Effects of Variable Dietary Sorghum Proportions on the In Vitro Digestibility of Nutrients for Ruminants

This study evaluated the effects of increasing proportions of sudangrass sorghum forage in ruminant diets, with or without polyethylene glycol (PEG), on rumen fermentation, gas and methane production, nutrient digestibility, and protein fermentation metabolites. Three experimental diets containing 20%, 40%, or 60% sorghum forage (S20, S40, and S60) were incubated in vitro with cattle rumen fluid. Incubations were performed with or without PEG used as a tannin-binding agent. After 24 h of incubation, gas and methane production, in vitro dry matter digestibility (DMD), neutral detergent fiber digestibility (NDFD), ammonia nitrogen concentration (N-NH₃), and volatile fatty acid (VFA) production and profiles were measured. Increasing sorghum inclusion resulted in a significant reduction in DMD (<i>p</i> = 0.0012). In contrast, NDFD increased (<i>p</i> = 0.0005), likely due to differences in lignin content among diets. Methane production was unaffected by the proportion of sorghum, despite the increasing tannin content. PEG supplementation significantly increased N-NH₃ concentration (<i>p</i> = 0.042) and isobutyric molar proportion (<i>p</i> < 0.0001), indicating enhanced rumen protein degradation following tannin neutralization. The total VFA concentration was not influenced by either sorghum level or PEG treatment. However, higher sorghum inclusion was associated with shifts in the VFA profiles toward higher acetate (<i>p</i> = 0.0023) and lower butyrate proportions (<i>p</i> = 0.0114). Overall, the results suggest that moderate levels of condensed tannins (CTs) in sorghum forage may alter rumen fermentation patterns without markedly reducing methane production. PEG supplementation further confirmed the biological activity of tannins, especially regarding protein metabolism. Therefore, sudangrass sorghum may be considered a viable forage option for ruminant diets, provided its inclusion level and tannin effects are carefully managed.