Made some minor adjustments to your shot and reposted. If I were the rider, and or owner of this horse, I wouldn't care too much about "blown highlights". Between the apparently black jacket, and white pants, I think you are exceeding the range of the camera sensor. But I wouldn't get too hung up on the technicalities of the shot, if you come up with a visually and reasonably pleasing photograph of your subject.

Visit my Nikonians gallery.

Attachment #1, (jpg file)