Corn processing, flake density, and starch retrogradation influence ruminal solubility of starch, fiber, protein, and minerals.

Five ruminally cannulated steers (body weight = 390 ± 7.86 kg) were used in three experiments to evaluate effects of corn processing, flake density, and starch retrogradation on in situ ruminal degradation. In experiment 1, corn was left whole or processed with no screen, ground through a 6-mm screen, or ground through a 1-mm screen. In experiment 2, we produced steam-flaked corn at four densities: 309, 335, 360, and 386 g/L. These four flake densities were sifted for 20 s through a 4-mm screen to produce two particle sizes within each flake density: sifted flakes (>4 mm) and sifted fines (<4 mm). In experiment 3, sifted flakes (335 g/L) were stored for 3-d at either 23 °C (starch availability = 55%) or 55 °C to induce starch retrogradation (starch availability = 41%). All samples for each of the three experiments were weighed into nylon bags and ruminally incubated for 0-h to estimate the soluble fraction. The residue remaining was analyzed for nutrient composition. In experiment 1, whole shelled corn had lesser (P < 0.01) ruminal solubility of all nutrients measured compared with ground corn. Corn ground with a screen (6 and 1 mm) had greater (P < 0.01) ruminal solubility of all nutrients measured compared with corn ground with no screen. Corn ground through a 1-mm screen had greater (P < 0.03) ruminal solubility of DM, total starch, CP, ADF, AHF, P, Mg, K, S, Zn, Fe, and Mn compared with corn ground through a 6-mm screen. In experiment 2, increasing flake density linearly decreased (P < 0.02) the soluble fraction of DM, total starch, CP, ADF, AHF, P, K, S, and Zn of sifted flakes. The soluble DM fraction of sifted fines tended to decrease (P = 0.06) linearly with increasing flake density. Total starch, CP, NDF, and Zn soluble fractions of sifted fines were not influenced by flake density. In experiment 3, storage of sifted flakes in heat-sealed foil bags at 55 °C for 3-d decreased (P < 0.04) the soluble fractions of DM, total starch, CP, NDF, P, Mg, K, S, and Fe. With each increase in the degree of corn processing, there was an increase in the solubility of nutrients. Increasing flake density can decrease ruminal solubility of flakes; however, the soluble fraction of sifted fines is not influenced as much by changes in flake density. Inducing starch retrogradation decreases ruminal solubility of starch, nonstarch OM, and minerals.