Ideal Amino Acid Balance Pattern for Swine and Its Application in Production

1. Concept

The Ideal Amino Acid Pattern (IAAP) is the counterpart of the Ideal Protein (IP). Both concepts share the same underlying principle: proteins whose amino acid composition and ratios perfectly match the requirements of animals (such as pigs). Specifically, IP refers to proteins that contain an optimal amino acid profile; IAAP, on the other hand, denotes the proportional relationship of each essential amino acid in the ideal protein relative to a reference amino acid—lysine. The primary reasons for selecting lysine as the reference amino acid are as follows: Lysine is the first limiting amino acid in pig diets based on cereal grains; nearly all lysine is utilized for synthesizing body proteins; it is easier to determine the lysine requirements of pigs at different growth stages than those of other amino acids; and chemical analysis of lysine content is simpler than that of sulfur-containing amino acids. The original concept behind IP or IAAP was the Chemical Score (CS), proposed by Mitchell and Block in 1946. This approach used the essential amino acid content of whole egg protein as a reference standard and compared it with other proteins under evaluation. By identifying the amino acid with the greatest deviation from the reference standard among all amino acids present in the protein, the percentage difference was taken as the Chemical Score of the protein being evaluated. By using the essential amino acid composition of eggs as the benchmark for assessing dietary proteins, this approach revealed the fundamental nutritional truth that protein nutrition is, in essence, amino acid nutrition. Thus, protein nutrition evolved from crude protein through digestible and utilizable protein and protein biological value to amino acid nutrition itself. In 1958, Howard first introduced the concept of IP, and it was officially adopted in production by the ARC in 1981. Subsequently, organizations such as the U.S. NRC (1988), the French AEC (1993), and Canada’s PSCI (1995) all successively adopted IP when establishing the amino acid requirements for pigs.

2. A brief overview of the research methods established by Pig IP or IAAP

There are many methods for studying and establishing IAAP, but the four most commonly used methods are as follows:

2.1 Analysis of the amino acid composition of pig carcasses

Based on studies of the amino acid composition of tissues in growing pigs, the theoretical basis for the IAAP method is that the amino acid composition of pig carcasses is highly correlated with their amino acid requirements. With the exception of newborn piglets, whose body proteins have a lower proportion of essential amino acids and a higher proportion of glycine and proline, the amino acid profile of body proteins in growing pigs does not show significant changes throughout the entire growth and fattening stage. Comparative analysis reveals that the amino acid composition of pig tissues closely matches the recommended amino acid pattern required by growing pigs; therefore, it is feasible to establish the IAAP using the amino acid composition values of body tissues.

2.2 Factorial Method

Pig amino acid requirements can be divided into maintenance needs and production needs. Based on the high correlation between tissue amino acids in pigs and their respective requirement levels, the amino acid requirements and the corresponding IAAP can be estimated by measuring the body’s protein deposition. For example, the ARC (1981) model is based on amino acid balance in lean muscle tissue.

2.3 Comprehensive Method

The optimal amino acid pattern is derived by summarizing estimated requirements for individual amino acids in pigs and data on the optimal lysine concentration in dietary protein. The NRC (1998) model was developed using this approach, drawing on data from a literature review.

2.4N Balance Test Method

The N-balance trial is an effective method for studying amino acid balance and nitrogen utilization in animals. This method includes the amino acid subtraction technique and the amino acid addition technique. The theoretical basis for both techniques is the linear relationship between amino acid levels in feed and pig performance. The principle of the amino acid subtraction technique involves sequentially subtracting a certain proportion (typically around 25%) of a specific amino acid from a diet containing all amino acids, then observing the animal’s response after each subtraction. Based on the animal’s responses—such as production performance, plasma urea nitrogen levels, and nitrogen deposition—the limiting amino acid is comprehensively identified. For instance, Fuller et al. (1990) established the IAAP based on the maximum nitrogen deposition observed in pigs. The amino acid addition technique, on the other hand, involves gradually adding synthetic amino acids to the diet—in increasing doses starting from a level below the requirement—for the amino acid under investigation, while ensuring that the requirements for other amino acids are fully met. The model developed by Baker et al. at the University of Illinois in the United States (1992) is based on data obtained from feeding pigs with purified diets supplemented with synthetic amino acids. In addition, there are other research methods, such as the traditional feeding trial method, the isotope tracing method, and the nutritional immunology approach; however, these will not be discussed in detail here.

3. Research Progress on IAAP in Pigs at Different Growth Stages

3.1 Raising Piglets with IAAP

There are relatively few published studies on the ideal amino acid pattern (IAAP) for weaned piglets. A recent study used DL-methionine (DL-Met), which has the same biological potency as methionine, and 2-hydroxy-4-methylthio-butanoic acid (HMTBA), a methionine hydroxy analog, as sources of methionine. The study employed three different methods to estimate the optimal ratios of true digestible sulfur-containing amino acids to lysine in weaned piglets. The results indicated that the average optimal ratios of true digestible sulfur-containing amino acids to lysine for lean-type weaned piglets weighing between 8 and 26 kg were 59.0% based on average daily gain (ADG) and 60.8% based on gain-to-feed ratio (G∶F) (Gaines et al., 2005). In practical production, weaning piglets—especially early weaning—which leads to dramatic changes in nutrition and environment, induces significant stress responses in piglets, including suppression of immune function and reduced disease resistance. As a result, piglets exhibit poor appetite, digestive dysfunction, diarrhea, and growth retardation. Therefore, studying the IAAP under stressful conditions in weaned piglets is of great significance. Li Jianwen et al. (2006) used the partial amino acid subtraction method to investigate the balance ratios among digestible lysine, methionine, tryptophan, and threonine in weaned piglets subjected to experimentally induced immune stress. The results showed that the ideal amino acid patterns for weaned piglets under normal and immune-stressed conditions differ significantly. Under stressed and normal conditions, the balanced patterns of digestible lysine, digestible methionine, digestible tryptophan, and digestible threonine were 100∶27∶29∶59 and 100∶30∶21∶61, respectively.

3.2 Growing and Finishing Pigs IAAP

Although it is generally believed that the amino acid profile of protein in growing pigs does not significantly change throughout the entire growth and fattening phase, it actually varies due to numerous factors, including pig breed, growth stage, and evaluation indicators.

3.2.1 IAAP in Growing-Finishing Pigs of Different Genotypes

Zhang Keying et al. (2000), through a study comparing the digestible lysine, methionine + cysteine, threonine, and tryptophan balance patterns in Large White dual-purpose lean pigs with body weights similar to those of the local Yayan pig breed in China, found that the digestible lysine, methionine + cysteine, threonine, and tryptophan balance patterns for Large White pigs and Yayan pigs were 100∶49∶72∶19 and 100∶78∶76∶21, respectively. This indicates that the IAAP of feed rations varies depending on the genotype of the pigs. The Yayan pig requires a higher proportion of methionine + cysteine, threonine, and tryptophan compared to the Large White pig.

3.2.2 IAAP in Growing-Finishing Pigs of Different Body Weights

According to the recommendations by NRC (1998), the relative proportions of sulfur-containing amino acids, threonine, and tryptophan to lysine all increase with the growing and fattening pigs’ body weight. However, the experimental results reported by Wang Jianming et al. (2001) showed that, except for tryptophan, the proportions of sulfur-containing amino acids and threonine relative to lysine tend to decrease significantly as body weight increases. The reason for this discrepancy between the two studies remains unclear. Nevertheless, some data indicate that the content of sulfur-containing amino acids tends to decline as pig body weight increases. For instance, Kemm et al. (1990) found that the cysteine content in the bodies of pigs weighing 20, 30, and 90 kilograms was 1711, 1519, and 1413 grams per kilogram of protein, respectively.

3.2.3 Growth and Finishing Pigs Fed with Different Protein Levels and Energy Intakes: IAAP

Bikker et al. (1994) found that dietary protein levels and energy intake affect the amino acid composition of pigs. These changes are likely attributable to differences in the proportions of muscle versus non-muscle tissues in the carcass, the ratios of blood to parenchymal tissues in organs, and the overall ratios between carcass and organ tissues throughout the body. Such changes, in turn, can influence the requirements for amino acids and the IAAP.

3.2.4 IAAP of Growing-Finishing Pigs Under Different Evaluation Indices

The results indicate that, when using production performance and meat quality as indicators, the optimal ratio of total sulfur-containing amino acids to lysine in the late-finishing phase of pigs is lower than the 0.65 recommended by NRC 1998. However, if the goal is to minimize fat deposition, this ratio should be 0.65 (Knowles et al., 1998). When taking optimal nitrogen retention and utilization as indicators, the ratio of essential amino acids to non-essential amino acids should be 50:50. Under low-protein conditions, the ratio of essential to non-essential amino acids becomes even more critical. Lenis et al. (1999) found that this ratio can be increased to 70:30 without compromising nitrogen utilization. In this case, the deaminated essential amino acids are efficiently utilized to synthesize non-essential amino acids.

3.2.5 IAAP in Growing-Finishing Pigs Under Different Rearing Conditions

Under conditions of Chinese livestock farming, while ensuring that other essential amino acids meet the required levels, the following amino acid ratio is considered relatively suitable: lysine 100, methionine + cysteine 65, threonine 65, tryptophan 20, and isoleucine 65. The ratio of essential to non-essential amino acids is 50:50 (Wu Xilin, 2006).

4. Practice and Existing Issues of Pig IAAP

A successful example of applying the ideal protein concept in practical production is the formulation of low-protein diets by supplementing monomeric amino acids into the diet. Studies have shown that, provided the lysine content of corn-soybean meal-based diets meets the requirements of growing pigs (20–55 kg), the protein level can be reduced by as much as 22% without compromising pig performance. However, under the same conditions, reducing the protein level by 22% would negatively affect the performance of finishing pigs (55–100 kg) (Tuitoek et al., 1997). Otto et al. (2003) demonstrated that by adding amino acids to achieve an ideal amino acid profile, reducing the crude protein level from 15% to 12% did not impair nitrogen retention. Yet when the crude protein level was lowered from 15% to 6%, the ileal digestibility of both essential and non-essential amino acids improved. Moreover, reports indicate that when diets are formulated based on apparent ileal digestible amino acids, the source of protein does not affect production performance, carcass quality, or body composition. On the other hand, lowering the Lys:DE ratio inhibits pig growth but has no impact on meat quality traits (Szabo et al., 2001). Thus, it is evident that, fully in line with the Ideal Amino Acid Pattern (IAAP), the crude protein level in diets can be reduced by supplementing monomeric amino acids. This approach not only conserves limited protein resources, exploits unconventional protein feed sources, and reduces nitrogen pollution from pig manure, but also helps minimize allergic reactions in piglets to protein sources. As an increasing number of essential amino acids become commercially available through chemical synthesis and are applied in actual production, there remains further room for reducing dietary crude protein levels. However, recent studies have shown that, beyond free amino acids, most dietary proteins are absorbed directly in the form of oligopeptides following digestion. The nutritional value of oligopeptides has been recognized in human nutrition; if dietary protein levels are too low, even the addition of essential amino acids may still impair animal growth. Therefore, when using the IAAP to formulate low-protein diets, it is crucial to carefully balance the ratio between monomeric amino acids and intact proteins. To date, research and application of the IAAP remain at an early stage. As more fundamental data—such as the true digestibility of amino acids and the body’s amino acid turnover rates—are accumulated, a more precise and dynamic IAAP will better serve the livestock and poultry industries.