Waste not, want not

We rescued an nine-year-old yellow Labrador Retriever, Bonnie a year ago. At the time she weighed in at a massive 44 kilos! She could barely walk, let alone run, and sat down every few feet when out walking. She is now a much healthier 32 kilos and it is wonderful to watch her running and swimming along with her friends.
Bonnie has been fed for the last year on a dry veterinary diet food, which has obviously been successful, but the problem is that she poos for England! Four or five times a day she delivers a pile of solid waste often strung together with grass, of which she scoffs copious amounts. Our spending on poo bags is astronomical! Does anyone know of a diet that would make this easier?
Chris Pace, by email