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Prediction of the apparent metabolizable energy of soybean meal based on a rooster model

Crinière, M., C. Picart , M. Hruby and C. Launay

Several digestibility studies were conducted with roosters to evaluate apparent metabolizable energy (AMEn) of 36 soybean meal (SBM) samples collected globally between 2008 and 2020 to support more reliable prediction of SBM nutritive value while formulating. The studies were conducted with 36 intact roosters to evaluate energy, apparent fat and organic matter digestibility (dFAT; dOM) and 36 cecectomized roosters for apparent ileal crude protein digestibility (dCP). For each SBM, wet chemical composition was known, especially moisture, crude protein (CP), fat (FAT), crude fiber (CF), ash and nitrogen solubility in KOH (NKOH), a processing quality criterion. The SBM origins were mainly from South and North America, and India. The average AMEn value was 2602 kcal/kg as fed (n: 36; SD: 199 kcal; min: 2209 kcal; max: 2923 kcal). This is higher in comparison to main tables (WPSA, 1989; Rostagno, 2017; CVB, 2021) and our previous data obtained prior to 2008. To predict AMEn, we used a factorial equation corresponding to the sum of the energetic contribution of each digestible fraction: CP, FAT and carbohydrates (CHO). The digestible fraction was calculated by multiplying the content of the fraction by its digestibility. The digestibility of carbohydrates (dCHO) was calculated from dOM, dCP and dFAT. For SBM AMEn, the main contributor is the digestible protein (about 75%). The average dCP value was 89.8 % (n: 27; SD: 1.7; min: 86.2%; max: 93.1%). The dCP data were subject to two factors ANOVA (linear for CF and quadratic for NKOH). The dCP was correlated to CF and NKOH (n: 17; R2: 0.77; SE: 0.8; P < 0.01). The CF affected negatively dCP. According to our model, SBMs with higher CF content have a reduced protein digestibility by up to 2.0%. The lower (overprocessing) and the higher (underprocessing) NKOH values reduce dCP as well. Overprocessing can reduce protein digestibility by up to 5% (i.e. 100 kcal/kg). These data confirm that the best dCP was obtained with a NKOH range between 75 and 85% N. The average dFAT value was 79.8% (n: 33; SD: 16.9). The average dOM value was 63.1 % (n: 14; SD: 4.4). The higher the proportion of crude fiber in carbohydrates, the lower dCHO. The use of these three predictive digestibility coefficients makes it possible to reduce the variability of the prediction of SBM AMEn (n: 26; MSE: 161 kcal).

In a factorial approach, digestible protein is the main contributor to SBM AMEn and it can be predicted accurately by CP, CF and NKOH solubility or other processing quality criteria such as protein dispersibility index. These data are easily analyzable by wet chemistry or NIRS. Further studies could evaluate digestibility of carbohydrates to improve the current SBM AMEn prediction.