Hey R00tb0x, I'm glad you're enjoying it! So that (along with a couple of the other fetishes) isn't currently implemented in v.08.1 but was set as framework for the piss bottles. They are included in version v.9 (whenever that is finished up). So to answer your question, no you aren't doing anything wrong, it just isn't there yet. Some of the WIP features (like the prison dialogue) was noted by some was missed unfortunately.
I do highly recommend popping in the revised code (should be in a link in this thread) that resolves the stress issue until v.9 drops if you're using the farm so the tits don't expand forever and over-stress your cattle.