Comparison of a three-step in vitro method and the mobile nylon bag technique for determining the intestinal digestibility of rumen-protected products in dairy cows

The objectives of this study were to develop a three-step in vitro method to assess intestinal digestibility of rumen-protected products in dairy cows, and compare this method with the mobile nylon bag technique. A 3 × 4 × 3 factorial experiment was performed to determine an optimal combination of pancreatin (0.8, 1.0, and 1.2 g/30 mL), sodium cholate (0, 1.0, 2.0, and 3.0 g/30 mL), and incubation length (8, 16, 24 h) for maximizing lipid digestion of a hydrogenated fat product. Following the preliminary study, a 2³ factorial experiment was conducted to investigate the effects of ruminal incubation (with or without 16 h rumen in situ incubation), gastric pH (pH = 1.0 or 2.0), and gastric acid incubation time (1 or 2 h) on digestion of the hydrogenated fat and a calcium salts of long chain fatty acids product. Once optimal culture parameters were determined, the developed in vitro method was compared with the mobile nylon bag technique. The pancreatin × sodium cholate and sodium cholate × incubation time interactions were significant in the intestinal digestibility of the hydrogenated fat product (P < 0.001), and the best case scenario was observed when the hydrogenated fat was incubated in 30 mL buffer solution containing 1.0 g pancreatin and 2.0 g sodium cholate for 24 h. A ruminal incubation × pH × incubation time interaction was detected for the intestinal digestibility of the hydrogenated fat (P = 0.002), and a ruminal incubation × pH interaction was observed for the calcium salts of fatty acids (P = 0.04). The digestion of hydrogenated fat was maximized when it was incubated in gastric pH 1.0 solution for 2 h following 16 h ruminal incubation. However, ruminal incubation had no effect on the intestinal digestibility of the calcium soap when incubated in gastric pH 1 solution (P < 0.05). The developed in vitro method had greater DM intestinal digestibility than the mobile nylon method (on average 0.78 versus 0.33), implying the mobile nylon bag method might have underestimated the DM intestinal digestibility of rumen-protected products. However, no significant difference was observed on intestinal digestibility of Met, Lys and choline in the rumen fat-protected products. Overall, the developed method could be used as an alternative approach for screening and evaluating intestinal digestibility of rumen-protected products.