Effects of particle size heterogeneity in milled maize and soybean meal on nutrient content and in vitro organic matter digestibility

The mean particle size of ground grains has effects on pig growth performance and gastrichealth. However the variation heterogeneity in particle size distribution may result in differentpig performances, even though the animal was fed the similar mean particle sizedmeals. The hypothesis about non-homogenous nutrient content distribution along with theparticle size distribution might be one of the key reasons to explain this phenomenon. Theobjective of present study was to identify the nutrient content among individual size fractionsof milled maize and soybean meal (SBM), and to relate it to the pig in vitro digestibilitycoefficients, thus gaining insight on how to improve grinding strategy. Maize and SBM wereground by hammer mill through a 2 and 6 mm screen size respectively, and sieved into 7fractions, from < 0.075 to > 3.36 mm. The nutrient composition including dry matter, ash,crude fibre (CF), crude fat, crude protein (CP), starch and in vitro digestibility of organicmatter (OM), CP (SBM) and starch (maize) were measured. The results show that the nutrientcomposition is significantly different (P< 0.001) among these fractional ground grains. Alarge difference in starch levels (754.2 vs 578.9 g/kg ) in maize was observed; CP contentof SBM increased when the sieve size gets larger. A significant difference (P< 0.001) in the invitro digestibility of OM and CP was also observed among the various particle size fractionsin both ingredients. However, the in vitro digestibility of starch did not differ significantly betweeneach size fractions in maize. The regression models relating the nutrient compositionand in vitro digestibility show that, the digestibility of OM was significantly, positively relatedto the starch level. Ash and starch content are positively related to the in vitro digestibilityof starch in maize. As for the SBM, a significant negative correlation was observed betweenCP and CF, crude fat and OM digestibility; ASH and CF had a negative effect onthe digestibility of CP, though crude fat showed a positive influence on the CP digestion.The relationships between the nutrient composition and pig in vitro digestion suggests thepig growth performance differences in practice may be explained by the nutrient contentamong different fractions. This indicates that the grinding strategies should also be consideredwhen designing the feeding formula / manufacturing the feed.