A Comparison of Techniques for Estimating Forage Digestion

The objective of this study was to compare in situ and in vitro methods for determining rate and extent of forage digestion in the rumen of beef cattle. Duplicate polyester (10 × 20 mm) or ANKOM F57 filter bags containing 5.0 or 0.5 g of ground (2mm) bromegrass hay, respectively, were incubated in situ or in vitro (ANKOM Daisy Incubator) for 3, 6, 9, 12, 15, 18, 24, 36, and 96 hours. Bags were incubated in the rumen of 3 ruminally cannulated Angus × Gelbvieh cattle consuming bromegrass hay, or in rumen fluid collected from these cattle. At each time point, duplicate bags were removed, rinsed with tap water and dried (55°C). Residues were analyzed for OM, NDF, and CP. Estimates of OM digestibility were higher (P≤0.03) for in situ and in vitro at 15, 24, 36, and 96 hours. At incubation times of 6 through 24 hours, NDF digestibility was comparable (P=0.10 to 0.44) between techniques, but differed (P≤0.03) at 3, 36, 48, and 96 hours of incubation. Forage CP digestibility determined in situ was greater (P≤0.01) than in vitro at all incubation times. Estimates of N fractions A, B, and C did not differ (P≥0.12) for either technique. However, estimated N degradation rate was eight fold higher (P=0.01) for in situ (8.1%/hour) than in vitro (1.6%/hour), resulting in greater (P=0.0001) estimated ruminally degradable protein of the forage for the in situ (73.6%) compared to the in vitro (53.8%) technique. Because estimates of forage digestibility differed depending on the technique employed, estimates of in situ or in vitro forage digestibility cannot be used interchangeably.