Evaluating Old and Novel Proxies for In Vitro Digestion Assays in Wild Ruminants

In vitro digestion assays provide useful data for wild ruminant ecologists. It remains highly constraining, however, to conduct these assays in the field. We evaluated various proxies for in vitro digestion assays that use ruminal liquor from wild ruminants. The ruminal liquor of freshly killed white-tailed deer (Odocoileus virginianus) was used to measure in vitro true digestibility (IVTD), total gas, and volatile fatty acids (VFA) production of dominant summer and winter forage. We evaluated the ability of 4 proxies to predict these digestibility variables: 1) IVTD, total gas, and VFA produced with cow ruminal liquor; 2) acid detergent fiber (ADF) concentration; 3) CO2 production from soil-forage mixtures; and 4) predictive models based on near infrared spectra (NIRS) of forages. Predictions using NIRS were best correlated with all deer digestibility variables (each r(2) > 0.93) with limited bias across forage species. Forage ADF and CO2 production from soil-forage mixtures were related with all digestibility variables (r(2) = 0.62-0.95). Digestion assays using cow ruminal liquor was relatively good for predicting IVTD (r(2) = 0.94) from deer ruminal liquor, but the weakest for predicting total gas (r(2) = 0.63) and VFA production (r(2) = 0.44). Calibrated NIRS models could substantially improve the prediction of in vitro digestibility variables. If access to NIRS technology or the ability to collect reference data with deer inocula for NIRS calibrations is impractical, forage ADF or CO2 production from soil-forage mixtures remain acceptable alternatives. Comparing the digestion kinetics and VFA production of wild and domestic ruminants is an innovative avenue for better understanding the adaptive ecophysiology of digestion by different herbivores. (C) 2016 The Wildlife Society.