Comparison between in situ and in vitro techniques to estimate rumen dry matter and neutral detergent fiber digestibility of typical feedstuffs for dairy cow nutrition

The objective of this study was to compare 2 techniques for assessing DM and NDF rumen digestibility of feedstuff: the in situ nylon bag technique and the Daisy II (Ankom Technology Corporation) incubator technique. In this study, we compared the estimated rumen digestibility of DM and NDF be-tween both techniques for wheat silage, corn silage, wheat hay, and alfalfa hay as forage feeds and ground corn, corn gluten feed, dry distillers grains, and canola meal as con-centrate feeds. In total, we compared 96 samples for DM and 85 samples for NDF digestibility at 9, 30, 120, and 240 h of incubation. We found a high correlation between the 2 techniques for both DM (r 2 = 0.90) and NDF (r 2 = 0.95) digestibility. However, for several feed-stuffs, comparing the 2 methods resulted in no consistent trend in digestibility, mainly for NDF, throughout the in-cubation times. We also analyzed the data by 2 categories: forage versus concentrate. The correlation between the digestibility of DM and NDF using both techniques was greater for concentrate than for forage feedstuffs (r 2 = 0.94 and r 2 ~0.90, respectively). We assume that the greater consistency in the texture and chemical composition of concentrate feedstuffs, compared with forages, accounts for the greater correlation observed. In conclusion, the correlation between the estimated rumen digestibility of DM and NDF in situ and with the Daisy II was high. How-ever, inconsistencies in NDF digestibility were evident between the in situ and Daisy II techniques across feed fami-lies, individual feedstuffs, and incubation times. These discrepancies highlight the limitations of both techniques and suggest that feed evaluation and ration formulation should rely on a single analytical method rather than mix results from different approaches.