My tamed Diplodocus seems to reject mejoberries and instead eat azulberries. I haven’t tried wild gaming a diplo with azulberries but I would recommend trying! Hope this helps. Also where do I find plant species Y? I can find x in swamp but I can’t find Y. In mountain bushes???
I found it super easy to knock it out rather than passive tame, just put a bunch of Tranqs in it give it some berries and depending on the level, a few narcotics. You don’t usually need many narcotics.
Standing under its belly definetly works after wasting 80 tranq arrows barely did any damage to a lvl 35 diplo. Just don’t let the food run minimally low otherwise you lose the tame effectiveness.... upvote !
When passive feeding, either keep your distance so that it can’t charge you, or make sure you’re not on a great hight as it will launch you off the side of the hill/mountain, and it’ll hurt, trust me, I’ve made that mistake once, and I never will again.