Tornadoes or other extreme weather events carry a chance of damaging or destroying your barns and pastures, so you have to pay for repairs or purchase a new one entirely.

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If you buy too much hay at once, some of it will get old and moldy before you get to feed it to your horses.

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Nigel's grooming antics spread to your horses, so there is a chance horses don't let the groom do her job after you pay her.

My annoying, but entirely realistic idea:

more realistic feeding and weight/condition system. These are competitive horse remember, they're all at different levels and different weights, so have different ideal diets.

you have to tailor a feed feed plan for each.individual.horse.

oh and buy a million types of feed because one of your mares won't eat mint, and your too stallion is allergic to that pricey new mash you just bought