It's unhealthy for a rabbit (or any animal, really) to gain weight fast.
If your rabbit is underweight, it should gain weight slowly and gradually and get back to health that way. Don't sacrifice your rabbit's health and safety just to make the right weight. If you have plans for the rabbit that requires a good weight (like placing the rabbit in a show, or breeding the rabbit), you'll just have to put those plans off.
The first step to helping your rabbit gain weight is to find out why it's underweight. If it has a parasite or disease, that problem needs to be treated by a vet first or else nothing else that you do will work and the rabbit will only get worse. See the related question below for more info about rabbit vets.
Once you know the rabbit is healthy, aside from being underweight, the vet will probably suggest a diet slightly higher in fats and proteins than normal for adult rabbits - a diet more similar to that of baby rabbits. You might use a pellet specifically designed for babies, and you might feed the rabbit alfalfa hay instead of grass hay (like timothy).
To make sure you do it safely, a weight gain plan should be guided by your vet.
Feed her her favorite food and give her a lot of it
Hey there! I have a rabbit.. and keeping it on his weight... You should Feed them... Well Half of a bowl... And maybe some little bit's off carrots.. When the Bowl is empty... Feel it.. up slightly.
Feed him less
Whatever you normally feed your rabbit is probably fine; Rabbit Chow, or whatever.
When weighing a rabbit after it has been feed the biggest improvement one might see is on the scale. The weight of the rabbit after eating should be higher than it was before eating.
Yes, but you should try to feed your rabbit only enough feed to keep it healthy and at the right weight for it's breed. Bored rabbits will eat whether they ae hungry or not. they will get fat.
you can feed them alfalfa or timothy hay
twice a day.
feed it pie
If your rabbit hates vegetables, try leafy greens. If she won't eat leafy greens, buy commercial rabbit pellets to feed her.
You should feed the rabbit more hay and dried foods, and if this doesn't work visit the vets.
No, it's not a good idea to feed your rabbit watermelon. Rabbit.org has a list of things rabbits can eat and what you should feed your rabbit every day.