What are dietary defiencies?

I brought a 5 year old mare which turned out to be pregnant. Last week she gave birth to a lovely foal. I know that a balanced diet is very important to the mare and foal and have been doing a lot of research on bone development problems in young horses but I am a bit baffled, Could you explain what the possible dietary deficiencies is that can cause developmental orthopedic diseases, what symptoms I should look out for and what should I feed in order to prevent this happening to my foal. Also if I want to put my mare into foal again, how should I feed her to prevent getting nutritional deficiencies.