Did their digestive system get rooted?

This isn’t even an uncle tier joke.

I dunno, it needs work, but it isn’t horrible. Not truly a dad joke in the most particular way of defining it, but the intent is there, and that counts more.

I think you might be better off just starting with "my neighbor tried to feed his donkey a 100% carrot diet.

It didn’t stick."

Keep it as linear as possible, keep the punchline as short as possible. Deliver it as deadpan as possible.

Might do it as “my neighbor tried to give his donkey nothing but carrots. It didn’t stick.”

Usually, unless you’re going for something more long form, where you’re telling a story that ends with the punchline making the entire thing a joke, keep any dad jokes punchy. Think about the way Steven Wright delivers. You hit with a punchy, one line hook setup, then deliver the KO in no more than five words.

It doesn’t have to be a donkey, btw. That imagery isn’t as recognizable as it used to be, the donkey with a carrot hanging in front of it. So, as long as you’re sure the audience knows the carrot/stick part, you can change things up.