Yesterday when I took it off her I was cleaning the grass out of it and actually had a good look inside it. I had thought that the inner sides of the muzzle part had become smooth from getting worn but when I looked closer I realised it was this solid black build-up which I'd never noticed since the whole thing was black to begin with. I started scraping it off and I don't know what possessed me but I put my face right in there and had a good smell - pooowheeee!!!! For want of a better description it smelt like very strong dog sh*t and made me gag straight away.
I took it inside and scrubbed it all off (whilst gagging) but even once it was clean and dry it still stunk. I also noticed that the rubber had worn very thin on the bottom and wasnt far off breaking apart so I threw the whole thing away and brought a new one today. I feel awful that she has had to smell that terrible smell for so long although she seems rather unimpressed with the new muzzle as the hole is much smaller!
So anyone that has one maybe take a closer look at it, I don't recommend smelling it though.