Soybeans grown in the northern United States are exposed to fewer growing days and hours of sunlight than soybeans grown elsewhere in the U.S. As a result, soybeans grown in the northern U.S. fix less nitrogen, and have a lower concentration of crude protein, than other U.S. soybeans. However, the concentrations of particular amino acids, particularly indispensable amino acids, are more important for the purposes of diet formulation than the concentration of crude protein. The concentration of amino acids in soybeans grown in different parts of the U.S. has not been determined.

The amount of amino acids in soybean meal that are available to the pig also depends on digestibility, but no research has been conducted to compare the digestibility of amino acids among soybean meal produced in different regions of the U.S. Therefore, an experiment was conducted to compare the standardized ileal digestibility (SID) of amino acids and the concentration of SID amino acids in soybean meal produced in different regions within the United States and fed to growing pigs.

Twenty sources of soybean meal were used in the experiment. Soybeans were sourced from crushing plants located throughout the soybean growing area of the U.S. For analysis, the crushing plant locations were separated into 4 zones: 1) northern growing area (Michigan, Minnesota, and South Dakota; 4 samples), 2) eastern growing area (Georgia, Indiana, and Ohio; 6 samples), 3) western growing area (Iowa, Missouri, and Nebraska; 7 samples), and 4) Illinois (5 samples).

Twenty-two growing barrows with an average initial body weight of 25.5 kg were fed one of 22 diets. The diets were based on cornstarch, sucrose, and soybean meal from each of the four zones. Soybean meal was the only source of amino acids in the diets. Ileal samples were collected and analyzed to determine ileal digestibility of crude protein and amino acids.

The SID of crude protein was greater (P < 0.05) in soybean meal from Zones 1 and 3 than in soybean meal from Zone 4, and the same as in soybean meal from Zone 2 (Table 1). Minor differences in the SID of some individual amino acids were observed, but there was no difference in the average SID of indispensable amino acids. The average SID of all amino acids was greatest (P < 0.05) in soybean meal from Zone 3 and least in soybean meal from Zone 2, but the difference was only 1.07 percentage points.

The concentration of SID crude protein was greater (P < 0.05) in soybean meal from Zone 2 than in soybean meal from Zone 3, but not different than in soybean meal from Zones 1 and 4 (Table 2). With the exception of minor differences in threonine and tyrosine concentrations, there was no difference in the amount of SID amino acids among soybean meal from the different zones.

• There were no differences in the average SID of indispensable amino acids, and only minor differences in the average SID of total amino acids, among soybean meal sourced from four different zones in the United States. 
• There were no differences in the concentration of SID indispensable amino acids, SID dispensable amino acids, or SID total amino acids among soybean meal from the four different zones.
• The growing area of U.S. soybeans does not influence the protein value of soybean meal fed to pigs.

Table 1. Standardized ileal digestibility (%) of crude protein and amino acids in soybean meal by growing pigs

^a-eMeans within a row lacking a common superscript letter are different (P < 0.05).

¹Zone = 1: northern growing area (MI, MN, and SD); Zone 2 = eastern growing area (GA, IN, and OH); Zone 3 = western growing area (IA, MO, and NE); Zone 4 = Illinois.

Table 2. Concentrations (g/kg) of standardized ileal digestible crude protein and amino in soybean meal

^a-bMeans within a row lacking a common superscript letter are different (P < 0.05).

¹Zone = 1: northern growing area (MI, MN, and SD); Zone 2 = eastern growing area (GA, IN, and OH); Zone 3 = western growing area (IA, MO, and NE); Zone 4 = Illinois.

This report is based on research by Kelly Sotak-Peper and Hans H. Stein, published in Sotak-Peper's Ph. D. dissertation.