I'm not the one proposing a ban on football.
Designated pitches might be workable. In Llandaff Fields there are pitches that are more out of the way than others. When I think of this issue and imagine where exactly I'd be walking my dog, I'm thinking of the path that goes from Llandaff fields changing rooms to black weir bridge. I feel like the areas around the path would have to be left free for traffic of all sorts...dogs, walkers, kids etc. Can you imagine dog walkers and pedestrians all having use that narrow stretch of path? I think it could be problematic.
But the answer is not a quick fix type of thing. The answer is to control who can have a dog. To ensure they are properly trained to handle a dog (including picking up it's shit). Heavier penalties for dog owners who don't follow the laws. Encourge people to report those who they see leaving crap...maybe a quick-dial report line. Threaten to take dogs away from owners if they are unable to take care of them properly. Make it unacceptable.
With regards football pitches. There should be an inspection before every match anyway to make sure the pitch is free of glass and other dangerous objects. Maybe the council could provide dog shit/glass/cans hoovers and antibacterial killer foam spray so that the pitch is completely free of foreign objects.
It takes time and it takes thought and organisation but that is what living together in the same space is about......cohabiting.....conviviality. Not just banning stuff when you don't like it. That's pathetic IMO. As pathetic an attitude as leavingdog shit on the floor.