Is a composition of fish meal and corn grain good for a cow?

No. Leave out the fish meal because it has a potential risk of containing prions that can make a cow sick with BSE. Corn itself is merely a starch, an energy source, and doesn't provide much nutrition. You're better off using corn as a supplement and utilizing good quality hay and/or pasture as a main food source for your cow. If a protein source is needed, use canola or soybean meal or something similar that is high in protein content.