Oh, they need to loose 5 lbs? Ok, here's more details because that's pretty serious:
The other thing you can do is compare k/cal of the two foods.
I did some research for you:
Purina OM dry has 276 k/cal (calories) per cup
Wellness Super 5 mix healthy weight has 325 k/cal per cup
That's a HUGE difference when you're talking about a Yorkie.
Vet's recommend rx diets like OM in cases that need major weight loss as fast as possible and/or when an owner is having a hard time regulating the proper feeding amount (it came with it's own little measuring cup I'm assuming?) Your vet also should have given you an 'OM chart' which will show you how much food/calories your dogs should be having a day and how much they should weigh. If he didn't, talk to him about how many k/cals your dogs are supposed to have per day, then you can do the math yourself or ask your vet to help you.
The #1 reason dogs fail to loose extra weight is owner compliance, so something like OM was created to not only provide a food really low in calories and fat, but also to help owners because we tend to think an adequate amout of food for our dogs is not nearly enough, especially with the high quality foods - they're really dense in calories so it can be deceiving. The other thing about OM is that it's designed to "expand" in the stomach to help the dog feel full longer.
The vet I worked for was a major stickler on weight and nutrition, and in cases where simply cutting back food amounts wasn't helping, he'd put them on OM temporarily to help the owners out. That being said, he suggested I use it for my Beagle a few years ago when she needed to loose about 5 lbs. She freaked out! She started ripping through the garbage which she had NEVER done before. I stuck with it for a week then talked to him and we came up with a plan to use low calorie Innova and some veggies (cooked carrots and green beans mainly) instead. It was less food amount-wise than the OM, but she seemed more content and still lost the weight.
Soooo, if you REALLY don't want to use the OM, you have to do some math and be really specific in measuring their food. No treats (trust me, they'll survive).
Hope this helps, we did alot of weight loss programs for dogs where I worked as a tech so I've learned ALOT.
Good luck and just stick to your guns, don't feel guilty, it's for their heath and you know what's best for them more than they do
