Acharya I. P., Schingoethe D. J., Kalscheur K. F. and Casper D. P. 2015. Response of lactating dairy cows to dietary protein from canola meal or distillers' grains on dry matter intake, milk production, milk composition, and amino acid status. Can. J. Anim. Sci. 95: 267-279. A study was conducted to determine the response of feeding two different crude protein (CP) concentrations [low protein (LP, 14.3% CP) and high protein (HP, 16.3% CP)] and sources of protein [canola meal (CM) and high-protein dried distillers' grains (HPDDG)] on dry matter intake, milk production and composition, and amino acid (AA) status of high producing dairy cows. Sixteen Holstein cows were used in a replicated 4×4 Latin square having a 2×2 factorial arrangement of treatments. All diets contained 55% forage (50% alfalfa hay and 50% corn silage) and 45% concentrate. Diets were formulated to contain LP with CM, LP with HPDDG, HP with CM and HP with HPDDG. Experimental feeding periods were 4 wk with data collected during weeks 3 and 4 of each period. Dry matter intake (24.6 vs. 25.9 kg d^-1 for LP and HP, respectively) was increased for cows fed the higher CP diets, milk yield was greater for cows fed HP diets (34.0 vs. 36.4 kg d^-1), and milk fat percentage was greater for cows fed HP compared with LP, but were similar between dietary protein sources. Feed efficiency calculated as fat-corrected milk and energy-corrected milk was greater for cows fed HP than LP, but was similar between sources. Dietary metabolizable protein of CM diets was utilized more efficiently than from HPDDG diets for milk protein production. Mammary gland extraction efficiency of essential AA indicated that Met was the first limiting AA for CM-based diets while Lys was first limiting for HPDDG-based diets. In summary, canola meal and distillers' grains are both good quality protein supplements for lactating cows.