Tredstep polos are awesome. You can feel a breeze through the whole shirt.
SugarCubes - Yes, I think they are great to ride in. I am very happy with them! I ride in the v-neck. I have used the polos for judging, and they definitely are cooler.
I will buy some more for myself too. 
If you are looking for more of a sun shirt option, the Ariat sunstoppers are my absolute favorite. They are light but so comfortable. I have the team one and wear it to the barn and school and like anywhere I can. It is delightfully comfortable while still being gorgeous.
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