Good Idea about making provisions for forks on the bottom. I have a little Mahindra Max28 that I can get in the cow camp to muck it out and move the feeder, etc.
Our cows like to play with stuff, so I want it heavy. I was considering filling the base with Gravel. Our biggest steer I caught pushing our RTV900 just for fun.
The base will not contain grain, as we mter the feed and feed daily in a rubber bucket.
Regarding PT lumber. Copper arsenic is no longer used. It is some Vinegar based compound in the stuff they use in play grounds. Not as good as the deadly insect treatment, but reasonably safe, especially if you have PT that is aged at least a few months.
Note the stats are pine strapping, the base and legs are PT.