Firstly never ever brush it unless its clean & suitably covered with mane & tail conditioner......... My absolute favourite is the Carr, Day & Martin one, I spray liberally after washing & wait for it to dry before carefully brushing out ..........
Also make sure there is nothing in the stable or field or stable that its regularly catching its tail on & put curry powder on if there is any chance a field companion is chewing it.
as above, wash and condition it before ever touching it with a comb or brush, to keep it full at the bottom. just shake it out every day to remove bedding, or gently remove things with your fingers only, don't risk breaking any hairs with comb or brush.
you could try rubbing cornucrescine (do they do a spray now or did i imagine it?) into the top to encourage growth, worth a try, but not great if it's a competition horse!
otherwise, you can get false tails to attach, if the tail's really scratty.
Equyss (sp??) mane and tail re-growth cream,available form horse health wessex, is FAB.
mix 50/50 with water and spray in to tail 2 or 3 times a week and over 4-6 months you will see a massive difference. so not a short term miracle but its literally doubled the thickness of my TB's tail in that time scale.
One of mine has a white tail, so I bought a tail bag to help keep it white rather than orangey yellow. One of the other bonuses is that its really thick & full now.
My friend keeps her reining horses tail in a plait all the time for the same reason - take it out once a week to wash it and brush it.
Also - I don't like using silicone based detanglers, I find the hairs get more brittle & break more easily (along with absorbing more stains) so I use this stuff instead
i had to wait for 6 months for one of my mares tails to grow. i was looking at everyone elses horses tails whilst warming up fel envious!LOL
naughty gelding belonging to YO chewed the damn thing near enough off, she looked like had a yearling tail. mares are now leccy fenced away from his teeth now.
i didnt brush it at all and now i try to wash it and condition once a week as she wees all over it and i think that also weakens the hair....
it still needs to grow longer but at least its thicker now...
mane is fine and grows back 100x quicker post pulling- how does that work?!