What is a first limiting amino acid?

To use crystalline amino acids in low-protein diets effectively, and to minimize nitrogen excretion, you must first understand their limitations.

The use of lysine and other crystalline amino acids in low-protein diets effectively reduces the excesses of other amino acids beyond a pig's requirement and reduces nitrogen excretion. That's common knowledge. But to effectively use those crystalline amino acids, it is important to know the order in which they are limiting in various feedstuffs and the magnitude of difference between them, says University of Kentucky swine nutritionist Gary Cromwell.

Protein levels can be reduced by 4% without reducing performance if threonine, tryptophan and methionine are supplemented along with lysine, according to Cromwell. A prediction model developed in 2003 by Kentucky researchers showed a 10% reduction in nitrogen (N) excretion for every 1% decrease in dietary protein.

L-threonine and L-tryptophan have considerable potential as supplements for pigs because they generally are next to lysine in their order of limitation. Until recently, the high cost of these two amino acids prohibited their usage. Recent developments in biotechnology, new fermentation techniques and other new technological advances, however, have brought these amino acids (especially threonine) into the marketplace, Cromwell says. Currently, threonine is about the same price as lysine-HCL. Tryptophan is still expensive (about 10 times the price of lysine and threonine), but much less is needed in diets.

DL-methionine or methionine hydroxy analog (MHA) is almost universally used in poultry diets because it is the bird's first-limiting amino acid, but is generally not in the upper category of limiting amino acids for pigs. An exception would be for complex diets containing dried blood products (plasma or cells) or dried whey.

Limiting Amino Acid Order

To use crystalline amino acids in diets, it is important to know the order in which they are limiting when various feedstuffs and combinations of feedstuffs are fed, and the relative “distance” (or magnitude of difference) between the amino acids in that order, explains Cromwell.

For example, tryptophan is the first-limiting amino acid in corn and methionine is the first-limiting amino acid in soybean meal, but lysine is first limiting in a corn-soybean meal blend.

“For years, nutritionists assumed tryptophan was the second-limiting amino acid in a corn-soybean meal blend, but now we know tryptophan and threonine are almost equally limiting in their order,” says the Kentucky animal scientist.

Interestingly, threonine is more limiting than tryptophan in a corn-soybean meal diet in young pigs, while tryptophan is more limiting in older finishing pigs. And it is important to note that supplementing an amino acid that is fourth-limiting is of no benefit, and may be a detriment, if the second- and third-limiting amino acids are deficient, Cromwell adds.

Table 1 shows the amino acids that are present in a corn-soybean meal diet blended to provide from 8% to 17% crude protein. By comparing the amino acids at each protein level, one can approximate the order that the amino acids become limiting for a 50- kg (110 lb.) pig.

Cromwell offers this example: Going from left to right in the table (decreasing protein and decreasing soybean meal) it is obvious that lysine is first limiting, threonine and tryptophan are next and methionine + cystine, isoleucine and valine follow. Ample amounts of arginine and leucine and almost enough histidine and the aromatic amino acids (phenylalanine + tyrosine) are present in an all-corn diet without further protein supplementation.

Determining Amino Acid Order

Several years ago, Cromwell and his colleagues developed a visual method shown in Figures 1-3, for determining the order in which amino acids become deficient, and the magnitude of the order. It is surprisingly precise and accurate, says Cromwell, providing the amino acid levels in the feedstuffs being tested are accurate.

The first example, Figure 1, shows the six most limiting amino acids provided by a blend of ingredients, such as a corn-soybean meal diet. For such a blend, the left vertical axis gives the percent of the requirement met by corn, and the right vertical axis gives the percent of the requirement met by a soybean meal (25%) and corn (72.5%) blend. The horizontal line gives the amino acid requirements for a 50-kg (110 lb) pig. The point at which the amino acid lines intersect the requirement line indicates the order that amino acids become deficient.

In other words, the order of limitation is determined by the order that the amino acids intersect the National Research Council (NRC) requirement line, going from right to left.

In this example, lysine is first limiting, followed by threonine, tryptophan the sulfur amino acids (methionine + cystine), valine and isoleucine.

A wheat-soybean meal blend is first limiting in lysine and second in threonine (Figure 2). In a corn-meat meal blend (Figure 3), tryptophan is first limiting, lysine is second and threonine is third limiting.

Table 2 shows the six amino acids in order of the most limiting to the least limiting in commonly used cereal grains, protein supplements and other ingredients. The requirements of a 50-kg (110 lb.) pig are used, and only the six amino acids that are most likely to be limiting — lysine, threonine, tryptophan, methionine + cystine, isoleucine and valine — are given.

The order in which the amino acids become limiting changes as pigs increase in body weight and their requirements lessen.

For example, the sulfur amino acids are second limiting in a corn-soybean meal diet for a 10-kg (22 lb.) pig, are third or fourth limiting in an intermediate-weight pig, and sixth limiting in a corn-soybean meal diet for a 120-kg (264 lb.) pig. The order of threonine and tryptophan also change as pigs increase in body weight, as do the order of isoleucine and valine.

This same procedure has been further refined by adapting the data to a digestible amino acid basis. Using digestible amino acids with this procedure is useful in determining which amino acids can be supplemented and at what levels they can be added to feed ingredients with certain poorly-digestible amino acids (such as threonine in meat meal or meat and bone meal).

Supplementation Economics

The general assumption is that protein can be reduced 2% in a corn-soybean meal diet if 0.15% lysine is added (0.192% lysine-HCL) for grow-finish pigs. Another way of expressing this is that 96.15 lb. of corn and 3.85 lb. of lysine-HCL is equivalent to 100 lb. of dehulled soybean meal in a ton of diet. Based on summer '04 prices of corn ($2.40/bu; $.043/lb.), soybean meal ($310/ton; $0.155/lb.), and lysine-HCL ($1.20/lb.), this amounted to a savings of about $6.75/ton of feed. At current prices of corn ($1.80/bu.), soybean meal ($175/ton), and lysine ($1.00/lb.), the savings is about $1.80/ton when lysine is used in the lower, protein diet.

What are the economics of further reducing dietary protein and using more amino acids? Averaged over several weight groups, the amount of soybean meal can be reduced by approximately 10.6% (equal to a 4% reduction in dietary protein) if the four amino acids are added at levels of approximately 0.38% lysine-HCl, 0.10% threonine, 0.035% tryptophan, and 0.02% methionine, as shown in Table 3.

Expressed another way, 199.3 lb. of corn, 7.6 lb. of lysine-HCl, 2.0 lb. of threonine, 0.7 lb. of tryptophan, and 0.4 lb. of methionine can replace 210 lb. of soybean meal in a ton of feed. Last summer, this type of diet was cost effective, resulting in a savings of $2.47/ton of feed. However, at current prices of corn, soybean meal and amino acids, this type of diet would not be economical to feed (an additional $7.95/ton), mainly due to the high cost of tryptophan.

It is important to know the limiting amino acids in cereal grains, protein supplements, by-product feeds and combinations of feed ingredients, and to understand the order that amino acids become limiting as dietary protein is reduced, concludes Cromwell. That knowledge is essential in order to utilize supplemental amino acids effectively in low-protein diets that minimize nitrogen excretion.