Growth performance of weaner pigs fed diets containing grains milled to different particle sizes. I. Sorghum

Sorghum is the third most important cereal in Australia, in terms of production, and it is a major feed grain (Mahasukhonthachat et al. 2010). Mahasukhonthachat et al. (2010) and recently, Nguyen et al. (2015), revealed particle size and mill type as the primary determinants of in-vitro digestion properties of grains. However, there are limited studies on the effects of grain particle size and mill type on animal performance. Using sorghum, this study investigated these effects on performance of weaner pigs, and tested the hypothesis that within an optimum particle size range, pig performance is not affected.

Sorghum (var. MR43) was milled with industrial-scale hammer (HM) and disc (DM) mills, in a randomised experiment with two replicates. Four screens (2, 3, 4, and 5 mm) and four disc gaps were used in the HM and DM respectively. Four additional treatments were obtained by mixing the finest (F) and coarsest (VC) sizes from the mills (HM F- DM F, HM F-DM VC, HM VC-DM F, and HM VC-DM VC). Experimental diets [15 MJ digestible energy (DE)/kg, 1 g available lysine/MJ DE, 220 g/kg crude protein, and 350 g/kg starch), consisting 49.8% of the milled sorghum, were fed ad libitum to weaner pigs for 21 d. A total of 289 weaner pigs (Large White × Landrace, PrimeGro Genetics) aged 28 days and having a bodyweight of 6.8 ± 0.1 kg (mean ± SD), were individually housed and used in three batches in a randomised block design, with some incomplete blocks. There were 20 diets from the 12 particle size treatments, and 24 pigs were used per treatment. Pigs and feed residues were weighed weekly to calculate average daily feed intake (ADFI), average daily gain (ADG) and feed conversion ratio (FCR). The Rivalea animal ethics committee approved (14N009) the animal experiment. The diets were analysed (Nguyen et al. 2015) for geometric mean particle size diameter (D_gw) and geometric standard deviation of mean particle diameter (S_gw). Statistical methods (ASReml-R) analogous to ANOVA were used (Butler 2009).

The D_gw of the milled sorghum ranged from 400–800 µm, with up to 50% of the particles being higher than 1000 µm in size. There was no pronounced (P > 0.05) mill effect on S_gw, and neither the mill nor particle size affected (P > 0.05) the pig growth (Table 1). Irrespective of the mill or particle size, the pigs consumed (ADFI) and grew (ADG) more with age (not shown).

Table 1.  Effects of particle size of milled sorghum on performance of pigs from 0–21 days after weaning

The D_gw of the diets (600–750 µm) was not significantly different (P > 0.05) from that of the milled sorghum, and, therefore, the ingredients did not influence the D_gw of the diets. Hence, the measured animal responses were mainly due to the particle size of the milled sorghum. The absence of significant mill and particle size effects suggests 400–800 µm as the optimum particle size range for sorghum fed to weaner pigs. Feed mills, therefore, need not grind sorghum below 400 µm for milling economy, and particle size above 800 µm might be undesirable for good performance of weaner pigs fed sorghum-based diets.