Might be really hard to find another yard that works for you. If the turnout, hacking, facilities and other mgt aspects are all OK, this supplement thing might have to be managed by you.
I would tell him the supplement is not being used up at the right rate (don't say you booby-trapped it... you could just say you checked the levels and noticed a significant discrepancy/the dosage instructions indicate you should have finished first tub two weeks ago, whatever).
Then ask him if you can bag up your feeds in advance, adding the supplement yourself to each bag to "avoid confusion".
A faff I know but I had to do this once (and ended up changing yards because even then, they then managed to give my feeds to another horse... giving my horse god-knows-what).
The biggest thing we pay are Ym for is trust, 'we trust that are horses are not locked in the stables 24-7' And you would give your horse a supplement if you didn't think he needed it. You have to talk to him, face to face. Good luck
A Yard Manager should be doing what the client requests (if it's not detrimental to the horses welfare), not what he "thinks is best". He is completely in the wrong and I would be seriously having more than a few words, especially as he has lied twice. TBH, I would be moving yards.
Good example - I have what I consider a slightly bonkers livery. She's lovely, but faffs like hell, has all the gear and no idea - completely over horsed with a 16.2hh warmblood and an ex racehorse who she is scared of. Her horses are fed supplements for this & that, turn out managed in a certain way because one horse is a bit arsey, if her mare is in, she has to have this toy and that toy at set times of the day to relieve boredom, certain rugs for this, turnout rugs for that, etc, etc.
IMO I think it's all a load of bollocks, in that her horses would be fine without all the supplements and bullshit, and they wouldn't fall apart if they went out in the wrong colour turnout rug...
BUTShe is the client, her requests are not detrimental to horses welfare & she is paying me to give her a service so I do what she wants without querying it. Just because I would do things differently (and do) with my own horses and think her way is a load of old bollocks, doesn't mean I have the right to do it my way and then lie about it.
You say he is trustworthy in every other way, but I don't think I could trust someone who I knew had lied to me at least twice. I think I would be looking to move yards. If you want to stay put, you are almost stuck with doing things his way, unless he has a boss you can complain to? It sounds like raising it with him previously hasn't done any good.
Good he might be, but still out of line! People interfering in how I keep my animals is one thing that makes me see red! He has every right to discuss his opinion with you. He doesnt have the right to ride roughshod over your choices! Im fuming on your behalf! <<needs to breathe>>
I don't go every day. He insists that he feeds as well, rather than the grooms, as he's really conscientious and likes things just so. He really is as well, in every other way but he doesn't particularly agree with this supplement, so just doesn't feed it (it's not weird in any way! It's just quite specialist and he prefers other supplements). I don't want to move yards. It's fab in every other aspect.
I would sure as hell be telling him he is a liar! You are paying him to look after your horse and he isnt! Ive seen this at a very expensive yard round here where I used to work. Its not bloody on! And I would be moving yards ASAP! Cheeky fecker! Can you bag up each meal seperately with the supplements added do all the YM needs to do is just tip it in? Or make up the meal before you leave each day?
My gelding has to have breathing supplements every day. He's on full livery and therefore fed everyday by the YM. The YM wasn't giving him the supplement, so I spoke to him about it and he started giving the supplement. I've recently realised that he wasn't administering it again, so I 'booby trapped' the supplement and sure enough, I 'proved' that it wasn't been given. I text the YM and he said, yes, the supplement is being given. Now I know that the YM is prone to the odd porkie, although trustworthy in every other way. So I have a dilemma. I'm not usually a chicken but I don't feel like I can have it out with him. How do I say, I didn't believe you so I set you up and proved you to be a liar! I'm not there everyday, so can't give it myself. Help!