Hello and welcome to the forum. We have quite a few threads on the forum about this subject. You can find them by using the search box. I've had a fair bit of experience of controlling weight in my dogs and have found that prescription diets aren't necessary. A long time ago I was advised by a company's helpline staff member that low fat and higher protein was the way to go. Sounds easy but actually it isn't because high protein often equals high fat. The way that I did it is to feed a light dry food and topped it up with lean protein. The food was circa 10% fat, 26% protein and I gave less than the bottom figure RDA. Suitable vegetables (cooked and pureed) can also be added but best to choose non starchy ones.
I think it is essential to get to grips with quantity because for a dog to be overweight, the owner hasn't really understood how much they actually need. Weighing food accurately is the only way to go, not least because if the dog fails to lose weight you can easily cut down by 10%, in stages.
It's useful to weigh the dog regularly to monitor progress - maybe every one or two weeks. Aim for a slow, gradual weight loss. 20lb is a lot and it could take a few months. Don't push the dog to exercise too much as it could strain him My understanding is that diet is better for weight loss than exercise. Plenty of time for the latter when the dog has got down to a sensible weight.
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