Soybean meal (SBM) is the most used source of protein in animal feeding. In this first article of a series of three, the authors review composition and nutrtional value for different species, as well as factors determining their variability.
Soybean meal (SBM) is the main source of protein and amino acids (AA) used in animal feed formulations worldwide, particularly for monogastric species. Soybean is a source of high-quality vegetable protein, with a less variable chemical composition than other protein sources. However, due to its high inclusion rates (especially in poultry diets), any lack of uniformity in SBM will have more relevance than a variation in any of the other protein sources added to the formula.
In numerous practical situations, the quality control of soybean meal and derivatives is limited to analyses of moisture, crude fibre, crude protein (CP), and urease activity.
All these parameters are relevant. They allow us to properly evaluate the nutritional value of the ingredient. However, these variables are subjected to variations depending on:
SBM accounts for the main cost of raw materials in a feed mill.
There is a need to improve the methods currently used for soybean quality control. Most soybean quality control programs focus on the determination of protein content, but not much on the energy value, which reduces their practical relevance.
The objective of present study consists of three parts:
Different products derived from raw soybeans are available on the market, depending on the type and level of processing (Figure 1).
In certain countries, such as Argentina, solvent extraction is not used to remove oil. Instead, a pressure processing in combination with high temperatures is carried out. In this case, the product obtained is the expeller soybean meal with 40-44% CP and 8-11% fat.
This pressure processing is normally performed in small plants, not always with good facilities. Hence, the products are less uniform, and a strict quality control program is needed.
From the processed and degreased soybeans, value-added products can be generated through:
The chemical composition of SBM is less variable than that of other protein ingredients commonly used in commercial feeds for monogastric animals. However, given its high inclusion level in animal feeds, a high degree of uniformity is required.
Table 1. Proximate composition of soybean meal (47% CP) according to various sources
An issue for consideration in practical situations is that most feed mills estimate that the AA proportion (or profile) in SBM is constant. Thus, when the protein level in the matrix is modified, the content of all AA is often modified proportionally. This leads to errors with high impact on production costs.
The availability of AA in soybean is negatively correlated with the presence of ANF, particularly TI and Maillard reactions.
Various studies have demonstrated that the content of essential AA is not proportional to the CP level, and the AA content in soybeans depends not only on its protein content but also on the country of origin of the beans. The SBM received in Europe are often from USA, Brazil/Paraguay or Argentina. In general, soybeans from Brazil/Paraguay contain higher CP levels than those from USA, and these contain higher levels than those from Argentina, provided that hulls are not added to the final product (Table 2).
Table 2. Proximate composition1 (% DM) of soybean meal according to the country of origin of the beans (García Rebollar et al., 2016).
The advantage in CP found in the SBM from Brazil/Paraguay is partly lost when the AA profile is examined, since these meals contain lower levels of Lys, Met, Thr and other essential AA per protein unit compared to SBM from Argentina and USA. Therefore, the advantage of a higher protein level is partly lost since the animals do not need protein, but digestible AA.
In the laboratory, CP is calculated by multiplying the N content of SBM by a conversion factor set at 6.25.
The inclusion of low levels of urea allows for raising the theoretical protein values quite impressively. It should be considered that urea is not toxic for monogastric animals, but its nutritional value for these species is zero.
The calculation of SBM energy value is important for the correct estimation of its nutritional value. However, there is no available methodology defined for estimating its value in different soybean products.
In practice, the metabolizable energy (AMEn; poultry) is determined using prediction equations which are based on the possible energy contribution of its immediate principles. The main disadvantage of this method is that it considers neither the possible variation in protein digestibility, nor the sugar content (often higher than 8-9%) (García Rebollar et al., 2016) or the presence of ANF. All these parameters are variable according not only to soybean processing but also to the origin of the beans.
SBM contains approximately 12% moisture, 7-13% NDF and 1.7% ethereal extract, while ash represents approximately 6%. These values depend fundamentally on the chemical composition of the original bean, its genetics, but also, and perhaps at a higher proportion, on the geographical region where the soybean was produced. The latitude, which is related to the daylight hours during seed growth and maturation, is a probably key factor to understand bean composition.
This artcle was originally published in nutriNews Spain, under the title «Uso de productos derivados de soja en alimentación animal . Parte I»
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