Understanding the water isotopes in feed products derived from grass is fundamental for tracing domestic animal products. Grass silage water was reported to have fewer heavy isotopes than fresh grass, but it is still unknown whether dew formation (either dewfall or dewrise), exchange with soil water, or other processes override the expected enrichment of heavy isotopes due to wilting. The isotopic variations of water (δ2H, δ18O) in fresh grass and cut grass during wilting on soil and on plastic were compared in this study. Drying enriched heavier isotopes, but this was overridden by three processes that finally caused low δ2H and δ18O values: (i) the adsorption of humidity from the surroundings, (ii) the exchange with humidity, and (iii) the depletion of heavy water isotopes close to organic surfaces, called the surface effect, which was the most dominant effect at the end of drying when the water content became low.
Keywords: dew; hydrology; isotopes; surface effect; water.