Influence of flake density and starch retrogradation on in vitro gas production kinetics, digestibility, and ruminal fermentation characteristics of steam-flaked corn.

Previous research has demonstrated that decreasing starch availability of steam-flaked corn by increasing flake density or increasing the degree of starch retrogradation influences in vitro gas production kinetics. However, it is unclear if increasing flake density or the degree of starch retrogradation influences end-products of in vitro ruminal fermentation (methane, volatile fatty acids, ammonia). The objective of this experiment was to evaluate the effects of increasing flake density and the degree of starch retrogradation on in vitro gas production kinetics, digestibility, and ruminal fermentation characteristics of steam-flaked corn. Three ruminally cannulated steers were fed a high-concentrate diet and sampled for ruminal digesta for an in vitro fermentation experiment with a 5 × 2 factorial arrangement of treatments. Steam-flaked corn was produced to flake densities of 257, 296, 335, 373, and 412 g/L by adjusting the rolls of a steam-flaker. Samples were stored for 3 d at either 23 °C to produce fresh steam-flaked corn or at 55 °C in heat-sealed foil bags to produce retrograded steam-flaked corn. In vitro fermentation vessels were incubated for 24 h and then assessed for fermentation parameters including dry matter digestibility, volatile fatty acid concentrations, and total gas and methane production. Increasing the degree of starch retrogradation decreased (P < 0.01) the rate of gas production across all flake densities of steam-flaked corn but did not decrease the extent of gas production. In vitro methane production, dry matter digestibility, and microbial biomass concentration were not influenced by increasing flake density or starch retrogradation. Increasing the degree of starch retrogradation decreased (P = 0.03) the molar propionate proportion and increased (P < 0.06) the molar proportions of butyrate, isobutyrate, and isovalerate and the acetate:propionate ratio. Enzymatic starch availability of steam-flaked corn was positively correlated with mean propionate proportion (r2 = 0.93) and negatively correlated with the mean butyrate proportion (r2 = 0.89). Results from the current study demonstrate that increasing the degree of starch retrogradation of steam-flaked corn decreased the rate of in vitro gas production and altered volatile fatty acid profiles in the ruminal fermentation media.