
Pooled SEM   3 .004
1All diets contained 20% CP and were fed to chicks from 8 to 22 days of age. Lysine and sulfur amino acid digestibility coefficients (%) for the LQ and HQ MBM were 71 and 62 and 92 and 82, respectively (Wang and Parsons, 11).


  1. Sibbald, I.R., 1986. Bulletin 1986-4E, Agriculture Canada.
  2. Parsons, C.M., 1991. Proc. Tech. Symp., National Renderer's Assoc.
  3. Wang, Xincheng, Castanon, Fructuoso, and Parsons, Carl M., 1997. Poultry Sci. 76:54-58.
  4. Wang, X. and Parsons, C. M., 1998a. British Poultry Science 39:113-116.
  5. Parsons, C.M., Castanon, F., and Han, Y., 1997. Poultry Sci. 76:361-368.
  6. Wang, X. and Parsons, C.M., 1998b. Poultry Sci. 77:834-841.
  7. Johnson, M.L. and Parsons, C.M., 1997. Poultry Sci. 76:1722-1727.
  8. Johnson, M.L., Parsons, C.M., Fahey, Jr., G.C., Merchen, N.R., and Aldrich, C.G., 1998. J. Anim. Sci. 76:1112-1122.
  9. Fernandez, S.R., Zhang, Y., and Parsons, C.M., 1995. Poultry Sci. 74:1168-1179.
  10. Rostagno, H.S., Pupa, J.M.R., and Pack, M., 1995. J. Appl. Poultry Res. 4:293-299.
  11. Wang, X. and Parsons, C.M., 1998c. Poultry Sci. 77:1010-1015.

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