Abstract
The mean particle size of ground grains influences both pig growth performance and gastric health although this effect is not consistent and the underlying reasons are not fully understood. The inhomogeneous nutrient content distribution in particles of different sizes and the characteristics of particles may be reasons for inconsistent findings. The objective of the present study was to determine the nutrient content and physical characteristics of individual size fractions of hammer-milled maize and soybean meal (SBM), and to relate it to in vitro digestibility. Maize and SBM were hammer-milled over a 6- and 2-mm size screen, respectively, and were sieved into seven fractions. Particle characteristics of the hammer-milled material were determined by dry sieving, wet sieving and image analysis methods; the nutrient composition including dry matter, ash, crude fibre (CF), crude fat (CFat), crude protein (CP), starch and in vitro digestibility of organic matter (OM), CP (SBM) and starch (maize) were measured and nitrogen-free-extract was calculated. The results show that the nutrient composition differed among fractions of ground maize and SBM (P < 0.001). A large difference in starch levels (754.2 vs 578.9 g/kg) of maize was observed between the various sieve size fractions whereas the CP content of SBM increased with larger sieve sizes. The in vitro digestibility of OM and CP was different (P < 0.001) among the various particle size fractions for both ingredients. However, the in vitro digestibility of starch did not differ between each size fraction in maize (P = 0.060). The regression models relating the nutrient composition and in vitro digestibility show that the digestibility of OM was positively related to the starch level (P < 0.001). As for SBM, CF (negatively) and CFat (positively) were correlated with OM digestibility (P < 0.001); ash and CF had a negative effect on the digestibility of CP, though CFat had a positive relation with the CP digestibility (P < 0.05). Using image analysis, the OM digestibility of different fractions of maize and SBM could be related to the projected perimeter (R2 = 0.933) and solidity (R2 = 0.704) of particles in a linear model. The presented data show that the nutrient composition and physical characteristics of materials among various size fractions of hammer-milled maize and SBM differ and may explain pig growth performance differences observed in commercial production.
Highlights
Feed and feed ingredient particle size are important in both feed manufacturing and the nutrition of pigs (Laurinen et al, 2000; Fastinger and Mahan, 2003; Huang et al, 2015; Zhao et al, 2019) and poultry (Xu et al, 2015; Zaefarian et al, 2016; Mtei et al, 2019; Bozkurt et al, 2019)
In order to compare the results of dry and wet sieving, the particle size distribution (PSD) obtained from dry sieving was recalculated into the 7 fractions used for wet sieving
Dry and wet sieving of the hammer-milled soybean meal (SBM) and maize resulted in different mass distribution patterns (Fig. 2) where a larger percentage of material was retained on the coarsest sieve and dissolved and colloidal matter accumulated in the water used in wet sieving
Summary
Feed and feed ingredient particle size are important in both feed manufacturing and the nutrition of pigs (Laurinen et al, 2000; Fastinger and Mahan, 2003; Huang et al, 2015; Zhao et al, 2019) and poultry (Xu et al, 2015; Zaefarian et al, 2016; Mtei et al, 2019; Bozkurt et al, 2019). By reducing the mean particle size of feed ingredients/diets, the di gestibility of nutrients and growth performance in pigs can be increased (Kim et al, 2005; Lahaye et al, 2008; Ball et al, 2015; Rojas and Stein, 2015) due to an increased surface area to volume ratio exposing more nutrients to digestive enzymes (Wondra et al, 1993). This influence of particle size on pig growth performance is, not consistent and the reasons are not fully understood. This influence of particle size on pig growth performance is, not consistent and the reasons are not fully understood. Lawrence et al (2003) found that growth performance of nursery pigs was not affected by the particle size of soybean meal (SBM). Li et al (2018) reported that for weanling and growing pigs, when the particle size of brown rice in the diet decreased from 800 to 600 μm, the apparent total tract digestibility of gross energy, dry matter (DM) and crude protein (CP) increased, with no further improvements observed in the 400 μm diet
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