I think this is quite unusual in adult horses, but could be a sign of not enough roughage in the diet, or possibly boredom? I dont think it's often a sign of malnutrirtion as such, though a broad spectrum supplement won't hurt. I would be looking more at amounts of turn out / forage though.
He has ad lib horsehage and daily turnout
We have had him blood tested and he is on a general vitamin supplement and is regularly wormed but he still eats his own poo?!
We have mentioned it to the vet but he wasn't sure and said he would look into it.
Hmm. In that case may be a habit then from sometime in the past when he was deprived of roughage? I would try feeding something with higher dry matter content than haylage eg hay or even some oat straw?
My old horse ate his poo!! He was 5 when I first realised why he was easy to muck out, he ate it with relish!! I hated it and worried like mad, however my vet was completely unpreturbed and said that 'some horses just develop a taste for it'!!
He was regularly wormed and had vit and min supplements and adlib hay and daily turnout. He was the picture of health and was a brill little eventer, he was retired at 7 due to lameness , but I did just learn to grin and bear his nastly little vice!!! So if you now he is nutritionally ok I think you need to try and accept it
I was at a racing yard one day when I saw someone mixing in some straw to a shaving bed. I ask what they were doing and the trainer said that they do it for all the poo eaters. He said in all his many years of experience it was the only cure. Basically I think it is usally boredom or lack of forage. A straw bed would work too.
I had a horse who did this (who I had for 8 years) and would get positively upset sometimes whilst you were mucking out, trying to dive into the wheelbarrow. It went in phases, sometimes not bothered at all, other times really desperate for it. People came up with all sorts of suggestions, but nothing made any difference.
It was a bit difficult because he was an extremely good doer and it was not possible to feed him ad lib. Even so, he still did it when there was hay to eat.
He was a quirky horse but always looked incredibly well - glossy coat and good muscle tone - and kept on vitamin and mineral supplements. I didn't like his poo eating but it clearly didn't bother him, so I tried to ignore it. Nothing else to be done!!