Female kelpies should weigh between 30 and 46#. Let's go in the middle and say she should weigh about 38#. She looks like a bigger dog from the photos so if you aren't sure what her ideal weight should be just call your vet and ask. They would know better as they have actually examined her. The ideal weight is important as that is what you will use to determine how much food to give her. If I used 38# as her ideal weight she should eat about 588 calories per day which includes her food and all treats.
Honestly, the best weight loss food is going to be a prescription from your vet. These foods are specially formulated to have high fiber and lower calories so that your dog can continue to eat a more normal amount of food. Then they won't be hungry and begging all day. Once they reach their ideal weight on that food, then you can transition them to a maintenance diet.
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https://www.royalcanin.com/products/vet/weight-loss-support )
If you don't want to go that route, they can lose weight on any diet but she just may not be able to eat much. If I had to pick an over the counter diet I would use this one:
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https://www.chewy.com/hills-science-diet-adult-large-breed/dp/42749 )
If you used that diet at 297 kcal/cup she would eat 2 cups per day. That would leave no room for treats.
You can refer to this article for more information about weight loss:
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https://vcahospitals.com/know-your-pet/creating-a-weight-reduction-plan-for-dogs )
Hope this helps. If you have more questions feel free to post again. Best wishes.