Introduction: Providing good quality forage in dairy cattle diet is vital for the dairy farm industry. Canola forage was newly added to dairy cows’ diet in Iran. The present study aimed to determine the effects of replacing corn silage with canola silage on feed intake, apparent digestibility, milk yield, milk composition, and thyroid hormone levels of dairy cattle in Mashhad, Iran. Materials and methods: Nine Holstein cows were used in this study and were allocated in a 3×3 change over Latin square design study. The trial consisted of 3 periods, each lasting for 20 days. The first 15 days were considered an adaptation period. Experimental diets were formulated to provide 33% of the total dry matter (DM) as forage. Canola silage was substituted with corn silage at levels of 0, 50, and 100% of DM. Results: Dietary concentrations of neutral detergent fiber (NDF) and acid detergent fiber increased when corn silage was replaced by canola silage. Replacing corn silage with canola silage caused a significant decrease in DMI. The apparent digestibility of DM and crude protein were not affected by dietary replacement of corn silage with canola silage; however, organic matter and digestibility of NDF decreased significantly. When corn silage was replaced by canola silage, there were no significant changes in milk yield, fat corrected milk, and milk composition in the investigated cattle, but milk protein changed significantly. The results indicated that 50% substitution of canola silage caused higher milk protein. Rumen fluid pH and its ammonia nitrogen concentration increased significantly in experimental animals. The concentrations of T3 and T4 decreased, whereas blood urea nitrogen concentration increased by dietary replacement of corn silage with canola silage. Conclusion: Canola silage can be substituted with corn silage, and feeding canola silage can be beneficial in some aspects.

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