PSXII-22 Effects of branched-chain volatile fatty acids and protein-energy supplementation on rumen fermentation and digestive kinetics in dairy × beef steers grazing native or cultivated pasture.

Abstract Branched-chain volatile fatty acids (BCVFA), are microbial metabolites derived from ruminal degradation of the amino acids leucine, valine, and isoleucine, including isovalerate, isobutyrate, and 2-methylbutyrate, respectively. Cellulolytic bacteria, including Ruminococcus flavefaciens, Bacteroides succinogenes, Ruminococcus albus, utilize BCVFA for microbial protein and fatty acid synthesis. However, the effects of BCVFA supplementation on rumen fermentation and digestive kinetics in beef cattle grazing different forage-based production systems remain unclear. The objective of this study was to evaluate the effects of BCVFA supplementation in two different forage-based production systems, native or cultivated (endophyte tall fescue, novel endophyte perennial ryegrass, orchard grass; Stockmaster; Barenbrug USA Tangent, OR 97389), on ruminal fermentation, and digestive kinetics of dairy × beef cross steers. Eighteen Holstein × Angus steers, weighing 382 ± 42 kg, fitted with rumen cannulas were assigned to a completely randomized design in a 2 x 3 factorial arrangement. Treatments included two forage-based production systems (native or cultivated), and three supplementation levels: no supplementation (Control); protein-energy supplement (SUP); and protein-energy supplement plus BCVFA (SUP+BCVFA). The BCVFAs utilized were 2-methylbutyrate and isobutyrate. Approximately 2 h before supplementation, steers were administered 200 mL of chromium-EDTA as rumen fluid phase marker. Whole ruminal content samples (250 mL) were collected before dosing and at 3, 6, 9 h post-dosing, with the pH measured at each time point. Rumen fluid samples were analyzed for volatile fatty acids (VFA), ammonia (NH3), and chromium-EDTA. A treatment × time interaction was observed for ruminal pH (P = 0.01), with mean values of 6.78, 6.59, and 6.47 for Control, SUP and SUP+BCVFA, respectively. No treatment effects were observed for VFA concentration (P > 0.05), but cultivated pasture resulted in higher VFA concentrations (122.2 mM) compared to native pasture (81.9 mM; P < 0.001). Propionate concentration was greater in native vs cultivated pasture (P = 0.003). A treatment × time interaction was detected for acetate, isobutyrate, butyrate, isovalerate, valerate, and acetate:propionate (P < 0.05). A treatment × time interaction was also observed for ruminal NH3 (P = 0.006), with overall concentration of 2.97, 6.45, 7.13 mM for Control, SUP and SUP+BCVFA, respectively. No treatment effects were observed for rumen fluid dilution rate, turnover time, or outflow (P > 0.05). However, pasture type affected dilution rate and turnover time (P < 0.05), with a higher dilution rate in cultivated pasture and a longer turnover time in native pasture. Overall, BCVFA supplementation in dairy-on-beef steers grazing native or cultivated pasture improved the ruminal fermentation parameters, particularly ammonia concentration and specific VFA profiles without significantly altering digestive kinetics.