Soybean meal is a valuable source of protein, amino acids and energy in poultry diets. In the United States, poultry consumes 49 percent of the total soybean meal produced. Swine and beef consume the next largest percentage of the ingredient. Because soybean meal is the primary protein source in poultry diets and poultry is the primary user of soybean meal worldwide, it is important for the nutritionist to keep up to date on soybean meal research.
The following summary is based on the presentations made at the International Poultry Scientific Forum held at 2014 edition of the International Processing & Production Expo (IPPE).
Soybean composition's effect on digestibility
According to Dr. Jan van Eys, a consultant with Global Animal Nutrition Solutions Inc. who presented a review of soy quality at a U.S. Soybean Export Council (USSEC) event, soybean meal contributes 50 percent of the digestible amino acids in poultry feed. The factors affecting the digestible nutrients in soybeans include: the origin of the grain; how much oil is left after crushing; anti-nutritional factors (heating); ash and non-starch polysaccharide composition.
Comparing the 1994 National Research Council (NRC) Poultry Recommendations value for ether extract of soybean meal (1 percent) to other references, van Eys notes that 1 percent is a low estimate of soybean meal oil content. He also notes that only 25 percent of soybean meal is undefined by the NRC and would fall into the category of non-starch polysaccharide (NSP), such as sugars, starch and fiber. This is the component of soybean meal most likely to improve digestibility and energy values with the use of exogenous enzyme technologies.
NSP and B-mannanases
The NSP component of soybean meal comprises of as much as 20 percent of soybean meal and includes oligosaccharides and some simple sugars. Oligosaccharides are very difficult for poultry to digest due to a void of endogenous or natural bacterial enzymes in the chicken's gastrointestinal tract. Thus the application of enzyme technologies, such as B-mannanases, can potentially improve energy utilization of soybean meal carbohydrates.
According to a multi-institution team investigating the effects of B-mannanase supplementation in laying hen rations fed a low protein and energy diet, the inclusion of the enzyme produced short-term feed intake increases. Feed intake dropped after 8 weeks, which the team hypothesized was due to improved feed efficiency with the mannanase treatments. They found no overall effects on egg production or egg quality; however, eggshell thickness was reduced in eggs from hens on the low-density diet.
Meanwhile, scientists from Texas A&M University also tested B-mannanase on growth performance and illeal digestible energy values in broilers. They estimated a 55 kcal/kg boost of digestible energy of the negative control diet due to mannanase supplementation and improved carbohydrate digestion.
Amino acids and digestibility
The most valuable components of soybean meal for poultry are the highly available essential amino acids. Soybean meal is a good source of poultry's second limiting amino acid, lysine, and other essential amino acids, threonine and tryptophan. While a significant source of sulfur amino acids, methionine and cysteine, most poultry rations require synthetic methionine added to a corn-soybean meal ration to meet the birds total sulfur amino acids (TSAA) needs.
A major part of the value of soybean meal in poultry rations is due to the high digestibility of soybean meal amino acids (> 90 percent) compared to other plant proteins (< 90 percent). There is a negative correlation between acid detergent fiber (ADF) components of soybean meal and amino acid digestibility -- with digestibility decreasing by almost 6 percent (from 90 to 84 percent) when ADF in soybean meal rises from 4.8 percent to more than 11 percent. A high ADF can decrease the overall protein and amino acid value of soybean meal for poultry.
Heat treatments sway amino acid quality
Digestibility of soybean meal amino acids are also greatly affected by heat treatments, which destroy the trypsin inhibitors. With such a high level of available amino acids and sugars present in soybeans, if soybeans are overheated to destroy trypsin inhibitor, the Maillard reaction occurs, which results in a binding up of amino acids to burnt sugars. Most plants try to adhere to the optimum temperature necessary to destroy trypsin without compromising amino acid quality.
To avoid compromising amino acid digestibility by over toasting, some anti-nutritional factors may still be present. The remaining ANF should have minimum effects on protein availability to the bird.
Keeping tabs on protein quality
According to van Eys, access to a consistent quality soybean meal product is essential for maximizing feed formulations for a quality ration. Data suggests soybean meal protein digestibility decreases as storage time increases. Freshness and age of soybean meal then becomes a factor when formulating poultry diets based on amino acid digestibility.
Several lab tests are good indicators of protein quality. These tests include potassium hydroxide (KOH) solubility, the most accurate indicator of protein quality in soybean meal; PD Index; urease activity; and TI activity.
Soybean meal protein and amino acid quality has helped fuel the U.S. poultry industry for many years and will continue well into the future.
Copies of Dr. van Eys's summary are available from the U.S. Soybean Export Council, and abstracts of the International Poultry Scientific Forum papers are available from Southern Poultry Science Society at http://www.ippexpo.org/ipsf/ .