Reasons why you need to give a butt bath your rabbit
It is absolutely not normal for a rabbit to have "poopy butt." Rabbits are prodigious self-groomers and if your rabbit has soft or dried fecal material on his butt, or is heavily urine stained, it is a clear sign for you to intervene AND get to the root cause. The material may not all be poop. It is often uneaten cecotropes that accumulate if not eaten promptly.
This accumulation happens for a few reasons:
The number one reason is being overweight. When bunnies are overweight, they cannot groom themselves properly. Rabbits need to be able to reach their rectum to ingest their cecotropes or "cecals". Compounding this is the fact that an overly rich diet can create an excessive volume of cecals, which the rabbit is too full-- or too fat-- to consume.
Cecals are soft and are to be ingested directly from the rectum the moment they are being eliminated. They are a critical part of their nutrition because they provide a source of vitamins. If a rabbit is unable to access his rectum while they are being produced then they get smeared around their rectum causing a large wet mass, which then is adhered to and dries on their fur. Due to their soft tongues, they are unable to remove it. This causing a severe skin irritation and a strong smell. If the bunnies are kept at their ideal weight, this situation does not occur if they are otherwise healthy.
Another reason is poor husbandry. If litter boxes are not properly setup with an absorbent substrate on the bottom and hay on the top, this allows the bunny to come in contact with his urine causing yellow feet and matted fur around their urogenital area.
As they get a little older, their usual pristine litter box habits may start to deteriorate. This can be due to arthritis. Arthritis can make it difficult to jump into the litter box and will cause them to urinate around their pen. Arthritis impacts their ability to eat their cecals because they are no longer able to stretch to reach their rectum.
Arthritis pain may cause them to alter how they position themselves to urinate, resulting in the failure of the urine stream to clear the body, and run down the legs instead.As arthritis progresses, your bunny may become less mobile and urinate and defecate where they sit. The accumulation of feces and urine causing skin inflammation and "scalding" which is then extremely painful. When this situation happens, a butt bath is absolutely imperative for the restoration of their normal skin health.
Far less common is trauma to the vagina or penis causing the failure of urine and fecal material to be properly eliminated. Similarly, if your bunny has an episode of severe soft fecal pellets, cecals, or diarrhea this will also necessitate a butt bath, and require medical attention. True diarrhea is (watery consistency) is rare and is a medical emergency.
The need for a butt bath SHOULD BE A RARE OCCURRENCE. Healthy rabbits do not need baths of any sort.