This study evaluated productive performance of cows fed four levels of dietary crude protein across four stages of lactation. After an initial 14-d covariate period, cows were fed treatment diets for 56 d. Dry matter intake and milk yield were measured daily and milk composition and body weight were assessed over two days each week. Data were composited by week and summarized statistically through categorical mixed model ANOVA and continuous regression analyses. For a more complete description of the methods, please refer to the published paper. Supplemental materials include: Supplementary Figure S1. Graphical description of the approach used to obtain the dietary CP concentration at the maximum predicted response (panel A) and the 95% confidence interval for the maximal response (panel B). Supplementary Table S1. Least square means of the interaction between stage of lactation and dietary CP levels for production performance and feed efficiency. Supplementary Table S2. Predicted range in dietary CP for which there was no difference in production responses with increasing DIM.