Hi Kevin,
I'm not sure that would quite accomplish what you want, but I think it might be a good idea anyway.
Chickens will eat grass, but it is not a large part of their diet, which means in order to kill off the orchard grass, you would need to leave them in one concentrated spot for so long that it would harm the soil structure and soil biology. I'm not sure about rhubarb, but the
chickens would not eat the trees, but they would scratch around them a lot. This could cause some damage to the
roots on such a young tree. It might not kill it, but I would worry about stunting their growth a bit. You could possibly use electric poultry netting to
fence around the trees and keep the chickens safe from predators at the same time. The good part of the chickens would be the manure and the disturbing the soil slightly (move them each day or so, rather than leaving them until the grass is killed) and eating the grubs that would turn into things that would eat the trees or fruit. I believe fall is the best time to de-grub the area with chickens, but again, the killing off of the grass is not something I would suggest using chickens for.
If you just want to keep the grass under control (e.g. mowed), could I suggest geese? They do get the majority of their diet from grass, and would fertilize without scratching. Much less potential damage to the trees.
If you need to kill the grass, I can only suggest either manually cutting the sod off the top or smothering it with
cardboard and woodchips.
