Particle size distribution, energy consumption, nutrient composition and in vitro ileal digestion characteristics of hammer milled maize and soybean meal affected by moisture content

Grinding is an important feed processing technology, determining physical and nutritional characteristics of ground materials, which affects nutrient digestion in animals and their growth performance. This study aimed to clarify if differences in moisture content (MC) lead to differences in nutrient composition over various particle size fractions after grinding that have potential to affect feed manufacturing characteristics or animal performance. Maize and soybean meal (SBM) with targeted MC of 120, 140 and 160 g/kg (adding no (0), 30 and 60 g/kg of tap water, respectively) were hammer-milled and the physical and chemical characteristics as well as in vitro apparent ileal digestibility (AID) of particle size fractions were determined. The mill was fitted with a 6-mm (maize) or 2-mm (SBM) sized screen, with milled material subsequently separated by dry sieving (size ranging from < 0.075 to > 3.36 mm) and each fraction was analyzed for its nutrient composition, morphology characteristics and in vitro AID of organic matter (OM) and crude protein (CP). For the latter assay, specific particle size fractions were additionally ground using a laboratory mill (1 mm screen). Geometric mean particle size diameter and energy consumption increased with increasing MC (P < 0.05). Chemical composition, physical characteristics and in vitro AID of particle size fractions were significantly different (P < 0.001). Moisture addition had no significant effect on nutritional and physical parameters except for solidity in maize, ash content, projected area, circularity, and solidity in SBM. Physical characteristics of particles, especially particle size affected in vitro AID of OM and CP most (P < 0.05). Additional grinding of samples before determination of in vitro AID increased the OM digestibility up to 0.684 in maize (P < 0.001). Additional grinding of particles larger than 0.595 mm increased in vitro AID of OM and CP (P < 0.05). In summary, increasing MC has limited effect on the breakage behavior of maize and SBM, but increased energy consumption during hammer-milling. In vitro AID measurement of fractionated particles appears to require material should be ground to pass a 0.595 mm sieve rather than the prescribed 1 mm.