Triple stack lower stalks are TOUGH, especially the irrigated varieties. This is what they are designed to be to support a 10 ft tall high yielding corn plant. As a planter attachment person, there are times when these stack lower stalks seem like they are made of steel...especially in the fall.

I graze triple stack corn and have for 3 years now. This is a particulaly palatable WI dryland variety and I still do not force them to eat the lower stalk.

If I was going to graze bt or triple stack corn I would run a NON-knife roll corn head as high as possible.  A chopping corn head will cut most of the palatable parts of the plant into little pieces which are then largely driven over by mega-axle load equipment. Not much feed value left standing, just steel rods. 

An old JD cast iron roll cornhead with almost worn out rolls works the best for grazing. Just enough roll left to get the ears and leave everything else as intact as possible for the cows. jmho.

Jim at Dawn