Xie, M., Tremblay, N., Tremblay, G., Bourgeois, G., Bouroubi, M. Y. and Wei, Z. 2013. Weather effects on corn response to in-season nitrogen rates. Can. J. Plant Sci. 93: 407-417. The response of corn yield to in-season nitrogen rate (ISNR) fertilizer applications in a temperate humid climate is conditioned to a great extent by prevailing weather conditions, which affect nitrogen use efficiency and raise the level of uncertainty for making management decisions. A better understanding of the effects of temperature, expressed as accumulated corn heat units (CHU), and precipitation would help to ensure that a “closer-to-optimal” nitrogen rate is supplied at side-dressing. A meta-analysis was performed using a database of nitrogen response trials conducted from 1997 to 2008 in 60 locations in the corn grain production area of Québec, in conjunction with a weather database. Meta-analysis is a statistical procedure for combining results from a series of studies that is used in many fields of research. It is used to assess treatment effect (also called effect size) in a group of studies or experiments. Corn yield response to ISNR was negatively correlated with overall CHU accumulation, but positively correlated with CHU accumulation before side-dressing. Responses also showed a stronger relationship with cumulative precipitation (PPT) before side-dressing than after side-dressing. High and evenly distributed precipitation before side-dressing tended to increase responses to ISNR. It can be concluded that low CHU, low precipitation and low precipitation evenness before side-dressing reduce the impact of ISNR on corn yield.