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The symbiosis between cool-season grasses and fungi of the genus Epichloë is widespread, but highly variable in natural ecosystems. Biotic and abiotic controls underlying its distribution patterns have been difficult to establish, but might be related to a greater capacity of infected plants to deal with drought and herbivory. In this study we examined the relationship between grazing intensity and symbiosis frequency in plant species differentially preferred by herbivores. In all the grass species found in the studied Patagonian steppe, we determined the infection frequency along a grazing gradient by sheep (ungrazed, moderate- and intense-grazing). Three out of five preferred species and one out of four non-preferred species presented the symbiosis. Within the group of species having the symbiosis, intense grazing decreased the infection frequency of the preferred species while the frequency of the non-preferred species remained constant. These changes of infection frequency were correlated with changes in tiller biomass of hosts suggesting that symbiosis frequency could depend on overall plant performance. Our results suggest that high grazing intensity could weaken the relationship between these endophytes and their palatable hosts, eventually making them even more vulnerable to further grazing. [ABSTRACT FROM AUTHOR]