Do you mean crushed maize or maize silage?
Silage has already begun the fermentation process - the same process which occurs in the cows rumen. It is therefore more digestable as a result. Starches also ferment to propionic acid which is a precursor to weight gain at the expense of milk fat. The best quality silage will still be about 38% NDF (fibre) which produces acetic acid on fermentation- which is a precursor for your fat test. Wheat is an acidosis risk because it it only about 12% NDF at 12.7 MJME whereas a top quality pasture silage will still be about 38% fibre.