The objective of this study was to characterize the effects of genomic predicted transmitting ability values for protein yield (gPTA) and concentration of metabolizable protein (MP) on milk production in first lactation cows. It was hypothesized that cows of different gPTA would respond differently to diets differing in MP concentration. To test this hypothesis, 56 genomically tested, first lactation Holstein cows were blocked into quartiles by gPTA and randomly assigned to a diet containing an adequate (ADMP; 16.5%CP, 30.3%NDF) or low (LOMP; 14.5%CP, 32.8%NDF) concentration of MP. Diets were formulated to contain identical base ingredients of corn silage (40%), alfalfa silage (20%), ground high moisture shell corn (15.5%), canola meal (8%), roasted soybeans (4%), dried distillers grains with solubles (2%), soybean hulls (1.5%), and a vitamin-mineral mix (2.5%). To these base ingredients, ADMP had an additional 6.5% canola meal and LOMP had an additional 6.5% soyhulls added. Cows were fed individually once daily and milked thrice daily, with intake and milk yield measured daily. Milk was sampled at 6 consecutive milkings weekly and body weight (BW) was measured twice weekly for 2 wk before and for 12 wk while consuming treatment diets. Pretreatment measurements were used for covariate adjustment in the statistical model, contrasts for diet and diet by gPTA block interactions were calculated, and P ≤ 0.05 was considered significant with 0.05 < P ≤ 0.10 trending toward significance. Cows consuming ADMP had higher DMI, BW, and production of milk and milk components than LOMP. Milk and milk protein yield increased linearly with gPTA block for cows fed ADMP, but there were very small differences in these measurements when fed LOMP (linear gPTA block × diet interaction P < 0.08). Yield of fat and energy increased and BW decreased linearly with gPTA block irrespective of diet. Feed and nitrogen efficiencies were higher with ADMP and with increasing gPTA, but these factors did not interact with each other. In conclusion, the expression of genetic differences in milk and protein yield was blunted by feeding LOMP, but not ADMP, in first-lactation Holstein cows.