Variations in the estimates of energy value in soybean meal

There are variations in the estimates of the energy value of soybean meal due to factors such as the variations in composition of the ingredient,  the methods used to analyse it, as well as the models used to estimate AMEn. This article anlayses the limitations of the models currently in use and proposes a more representative model.

Introduction

Soybean meal (SBM) is the main protein source in diets of monogastric animals worldwide. It also provides a considerable energy value in standard broiler diets (Mateos et al., 2019), accounting for up to 18% of total energy required by the bird.

The energy content of soybean meal mainly depends on:

• content and quality of the protein fraction (Thakur and Hurburgh, 2007; Serrano et al., 2012).
• content of sucrose (Berrocoso et al., 2014; Ravindran et al., 2014).
• proportion of oligosaccharides (Coon et al., 1990).
• fibre content (Dilger et al., 2004; Ravindran et al., 2014). 

Different approaches to estimating AMEn of soybean meal (SBM)

Many research institutions investigating the nutritional composition of raw materials (Table 1) have reported values of apparent metabolizable energy corrected for nitrogen (AMEn) for soybean meal (47% CP). AMEn values vary between 2,180 and 2,360 kcal/kg (Table 2).

Although the research institutions cited in Table 2 consider SBM nutritional value to be constant, it actually can vary based on a number of factors, such as:

• The origin of the soybean (Lagos and Stein, 2017; Cámara et al., 2019)
• Environmental and agronomic conditions of the area planted with soybean (Karr-Lilienthal et al., 2004)
• Growth and harvest time, storage, and heat processing of soybean (Mateos et al., 2019; García-Rebollar et al., 2016)

Prediction equations to determine Soybean meal energy value have been developed. However, it is important to consider that SBM chemical composition varies between research institutions.

Consequently, the results of these equations are affected by the values used by each research institution.

The list of equations used to estimate AMEn values in poultry production includes the one published by WPSA in 1989 (European Table of Energy Values for Poultry Feedstuffs). García-Rebollar et al. (2016) observed that this methodology is mainly based on the energy contribution of:

• Crude protein (CP)
• Ether extract (EE)
• Nitrogen-free extract (NFE)

… without taking into consideration:

• amount or type of sugars
• presence of antinutritional factors (ANF)
• digestibility of CP and amino acids (AA)

Comparison between the AMEn values reported by different institutions

The present article presents a comparison between the AMEn values reported by different research institutions (Table 3). They were estimated using three prediction equations for SBM energy value in poultry production (particularly roosters), recommended by WPSA (1989) and CVB (2018).

The prediction equations

For calculation of AMEn values, the estimated ratio between EE and EEHA (0.59) was based on the data published by CVB (2018).

Disadvantages of the prediction equations

Prediction equations are useful tools to examine AMEn values in poultry production. However, such equations are general and do not consider factors that could affect the nutritional value and the quality of soybean meal. This may result in possible overestimation or underestimation of its energy content (Serrano et al., 2012; Mateos et al., 2019).

Effect of the methods to analyse SBM components on predictions

A potential problem when it comes to calculating the energy value using these equations is that the method used to analyze the main components can affect the estimates. For example, the EE content can vary according to the method of processing, showing a higher value for SBM when a prior acid hydrolysis is used.

Consequently, the energy coming from the lipid fraction of SBM can be different between samples, even when SBM has the same protein and EE contents.

Equations are not updated

A further observation is that the WPSA equation, which is widely used to estimate AMEn in poultry production, is 32 years old, and it may not be as precise as currently required (Mateos et al., 2019). This equation uses the nitrogen-free extract (NFE) as a main variable to estimate the energy content. This variable does not have a clear meaning, since it is calculated as the difference between 1,000 and the proximal content of the analysis.

Ravindran et al. (2014) and Lagos & Stein (2017) observed a higher CP digestibility and higher sucrose content for the SBM from USA, compared to SBM from Brazil (BRA), Argentina (ARG) and India.

Another possible energy contribution that is not estimated by this equation is the content of oligosaccharides (mainly estachiose). Although it is considered as an antinutritional factor and cannot be digested by monogastric animals, it can provide an extra energy contribution (especially in adult animals) through fermentation (Coon et al., 1990).

Developing new models 

Due to the variations in SBM energy content reported in different studies (García-Rebollar et al., 2016; Mateos et al., 2019), further work is required to develop a model or equation for energy estimation in SBM. This will allow to have a clearer and less variable estimate of the AMEn content in this ingredient.

Currently, a model is sought in which the contributions of AMEn (kcal/kg) by each of the nutritional fractions of SBM are considered. For example, the protein, fat, fiber (CF, NDF and soluble fiber), sugars, and oligosaccharides fractions.

It is also important to mention that, according to the method used to determine EE content, differences in the results for fat fraction may exist (NRC, 2012). Therefore, in the model to be developed, it is suggested to use only EE after acid hydrolysis (HCl)

Model to be developed at the Polytechnic University of Madrid

References are available on request

This article was originally published in nutriNews Spain, under the title Variación en la estimación energética de la harina de soja