Food item categorization should reduce the cost of information processing by herbivores when selecting their diet in complex environments. We assessed the ability of sheep to categorize food items by offering them ryegrass (Rg) and fescue (Fe) in pots cut tall (T) or short (S). Ewes’ preferences were tested in three binary choices, RgS–FeT, RgS–FeS and FeT–FeS, before and after aversive conditioning against RgT. After conditioning, the ewes decreased their preference for RgS, but their choice between tall and short fescue was unchanged. Thus the ewes generalized their aversion to the species but not to the sward height. Comparing the choices between the two species offered at the same height showed choices were similar between RgS and FeS here and between RgT and FeT in Ginane and Dumont (2006). We conclude that sheep can use species-based, open-ended categorization when selecting their diet, while other plant characteristics, such as sward height, are not used to define a category, despite their importance in diet selection.