I had some grapes in the fridge getting a little old, but still sweet; so thought I'd try them on the minis. Out of 15 equines, only 3 would even sample them, and only two liked them; the rest turned away upon first smell. Bonny sampled, and I thought she was interested, but she walked away. Both Dolly and Belle went nuts for a little bit of grapes; had a mix of white and red (the red were sweeter in this batch).
I've tried other fruits and veggies in the past, nothing really trips their trigger in the way of fruits and veggies; but, there are a few commercial treats that they flip for.