Last year I planted about 100 2-year old seedling Sugar Maples in a marginal hay field in order to develop a sugar bush. I would like to broadcast fertilize them. What is the recommended fertilizer and application rates?

The Ontario Maple Syrup Producers Association collaborated on the publication A Guide To Maintaining Sugar Bush Health and Productivity, which contains a chapter on establishing maple orchards (pg. 65) that would be a useful read for you. It does not include specific application rates, because results are so dependent on the characteristics of the soil in a given area. Testing the soils will be an important step before determining what amounts to use. In general, sugar maple will respond well to increases in calcium. In forest soils there appears to be a long lasting benefit from applying lime at a rate of around 1 ton per acre. Aside from application of lime, I wouldn’t recommend putting down any other fertilizers without knowing the soil status and if it will actually be needed. What may be more important than fertilizer is using tree tubes and fencing to protect the seedlings from browsing rodents and deer.