Fluorine has no known physiological role in grasses, yet is an essential nutrient for grazing animals. The evolution of an uptake mechanism for F must be examined in the context of the coevolution of grasses and grazers. The objective of this study was to determine whether uncultivated grasses buffer the uptake of F in a range acceptable for grazing animals and whether cultivated grasses have lost this buffering mechanism. The uncultivated grasses big bluestem (Andropogon gerardii Vitman), little bluestem (Schizachyrium scoparium (Michx.) Nash) and switchgrass (Panicum virgatum L.), and the cultivated grasses wheat (Triticum aestivum L.), rye (Secale cereale L.) and oats (Avena sativa L.), were grown in a greenhouse and watered with NaF at F concentrations in the range 0–9 mg/kg. Total F concentrations of plant shoots were determined by the alkali-fusion method. Average F contents for uncultivated grasses (29 mg/kg for big bluestem, 26 mg/kg for little bluestem, 25 mg/kg for switchgrass) were below the upper limit for dairy cattle (40 mg/kg), while average F contents for cultivated grasses (51 mg/kg for wheat, 97 mg/kg for rye, 99 mg/kg for oats) were well above the upper limit for cattle. For the uncultivated grasses, plant F content did not correlate with F concentration of irrigation water (R² = 0.13 for big bluestem, R² = 0.14 for little bluestem, R² = 0.22 for switchgrass), while correlations were moderate to good for cultivated grasses (R² = 0.61 for wheat, R² = 0.52 for rye, R² = 0.52 for oats).