Thanks folks - Shygirl - yes, her teeth are poor due to her age. When we asked the vet whether it was harmful to the dog for it to eat horse poo, he said 'No, it's just stewed greens!', so I'll assume it's not harmful to her. I will increase her mash supplement and see if she stops. Her breath is scarcely the most fragrant at the best of times but at the moment - Yewch!