Background/Question/Methods Climate variability is a major structuring factor of grassland ecosystems, yet there is great uncertainty in how it affects grazing herbivores. We determined how variation in precipitation affected the weight gains of bison calves and yearlings in two North American tallgrass prairies. These records are the longest continuous records in the world of weight gain for non-domesticated grazing herbivores. In addition, for one site we analyzed the effects of climate on the flowering of three major grass species over a 12-year period. Results/Conclusions Across years, lower midseason precipitation consistently increased bison weight gain, while lower late-summer precipitation generally decreased weight gain. Low midsummer precipitation decreases grass flowering whic increases the nitrogen concentrations of biomass available to grazers. Burning in infrequently-burned watersheds increased the weight gain of the bison, though the effect was reduced by high mid-summer precipitation. These results show that predicting the effects of climate change on grazers must incorporate both the timing and magnitude of changes in precipitation and their effects on both the quantity and quality of biomass. In mesic grasslands, changes in the timing of precipitation might be more important than changes in the amounts, while increased midsummer precipitation can actually reduce the weight gain of grazers.