Slow feeders have been an essential tool for horse owners, for years now, looking to optimize their equine companions' health and well-being. And it has been essential tool for us also. That is the reason why Heica feeders exists.
Young horses are known of nibbling and fussing. They are active and have energy and the best way to ease that is to let them play in a big herd with different age of horses.
Besides this we recommend that you serve the hay in activation style ie. form the haynet. Maybe there are less fuzzing and nibbling with humans.