Silage is an important dietary water source that influences the oxygen and hydrogen isotopic composition of domestic herbivores and their products. Silage sampled fresh from the silo had¹⁸O- and ²H-depleted tissue water when compared with fresh pasture grass sampled around midday during the silage-making seasons. During exposure in the feed bunk, silage water became increasingly enriched in ¹⁸O and ²H. When δ¹⁸O was plotted against δ²H, the slope of the regression was less during daytime than during night-time. Exposure to ¹⁸O- and ²H-enriched or -depleted water vapor inside sealed glass containers led to strong changes in the isotope composition of silage water. The results resembled predictions from the Craig–Gordon isotope model of evaporation. The atmospheric conditions during exposure (relative humidity, exposure time, and isotopic composition of the air vapor) in the feed bunk thus strongly affect the isotopic composition of silage water ingested by domestic herbivores.