I think first of all that the barn owner is going to be fielding a lot of questions about the increase. They probably already feel uncomfortable about it. My mom would say “It can’t hurt to ask” but she’d be wrong. It CAN hurt to ask.
The truth is they have the right to increase it as they see fit, or even to stop boarding altogether.
What other vendor and providers do you work with who are raising prices? Are you asking them to justify those to you? What is it about a boarding arrangement makes someone more likely to ask these question versus say why the cost of whole wheat flour went up more than the cost of bread flour by percentage at the supermarket?
If the situation is that you cannot pay it, that is a little different, and then you have to have a conversation. If the situation is that you will not pay it, and will move because of it, then also a conversation is in order.
But if you’re just feeling that it’s unfair because the percentage is different (especially if it’s a relatively small flat rate increase to all prices), I think it’s best just to chill. For whatever reason, humans like round dollar increments so if everyone’s board is going up say $50 that’s not super unusual.
FWIW I’m paying 20% more for hay for my animals at home and that’s not counting the cost of the gas to pick it up. Wages are also rising. You don’t want your barn owner to add up their hours and realize they could make more money renting out a room and stalls as an AirBnB and working at In-N-Out Burger instead of taking care of your horse.
A place that is comfortable and safe that takes good care of your horse is a jewel. Pasture board isn’t as much of a cost saver as you may be thinking, especially when the full cost picture is calculated including the cost of the land and how else it might be used, not to mention the extra labor to maintain that area.