Mixer and loose poos?

I have recently (last couple of days) started giving Cooper more mixer with his raw food to fill him up/bulk him out a bit more, but I noticed last night that his poo was very loose (looked firm but wasn't when I picked it up).
Harry has the same food, except less mixer and his poos were fine.

Is it likely that it's the mixer that is making him loose?

He has been on the same diet for a while now, although yesterday he did have a few pieces of Eukanuba kibble that I gave as training treats which he had not had before (Harry had the same amount)

..simple answer if its cereal based.. yes

It's this one they have

http://www.foldhill.co.uk/product_de...=40&species=1#

Could be the mixer or the Eukanuba, one way to find out - cut one out and see if symptoms still continue. Lots of animals are fine with some foods in small quantities but larger amounts will have a more obvious effect. For example, my two are fine with the small amount of lamb in their lamb & tripe mince, but if I give them a lamb breast each, they have terrible wind the next day!

He's not had any Eukanuba today so I can see if it's that, but he literally only had 2 pieces just to see if he would eat it.
Tomorrow he won't be getting any mixer to see if it's that, but I'm not sure if I will be able to tell that quickly

Why add dryfood or mixer to a raw diet?

If the dog needs building up try giving an extra wee raw feed each day. Build the dog up gradually with the good stuff rather than filling it with junk.

WHy add mixer to a raw diet????

Hows he today in the poop department?

I give him mixer because it would cost me a fortune to feed him with only raw,unfortunately I don't have a friendly butcher who likes to give me free things. (before anyone jumps on me he gets enough raw food for a 'normal' dog but he eats a lot as he is allowed to eat as much as he likes, the mixer fills him up more than raw food).

I have started adding some extra tripe to his food but he just seems more hungry than usual

I haven't seen him poop yet today (someone else let him out this morning) but I am taking him up to see the horses this morning so hopefully we can see if the extra tripe is hardening his poops any.

Not going to jump on you and think it's a bit unfair that people have - not everyone switches to raw to go "carb free", some just prefer having more control of the diet, being able to know what you're giving them, etc. (plus some dogs just don't enjoy kibble but love raw), plenty of reasons to switch to raw and some of these will include adding some form of carbs.
If it's an intolerance to grain then this may be the issue. You could feed mashed potato instead - boil and mash it up, but obviously skip out the milk, butter, seasoning etc.! You could feed the potatoes whole if you wanted but someone on another dog forum recently had their dogs operated on for symptoms of an obstruction, and it turned out they had a potato lodged in their throat! Strange but true, so I'd definately stick with the mashed potato if your dog tends to bolt down food rather than chew.
Alternatively, have you tried veg instead of mixer to bulk the food out and fill him up?