Horse scouring?

[Nannon]

My livery yard owner has moved the horses in our patch over to a new field which is ex dairy grass, however it's going through my TB at a rate of knots - serious projectile bright green poo! His oats and food are coming straight out the other end etc and he's just lethargic and hard work in the school, not like he is usually. I've been getting him in for 4 or 5 hours the last few days and letting him have plenty of hay to try and bulk it out a bit - I have no idea what else to try and when I go back to work in the week I won't be able to just get him in for more than 2 or 3 hours.
Any suggestions?!

 

[amandap]

Really he needs to be off that grass by the sound of it. Can he go back to the old field or be in longer?
Yea sacc, activated charcoal and some salt if you don't already feed it might help but the grass (? sugars) is irritating his gut by the sound of it, so as little time on it as possible or strip graze or muzzle till he adjusts or the grass is less fierce for him.