Unless it's been recently changed - the boost starts automatically when you get back from the fair - if you are in first you need to plan for it before you go.

Here is the basic check list and priority of what animals I put in my Prized Barn. I've grouped them into low - medium -high. Depend on your needs, they may differ, but this gives you a good starting point.

It's important to always put your best XP producing animal in the Prized pen. For example, when putting your rabbits in the pen, look for the Harlequins. They will generate your best XP, along with the desired outputs.

If you have the Level 100 barn and looking to maximize your blue ribbon count, dropping all non-blue ribboned prized animals in there would be ideal. Just remember to remove them all once your ribbon has been reached.

Managing the prized animal barn can take a bit of work, but it is well worth it. Feeding it every day is important XP, and you should never want for prized animal goods.

Low Priority (1-3)

Pigs -truffles
Goats - goat cheese
Ducks, geese - white feathers
Cows - swiss cheese
Deer (some)- antlers
Peacocks - peacock plume
Ostrich - ostrich plume
Pheasant - quill
Mini goat - cashmere fine wool

Medium Priority (3-6)

Chickens - brown eggs useful in many crafts
Donkeys - burlap often needed in quests
Buffalo, Deer - fine fur needed in quests
Rabbits - fine wool, used often
Sheep - fine fleece, used often

High (6+)

Horses - many saddles are often needed for OB
Otters - mussel shells, kiln crafting item which gives high XP
Turtles - seaweed is always for the OB crafts

Priority (fill in the rest of the barn)

Any animal which you have not blue-ribboned.
Any other animal with high XP payout (32+) i.e. polish sliver-laced chickens