Climate variability is a major structuring factor in grassland ecosystems, yet there is great uncertainty in how changes in precipitation affect grazing herbivores. We determined how interannual variation in the timing and magnitude of precipitation affected the weight gain of free-roaming bison in their first and second year. Bison weights were analyzed for 14 years for Konza Prairie, Kansas, and 12 years for Tallgrass Prairie Preserve, Oklahoma. Greater late-summer precipitation increased bison weight gain. For every 100 mm precipitation, weight gain increased 6.4–15.3 kg depending on age classes and site. In contrast, greater midsummer precipitation decreased weight gain. For every additional 100 mm precipitation, weight decreased 9.7–17.3 kg depending on age class and site. The decreased weight gain of bison with greater midsummer precipitation was associated with increased grass stem production during the period for each of three dominant grasses at Konza Prairie. Although greater stem production increases the quantity of aboveground biomass, it should decrease the overall nutritional quality of biomass to grazers, which would reduce weight gain. With offsetting effects of mid- and late-summer precipitation on weight gain, 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.