Use of threonine improves amino acid balance of swine diets
In comparison to protein ingredients, crystalline amino acids enable more precise adjustment of dietary amino acid balance. It is known that threonine is the second limiting amino acid in most swine diets. For diets that have already been supplemented with L-lysine HCl, threonine co-acts with lysine as the first limiting AA, which means an addition of threonine and higher level of lysineHCl can further improve amino acid balance and lower feed cost. However, such practice is rarely known in China, the addition of threonine is minimal, and the spread of this knowledge has a long way to go.
If crystalline threonine is not used, meeting the requirement of threonine in the diet will rely only on the use of higher levels of protein ingredients. However, protein ingredients contain not only threonine but also other indispensable and dispensable amino acids. The consequence of using protein ingredients to meet threonine requirement is that amino acid balance is under-optimized, the kinds and extents of excessive amino acids are greater, and the cost of feeds is uneconomical. Therefore, threonine can be considered as a hurdle or bottleneck that formulators must face.
Similar to the benefit of using lysine-HCl in diet formulation, the use of threonine may reduce wastage of both dispensable and indispensable amino acids, or reduce the level of dietary crude protein. It has been known to western swine producers that the inclusion of crystalline amino acids can reduce dietary crude protein levels without jeopardizing animal performance or, in some circumstances, even improve it.
This article compares the degree of amino acid balance of swine diets in four countries in Europe and Asia, assesses problems and potentials existing in current Chinese formulation practices, and gives examples to show the benefits of using crystalline threonine to improve amino acid balance in swine diets.
1. Poorer amino acid balance in Chinese swine diets leads to more wastage of indispensable amino acids
A survey conducted by Ajinomoto Animal Nutrition Group in 1999-2001 on 379 typical swine diets in 21 countries showed there are clear differences in amino acid balance among those main swine producing countries and regions. Table 1 lists average dietary levels of crude protein and lysine in pig starter diets in China, France, Germany and Thailand.
Table 1. Dietary levels of crude protein and lysine in pig starter diets in different countries
An important feature of the Chinese swine diets is that the level of the first limiting amino acid, lysine is relatively low compared with those in the other countries. The lysine level of Chinese pig starter diets is 17.4% lower than that in France and Thailand, 8.3% lower than that in Germany, but the level of crude protein is 3.2% higher than in France and 5.2% higher than in Germany. This is an example that much stress has been placed on crude protein in China. Nutritionists should give up the out-dated practice to determine dietary crude protein level empirically. On the contrary, the dietary crude protein levels should be decided specifically for each formula by least-cost formulation programs.
Despite that the crude protein level in China is lower than in Thailand, the lysine level in Thai diets is much higher. Thus, if we judge the degree of amino acid balance based on the ratio of lysine/protein (ideally near 6.5%), the Chinese swine diets ranks the lowest among the four countries, being equivalent to 88.5% of Thailand, 88.1% of Germany, and only 82.2% of France.
The greatest deviation from the ideal ratio of lysine/crude protein indicates a common malpractice in designing diets in China, i.e., over-emphasis on crude protein whilst neglecting amino acid balance. Such practice causes unbalance of amino acids in the formulation and leads to wastage of protein or amino acids. Based on the data in Table 1, one could estimate that for each hog slaughtered in China, the amino acid consumption would be at least 20% higher than in France.
2. More waste of amino acids in Chinese swine diets
Table 2 lists the ratios between indispensable amino acids and lysine in swine diets. For comparison, the ideal protein model by Chung and Baker (1992) from the University of Illinois is also listed.
Table 2. Comparison of dietary amino acid balance in pig starter diets in different countries
1Ideal ratio of Kansas State University (1998).
If the level of an indispensable amino acid meets the requirement set by ideal protein, the performance of the pig will not be affected by this amino acid, and the amount that is in excess to the requirement will be wasted. Judging from the ratios of indispensable amino acids to lysine (ideal protein), all dietary indispensable amino acids (except lysine) in China diets are oversupplied in greater degree than in other countries listed. Compared to France, the excess is Met+Cys: +22.5%,Thr: +9.6%, Trp: +11%,Ile: +27.6%, Val: +27.9%, Phe+Tyr: +30.3%,His: +34.3%, Leu: +47.2%, and Arg: +26.7%. Compared to Germany, it is Met+Cys: +9.1%, Thr: +6.3%, Trp: +5.3%, Ile: +23.3%, Val: +19.2%, Phe+Tyr: +23.5%, His: +23.7%, Leu: +36.8%, and Arg: +21.1%. Even compared Thailand, the indispensable amino acids in Chinese pig starter diets are excessive (Met+Cys: +9.1%, Thr: +3.0%, Ile: +8.8%, Val: +13.0%, Phe+Tyr: +12.7%, His: +17.5%, Leu: +27.9%), with exception of Trp: +0% and Arg: -6.4%.
As shown in Table 2, the dietary level of isoleucine in both China and Thailand exceeds the levels set by the ideal protein model, while this level in France and Germany just satisfies the requirement based on ideal protein. However, in all these four countries, the dietary levels of valine, phenylalanine and tyrosine, histidine, leucine and arginine all exceed the requirements by ideal protein, indicating widespread wastage of these amino acids in swine feeds. This has to do with the absence of supply of these amino acids as crystalline, particularly isoleucine and valine in the market place.
It is interesting to note that the ratio of Met+Cys to lysine in both Germany and France seems to follow the ideal protein model by Kansas State University (Met+Cys/Lys: 55%), while China and Thailand seem to follow the model proposed by University of Illinois (Met+Cys/Lys: 60%).
3. Use of threonine improves amino acid balance and further reduces formulation cost
Swine diets in Europe (France and Germany) achieved a "no-excess" balance of indispensable amino acids except valine, histidine, leucine and arginine. This could be attributed to the common use of crystalline threonine in this region. Since today free isoleucine is unavailable to the feed industry, the wastage of certain amino acids limiting after isoleucine seems unavoidable for the time being.
In contract, the amino acid balance in Asia (China and Thailand) is relatively poor, and there is even considerable wastage in threonine, tryptophan and isoleucine whose wastage is rarely seen in Europe and the extent of wastage in other amino acids examined are greater than in Europe. This is probably due to the limited use of threonine in Asian diets. The application of these four commercially-available free amino acids (Lys, Thr, Met and Try) has not become a popular practice in diet formulation, particularly threonine. The confinement of the use of crystalline threonine has become a bottleneck to further improve amino acid balance and has led to more wastage of protein sources in China.
As mentioned previously, threonine is the 2nd limiting amino acid in swine diets. When the formulation program picks up lysine-HCl in the formula, the ranking of the most limiting amino acid changes from lysine only to lysine and threonine simultaneously. Under such circumstance, if there is no choice of crystalline threonine, better amino acid balance will be impossible, and a greater reduction of amino acid wastage will not be obtained. Therefore, the use of crystalline threonine in diet formulation is critical to improve amino acid balance, reduce amino acid wastage, and lower formula cost. Because threonine has become co-limiting with lysine, one may argue that without using crystalline threonine, the dietary amino acid balance will never be further improved.
Application of ideal protein model to re-optimization of 130 swine diet formulas collected from field in China showed that the cost of feed was greatly affected by allowing the choice of crystalline threonine in the formulation process. During the formulation, the level of crude protein was determined by the formulation software itself, and all the indispensable amino acids reached the level of ideal protein model. Results showed that the addition of threonine improves amino acid balance and reduces formulation costs irrespective of growth phases (Table 3).
Table 3. The effects of applying ideal protein model in least-cost formulation on dietary amino acid balance and formula cost
1Stands for standardized ileal digestible amino acids.
2 Exclusive of vitamins, minerals and medicines, major ingredient prices in RMB yuan/kg:
corn 1.27,soybean meal 2.82, fishmeal 5.71, Lysine-HCl 23.2 and threonine 32.5.
As shown in Table 3, corresponding to the addition of crystalline threonine in the diets, crude protein level decreased on average by 1.23-1.04%, and formula cost lowered on average by 70-21 yuan/t. Inclusion of crystalline threonine improved overall amino acid balance (judging by the ratio of lysine/crude protein) by 6.1%, 6.2% and 7.1% for starter, grower and finisher, respectively. For the formulas containing added threonine, the level of the most probable limiting amino acids after threonine such as tryptophan and isoleucine can still meet requirements of ideal protein. Thus, use of crystalline threonine resulted in improvement in amino acid balance and reduction in wastage and hence, formula cost without causing amino acid deficiency.
4. High protein feed is of high quality: A wrong perception
It must be pointed out that a prevailing practice in diet formulation in China is to set up high levels of crude protein while neglect threonine and other indispensable amino acids, if not lysine. This is an out-dated and wrongful practice. Such practice not only prevents the utilization of the latest research findings to improve amino acid balance, but also misleads farmers to value diets that are rich in crude protein as high quality feeds for their animals. This misperception must be corrected.
First, a comparison of crude protein level to judge the quality of feeds is meaningful only when diets have the same level of dietary lysine and available energy. But the conclusion of this comparison may be unexpected to many: the ones high in crude protein are often of poorer quality. This is because to reach the same levels of dietary lysine, use of ingredients poor in amino acid profile and digestibility will ultimately lead to higher levels of crude protein than use of ingredients with good amino acid profiles and digestibility. Therefore, diets higher in crude protein may not necessarily be of qood quality.
Secondly, more wastage of amino acids in high crude protein diets means a lower dietary level of net energy (NE). It is known that dietary NE content is negatively correlated to its crude protein level. Diets differing in 1 percentage unit crude protein represent a net energy difference equal to 2.5 kg vegetable oil/t of diet (Qiao, 2003). This may explain partly why hogs fed on high protein diets tend to be leaner.
Thirdly, high protein diets are unfriendly to the environment. Not only will these diets increase the emission of ammonia and hydrogen sulfide in the farmhouse that may lead to harmful effects on animal health and performance, but also will these diets cause greater pollution to environmental resources like water and air. According to Kerr (1995), growing one hog will produce 3.6 kg nitrogen (N) waste of which 60% is released in the manure and 40% emitted into the air. As a rule of thumb, for every 1% reduction in dietary crude protein, the N excretion is lowered by 10%. Thus, 1% reduction in dietary crude protein will reduce N emission as ammonia by 0.18 kg/hog and non-gaseous N as urea by 0.46 kg/hog. Conservatively, suppose 500 million hogs are produced in China per year, a 1% reduction in crude protein would avoid N emission into the air as ammonia by 90,000 ton per year and N excretion into the earth stratum as urea by 230,000 ton per year.
Fourthly, high crude protein diets severely waste protein resources. In the year 2003, the total feed output in China was equivalent to compound feed of about 160 million tons. Had a 1% reduction in crude protein (equivalent to lowering 2.3% soybean meal inclusion in feeds) been achieved, a total saving of 3,680,000 tons of soybean meal would have been realized. Needless to point out the significance of this saving to a country short of feed protein resources.
Amino acid balance depends not only on the advancement of animal nutrition research, but also on the availability of commercial supply of free amino acids. Increasing understanding of dietary amino acid balance and industry ability to supply commercial amino acids will facilitate more free amino acids to be utilized by feed producers, which will further reduce dietary crude protein levels as well as formulation cost of the feed.
Starter Pig Recommendations. In: The Kansas Swine Nutrition Guide, Kansas State University Agricultural Experiment Station and Cooperative Extension Service. 1999.
Chung T K, Baker D. H. Ideal Amino Acid Pattern for 10-Kilogram Pigs. J. Anim. Sci. 1992. 70: 3102-3111.
Kerr B J, Nutritional strategies for waste reduction management: nitrogen. In: Longenecker J B and Spears J W, New Horizons in Animal Nutrition and Health. The Institute of Nutrition and The University of North Carolina, 1995.47-68.
Qiao Y R. Advance in lower protein diet research for pig and broiler. Feed Industry. 2003. (11)17-21.