i have not heard about alfalfa having high nitrates and being bad for horses
corn, sorgum, and silage have high nitrates and can cause what is known as brown blood disease- which is fatal (look under the norwood maples thread for some good lists of toxic plants)
the reason you do not want to feed
only alfalfa is that it has an imballance in the calcium/ phosphorus ratios- this can be a praticular problem in growing horses... that being said it is an excellent source of protein- like others have said a mix of grass and alfalfa is a good forage source.... see
www.safergrass.org for a good discussion on hay/ grass- particularly for overweight horses
kelly