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<0.01) and RDMD (P<0.05). There was compensatory DM degradation post-ruminally from germplasm with low RDMD values (wx1(Oh43) and Oh43), thereby reducing inbred differences for TDMD. The influence of advancing maturity on degradabilities was greater for wx1, su2 and Oh43, than for o2 and fl2 mutations which have inherently soft endosperm properties. When compared to freeze drying, oven drying at 40 °C reduced the A Fraction, particularly for early harvested samples. The ranking of inbreds for decreasing A Fraction, RDMD and TSTARCHD was o2(Oh43) > fl2(Oh43) ≥ su2(Oh43) > 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.