Effects of propionic acid inclusion and ensiling time on in situ ruminal starch degradability of high-moisture flint corn grain silage

This study evaluated the effects of propionic acid inclusion (PAI; 0, 0.25, and 0.50% of fresh matter) and ensiling time (ENT; 30, 60, 90, 120, 150, and 180 d) on the chemical composition, fermentation profile, DM losses, and in situ ruminal starch degradability of high-moisture flint corn grain silage (HMC). Rolled corn grain with 45% moisture and 80% vitreousness was ensiled in 10-L mini silos at a packing density of 1,300 kg/m³. The experimental design followed a 3 × 6 factorial arrangement with 3 replicates per treatment, totaling 54 silos. Samples were analyzed for chemical composition, ammonia-N, pH, organic acids, and ruminal starch degradability. Ruminal degradability was assessed using a nylon bag technique incubated in 3 rumen-cannulated Holstein cows, and a nonlinear modeling approach was applied to estimate starch degradation kinetics. For mycotoxin analysis and ruminal starch degradability, the 0-d (non-ensiled) sample was also included. Data analysis employed the NLMIXED procedure of SAS, with the Newton-Raphson optimization algorithm and adaptive Gaussian quadrature for likelihood integration. Contrasts were conducted to compare PAI versus control treatment (0% PAI) and between the PAI levels. An interaction between PAI and ENT was observed for DM losses, with the greatest losses (>6%) occurring in silages treated with 0.50% PAI at 60 d ENT. Silages treated with PAI had lower final pH values than the control, although PAI did not improve fermentation profile. Crude protein decreased, and ammonia-N increased with longer ENT. Ruminal starch degradability improved with ENT, but PAI reduced both the degradation rate and the maximum extent of starch degradation. At 180 d, estimated degradation rates were 100%/h for the control treatment, 80%/h for 0.25% PAI, and 70%/h for 0.50% PAI. In conclusion, under the conditions of this study, PAI did not improve silage fermentation profile and negatively affected starch degradability in the rumen. Conversely, prolonged ENT improved starch availability. Therefore, long-term ENT (180 d) is recommended to optimize the nutritional value of HMC, particularly in flint-type corn grain.