Influence of single-gene mutations, harvest maturity and sample processing on ruminal in situ and post-ruminal in vitro dry matter and starch degradability of corn grain by ruminants

Combined effects of single-gene mutations (G), harvest stage (HS) and sample drying technique (DT) on the proportion of dry matter (DM) and starch degraded ruminally in situ and post-ruminally in vitro were evaluated using four near-isogenic lines in Oh43 inbred background: floury-2 ( fl2), opaque-2 ( o2), sugary-2 ( su2), waxy-1 ( wx1) genes and normal Oh43. The inbreds were grown at the University of Wisconsin West Madison Research Station (Madison, WI, USA) during the summer of 2002 in three row plots of 3.04 m × 0.76 m, in a randomized complete block design with three replications. Harvesting was at four stages (HS1 = 1/2 milk-line; HS2 = 5 d post HS1; HS3 = 10 d post HS1; and HS4 = black layer) with samples split for oven drying at 40 °C for 72 h and freeze drying for approximately 60 h. Dried kernels were ground through a Wiley mill (6 mm screen) for measurement of zero hour DM solubility (i.e., A Fraction) and ruminal in situ DM degradability (RDMD) after 14 h incubations (1.5 g/bag × 8 replicates in 5 cm × 5 cm bags of 50 μm pore size) using two steers. Residue from the 14 h bags proceeded to an 8 h enzymatic post-ruminal degradation, from which the post-ruminal residue was oven dried at 62 °C for 48 h and DM and starch contents determined to provide estimates of total tract DM degradability (TDMD) and total tract starch degradability (TSTARCHD). Three-way interactions for G × HS × DT were observed for the A Fraction (P wx1(Oh43) ≥ Oh43. Results identify key properties of corn grain germplasm and their relationships with ruminal in situ and post-ruminal in vitro DM and starch degradability measurements that may be used in advanced corn breeding efforts.