Silica in grass leaves has been proposed as a defence against herbivores and as a key driver of grass-grazer co-evolution. It is thought to lead to increased abrasiveness of foliage, so wearing down herbivore teeth and mandibles and deterring feeding. However, at present there is little direct evidence to support these ideas, and the effects of silica on foliar digestibility and herbivore performance have not been tested experimentally. In this study, we examined the effects of manipulating silica levels on the abrasiveness of grasses and on the feeding preference and growth performance of specialist grass-feeding herbivores, both mammalian (voles) and insect (army worms). Elevated silica levels did increase the abrasiveness of grasses and deter further herbivory. Silica also reduced the growth rates of both types of herbivore by reducing the amount of nitrogen they could absorb from foliage. For voles, this reduction in body mass has implications for their fecundity and population dynamics. In insects, these effects were linked to the impact of silica on mandible wear: the highest silica levels produced the greatest reduction in mandible length and the greatest reduction in nitrogen absorbed. Switching to a low silica diet could not reverse these effects: the anti-herbivore impacts of silica are severe, long-term and irreversible.