In the most dairy farms in Iran, forage portion is consisting of corn silage and alfalfa hay. Therefore, optimal placement of these two forage source which could improve feed intake and milk production can be a useful and important guide for making decisions in the dairy industry. Therefore, the purpose of this experiment was to evaluate the effect of different ratios of alfalfa hay to corn silage on production and reproductive efficiency in early and mid-lactation Holstein dairy cows in summer. The experimental design was a completely randomized design with two treatments in two separated halls, that each owns 60 Holstein dairy cows. The average MY(47.5 ± 0.5), DIM(40 ± 15) and parity (2.48) were about to be equal for cows in each hall. Cows were randomly assigned to the experimental diets. Concentrate to forage ratio was 60 to 40 for two experimental diets. First experimental diet containing 50% corn silage as forage source and 50% alfalfa hay and the ratio of corn silage to alfalfa hay for second experimental diet was 75:25. Rations in terms of NEL(1.7 Mcal per kg DM), CP(18%), NDF(28.5 ± 0.2%) and NFC(42 ± 0.3%) were equal to each other. Cows were fed mixed (TRM) twice a day (5 AM and 5 PM). Cows within each hall had free access to water and rock salt. DMI of cows fed 75% corn silage diet was greater, and tend to be significantly higher ( P =0.06) in early lactation, and in the middle and whole of lactation DMI of cows that were fed 75% corn silage diet was higher than 50% corn silage diet ( P 0.05). The MY( P 0.01) and ECM( P =0.05) was also higher for 75% corn silage diet than 50% corn silage diet. While, due to higher milk fat content in dairy cows consuming 50% corn silage diet ( P 0.05), but 3.5% fat-corrected milk were not affected by treatments ( P 0.05). About the other components of the milk in all time periods, the lactose content of milk tend to be significantly affected by treatments ( P =0.07) and fat to protein ratio of milk was significantly higher for 75% corn silage diet than 50% corn silage diet ( P 0.01). Although the percentage of milk protein were similar among treatments, but milk protein yield in mid-lactation ( P 0.05) and total test period was higher for 75% corn silage diet than 50% corn silage diet ( P =0.07). BCS and its changes during experimental period were also affected by treatments, so that cows consuming diets containing 75% corn silage as source of forage, had higher BCS( P 0.01) and in early lactation these cows lost less BCS ( P 0.05). About the digestibility of dietary nutrients, the results showed that 75% corn silage diet in the early lactation had higher digestibility than 50% corn silage diet ( P 0.05). Feed efficiency (ECM per DMI) was significantly higher for 75% corn silage treatment than the other treatment ( P =0.05). Among the blood parameters, only BUN was affected by treatments and in early lactation and total lactation was significantly higher for 50% corn silage diet than 75% corn silage diet ( P 0.05), but in mid-lactation was not affected by treatments ( P 0.05). In this study the reproductive parameters were numerically higher for 75% corn silage diet, but they were not affected significantly by the experimental diets. The results of these experiments showed that increasing the portion of corn silage from 50% to 75% as source of forage for dairy cows, without affecting the reproductive parameters, can improve the productivity of Holstein cows. Key words : dairy cattle, corn silage, alfalfa hay, performance, reproduction.