How do I get kittens to stop weaning off of their mother? They're too old.?

The kittens in question are about 3 months old and are killing the mother cat. She's losing weight, although she eats, and she can,t gain it back, no matter what I feed her. The kittens eat solid food, and I've been giving the mommy tuna and beef to try and fatten her up. How can I stop this? I'm worried about Mommy cat. We can't always be here to pull the kittens away from her.
If you cannot separate mom from her babies, you could put a stocking on her body so that the kittens can be with her but not nurse. Cut the toes out of a sock that will fit her. This is what breeders do when there is a medical reason why a mom with milk shouldn't nurse (medications, incompatible blood-types, etc.)

As for putting more weight on mom, free-feed her dry kitten food, a brand as high in protein and fat as you can find. Eukaneuba is a brand that is widely available.

Good luck.
Not sure you can without separating them. If the mother wasn't okay about it then she wouldn't let them do it.
Make sure there's always water available for the kittens to drink (I'm sure you are anyway) and keep feeding the mother up. The milk will get less as they suckle less.
Maybe separating the kittens from the mother for part of the day would help.

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