Detecting differences in starch digestibility using in vitro methods among corn hybrids harvested at silage maturities

The objectives were to 1) determine whether genetic differences can be detected for carbohydrate fermentability among corn hybrids at silage maturities when the effects of drying and grinding are eliminated and 2) determine if in vitro starch digestibility (IVSD), a routine laboratory method used by commercial laboratories for hybrid evaluation, can detect differences when DM is controlled. Samples of whole corn plants from 3 hybrids (Brevant): B99B79SX (BMR, n = 7), B96T79SX, standard (STAN, n = 6), and B95U78SXE, floury-BMR (FL, n = 7), that had similar range of DM were selected. Sets of 8 ears were harvested concurrently and kernels removed from the ears by hand. Samples of undried kernels were quartered (QKERN) and in vitro gas production (IVGP) was measured for 120 h. More gas was produced by QKERN of FL than of STAN and BMR from 9 to 18 h (P < 0.05), and QKERN of FL produced more gas than BMR through 21 h (P < 0.05). The QKERN of FL had a shorter lag than STAN or BMR (P < 0.001) and a faster rate of gas production than BMR (P < 0.01), establishing that genetic differences are present at silage maturities. To determine whether the routine laboratory method can detect these differences, kernels (GKERN) and whole corn plants (GWP) were dried and ground to pass a 4-mm screen to measure IVSD after 3, 8, and 24 h. Particle size of GKERN was determined by dry sieving using a set of 13 sieves. For both GKERN and GWP, IVSD was lowest for BMR and highest for FL after 8 h. The IVSD8h of GKERN decreased as particle size increased at a similar rate for the 3 hybrids but the intercepts were different, with BMR being lowest and FL highest. Similarly, the IVSD8h of both GKERN and GWP decreased with increasing whole plant DM at a similar rate for the 3 hybrids and the magnitude of difference for IVSD8h was larger for the range in DM than the range in intercepts for the hybrids (0.123 vs 0.071 and 0.117 vs. 0.100, for GKERN and GWP, respectively). Plant maturity, or DM, has a larger effect on IVSD than hybrid type, and should therefore be controlled when evaluating genetic differences.