Growth performance of weaner pigs fed diets containing grains milled to different particle sizes. II. Field pea

Various studies have highlighted the importance of grain particle size on growth performance of pigs (Choct et al. 2004; Montoya and Leterme 2011). However, the studies concentrated on cereals, used one mill type, or had an insufficient number of treatment levels to probe the performance-size relationships. Field pea is low in anti-nutritional factors, and it is an important protein source in pig feeds (Nguyen et al. 2015). Hammer-, disc- and roller-mills are mainly used in pig feed manufacture, and mill types can influence growth performance (Choct et al. 2004). Using commercial mills to replicate field situations, this study investigated how weaner pigs responded to diets containing hammer- and disc-milled field peas of different particle sizes. The hypothesis tested was that an optimum particle size range exists, within which, growth performance is independent of particle size.

Field pea (var. Walana) was milled, in two replicates, using commercial hammer (HM) and disc (DM) mills, in a randomised design with four screen sizes (2, 3, 4, and 5 mm) and four disc gaps respectively. The finest (F) and coarsest (VC) sizes from the mills were mixed for four additional treatments: HM F- DM F, HM F-DM VC, HM VC-DM F, and HM VC-DM VC. A total of 20 milled grains, but 12 treatments, were used (30%) to formulate the experimental diets [14 MJ digestible energy (DE)/kg; available lysine/DE, 0.09 g/MJ DE; 430 g/kg starch, 190 g/kg crude protein) for weaner pigs [Large White × Landrace, PrimeGro Genetics; 28 days of age and weighing 7.3 ± 0.10 kg (mean ± SD)]. After adaptation for 6 days on a commercial diet, a total of 400 pigs in two batches were individually housed and fed the 20 diets ad libitum over a 21-day period using a randomised block design with some incomplete blocks. Hence, there were effectively 20 pigs per diet (or 33 pigs per treatment), and the 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 (13N023C) the animal experiment. The diets were analysed for geometric mean particle size diameter (D_gw) and geometric standard deviation of mean particle diameter (S_gw) as before (Nguyen et al. 2015). Statistical methods (ASReml-R) analogous to ANOVA were used (Butler 2009).

Table 1 shows that the D_gw of the milled pea ranged from 600–800 µm, and had up to 45% of the particles greater than 1000 µm. The disc-milled pea had a wider D_gw range (200 µm) with the mill settings than the hammer-milled pea. With age, the pigs’ ADFI and ADG increased (not shown), but their growth performance was not significantly (P > 0.05) affected by the mill type and particle size from 0–21 days (Table 1). The D_gw of the diets was from 500–700 µm, and not different (P > 0.05) from the D_gw of the milled pea. Hence, the diet ingredients were not coarser than the milled pea, whose particle size can, therefore, be inferred to solely affect the measured growth of the pigs. In view of the absence of significant effects of the diets on the growth performance of the pigs, it is suggested that the particle size range (600–800 µm) of the milled field pea is an optimum range at the 30% inclusion for weaner pigs. Feed mills should take cognizance of this range to guide their milling operations, during feed manufacture.

Table 1.  Effects of particle size of the milled peas on performance of pigs after weaning