Pig feed ingredients affect enzyme diffusion coefficients

Pig diets are mainly manufactured from grains that are milled to various particle sizes. Particle size affects grain and feed digestibility (Huang et al. 2015; Nguyen et al. 2015), and although animal studies typically use complete feeds (milled grains and ingredients), in vitro studies have concentrated on milled grains to calculate the enzyme diffusion coefficients. These are proposed to control particle size-digestibility relationships (Al-Rabadi et al. 2009; Tinus et al. 2012). Ingredients in feed supplement grains to achieve a balanced nutritional profile, and can affect diet digestibility. This study examined how ingredients affected enzyme diffusion coefficients, with the hypothesis that the coefficients would be unchanged in pig diets.

Sorghum (var. MR43) and field pea (var. Walana) were each milled and mixed to different particle sizes in duplicate to make 20 diets. The sorghum diets contained 50% of milled sorghum, with dehulled oats (10%), whey powder (10%), lupin kernels (5%), canola meal (5%), soybean meal (5%), meat meal (7%), fish meal (3%), blood meal (2%), tallow (2%), and mineral/vitamin mixes as the ingredients. The field pea diets contained 30% of milled pea, and the ingredients were soft wheat (29%), barley (23%) meat- and bone-meal (7%), soybean meal (7%), tallow (2%), and mineral/vitamin mixes. In vitro digestion and geometric mean particle size diameter (D_gw) of the milled grains and diets, were analysed (Nguyen et al. 2015). Based on Tinus et al. (2012), the digestograms were described by a modified first-order kinetic model to obtain the rates of starch (K_ST) and protein (K_PR) digestion. Diffusion coefficients (D_IFF) were obtained from relationships 1/K_ST α (D_gw²/D_IFF) and 1/K_PR α (D_gw²/D_IFF) by regression. Minitab™ statistical procedures were used.

The ingredients did not materially change (P > 0.05) the D_gw of the diets, which was within 10% of that of the milled grains (field pea: D_gw-grain = 1.07 D_gw-diet; sorghum: D_gw-grain = 0.91 D_gw-diet, R² > 0.4, P < 0.01). However, the starch and protein contents (g/100g; mean ± SD) were for the sorghum: grain - 13 ± 0.3 protein, 58 ± 0.6 starch, diet - 23 ± 0.5 protein, 33 ± 0.7 starch, and for the field peas: grain - 22 ± 0.1 protein, 42 ± 0.1 starch, diet - 20 ± 0.4 protein, 38 ± 1.1 starch. The kinetic model adequately described (R² > 0.9; P < 0.001) the digestograms, and 1/K_PR, 1/K_ST and D_gw² were significantly (P < 0.01) related (Fig. 1). For the diets and milled grains, the protein digested faster than the starch, and the inverse square relationship is consistent with digestions being rate-limited by enzyme diffusion within particles. However, the ingredients changed the diffusion coefficients (cm² s^–1) by 30-400% (sorghum: grain – 270 for protein, 3 for starch, diet – 210 for protein, 6 for starch; field peas: grain – 530 for protein, 2 for starch, diet – 110 for protein, 4 for starch). The ingredients reduced the protein coefficients and increased the starch coefficients. It is suggested that the protein-containing ingredients digested more slowly than the field pea or sorghum protein, whereas the starch-containing ingredients digested faster than the field pea or sorghum starch. Starch-protein interactions exist in field peas and sorghum, and limit starch digestion (Tinus et al. 2012; Nguyen et al. 2015). While the inverse square relationship between particle size and rate of digestion holds for diets and grains, pig feed ingredients affect the values of apparent enzyme diffusion coefficients.

Fig. 1.  Typical relationships between the rates of digestion and particle sizes of the milled grains (■) and diets (□) showing starch digestion in the field peas (A) and protein digestion in the sorghum (B). Error bars were omitted for clarity (predicted –).