Considering different scenarios of future trends in climate, several authors have found that the impact that climate change will have on agriculture will most likely be negative. Most of these studies consider regions with low level of irrigation and do not control for purchased farm inputs. An important step towards understanding the evolution of agricultural production is to carefully estimate the effect that different temperatures and precipitation have on agricultural productivity considering also inputs under farmers control and the farmers profit-maximizing behavior. This research develops a county level biomass production function for an 800-mile climatic gradient from the Rocky Mountains to the Mississippi River (41N). Our results quantify the critical effects that high temperatures have on agricultural productivity in the region, after controlling for irrigation, other managed inputs, soil characteristics, precipitation, and technological change. We find a negative and increasing (nonlinear) effect of temperatures over 30 C on crop yields; a full day of temperatures between 30 C and 35 C decreases expected yield by 1.7% and a day of temperatures over 35 C decreases yields by 23.1%. In addition, converting rainfed crops to irrigated crop will produce a sharp decrease in the negative impact of the higher temperature interval. Acknowledgement : Support is acknowledged from the Agricultural Research Division s Strategic Investments: Enhancing Interdisciplinary Teams Program Forty-First Parallel Agro-Ecosystem Sustainability and Productivity, University of Nebraska, and from USDA, NIFA NEB-24-164 and NEB-24-172.