Livestock in the Amazon is mainly limited by the lack of forage biomass production in the dry season. The sensitivity of pasture to drought depends on pasture management and on certain natural factors that enable the grass cover to stay green for longer under low rainfall. The sustainability of livestock production systems depends on identifying these natural factors to make better use of their forage production potential without consuming additional water resources or resorting to deforestation. We built an index of pasture dependence on rainfall at the beginning of the dry season by calculating the slope of the linear regression between the MODIS enhanced vegetation index (EVI) and monthly cumulative rainfall obtained from the Tropical Rainfall Measuring Mission (TRMM). The index was calculated in Paragominas municipality. A 15-year time series (2000-2014) was used to assess the sensitivity of pasture to drought. A map of geomorphological units was overlaid on the index map to check whether the soil and the landform influence drought sensitivity at a large scale. Drought resistance was shown to be higher in lowland areas with loamy sand with a shallow groundwater table and on plateaus covered by fertile sedimentary clay. The escarpments with a ferruginous duricrust and mottled clay were shown to be the most sensitive to drought. These results underline the importance of natural factors in pasture productivity in tropical humid areas.