The objective of this study was to evaluate the interaction effects of photoperiod management and nutritional management on milk production of lactating dairy cows. We hypothesized that feeding a high grain diet increases milk yield of cows exposed to long photoperiod to a greater extent compared with those exposed to short photoperiod. The study was conducted at a tie-stall barn with metal halide light fixtures. Thirty mid-lactating cows (109 ± 37 d in milk; mean ± SD) were exposed to long photoperiod (LP; 16 h/d light; n = 15) or short photoperiod (SP; 8 h/d light; n = 15). After 30 d of light adaptation, cows within each photoperiod treatment were fed 3 diets containing steam rolled barley grain at 15, 25 and 35% of dietary dry matter (LG, MG, and HG, respectively) in a 3 × 3 Latin square design. Dry matter intake was greater for cows fed the HG diet compared with those fed the LG diet (22.7 vs. 20.4 kg/d; P < 0.001) whereas it was not affected by photoperiod treatment. The interaction effects between photoperiod and dietary treatments were significant for yields of milk (P = 0.03), milk fat (P < 0.01), and milk protein (P = 0.02). Feeding the HG diet increased milk yield by 4.0 kg/d compared with the LG diet for cows exposed to SP treatment (30.5 vs. 26.5 kg/d, P < 0.05) whereas the increase was 2.2 kg/d for cows on the LP treatment (30.4 vs. 28.2 kg/d, P < 0.05). Likewise, the HG diet increased milk protein yield compared with the LG diet to a greater extent for cows on the SP treatment (1.03 vs. 0.84 kg/d, P < 0.05) than those on the LP treatment (0.99 vs. 0.90 kg/d, P < 0.05). However, dietary grain content did not affect milk fat yield for cows on the LP treatment while the HG diet increased milk fat yield compared with the LG diet for cows on the SP treatment (1.25 vs. 1.10 kg/d, P < 0.05). These results indicate that milk production responses to high grain diets may be greater for cows exposed to short photoperiod, and that dietary grain content can be reduced without negatively affecting milk fat yield if lactating dairy cows are exposed to long photoperiod.