I think seed companies put out a height rating on hybrids - at least some companies do.  Have you paid attention to this when selecting hybrids you buy?

I think I see plant height can be related somewhat to planting date.  Same hybrid planted early can be somewhat shorter than the same hybrid planted weeks later.  I am not saying you will see huge differences, but I think there will be incremental differences.

Amount of rainfall during the vegetative stages can affect final height.   

In the end, I think you need to pay attention to height ratings when you select your hybrids to buy.

You are probably correct in that hybrid was probably better marketed for the silage market.

Those are my thoughts.